Chapter Two
Structural and Functional Anatomy
Leonardo da Vinci once said that in addition to being a work of art, the human body is also a marvel of engineering. Da Vinci’s statement is particularly true when it comes to the anatomical structures necessary to allow for bipedality, since walking on two legs presents an engineering conundrum: during early stance phase the lower extremity must be supple in order to absorb shock and accommodate discrepancies in terrain, while the latter portion of stance phase requires that these same structures become rigid so they can tolerate the accelerational forces associated with the propulsive period. This is in contrast to quadrupeds, who have the luxury of being able to absorb shock with their forelimbs while their hindlimbs serve to support and accelerate (picture a cat jumping on and off a ledge). The human body is able to accomplish these contradictory functions through a series of intricate articular interactions that allow the same anatomical
structures to behave differently during the early and latter phases of gait. For example, the bones of the foot and ankle play a vital role during early stance as lowering of the medial arch creates a parallelism of the midtarsal joint axes that effectively unlocks the articulations of the midfoot allowing the bony structures to shift in order to accommodate surface irregularities (Fig. 2.1). Lowering of the arch is also indirectly responsible for shock absorption at the knee because it causes the talus to slide medially down the calcaneus (arrow A in Fig. 2.1), causing the ankle to twist inwardly as it follows the talus. The resultant internal rotation of the ankle forces the tibia to rotate beneath the femur, allowing the knee to flex (i.e., the knee is not a pure hinge joint since the tibia must internally rotate in order for the knee to flex properly). Knee flexion, in turn, allows eccentric contraction of the quadriceps muscle to dampen large amounts of vertical forces. To assist the knee in absorbing shock, the gluteus medius muscle is lowering the
Figure 2.1. Osseous anatomy of the foot and ankle with an anterior view of the right talus and calcaneus pictured in the lower corners. Notice how pronation of the midtarsal joint (lowering of the medial arch) creates a parallelism of their shared axes while supination (arch elevation) creates a malalignment of these same axes. The improved axis alignment associated with lowering of the arch increases the range of motion available to the joint by more than 10°. Conversely, supination effectively locks the joint, as the axes no longer converge. This action is comparable to shifting the alignment of hinges on a door: when properly aligned, the parallelism of their shared axes allows the door to open without resistance while even a slight alteration in the position of one of the hinges will make it difficult to open the door.
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Skipping forward
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Chapter Two Structural and Functional Anatomy is flexed in the acetabulum) was 3 times body weight. This means that the hip is designed in a fashion similar to the patellofemoral joint, in which surface area increases as joint forces increase (refer back to Fig. 2.55). In the majority of daily activities, the greatest forces are placed on the hip when the hip is flexed, so the paradoxical alignment where both the femur and the acetabulum project forward is not a defect, but rather, a necessity since it distributes pressure over a larger area as forces increase with hip flexion. After 7 million years of natural selection favoring metabolic efficiency, there are few mistakes. The Pelvis The pelvis forms a stabilizing ring in which the innominate bones connect posteriorly to attach to the sacrum (forming the two sacroiliac joints) and anteriorly to the fibrocartilaginous pubic symphysis (forming a classic amphiarthrosis joint). To effectively deal with the forces of upright posture, the pelvis has successfully remodeled itself both in size and shape. The most obvious change in the appearance of the innominates is the surface area of the ilia has appreciably increased in order to accommodate the larger and more functional gluteus medius and iliacus muscles (Fig 2.76). In contrast, the length of the ischium has substantially shortened secondary to the decreased dependency on the hamstrings during bipedal gait. In addition to changes in the shape of the innominates, significant changes have also occurred in the sacrum. While chimpanzees and apes possess pelvic rings with extremely stiff sacroiliac joints and fused pubic bones (i.e., they lack a pubic symphysis), evolutionary remodeling has converted the sacrum from the long, thin, rectangular shape present in chimpanzees to the more triangular, keystone appearance currently seen in Homo sapiens (Fig. 2.77). In a thorough review of the evolution
Figure 2.77. Compare the keystone-shaped human sacrum with the rectangular chimpanzee sacrum.
of the lumbar spine and sacrum, Abitbol (78) notes that old-world monkeys possessed 7 lumbar vertebrae and the transition to bipedality produced sacralization of the lower two vertebrae allowing for the formation of our modern-day 5 fused sacral segments. Besides lengthening the sacrum (thereby increasing surface area of the sacroiliac joints), the sacralized lumbar vertebrae also made the upper sacrum significantly wider and allowed for the development of the sacral alae or wings. According to Abitbol (78), widening of the upper sacrum associated with sacralization was an essential prerequisite for bipedality because it allowed the sacrum to function as a firm base of support for the trunk during upright posture. The forces associated with the transition into upright posture also produced a significant remodeling of the sacroiliac articular surfaces. Compared with the chimpanzee, Homo sapien sacroiliac joints have transitioned from the long, narrow “archipelago-like� shapes seen in primates to the significantly larger, more compact inverted L-shaped structure present today (79) (Fig. 2.78). The more vertical upper arm of the L-shaped
Figure 2.76. The lateral view of the chimpanzee pelvis and a modern human pelvis. Notice the ilia of the chimpanzee pelvis are narrow and positioned in the frontal plane, while the ilia of the human pelvis flare forward and are significantly wider, providing greater surface area for the origins of the gluteus medius, minimus, and the iliacus muscles.
Figure 2.78. The articular surface area of the sacroiliac joint (SI) in chimpanzees is greatly reduced compared with the human sacroiliac joint (shaded areas).
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Human Locomotion: The Conservative Management of Gait-Related Disorders articulation is contained in the first sacral segment while the nearly horizontal arm lies within the second and third sacral segments (80, 81). Besides changing the physical shape of the sacroiliac joint, upright posture also resulted in a considerable increase in the sacroiliac joint’s surface area. As noted by Lovejoy and Latimer (79), after adjusting for size differences between the chimpanzee and human, a modern sacroiliac joint possesses approximately 20% more articular surface area than a chimpanzee’s. This increase in area allows for improved dissipation of forces and enhanced storage of energy in the ligamentous restraining mechanisms because a larger area means a greater circumference, which in turn means greater ligamentous support and hence improved dissipation of energy. To calculate the degree to which soft tissue restraining mechanisms can control a joint, Lovejoy and Latimer (79) refer to an engineering index that compares the ratio between the perimeter of a joint with the square root of its area and determined that the human sacroiliac joint is significantly less rigid than a chimpanzee’s sacroiliac joint. The increased flexibility resulting from the enlarged sacroiliac perimeter allows the more abundant soft tissue restraining mechanisms to better dampen kinetic energy associated with sacral motion. Interestingly, even when adjusted for size differences, Lucy had almost the identical surface area and perimeter ratio as modern humans (79), suggesting the changes in sacroiliac joint shape and size occurred quickly in response to occasional bipedality. Classification of the sacroiliac joint as a synostosis, syndesmosis or diarthrosis is difficult, since the joint’s
classification changes with age and varies throughout different areas of the joint; e.g., the L-shaped articular component is a diarthrodial synovial joint while the larger superoposterior portion of the sacroiliac joint is a fibrous syndesmosis. This latter section, known as the interosseous region of the sacroiliac joint, is especially important in providing stability because it houses the powerful interosseous ligament. Consisting of deep and superficial sections, the interosseous ligament fills the gap formed between the posterior sacrum and ilium and is comprised of short, dense, extremely strong multidirectional fibers (Fig. 2.79). In their detailed study of the topography of the interosseous region, Rosatelli et al. (82) note that 100% of cadaveric subjects greater than 55 years old possess moderate to extensive ridging of the interosseous region, and 60% of these subjects possess a significant bony ridge in the central portion of the interosseous region. The authors state that this bony ridge converts the one time syndesmotic interosseous component into a “partial synostosis,” because this bony ridge essentially fuses the sacroiliac joint. The capacity of the interosseous region to stabilize the sacroiliac joint was demonstrated in a cadaveric study by Miller et al. (83), in which the pubic symphysis was removed and all of the supporting muscles and ligaments were released prior to attempting to move the sacrum on the ilium (the interosseous ligaments were left intact). Even with all the supporting ligaments cut and the pelvic ring no longer intact, there was little change in sacroiliac motion. This research makes it clear that the interosseous region of the sacroiliac joint plays a vital role in stabilizing the pelvic ring during upright postures.
Figure 2.79. The interosseous region of the sacroiliac joint (shaded area in A) houses the extremely powerful interosseous ligaments (see transverse section B). Redrawn from Bernard T, Cassidy J. The Sacroiliac Joint Syndrome: Pathophysiology, Diagnosis and Management. In: Frymoyer J (ed). The Adult Spine: Principles and Practice. New York: Raven Press Ltd 1991.
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Chapter Two Structural and Functional Anatomy Unlike the frequently ignored interosseous region, the articular synovial component of the sacroiliac joint has been extensively studied. Before puberty, the sacral side of the sacroiliac joint contains smooth, creamy colored hyaline cartilage that articulates with a thin, roughened portion of cartilage on the iliac side. Frequently identified as fibrocartilage, the iliac portion of the joint is in fact covered with hyaline cartilage (84). In an MRI study of 114 healthy children, Bollow et al. (85) determined the average width of the sacroiliac joint is 4 mm, with 3 mm of cartilage on the sacral side and 1 mm of cartilage on the iliac side. Shortly after puberty, irregularities and fissures begin to develop (particularly on the iliac side) that increase the coefficient of friction and stiffen the joint. According to Bernard and Cassidy (86), during the second decade of life a “crescent-shaped ridge develops on the entire length of the iliac surface with a corresponding depression on the sacral side.” The authors claim that this ridge limits motion to x-axis rotation so the sacroiliac joint moves primarily in the sagittal plane. The presence of this ridge has been described by others (73, 87) but is at odds with a study by Solonen (88), who performed a detailed analysis of 30 sacroiliac joints and found no evidence of this ridge. In a more recent study, Dijkstra et al. (89) failed to find a consistent ridge but noticed the sacral surface tends to be concave while the iliac surface tends to be convex. Regardless of the presence of the ridge, the surface irregularities vary in height between 2 and 11 mm and are always reciprocal in form. With advancing age, these irregularities become exaggerated, and there is evidence of osteoarthritis on the iliac portion of the joint by the third decade of life and on the sacral side of the joint by the fourth decade (86). Vleeming et al. (90) suggests that the significant ridging present between a sacrum and ilium increases the coefficient of friction, which helps to stabilize the joint. In an autopsy study of cartilage and subchondral bone thickness in 15 adult sacroiliac joints (average age 52 years old), McLauchlan (84) measured 1.8 mm of cartilage on the sacral side and 0.8 mm of cartilage on the iliac side, with no difference between men and women. Although the sacral cartilage was twice as thick, it possessed lower cell density and its supporting subchondral bone was significantly softer and was comprised of porous, cancellous bone. Conversely, the hyaline cartilage on the iliac side possessed more chondrocytes and fewer proteoglycans, but its subchondral bone was nearly twice as dense as the sacrum’s subchondral bone. This raises the interesting question as to why there would be such a difference in bone density and cartilage thickness between the two sides of the sacroiliac joint since they are exposed to the same forces. It seems possible that the soft cancellous bone of the sacrum works like a “cushion” to dampen vertical loads traveling between the torso and pelvis, while the rigid subchondral bone of the ilium plays no role in shock absorption and
functions to transfer vertical forces between the torso and lower extremities. The softness of the sacrum’s subchondral bone is evidenced by the frequency of sacral versus iliac stress fractures in long distance runners. It also explains the delayed development of osteoarthritis in the sacral versus iliac cartilage, since studies have shown that osteopenia and osteoporosis actually protect against the development of osteoarthritis because softer bones absorb vertical forces more effectively (91). The improved shock absorption associated with the softer bones apparently delays the progression of osteoarthritis. In addition to the strong support provided by the interosseous ligament, motion in the sacroiliac joint is also controlled by a series of capsular and extracapsular soft tissues. Anteriorly, the sacroiliac joint is covered by an extension of the capsule known as the anterior sacroiliac ligament. Because it is so thin, this ligament is relatively weak and provides little resistance to motion. Posteriorly, the joint is protected by the superficial component of the interosseous ligament (which attaches to the sacral crest at S1 and S2) and by the long dorsal sacroiliac ligament. This important ligament originates from the lateral sacral crest at S3 and S4 and traverses superolaterally to attach to the inner portion of the iliac crest and the PSIS. The ligament is reinforced directly below the PSIS with fascia from the gluteus maximus muscle, which makes the ligament so “solid and stout that one could easily think a bony structure is being palpated” (92). Besides receiving fibers from the gluteus maximus muscle, the long dorsal sacroiliac ligament is also reinforced with fibers from the thoracolumbar fascia, the erector spinae and the multifidus muscles. With these powerful stabilizing fascial attachments, the long dorsal sacroiliac ligament plays a key role in stabilizing the sacrum by limiting the range of counternutation, essentially locking the joint to protect against additional motion (Fig. 2.80).
Figure 2.80. The long dorsal sacroiliac ligament (A) limits counternutation of the sacrum (arrow).
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Human Locomotion: The Conservative Management of Gait-Related Disorders Immediately inferior to the long dorsal sacroiliac ligament are the sacrospinous and sacrotuberous ligaments. The sacrospinous ligament connects the lower sacrum to the ischial spine and is intricately connected to the coccygeus muscle (93). Although the biomechanical function of this ligament is rarely described, it may play a role in limiting frontal plane motion of the sacrum since its lower fibers are well-situated to limit Z-axis rotation. Unlike the rarely discussed sacrospinous ligament, the sacrotuberous ligament has been extensively studied. This large, complicated ligament is comprised of separate lateral, medial and superior fibrous bands. According to Willard (94), the lateral band is reinforced with fibrous attachments from the piriformis muscle and runs nearly straight up and down connecting the lateral aspect of the ischial tuberosity with the posterior superior iliac spine (PSIS). The medial band is made of fibers that spiral so their medial components run superiorly to attach the sacrum, while their lateral components twist to attach on the lower sacrum and coccyx (Fig. 2.81). The twisting of the medial band of the sacrotuberous ligament allows for the springlike storage of energy when exposed to large tensile forces. The superior band of the sacrotuberous ligament receives strong fibrous attachments from the gluteus maximus muscle and traverses the entire length of the sacrum, essentially connecting the coccyx to the PSIS. The body of the sacrotuberous ligament is reinforced by tendons from the multifidus muscles, which run between the superior band of the sacrotuberous ligament and the long dorsal sacroiliac ligament. As noted by Vleeming (92,95), the sacrotuberous ligament also receives fibrous reinforcement from the bicep femoris muscle. Detailed cadaveric dissections reveal the bicep femoris attaches either directly to the sacrotuberous ligament, or indirectly via a fibrous bridge along the ischium connecting the bicep femoris to the sacrotuberous ligament (92). Whether attached directly
or indirectly, tension in the bicep femoris muscle increases strain in the sacrotuberous ligament (92). Because of its significant size, spiraling fibers, and numerous muscular and fascial attachments (e.g., piriformis to the lateral band, gluteus maximus to the superior band and bicep femoris to the entire complex), the sacrotuberous ligament plays a key role in preventing excessive nutation of the sacrum (arrow in Fig. 2.81). The powerful sacroiliac restraining ligaments and muscles, when coupled with the keystone-shaped sacrum provided by sacralization of the previously lumbar vertebrae, allow for what Snijders et al. (96) refer to as the “form and force closure system,” in which the shape of the sacrum provides form closer while the ligamentous/ muscular restraints create the force closure (Fig. 2.82). When working together, the form and force closure system creates a self-locking mechanism that provides significant stability against vertical loads while lessening muscular effort. The efficacy of the sacrum’s self-locking mechanism was demonstrated by Gunterberg et al. (97), when they vertically loaded a cadaveric spine and noted the sacrum could resist a downward shear force of 4,800 N without being damaged. Obviously, the ability to tolerate such large axial forces played an important role in the development of bipedality. Although the anatomical factors controlling motion are well understood, the actual degree of motion present and the specific location of the sacroiliac axis of motion has been the subject of considerable debate. Using technologies available at the time, early researchers suggested the sacroiliac joint possessed a minimum of 6° to 10° of rotation (98,99), 5 mm of translation (100) and moved about a fixed axis that varied in location from the pubic symphysis to a point 10 cm below the sacral promontory (101). In a simple yet clever attempt to confirm the actual location of the sacroiliac joint axis of motion, Wilder et
Figure 2.81. The sacrotuberous ligament is comprised of separate medial (M), superior (S) and lateral (L) bands. The piriformis muscle has a fibrous slip connecting to the lateral band while fibers from the bicep femoris muscle blend with the insertion of the sacrotuberous ligament. Before attaching, the different fibers of the sacrotuberous ligament twist (circle), allowing this ligament to store energy like a spring to limit nutation of the sacrum (arrow).
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Chapter Two Structural and Functional Anatomy
Figure 2.82. Form closure (left) and force closure (right) of the sacroiliac joint. Redrawn from Vleeming et al. (102).
al. (101) used a carpenter’s contour gauge to reconstruct the functional topography of the articular component of the iliac bone and determined that because the articular surfaces possess significant elevations and ridges, rotation could not occur about a single axis without considerable separation of the joint. They determined that the sacroiliac joint must separate 7.25 mm in order to allow for frontal plane motion, and 3.4 mm to allow for sagittal plane motion. Such large separations completely ruled out the possibility of frontal plane motion and suggested that sagittal plane motion was possible but was most likely limited. Using data obtained from the topographical analysis, Wilder et al. (101) presented a “best fit axis” for sagittal and frontal plane motion and noted that because motion was limited by joint irregularities and ligamentous tension, the sacroiliac joint functions as a “shock absorbing structure,” with energy absorbed by the ligaments as the sacrum shifts over the corresponding bony irregularities in the ilium. Because the sacroiliac joint moves with significant translations over bony ridges, Vleeming et al. (102) compared it to the friction devices used for painting ceilings (Fig. 2.83). Despite the presence of a single fixed axis of motion in this model (i.e., the screw), this simple analogy is consistent with the motions described by Wilder et al. (101) and provides a good example of how motion may be limited by irregularities in a joint’s surface. Debate over the degree of motion possible in the sacroiliac joint continued until the development of roentgen stereophotogrametric analysis (RSA). As mentioned previously, this technique involves surgically implanting 0.8 mm tantalum beads into the bony structures on each side of a joint and incorporating specially designed X-rays to precisely determine 3-dimensional motions between the beads. Because error rates associated with RSA are so small (less than 0.15° for rotation and 0.1 mm for translation) this technique has become the gold standard for evaluating joints with small ranges of motion (103). In the first study using RSA to evaluate sacroiliac motion, Egund et al. (104)
Figure 2.83. Friction device analogy used to represent function of the sacroiliac joint. Redrawn from Vleeming et al. (102).
evaluated 4 patients as they moved through a series of 7 different body positions and determined that the sacroiliac joint moved a maximum of 2°, and that motion occurred about a transverse axis passing through the iliac bones near the iliac tuberosity. The location of this axis was consistent with prior research (105, 106) and was even consistent with Vleeming’s friction device analogy (Fig. 2.83). Interestingly, the axis location described by Egund et al. (104) correlates to the exact point in the interosseus region described by Rosatelli et al. (82) where an osteoarthritic bony ridge develops relatively early in life. Since the point in which an axis passes through a joint is exposed to the greatest shear force (e.g., the medial tibiofemoral joint), it is possible that the accelerated rate of degeneration present in the interosseous region could be explained by the passage of the axis. In subsequent studies of in vivo sacroiliac joint motion incorporating RSA, Sturesson et al. (103) evaluated 21 females and 4 males (ages 18 to 45) that had been previously diagnosed with either unilateral or bilateral sacroiliac instability and had them perform specific tasks, such as sit to stand, supine to stand, and standing to prone with one leg hyperextended. Using RSA, the authors determined that with sit to stand the ilia rotate backwards in relation to the sacrum an average of 1.1° (which is consistent with Snijders et al. [96] self-locking mechanism) and when moving from a standing position to lying prone with one leg hyperextended (intended to mimic a “big step” while walking), the sacroiliac joint moved through 59
Human Locomotion: The Conservative Management of Gait-Related Disorders an average of 2.2° of motion. The authors note that even though sacroiliac motion occurred in all 3 planes, 90% of the motion occurred in the sagittal plane about a transverse axis. The authors also emphasized that even though the reported ranges were low, they may have overestimated actual ranges because the patient population they studied possessed potentially hypermobile sacroiliac joints. In a follow-up study designed to critically evaluate Snijder’s form and force closure theory, Sturesson et al. (107) again evaluated patients suspected of having hypermobile sacroiliac joints, only this time they were asked to stand on one leg while the other leg was maximally flexed. Subsequent RSA analysis revealed the addition of extreme hip flexion during unilateral stance produced insignificant changes in sacroiliac motion because the ilia and sacrum moved as a fixed unit. Sturesson et al. (107) claim that this confirms the presence of a form and force closure system, in which the sacroiliac joint self-locks while standing upright. While RSA has repeatedly determined that sacroiliac joint motion is small and occurs primarily in the sagittal plane, Smidt et al. (108-110) performed a series of cadaveric and in vivo studies in which they found sacroiliac ranges of motion that were almost 10 times greater than those reported by Sturesson et al. (103,107). In one study, they evaluated gymnasts in a half-kneeling reciprocal straddle position (i.e., kneeling with one hip maximally flexed with the opposite hip extended) and found an average of 36° of motion between the innominate bones (110). In another in vivo study of young healthy volunteers, Smidt et al. (108) report a mean range of 9° of sacroiliac motion when subjects were placed in the reciprocal straddle position. In yet another study, Smidt et al. (109) implanted radioopaque markers in 5 left and 5 right elderly cadaveric sacroiliac joints and positioned them on their sides as CT scans were performed while the cadaver’s hips were moved into extremes of flexion and extension. Despite the advanced age of the cadavers, the authors found up to 17° of sagittal plane motion and 8 mm of translation of the PSIS relative to the sacrum. Smidt et al. (109) emphasized that their measurements were accurate because they used extreme ranges of hip motion. To test the theory by Smidt et al. (109) that extreme hip positions are necessary to produce the full range of possible sacroiliac motion, Sturesson et al. (111) surgically implanted tantalum balls in 6 women with potentially hypermobile sacroiliac joints (disabling posterior pelvic pain of long-duration following pregnancy) and incorporated RSA to evaluate in vivo sacroiliac motion while the patients stood in a reciprocal straddle position (the left hip was maximally flexed while the right hip was maximally extended). As with their prior studies, Sturesson et al. (111) again found only 2° of sacroiliac motion with almost all of it occurring in the sagittal plane. Although unable to determine why Smidt et al. (108-110) found such
large ranges of motion, the authors emphasize that in order to accommodate the 17° of sacroiliac rotation described by Smidt et al. (109), the pubic symphysis would have to move through a range of 8 cm, which could not occur without dislocation of the symphysis. They further reference the axis of motion described by Egund et al. (104) and claim that the 8 mm of translation described by Smidt et al. (109) would produce subluxation of the sacroiliac joint. Additional support of the ranges reported by Sturesson et al. (111) was supplied by Kissling et al. (112), who embedded stainless steel rods in the ilia and sacrum of healthy volunteers and, using stereophotogrametric methods, determined that the sacroiliac joints moved through a 3° range of motion when the lumbar spine moved from full flexion to full extension. After reviewing state-of-the-art in vivo motion analysis, it becomes clear that the sacroiliac joint moves through extremely small ranges of motion that occur primarily in the sagittal plane. Before fusing later in life, the sacroiliac joint plays a minor role in shock absorption (minor compared to the knee) and a more important role in self-locking to stabilize the pelvic ring as vertical forces peak. A single axis of motion is not possible and the joint moves with a combination of gliding, tilting, nodding, rotation and translation, which is determined by its unique blend of ridges and grooves. The Spine The typical modern human spine contains 7 cervical, 12 thoracic and 5 lumbar vertebrae. Variation in the number of lumbar vertebrae is not uncommon: 3% of the population have 4 lumbar vertebrae and 5% have 6 (78). During upright posture, the spine serves to support and balance the skull and assists in the transfer of forces between the scapulae and pelvis. The spine also plays an important role in the dampening of ground-reactive forces following heel strike. Functionally, each vertebra is divided into 3 distinct components: the vertebral body, the pedicles, and the posterior elements (Fig.2.84). Excluding the first two cervical vertebrae, the remaining vertebral bodies are separated by intervertebral discs consisting of a central nucleus pulposus (a thick, viscous gel made of water and mucopolysaccharides) surrounded by a series of concentric layers of fibrous tissue known as the annulus fibrosis. To enhance support, each layer is connected with collagen fibers that reverse their angle of attachment at each alternating layer thereby providing added protection against twisting motions (Fig. 2.85). The annular layers become more distinct when moving towards the periphery and are attached at their outer margins to the vertebral bodies via small ligaments known as Sharpy’s fibers. The posterior elements consist of transverse processes, laminae and spinous processes (all of which serve as attachment points for ligaments and muscles), along with the superior and inferior facets, which are 60
Chapter Two Structural and Functional Anatomy
Figure 2.85. Anterior view of the intervertebral disc demonstrating how collagen fibers in the annulus fibrosis alternate directions providing significant protection against rotational forces.
Figure 2.84. Components of a lumbar vertebra. Lateral (A) and superior views (B).
connected by the pars interarticularis. The primary movements available in the cervical, thoracic and lumbar regions differ as the relationships between the facets and the cardinal planes vary (Fig. 2.86). Asymmetry between the right and left facet joints is more prevalent in the lumbar spine, making that region more prone to aberrant segmental motion. While the last 7 million years of evolution have produced modest changes in the shape and function of the cervical and thoracic spine, the lumbar spine was forced to remodel rapidly in order to tolerate the forces of upright posture. The most obvious change is the gradual increase in width of each successive vertebra when moving proximal to distal (Fig. 2.87). This distal increase in width is not present in apes and the added surface area allows for an improved distribution of pressure between the lower vertebral bodies. First noted in Homo erectus, the increased distal vertebral width possibly developed in an attempt to better dissipate the increased ground-reactive forces associated with endurance running (113). As stated in chapter 1, the single most important anatomical change necessary to accommodate the stresses of bipedality was the development of the lumbar lordosis. While our chimpanzee relatives are unable to extend their
Figure 2.86. The lumbar facets (A) rest in the sagittal plane while the thoracic facets (B) rest in the frontal plane. The cervical facets (C) are angled 45째 to the transverse plane allowing for fairly large ranges of motion in all directions.
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Human Locomotion: The Conservative Management of Gait-Related Disorders
Figure 2.87. The lumbar vertebrae become gradually wider when moving from L1 to L5.
lumbar spines beyond straight (the shape of their posterior elements and the approximation of their iliac crests and ribs create physical barriers that limit motion), the modern human lumbar spine extends well beyond neutral, allowing the upper body to effectively balance over the pelvis. Because chimpanzees possess such rigid lumbar spines, which provide invaluable protection against excessive spinal motion while they twist through trees, they can only stand upright by excessively flexing their hips and knees (refer back to Fig. 2.65). Lumbar lordosis is essential for bipedality because it allows the more proximal segments of the body to be stacked almost directly on top of the more distal segments, thereby lessening the large extensor moments that would otherwise be necessary to maintain upright posture (Fig. 2.88). The primary anatomical factor responsible for the development of the lumbar lordosis is the progressive increase in interfacet distances when moving from the first to the fifth lumbar vertebra (Fig. 2.89). This pyramid configuration is not present in chimpanzees and the distance between chimpanzee facets actually decreases when moving down the lumbar spine, creating a physical block that limits extension. According to Lovejoy and Latimer (79), the progressive widening of the distal interfacet distances in humans allows the facets of the lower lumbar vertebrae to “imbricate, thereby avoiding direct contact with intervening structures such as the laminae or the pars interarticularis.� The increased distal interfacet distances explains why the lower lumbar vertebrae possess gradually increasing ranges of extension (e.g., L1-2 has a combined
Figure 2.88. Gravitational line of the body (passage points through body indicated on right). Notice how lordosis of the lumbar spine (arrow) positions the upper body over the pelvis, balancing the proximal segments on top of distal segments. Over the past 7 million years, natural selection has favored individuals with narrow anteroposterior diameters because they have shorter lever arms for the back extensors to balance the center of mass during upright activities. This contrasts with Neanderthals, whose larger A/P diameters (helpful for maintaining heat in cold climates) made bipedality metabolically more expensive because the back extensor muscles had to work harder to fight the anteriorly displaced center of mass.
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Chapter Two Structural and Functional Anatomy the lowest 3 lumbar vertebrae. The authors convincingly argue that the 3:2 female/male wedging pattern (which is not present in chimpanzees) provides an evolutionary advantage in that it allows pregnant females to increase their lumbar lordosis with less intervertebral motion. While the ability to efficiently increase lordosis during pregnancy may not seem like a driving force for natural selection, remember the early hominid females spent most of their adult life either pregnant or lactating (116). As a result, the metabolic advantage associated with exaggerated lordosis coupled with the potential for reduced pain secondary to decreased spinal shearing would provide a powerful evolutionary advantage favoring the dorsal wedging of L3. Because pregnancy increases mass of the abdomen by more than 30% (117), it displaces the center of mass anteriorly, which significantly destabilizes the entire body as it increases spinal shear and torque in the low back and hip. Dorsal wedging of the third lumbar vertebra appreciably lessens spinal torque as it extends the female’s upper body posteriorly, shortening the enlarged abdomen’s lever arm thereby reducing strain on the posterior musculature (Fig. 2.90). Because the pregnant female’s facets are significantly compressed even with the dorsal wedging of L3, evolutionary remodeling of the posterior elements also provided women with a 14% increase in facet surface area (which more effectively distributes pressure) and the facets themselves are oriented 13% closer to the frontal plane (115). These combined factors lessen twisting of the pars interarticularis and reduces the risk of spinal injuries associated with prolonged extension of the lumbar spine. As stated by Whitcome et al. (115), the decreased shear and increased efficiency associated with the dorsal wedging of
Figure 2.89. Posterior view of the lumbar spine demonstrating the gradual increase in interfacet distance upon moving from L1 to L5.
range of flexion/extension of 12°, while L5-S1 has a combined range of 17° [114]) and why the lower lumbar vertebrae play a more important role in the formation of lordosis. The pyramidal facet configuration was first noted over 3.1 million years ago in Australopithecus afarensis confirming that lordosis of the lumbar spine was “an ancient, primary adaptation to bipedality” (79). In a beautifully written paper reviewing the evolution of lordosis, Whitcome et al. (115) describe a sexual dimorphism of the lumbar spine in which males were shown to possess a dorsal wedging of the lowest two lumbar vertebrae (which is responsible for the shape of the entire lordosis), while females possess a dorsal wedging of
Figure 2.90. Normally, the body’s center of mass falls slightly posterior to the center of the femoral head and the upper body is balanced over the pelvis (A). Because pregnancy displaces the center of mass anteriorly (B), the back extensors must counter the force generated by the anteriorly displaced center of mass by tensing vigorously (large arrow in B). This increases the metabolic cost of locomotion and creates a significant posterior shear on each lumbar vertebra (small arrows in B). Wedging of the third lumbar vertebra in females (D), allows for an increased range of lumbar lordosis, which displaces the upper body posteriorly (arrow in C), effectively displacing the center of mass into a more balanced position (C). Modified and redrawn from Whitcome et al. (115).
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Human Locomotion: The Conservative Management of Gait-Related Disorders L3 and modified posterior elements would have played a major role in natural selection since low back pain and fatigue would have “limited foraging efficiency and the ability to escape predators,” leaving the pregnant female more prone to malnutrition and/or death. They note that the sexual dimorphisms in L3 and the posterior elements were present in the earliest Australopithecus africanus, confirming that natural selection strongly favored its development (115). In order to describe movement, modern human lumbar vertebrae are grouped into 5 functional spinal units (FSUs) made up of neighboring vertebra and their associated intervertebral disc: the first unit is between L1 and L2 while the fifth unit is between L5 and S1. Obviously the number of units changes in individuals possessing 4 or 6 lumbar vertebrae. While most authorities claim the primary function of the intervertebral disc is to act as a shock absorber, Farfan (118) suggests that the discs play only a minor role in shock absorption and that it is the vertebral bodies themselves that function as the spine’s shock absorbers. He claims their powerful sides, which are reinforced with thick cortical bone, strongly resist deformation while the thin superior/inferior end plates (comprised of less than 0.6 mm of cortical bone) bulge in and out allowing the cancellous bone within the vertebral bodies to dampen vertical force (Fig. 2.91). Vertebral body cancellous bone is well-designed to dampen forces as it possesses thick vertical columns of trabeculae bone that are reinforced on each side with smaller, more flexible horizontal trabeculae (119). When compressed, the vertical columns begin to bend but support provided by the horizontal fibers serves to lessen the degree of bending thereby allowing the vertical columns to spring back to their original position. When sufficiently stressed by the excessive buckling of the vertical columns, the horizontal trabeculae fracture, but their extremely small diameters allow them to be rapidly
repaired with minimal osteoblastic activity. This allows the vertebral bodies to act as a self-repairing shock absorbing system in which buckling of the end plates bend the vertical trabeculae, which are protected by the frequently damaged but easily replaced horizontal trabeculae. Unfortunately, because they are fewer in number and smaller in size, the transverse trabeculae are first to be affected by osteoporosis, which has a profound effect on the mechanical stability of the spine (120). Despite its moderately unimportant role as a shock absorber, the intervertebral disc plays a significant role in distributing pressure evenly over the entire end plate (failure to do so results in the formation of Schmorl’s nodes) and in physically separating the facets thereby allowing for greater motion; i.e., the thicker the disc, the greater the mobility. When working properly, the reinforced lamina of the annulus maintains the nucleus in a central position making the intervertebral disc extremely tolerant of vertical forces. The belief that upright posture is somehow responsible for the prevalence of disc herniations in modern society is unfounded because the lumbar intervertebral discs, when maintained in a neutral lordosis when standing upright, are remarkably tolerant of even extreme vertical forces. It is only when the spine is flexed that the discs are prone to failure. In fact, McGill (121) claims that it is almost impossible to herniate a disc without full flexion of the lumbar spine. McGill (121) also refers to an interesting personal communication with Adams, who suggests an alternate role for a healthy disc is to allow the compressive loads associated with daily activities to increase intradiscal hydrostatic pressures. The increased disc pressure is important since it functions to inhibit the ingrowth of blood vessels and nerves. This is clinically supported by the increased presence of nerve and vascular tissue in degenerated discs (121), which have been implicated in the development of discogenic pain syndromes. While the vertebral bodies and intervertebral discs work together to absorb shock, distribute pressure and physically separate the vertebrae, the posterior elements play important roles not only in defining movement patterns, but also in stabilizing the spine through their numerous muscular/ligamentous attachments. The lumbar spine is stabilized by 7 primary ligaments (Fig. 2.92), with the fourth and fifth lumbar vertebrae receiving additional support from the powerful iliolumbar ligament (Fig. 2.93). Because of the orientation of its fibers, the iliolumbar ligament limits transverse and frontal plane motion of the fifth lumbar and plays an important role during the gait cycle as it provides significant protection against excessive anterior translation of L5 on the sacrum. When stressed in flexion, the pedicles separate while the ligamentum flavum, which contains 60 to 80% elastic fibers (122), stretches to protect the posterior spinal cord without interfering with joint motion. Because of
Figure 2.91. Side view of lumbar vertebrae demonstrating how the thin vertebral end plates bulge in and out (arrows), allowing vertical forces to be absorbed by the cancellous bone (inset).
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Chapter Two Structural and Functional Anatomy its distance from the axis of motion and the direction of its fibers, the supraspinous ligament provides the most significant resistance to full flexion (123). In contrast, the interspinous ligament possesses 3 bundles that angle in such a way that they are unable to limit flexion effectively; i.e., the upper fibers move forward thereby reducing tension throughout the interspinous ligament while the spine flexes. According to Heylings (124), the interspinous ligament functions similarly to the collateral ligaments of the knee in that it allows for large ranges of sagittal plane motion while protecting against transverse and frontal plane shear. This clever design allows the interspinous ligament to unfold as it guides the facets through large ranges of motion, essentially behaving like a Japanese fan wedged between adjacent spinous processes. Although this ligament plays an important protective role during the majority of spinal motion, during terminal flexion, its fiber orientation induces anterior spinal shear on the proximal vertebra that may increase the potential for disc injury (121). Whereas ligaments play a principle role in limiting lumbar flexion, extension is limited primarily by facet imbrication (Fig. 2.94) and, to a lesser extent, by tension
Figure 2.92. The primary ligaments of the lumbar spine. The interspinous ligament is divided into ventral, middle and dorsal components. Redrawn from McGill (121).
Figure 2.93. Anterior (left) and posterior (right) views of the various bands of the iliolumbar ligament.
in the anterior disc and anterior longitudinal ligament. On occasion, bony contact between spinous processes and/ or laminae may serve to limit motion. The laminae and pedicles are surprisingly elastic and they have been shown to bend up to 3째 with even modest daily activities (125). Imbrication of the lumbar facets during extension markedly stabilizes the spine. In a detailed cadaveric analysis of the torsional stiffness of 15 L2/3 and L4/5 functional spinal units, Garges et al. (126) preloaded each unit by applying 300 N of axial compression (to mimic weight of the upper body) and applied cyclic loads while the motion segments were placed in varying degrees of flexion and extension. The authors conclusively demonstrate that extended lumbar vertebrae are stiffer, generated higher torque values and can absorb more
Figure 2.94. Approximation of the facets (arrows) limits continued extension of the lumbar spine.
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Human Locomotion: The Conservative Management of Gait-Related Disorders energy than flexed vertebrae. The authors stress that the added stiffness associated with spinal extension provides significant protection against disc injury, emphasizing the biomechanical importance of maintaining the lumbar lordosis. Depending upon the height of the intervertebral disc, ranges for intervertebral lumbar flexion/extension vary from 12° to 17°, with the lower vertebrae being appreciably more mobile (114). Because of the location of the axis of motion, flexion produces a slight anterior translation of the upper vertebra, while extension produces a slight posterior translation of the upper vertebra (typical ranges of translation vary from 1 to 3 mm). As with most axes, the axis for sagittal plane motion is constantly shifting and it moves from the anterior aspect of the intervertebral disc during flexion to the posterior disc with extension. At L5-S1, the axis for flexion tends to be located in the center of the body of L5 and shifts to the L5-S1 intervertebral disc during extension (127), with greater ranges of translation occurring during extension. In the frontal plane, lateral flexion of the lumbar spine is limited by muscular/ligamentous tension on the side contralateral to the direction of motion (primarily the quadratus lumborum/intertransversarii muscles and the intertransverse ligaments). Typical ranges for lateral flexion between the first through fourth lumbar vertebrae average 6° to 8°, while tension in the iliolumbar ligament limits lateral flexion of L5 to approximately 3° in each direction (114). Because of the oblique shape of the facets, lateral flexion is an impure motion in that side bending is accompanied with a significant degree of rotation. Originally noted by Lovett in 1903 (128), the coupling of lumbar motion occurs in such a way that when the lumbar spine is in a neutral and/or flexed position, lateral flexion is accompanied by ipsilateral rotation, and when the lumbar spine is extended, contralateral rotation occurs. For example, side bending to the right when in a neutral or flexed position is accompanied by right rotation of the functional spinal unit (Fig. 2.95), while the same motion when the spine is extended would cause the vertebrae to rotate to the left. These patterns of coupled motion have been supported by Fryette (129) and Kapandji (130) and are important clinically since they are used to design treatment plans for the restoration of lumbar motion using manual therapy techniques. In fact, a recent study of 369 physical therapists showed that 93% incorporated motion-coupling rules when treating mechanical lumbar spine disorders (131). Unfortunately, in an extremely thorough review of the literature related to coupled motions in the lumbar spine, Legaspi and Edmond (132) conclusively demonstrate that even though coupling of lumbar lateral flexion and rotation does occur, there is so much individual variation in the pattern of coupled motions that it is impossible to claim that any single pattern is predominant. The authors go on to state that clinicians should “consider eliminating the use of
Figure 2.95. Posterior view of two lumbar vertebrae. According to the classic theory of coupled motions, when the spine is in a neutral or flexed position, lateral flexion to the right (small arrow in facet) is coupled with right rotation of the superior vertebra (circular arrow around axis).
the concept of coupled motion patterns in their evaluation and intervention for patients with lumbar spine conditions.” The failure of any single rule to predict coupled motions associated with lateral flexion is also true in the thoracic (133) and cervical regions of the spine (134). Because the facets in the lumbar spine are situated near the sagittal plane, transverse plane motion is limited, particularly when the lumbar spine is extended since approximation of the facets effectively limits motion. Conversely, because flexion separates the facets, larger ranges of rotation are available when the spine is fully flexed (135). According to White and Panjabi (114), the first 4 functional spinal units are capable of 2° of rotation in each direction, while the lumbosacral articulation, with its powerful iliolumbar ligament, is capable of only 1° of rotation in each direction. Regarding the axis of motion, Bogduk (136) claims that pure axial rotation is only possible during the first 3° of rotation with the axis of motion running vertically through the posterior vertebral body. He notes that motion is limited by tension in the annulus (where fibers that are aligned with direction of rotation are under stress) and by compression in the contralateral facet and tension in the ipsilateral facet. Continued forceful rotation causes the axis of rotation to shift to the contralateral facet, which drastically limits further motion. Bogduk (136) claims the intervertebral discs provide 35% of the resistance to rotation while the posterior elements provide the remaining 65%. As mentioned, the powerful iliolumbar ligament appreciably limits rotation at L5-S1, thereby providing important stability against torsional strains associated with bipedalism. 66
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Chapter Two Structural and Functional Anatomy
Figure 2.119. Interossei. The tendons of the 7 interossei muscles (there are 4 dor足sal and 3 plantar interossei) pass very closely to the transverse metatarsophalangeal joint (MTJ) axes and are therefore able to exert a mild plantarflectory force at these axes. However, their tendons possess significant lever arms to the vertical metatar足sophalangeal joint axes that allow for the development of equally strong adduc足tory/ abductory moments. These moments are resolved into a compressive force necessary for transverse plane stabilization of the lesser metatarsophalangeal joints.
Figure 2.120. The popliteus muscle effectively controls transverse plane tibiofemoral motion as it possesses a long lever arm to the vertical axis of the tibiofemoral joint (B and C). Because it wraps anteriorly around the lateral knee (A), the popliteus muscle also effectively controls anterior glide of the femur on the tibia and/or posterior glide of the tibia on the femur. Figure A demonstrates how the medial and lateral gastrocnemius muscles, along with the plantaris muscle, possess significant lever arms for controlling sagittal plane motion of the knee. The semimembranosus muscle (D) attaches to the posteromedial corner of the knee and in addition to displacing the medial meniscus posteriorly upon knee flexion, the semimembranosus attachment contains 5 different expansions that allow it to stabilize the knee against rotational instability: 1=pars reflexa, 2=direct posteromedial tibial insertion, 3=oblique popliteal ligament insertion, 4=expansion to posterior oblique ligament and 5=popliteus aponeurosis expansion. These multiple attachment points allow the semimembranosus muscle to function as an important synergist to the anterior cruciate ligament in preventing anterior translation of the tibia on the femur and valgus stress at the medial knee. Figure D was redrawn from Sims W, Jacobson K. The posteromedial corner of the knee. Am J Sports Med. 2004;32:337.
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Figure 2.121. Lateral view of right hip and thigh. Because it is located anterior to the tibiofemoral joint’s axis of motion, vastus lateralis is a powerful extensor of the knee while rectus femoris possesses a significant lever arm for controlling both hip and knee sagittal plane motions. Sartorius, which inserts into the medial aspect of the proximal tibia, functions as a hip and knee flexor and tibial internal rotator. The gluteus maximus muscle is divided into upper and lower portions with the lower portion possessing a significant lever arm for stabilizing the hip in the sagittal and transverse planes. The upper portion, which represents approximately 60-70% of the muscle, blends into the iliotibial band and is an important frontal plane stabilizer of the hip. The anterior aspect of the iliotibial band receives fibers from the tensor fasciae latae, which serves to balance the force generated by the posterior pull of the upper gluteus maximus. The iliotibial band also receives support from the fascia covering the gluteus medius and has a significant fibrous attachment to the tubercle of the iliac crest. The bicep femoris long head crosses both the hip and knee joints and possesses a long lever arm for controlling sagittal plane motion in both joints. The short head of the bicep femoris muscle also attaches to the proximal fibula and possesses a longer lever arm for controlling sagittal plane knee motion.
Figure 2.122. Posterior view of right lower extremity. Because the piriformis, obturator internus, superior/ inferior gemellus, quadratus femoris and lower gluteus maximus are located posterior to the hip axis of motion, they are powerful stabilizers of transverse plane motion. Conversely, because the iliotibial band, gluteus medius, gluteus minimus and the upper fibers of gluteus maximus attach superiorly to the hip axis of motion, they are powerful abductors of the hip. The significant lever arms afforded the hamstring musculature (semimembranosus, semitendinosus, and the long and short heads of bicep femoris) to the sagittal plane axis for knee motion makes these muscles effective flexors of the knee. Since most of the hamstring muscles cross the hip joint (the short head of the bicep femoris does not) they are also important stabilizers of sagittal plane hip motion.
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Chapter Two Structural and Functional Anatomy
Figure 2.123. Anterior view of right lower extremity. The adductor muscles originate from different points on the pubic bone and possess significant lever arms for adducting the femur. Their role in controlling transverse plane motion is controversial and will be discussed in the following illustration. The quadriceps femoris muscles (rectus femoris, vastus intermedius, vastus lateralis and vastus medialis obliquus) insert anterior to the sagittal plane axis for knee motion and are therefore powerful extensors of the knee. Because the semitendinosus, gracilis, and sartorius muscles (collectively referred to as the pes anserine) insert medial to the vertical axis of knee joint (not illustrated), they are important transverse plane stabilizers of tibiofemoral motion.
Figure 2.124. Side view of right innominate and femur with hip in a straight (left) and a flexed (right) position. Notice how when the hip is in a neutral position, the origins of all the adductors are positioned anterior to the hip axis of motion allowing them to function as flexors of the hip. When the hip is flexed (or, as in this case, when the innominate is flexed), the adductor origins are displaced slightly posterior to the hip axis of motion causing them to behave as weak extensors of the hip.
Figure 2.125. The right femoroacetabular joint as viewed from above. When the hip is internally rotated (A), the tendons of obturator externus (oe) and the adductors (add) are positioned anteriorly to the hip’s axis of motion and therefore behave as internal rotators of the hip. When situated in a midline position (B), the obturator externus tendon is situated behind the hip’s axis of motion (making it an external rotator of the hip) while the adductor musculature continues to be positioned in front of the axis of motion (making them internal rotators of the hip). When the hip is in a fully externally rotated position (C), the tendons of obturator externus and the upper adductors are located behind the hip’s axis of motion causing them to behave as hip external rotators.
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Figure 2.126. Posterior view of right hip and pelvis. When the hip is in a neutral position, the piriformis, superior /inferior gemellus, obturator internus/externus and the posterior fibers of gluteus medius have significant lever arms for externally rotating the hip.
Figure 2.127. Superior view of right hip. Gluteus maximus possesses the longest lever arm for producing external rotation of the hip (A) while gluteus minimus possesses the longest lever arm for producing internal rotation at the hip (B). Because gluteus medius and minimus possess fibers that pass anterior and posterior to the hip axis of motion, they are capable of both internally and externally rotating femur, depending upon the location of the specific fiber (the central fibers of gluteus medius and minimus function primarily as abductors while the more forward and backward fibers function as internal and external rotators, respectively).
Figure 2.128. The iliopsoas consists of the two separate muscles: iliacus and psoas major. The iliacus originates from the upper two thirds of the iliac fossa while the psoas major originates from the anterior surface of the transverse processes, vertebral bodies, and intervertebral discs from T12 to L5. Both insert into the lesser trochanter of the femur where, because of their significant lever arm to the hip axis of motion, they function as powerful flexors of the hip. When the hip is maintained in a fixed position during static stance, iliacus produces an anterior tilt of the innominate while psoas major flexes the lumbar spine. The psoas minor muscle is a fairly unimportant muscle and is absent in more than 40% of the population. Because of its distance to the axis for sagittal plane spinal motion, psoas minor functions to flex the lumbar spine and tilt the pelvis posteriorly.
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Chapter Two Structural and Functional Anatomy
Figure 2.129. Quadratus lumborum and the intertransversarii muscles. The quadratus lumborum muscle is divided into superficial lateral iliocostal fibers (A), medial deep diagonal iliolumbar (B), and lumbocostal fibers (C). The intertransversarii muscles attach to adjacent transverse processes and are typically absent between L5 and the sacrum. Quadratus lumborum is a powerful lateral flexor of the lumbar spine and has a small lever arm for controlling sagittal plane motion (see next illustration). The quadratus lumborum muscle blends with the iliolumbar ligament, which contributes to dynamic stability of the lumbosacral junction. Modified and redrawn from Eisler P. Die muskeln des stammas. Gustav Fisher, Jena, 1912.
Figure 2.130. During lateral flexion lumbar spine, the superficial lateral iliocostal fibers of the quadratus lumborum (A) have a significant lever arm to the frontal plane axis of motion of the lumbar spine (small circles between vertebrae). The deep diagonal iliolumbar (B) and lumbocostal fibers (C) of quadratus lumborum possess slightly shorter lever arms but are still important lateral flexors. When tensing bilaterally, quadratus lumborum has the ability to create a compressive load that significantly stabilizes the lumbar spine. During sagittal plane motions, quadratus lumborum possesses a small but significant lever arm for stabilizing the lumbar spine (the superficial lateral iliocostal fibers of the quadratus lumborum have a particularly long lever arm when the lumbar spine is extended (D). Partially modified and redrawn from Travell J, Simmons D. Myofascial Pain and Dysfunction, The Trigger Point Manual, The Lower Extremities (Volume 2). Baltimore;Williams and Wilkins 1992:36.
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Figure 2.131. Iliocostalis lumborum, longissimus thoracis and latissimus dorsi. These muscles originate from the dorsal surface of the sacrum, blending with the lumbodorsal fascia (A). Iliocostalis and longissimus attach to the lower 6 ribs while the latissimus dorsi muscle attaches to the medial edge of the intertubercular groove of the humerus. These muscles have significant lever arms for stabilizing the lumbar spine and play an important role in limiting sagittal plane motion. Iliocostalis and longissimus possess subdivisions that attach to the lumbar spine.
Figure 2.132. Subdivisions of iliocostalis lumborum and longissimus thoracis: longissimus thoracis pars lumborum (left) and iliocostalis lumborum pars lumborum (right). Note that iliocostalis lumborum pars lumborum has no attachment to the fifth lumbar vertebra. As demonstrated in the next illustration, these muscles have significant lever arms for controlling sagittal plane motion of the lumbar spine.
Figure 2.133. Longissimus thoracis pars lumborum (solid lines) and iliocostalis lumborum pars lumborum (dotted lines) possess extremely long lever arms for controlling sagittal plane motions. McGill (121) emphasizes that the lumbar portions of iliocostalis and longissimus are the most powerful extensors of the lumbar spine and possess lever arms to the axes of motion (small circles) that may exceed 10 cm (4 inches). Notice how the axes of motion for flexion are displaced anteriorly compared to the axes for extension (compare x and y). The forward shifting of the axes allows the muscles to maintain their mechanical advantage during sagittal plane motion. Because the lower fibers of these muscles possess a more angled line of application of force (B), they create more of a posterior pull allowing them to stabilize the more mobile lower lumbar vertebrae. Redrawn from McGill (121).
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Chapter Two Structural and Functional Anatomy
Figure 2.134. The multifidi and long and short rotatores. The multifidi are complicated muscles that originate from the sacrum, interosseous ligament, lumbodorsal fascia, medial edge of iliac crest, and the transverse and mammilary processes of the lumbar spine and insert into the spinous processes one to three vertebrae above. The rotatores are small muscles that arise from the transverse processes and insert into the lamina above. These deep muscles play an important role in stability as the multifidi possess a line of action that compresses the lumbar vertebrae while maintaining a moderate lever arm for extending the lumbar spine. Because they span such short distances, the multifidi essentially behave as variable length ligaments and play an important role in fine-tuning subtle perturbations in joint motion. The long and short rotatores control transverse plane rotation but their most important function may be in providing proprioceptive input to the central nervous system regarding subtle changes in joint position (121). Redrawn from Travell J, Simmons D. Myofascial Pain and Dysfunction, The Trigger Point Manual, The Lower Extremities. Baltimore;Williams and Wilkins 1992:642.
Figure 2.135. Transverse section of torso just above the iliac crest (inset on left). The anterior abdominal fascia (A) blends with the posterior lumbodorsal fascia (B) to form an anatomical hoop (dashed arrows) that stabilizes the lumbar spine. Notice how the lumbodorsal fascia (LDF) blends between the supporting musculature (inset circle). The transverse abdominus (TA) and the internal oblique muscle (IO), when tensed, apply significant tension to the hoop while the external oblique muscle (EO) has an 11 cm lever arm for controlling lateral flexion of the lumbar spine (121). Abbreviations: LD= latissimus dorsi, QL=quadratus lumborum, ES=erector spinae, Mu=multifidus, Ps=psoas, RA=rectus abdominus, EO= external oblique.
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