Chapter Six
Foot Orthotics Despite numerous methods of modifying motion during the gait cycle (e.g., gait training, agility drills, isokinetic exercises), the most popular technique to alter lower extremity movement is with the use of custom and prefabricated foot orthotics. Both custom and prefabricated orthotics are available in a variety of materials, ranging from the relatively rigid graphites to the more flexible leather laminates. Foot orthotics are particularly commonplace in the sporting community, where they are used to treat a wide variety of injuries, including patellofemoral pain syndrome, plantar fasciitis, and Achilles tendinitis. In spite of their clinical popularity, there continues to be significant controversy regarding their exact mechanism of action. Researchers argue as to whether orthotics are effective because they improve skeletal alignment, lessen lower extremity rotation, decrease velocity of motion and/or alter the mechanical efficiency of the stabilizing muscles. Furthermore, practitioners have strong opinions when it comes to deciding which casting technique should be used when fabricating an orthotic: some claim orthotics should only be made from off weight-bearing neutral position casts, while others claim only full weightbearing foam step-in techniques are appropriate. Even the most fundamental questions concerning orthotic material selection, fabrication techniques, and the clinical indications for specific additions remain unanswered. Obviously, this controversy greatly affects the consistency in which orthotic treatment protocols are applied, which in turn has a negative effect on clinical outcomes. Fortunately, in the past few years, advances in 3-dimensional imaging have allowed for an improved understanding of the various mechanisms responsible for orthotic efficacy. In many situations, the results have been surprising. The purpose of this chapter is to review the different ways foot orthotics affect lower extremity kinematics and relate this information to the design and fabrication of the orthotics themselves. Specific orthotic casting methods, materials, orthotic additions, sport specific variations, and in-office manufacturing techniques will be reviewed in detail.
support a pronated foot with an orthotic, the calcaneus will become more vertical (i.e., straighter), causing the lower extremity to rotate externally, thereby improving alignment at the knee. These coupled movements have been theorized to continue along the entire kinetic chain, with some authors claiming that foot orthoses may help to lessen low back pain (1). Surprisingly, a significant body of information suggests that this may not be the case, as most 3-dimensional research suggests that orthotic intervention produces very little change in frontal plane rearfoot motion during the gait cycle (2-7). In an exceptionally detailed study, Stacoff et al. (6) surgically implanted intracortical pins into various bones of the lower extremity (which is the gold standard for studying 3-dimensional motion) and concluded that orthotics produce little to no change in rearfoot eversion during the gait cycle. More recently, Nawoczenski et al. (7) performed an interesting 3-dimensional analysis and determined that individuals with low arches actually have increased ranges of rearfoot eversion while wearing orthotics. The inability of orthotics to modify frontal plane movement of the rearfoot caused investigators to come up with alternative theories to explain the mechanisms responsible for the beneficial clinical outcomes associated with their use. Messier and Pittala (8) suggest that orthotics work because they alter the velocity of pronation, while others claim orthotics work because they decrease tibial rotation and/or decrease impact and loading rates of vertical ground-reactive forces (9,10). It is also suggested that orthotics work because they improve proprioception, decrease ankle inversion moments, decrease genu valgum and/or decrease external rotation moments at the knee (11, 12). To add to the controversy, some experts question whether the effectiveness of orthotic intervention is related to the actual shell of the orthotic or the applied post material (13). In 2003, Mundermann et al. (13) tested these theories by performing a detailed 3-dimensional examination comparing a range of kinetic and kinematic variables in 21 pronators wearing one of four test conditions: a full-length varus post, a custom molded orthotic shell, the same shell with a varus post, and an unposted flat insole. After having the subjects fitted with reflective skin markers, kinematic data was recorded with 7 high-speed cameras and groundreactive forces were measured using a force plate. The
Mechanism of Action It has been a long-held belief that the main reason orthotics work is because they improve skeletal alignment. In fact, the origin of the word orthotic stems from the Greek word ortho, meaning straight. It seems intuitive that if you 301
Human Locomotion: The Conservative Management of Gait-Related Disorders subjects then ran 200 meters on an indoor track in each of the four test conditions as researchers measured 15 different variables that included maximum foot eversion, maximum foot eversion velocity, tibial rotation (range and velocity), foot inversion (range and velocity), ankle inversion moments, knee abduction moments and vertical impact forces. The results were interesting as the post, shell and post, and shell alone all altered different parameters of gait compared to the flat insole control. Specifically, the post alone condition significantly decreased rearfoot eversion during the contact period, and slightly decreased tibial internal rotation. In addition, the post alone appreciably lessened the maximum ankle inversion moment, which would decrease strain on the muscles responsible for decelerating subtalar pronation during the contact period; e.g., tibialis posterior. When used by itself, the custom molded polypropylene shell (molded from a neutral position positive model) had different actions. Although it had no effect on frontal plane motion of the rearfoot (range or velocity), it markedly lessened the maximum ankle inversion moment, which as mentioned, decreases strain on tibialis posterior. The shell also appreciably reduced the range of tibial internal rotation and, more importantly, increased the angle of maximum foot inversion during the propulsive period. The authors state that this finding suggests that molding plays a significant role during late stance phase. Also of note, the custom molded shell significantly improved shock absorption as it decreased the vertical impact peak and vertical loading rates present during early stance. Mundermann et al. (13) speculate that custom molding of the orthotic shell increased shock absorption by providing a larger contact area between the foot and the orthotic. They go on to reference Perry and Lafortune’s (14) belief that the unaltered range of rearfoot eversion associated with using the shell only might allow the subtalar joint to act as a more efficient shock absorber. When analyzed with the post and shell condition, the subjects consistently demonstrated the same kinetic and kinematic changes present with the shell alone; i.e. decreased vertical force, minimal changes in contact period frontal plane motion of the rearfoot, a decreased inversion ankle moment and an increased maximum foot inversion angle during the propulsive period. In a follow-up study published the same year by the same authors, Mundermann et al. (15) again studied the effect of post vs. shell vs. control in 21 pronators, only this time they also included EMG data of muscle function and evaluated comfort in each of the test situations with a visual analog scale. The findings regarding kinetic and kinematic data were similar to the previous study in that the post alone significantly decreased maximum foot eversion, slightly increased vertical impact peak and improved ankle inversion moments. Conversely, the shell alone improved shock absorption, lessened ankle inversion moments and
allowed for greater supination during propulsion. Again, when the shell and post were combined, the effect of the shell was dominant. What is interesting about this study is that comfort was significantly higher for the shell alone compared to all other conditions. The authors state, “in general, custom molding of foot orthoses appears to be a feature that increases comfort.” In both studies, foot orthoses produced significant and systematic changes in kinetic, kinematic, comfort and EMG variables. Although these outcomes conflict with prior research suggesting orthotics do not significantly alter motion (6,7,16), Mundermann et al. (15) claim the controlled nature of their studies made for more accurate outcomes. The conclusion that custom molding improves comfort is clinically supported by the fact that studies comparing efficacy of custom versus prefabricated orthotics often report lower dropout rates for the individuals wearing custom orthotics (17). In an in vivo study of foot pressures associated with different orthotics, Redmond et al. (18) evaluated plantar pressures in 22 healthy individuals as they wore either thinly-soled athletic shoes alone, flat stock insoles modified with 6° varus posts or functional foot orthotics made from neutral position off weight-bearing plaster casts that were manufactured with 6° varus posts. The authors note that plantar pressures in subjects wearing the posted stock insoles differed only slightly from the shoe condition but the custom foot orthotics significantly lowered all pressure related variables with reductions ranging from 14 to 21%. The total contact area was increased by 38% in the custom orthotic group and 16% in the posted insoles. Maximum heel forces were reduced 8% in the custom orthotic group as the rearfoot contact area was increased. The authors note that despite the varus posts, neither the custom orthotics nor the posted insoles shifted force laterally and the custom orthotics worked primarily by supporting the arch. By elevating the arch, the custom orthoses increased maximum force displaced beneath the arch but because the closely fitted contours of the custom orthotics spread forces evenly over the entire arch area, there was no significant change in pressure peaks through the midfoot. In another interesting evaluation of 3-dimensional motion with custom orthotics, Williams et al. (19) studied the effect of standard foot orthoses and inverted orthoses on lower extremity function (details concerning the manufacture of inverted orthotics will be discussed briefly). The standard foot orthotics were made from graphite shells with 4° varus posts, while the inverted orthotics were made from the same materials but possessed extremely high post angles; i.e., the rearfoot of the orthotic shell was inverted either 15° or 25° (individuals with less than 10° resting calcaneal stance eversion were posted at 15° while those with greater than 10° calcaneal eversion were posted at 25°). Despite the extremes of these post angles, the subsequent 3-dimensional gait analysis revealed absolutely 302
Chapter Six Foot Orthotics no differences in frontal plane movements of the rearfoot as the maximum degree of calcaneal eversion was unchanged. In fact, almost half of the subjects in both orthotic groups actually everted farther than the controls and frontal plane kinematics were quite variable. Even though they had no effect on rearfoot motion, the inverted orthotic devices significantly decreased the inversion moment at the ankle (54% in 10 of 11 subjects), while the standard orthotic decreased the inversion moment by 27% (in 8 of 11 subjects). This translates into a considerable reduction in muscle strain placed on the medial leg musculature with both devices. In perhaps the most detailed 3-dimensional study performed to date, McLean et al. (20) evaluated lower extremity biomechanics in 15 female runners fitted with custom orthotics. The orthotics were fabricated from neutral position off weight-bearing casts and the orthotic shells were made from polypropylene. Each orthotic was intrinsically posted to 5° varus and were manufactured by an accredited orthotic laboratory. To minimize the effect that pain might have on lower extremity motion, the authors chose to study only non-pronators (i.e., less than 10° of rearfoot eversion). The authors emphasize that prior research on frontal plane rearfoot motion has been inadequate in that only maximum values for various dynamic variables have been recorded and to get a true picture of the effect of orthotic intervention, dynamic variables must be recorded at discrete points of stance (i.e., 5%, 10%, 15%, and 20% of stance phase). They suggest this time frame is essential to evaluate accurately the effect of orthotic intervention. After recording joint and kinematic data, the authors calculated mean values for the rearfoot eversion angle, rearfoot eversion velocity and the ankle inversion moment at discrete points during stance for the orthotic group and the control group. Using this detailed approach, McLean et al. (20) demonstrated that orthotic intervention significantly decreased maximum rearfoot eversion from 15 to 50% of stance phase, and that rearfoot eversion velocity was significantly decreased both immediately following heel strike and at 10% and 15% of stance phase. In addition, the orthotic group exhibited a reduced ankle inversion moment from 15 through 70% of stance, which the authors claim lessens strain on the tibialis posterior muscle as it decelerates subtalar joint pronation. By combining data from the previously discussed studies (15,16,19,20), it becomes clear that one of the primary effects of orthotic intervention is to alter the inversion moments at the ankle thereby reducing strain on the medial lower leg musculature. The beneficial effect of an orthotic post can be related to its ability to lessen the angular velocity and range of rearfoot eversion during the very early stages of stance, while the beneficial effect of the orthotic shell most likely happens later in stance phase when it acts to increase the angle of maximum foot inversion.
Regarding the effect of orthotics on tibial rotation and knee mechanics, the literature again reveals inconsistent outcomes. Some studies demonstrate that orthotics increase knee extension moments (20) and increase external rotation moments (21) (both of which may increase the potential for knee injury), while other studies have demonstrated a reduction in frontal plane motion at the knee and decreased tibial rotation velocities (22). In their classic evaluation of the effect of rearfoot pronation on knee kinematics, Lafortune et al. (3) surgically inserted intracortical rods into the proximal tibia, distal femur and central patella of 5 volunteers. Target clusters attached to these rods allowed researchers to measure the exact degree of rotation present between the bones of lower extremity as subjects walked on a treadmill wearing sneakers fitted with either a full-length 10° varus midsole, a full-length 10° valgus midsole or a neutral midsole. When wearing the valgus posted midsole, internal tibial rotation increased 4° more than with the varus midsole but at the tibiofemoral joint, there was no consistent difference in the pattern of internal/external rotation between the various conditions. This means that as the tibia was forced into internal rotation by the valgus post, the femur rotated the exact same degree so that no net rotation occurred at the tibiofemoral articulation: all of the transverse plane motion induced at the foot by the valgus post traveled through the knee and was absorbed by the hip. The patella in this situation is particularly prone to injury, as the internally rotating femur would drive the lateral femoral condyle into the lateral patella facet. The researchers also noted the surprising observation that when the foot was forced to pronate by the valgus wedge, the tibia shifted medially an additional 2 mm relative to the femur. This is significant because a medial shift of the tibia relative to the femur will increase tensile strain placed on the anterior cruciate ligament, which could explain the connection between foot pronation and ACL injuries (23). In an important study evaluating the effect of medial posts on 3-dimensional knee and ankle motion, Joseph et al. (24) measured rearfoot pronation and knee valgus as 10 female athletes performed drop jump landings from a 31 cm platform while wearing sneakers fitted with full-length 5° varus wedges. Unlike the previously mentioned study by Lafortune (3), this study quantified each individual’s foot type by measuring navicular drop. Because of the controversy regarding the rationale for prescribing posts, the researchers had all participants use 5° varus posts. The subsequent 3-dimensional evaluation revealed the varus posts produced significant reductions in knee valgus and rearfoot eversion at initial contact, and moderate reductions later in the gait cycle. The authors suggest that because the posts reduced rearfoot eversion and knee valgus during the contact period, they may help prevent ACL injuries, which often occur early during stance phase as the knee is near full extension (25). One of the most interesting findings of 303
Human Locomotion: The Conservative Management of Gait-Related Disorders this study was that individuals with the greatest degree of pronation had the straightest knees during the drop jump. This is at odds with a study by McClay and Manal (26), in which overpronators landed with greater knee valgus when running. Joseph et al. (24) suggest that excessive foot pronation may actually protect the knee as an overly mobile pronated foot may act to absorb ground-reactive forces during contact thereby minimizing the need for knee movement during dynamic maneuvers. Overall, the results of this study are consistent with a study by Eng and Pierrynowski (22) in which posted orthotics significantly reduced frontal plane knee motion during the propulsive period of walking and running. It is clinically significant that any study evaluating the connection between rearfoot pronation and knee valgus should always take into account the individual’s arch height, because height of the medial longitudinal arch significantly affects the transfer frontal plane rearfoot motion into transverse plane tibial internal rotation; i.e., high arches increase the transfer of frontal plane motion into tibial rotation while low arches decrease the transfer (27). These factors are significant when attempting to control knee motion with orthotics since the clinical connection between rearfoot pronation and knee injuries is less predictable in low-arched individuals; e.g., varus and valgus posts used to treat lateral and medial knee osteoarthritis (respectively) are less likely to be effective when used on hypermobile, pronated feet. As a result,
comprehensive management of dynamic knee disorders should include a thorough evaluation of hip extensor strength, because a strong gluteus maximus is capable of controlling tibial internal rotation during stance phase (28). Orthotic Casting Techniques In order to create the most functional foot orthotic possible, it is crucial that the negative impression accurately captures specific aspects of foot morphology, including height and contour of the medial longitudinal arch, forefoot and rearfoot widths and the location of the first ray. It is important that these measurements be repeatable, otherwise it would be impossible to obtain consistent clinical outcomes. The most popular techniques for obtaining negative impressions are the neutral position off weightbearing plaster casts and the semi-weight-bearing and full weight-bearing foam step-ins. Less common techniques for capturing a negative foot impression include partial weight-bearing and off weight-bearing laser and light scans, contact digitized scanning, various pressure mats and in-shoe vacuum techniques. Methods for performing the different casting techniques are summarized in the following illustrations (Figs. 6.1-6.6). Despite the numerous options for obtaining negative foot impressions, there is significant controversy regarding which technique is clinically most efficacious. For example, proponents of plaster casting claim that only the off weightbearing plaster casts accurately capture angular forefoot/
Figure 6.1. Legend on next page.
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Figure 6.1. Off weight-bearing neutral position plaster casts. As the name implies, this technique involves taking a plaster cast with the patient’s foot maintained in a neutral position. This process requires 4 strips of extra-fast-setting plaster splints (each strip is folded in half), a tray of warm water and a towel to clean up the mess. First, the patient’s lower extremity is rotated so that the foot rests in a vertical position. Although this procedure may be done with the patient prone or supine (which produce comparable results [79]), for simplicity, only the supine technique is illustrated. With the foot in a vertical position, the talonavicular joint is placed in its neutral position, and a firm dorsiflectory force is placed on the fourth and fifth metatarsal heads (A). The patient is instructed to keep the foot as close to this position as possible while the plaster is being applied: this prevents buckling of the plaster when the foot is later repositioned and loaded. A plaster splint is prepared for application by folding a dry splint into the palm of the hand while pinching the free end between the thumb and index finger (B). The plaster strip is then submerged into a tray of warm water for approximately 3 seconds, removed from the water and gently squeezed. The plaster of Paris is then mixed thoroughly through the cloth by repeatedly squeezing the wet plaster strip in the palm of the hand. It is important that the free end be firmly held throughout this process, as this allows the practitioner to unfold the wet plaster strip. Once the plaster has mixed with the cloth, the plaster strip is opened and the upper border of the strip is folded approximately 6 mm, thereby creating a lip along the entire upper edge (C). The plaster is then applied to the foot by wrapping it around the heel (D) and “tacking” it down to the top of the first and fifth metatarsal heads. The hanging plaster is then smoothed against the medial arch (E), and then overlapped by the lateral strip. The small V-shaped flap that forms at the base of the heel is then smoothed against the cast (F). The second piece of plaster is applied by draping it over the forefoot (G) (the plaster on the dorsal foot is tacked against the previous strip) and folded as illustrated in H and I. The patient is then asked to relax, and the forefoot is loaded by firmly grasping the proximal phalanges of the fourth and fifth digit between the thumb and index fingers (J). Using this grip, a slight long axis traction is applied to the toes as the practitioner pushes the base of the proximal phalanges horizontally, in a plane parallel to the examining table, until firm resistance is noted when the lateral column locks. To prevent the toes from dorsiflexing, pressure from the index finger is applied to the distal aspect of the proximal phalanx while the thumb applies a horizontal force at the base of the proximal phalanx (K). This position is maintained with the talonavicular joint in its neutral position until the plaster hardens. An alternate (and easier) method for loading the lateral column is to have the practitioner place the distal phalanx of his or her thumb directly over the fourth and fifth metatarsal heads. While maintaining talonavicular congruency, the thumb pushes horizontally along a line paralleling the table until the lateral column locks. The only problem with this technique is that the thumb leaves a small “dent” in the plaster that must be removed by the laboratory. The finished cast is removed by pinching the skin on the dorsum of the foot (white arrows in L) and pulling down on the heel. The practitioner then carefully pushes the cast forward, gently shaking it until the cast glides off the forefoot (M). The accuracy of the negative impression may now be evaluated by placing the impression on a level surface and noting the frontal plane position of the heel: The neutral foot will present with the rearfoot bisection perpendicular to the examining table (N). If insufficient pressure was used while loading the lateral column, a false forefoot varus alignment will be present (O), while a plantarflexed first ray will produce a cast in which the rearfoot is inverted (P). The most important criteria to consider when evaluating the negative model is that the plaster impression should match the shape of the neutral position foot in every detail. Deviation from the anticipated shape most commonly results from insufficient loading of the lateral column, taking the impression with the talonavicular joint supinated or pronated (which will result in a false forefoot varus or valgus, respectively), and/or faulty use of the suspension technique (inappropriate dorsiflexion of the toes will result in a plantarflexed lateral column). Other factors to consider when evaluating the negative casts are that the transverse plane relationship between the forefoot and rearfoot should exactly match the patient’s foot; i.e., an individual with a rectus foot should present with a straight lateral column (Q) while an individual with a metatarsus adductus should demonstrate a medial angulation of the forefoot (R). If the suspension technique is used, the thumbprint should always parallel the sulcus (S), should not contact the third digit, and the thenar eminence should not be imprinted on the lateral aspect of the cast. Furthermore, the fourth and fifth digits should be neither dorsiflexed nor plantarflexed (T). Finally, the contour of the lateral arch (U) should duplicate the contour of the patient’s neutral position foot. Examination of the interior of the cast should reveal well-defined skin lines, and the plantar impressions of the first and fifth metatarsal heads should be clear. If for any reason the neutral foot and the negative impression do not match, the cast should be retaken.
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Figure 6.3. Partial and off weight-bearing laser scans. To perform partial weight-bearing laser scans, the patient’s foot is placed on the scanner while the heel is brought to a vertical position with the talonavicular joint maintained in a neutral position. This occasionally requires the assistance of another person. The off weight-bearing laser scan is performed by using the suspension technique to maintain the patient’s foot in a neutral position while the foot is being scanned. Data samplings of the X-, Y-, and Z-coordinates are then recorded from the patient’s foot. The computer analyzes this information and creates a graphic display of the foot on a computer screen. Note that laser scans differ from less sophisticated light scans, which approximate arch shape by comparing varying degrees of brightness and shadows from a digital image. The next step in creating a negative model from a laser scan involves manipulating the graphic image to allow for the soft tissue expansion associated with weight-bearing and/or for the addition of virtually any modification including balances, forefoot platforms, pocket accommodations, deep heel cups, etc. Once the image has been modified to the desired shape, it is sent over a computer network to a milling area. The milling machine resembles a drill press that exactly duplicates the final computer image out of an intermediate material. This intermediate serves as a positive model upon which any shell material may be molded, e.g., leather, Plastazote, graphite, etc. If a plastic orthotic is desired, first the top is milled to the desired shape and then the material is inverted to mill the bottom. As with designing the superior surface, it is possible to mill the plantar surface into any shape, thereby incorporating intrinsic/extrinsic forefoot and/ or rearfoot posts, medial grind-offs, etc.
Figure 6.2. Neutral position semi-weight-bearing polystyrene foam step-in impression. Method: With the patient standing next to the examining table, the feet are positioned at their proper angle and base of gait, as determined during the gait evaluation. The patient is then asked to sit, and the practitioner places a tray of polystyrene foam under each foot. While maintaining talonavicular congruency, a firm downward force is applied, first on top of the knee, then on top of the metatarsal heads and toes (A). The finished impression must be at least 2 inches deep so the laboratory can fill it with enough plaster to obtain an adequate positive model of the patient’s foot.
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Figure 6.4. Contact digitized scanning. Developed by AmFit Orthotic Laboratory, this technique involves having the patient step on a tray containing a large number of 4 mm wide pistons that are maintained in an elevated position by a stream of controlled air pressure. As the patient’s foot displaces the pistons, a computer analyzes the information and produces a 3-D image. As with the laser scans, the image obtained from the data can be modified to allow for the addition of a wide range of posts and/or balances. The finished image is sent to a milling machine and converted into an orthotic. Although orthotics manufactured with the contact digitizing technique were initially made only from a compressed EVA material, manufacturers are now able to make orthotics out of a variety of materials, including graphite. Drawn from a photograph in www. Amfit.com.
Figure 6.5. Pressure mats. This method of scanning involves having the patient walk over a specially designed mat that relays information regarding pressure distribution and displacement to a computer. This information is useful when creating an orthotic because it allows the practitioner to accommodate the altered pressure patterns with specific orthotic modifications (e.g., metatarsal pads, varus/valgus posts and/or additional arch support). Drawn from a photograph in www.digitalorthotics.com.
Figure 6.6. In-shoe vacuum cast. This technique was developed to create orthotics that precisely fit specific shoes. To perform an in-shoe vacuum cast, the foot is wrapped in plaster and placed in a plastic bag. The cast is then vacuum-molded to the patient’s foot while he/she is wearing a specific shoe. The talonavicular joint is typically maintained in its neutral position, and the forefoot may or may not be loaded. Because of difficulties associated with positioning the foot, this technique has serious drawbacks and it most often captures a picture of the foot with the forefoot adducted and the midtarsal joint supinated (80).
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Human Locomotion: The Conservative Management of Gait-Related Disorders rearfoot relationship while critics argue that off weightbearing plaster casting captures an elevated picture of arch height that, when converted into the positive model, could lead to iatrogenic injury as the elevated arch would block the range of pronation necessary for shock absorption. Advocates of foam trays claim that weight-bearing foam impressions more accurately capture soft tissue expansion associated with weight-bearing and therefore allow for the production of a more comfortable orthotic. In spite of this long-term controversy, few studies have compared the reliability and accuracy of the different casting techniques. In 2002, Laughton et al. (29) evaluated the 4 most common methods used for obtaining negative foot impression: neutral off weight-bearing plaster casts, neutral partial weight-bearing step-ins and neutral partial and off weight-bearing laser scans. Each impression was repeated 3 times on 15 subjects and measurements were made regarding rearfoot and forefoot width, forefoot/ rearfoot angular relationships and arch height. This information was then analyzed for reproducibility and accuracy; i.e., were the measurements from the repeated casts reproducible, and did the resultant cast measurements accurately represent measurements from the foot. As expected, plaster casting produced the most reliable measurements of forefoot/rearfoot relationships while foam techniques were least reliable. The plaster cast technique consistently captured a slightly narrower picture of rearfoot width (compared to true rearfoot which was measured with the subjects seated with feet flat on the floor) while all other techniques captured a rearfoot width significantly wider than the actual foot. In fact, the off weight-bearing laser scanning and foam impressions captured a rearfoot width almost a full centimeter greater than actual rearfoot width. The authors claim that this could influence both fit and comfort of the resultant orthotic. The biggest surprise of the study was that the off weight-bearing plaster casts produced one of the lowest measures of arch height. It was expected that the partial weight-bearing methods would capture significantly lower arch heights, but the partial weight-bearing neutral position laser captured the highest arch height (3.8 cm at the apex) while the off weight-bearing laser captured the lowest (2.0 cm at apex). The 1.8 cm difference between these two measurements is unacceptable since the resultant arch height in the final orthotic would be either dangerously high or too low to function properly. The authors state that the open space between the foot and the glass of the scanning bed may have resulted in “increased noise from background light,” which may have distorted the image. Overall, the plaster impressions gave a more reliable measure of arch height than either the foam or the off weight-bearing laser scanning. The authors claim that this is due to the repeatable way in which the plaster is contoured to the arch during the casting process. The low reliability of arch height measurements with the foam
techniques most likely resulted from the inconsistencies in the way the foam displaced the soft tissues of the arch. Because of the low reliability and often extreme distortion of arch height, continued use of neutral position semi-weight-bearing foam step-in technique should be reconsidered, because it captures a grossly elevated picture of arch height (29). The positive model made from the resultant cast could result in significant contusion of the soft tissues and possibly block the range of pronation necessary for shock absorption (regardless of the shell material used). To demonstrate the differences between various casting techniques, figure 6.7 illustrates variation in the captured shape of the medial longitudinal arch when the same person was cast with a semi-weight-bearing foam step-in and an off weight-bearing neutral position plaster cast. In an award winning cadaveric study that inadvertently argued in favor of foam step-in techniques, Kogler et al. (31) surgically implanted reluctance transducers inside the plantar fascias of 7 cadaveric limbs and proceeded to apply 900 N downward force into the proximally amputated tibiae in various test conditions; i.e., barefoot specimen, specimen with shoe and specimen with shoe and 5 different test orthotics made from different casting techniques. The efficacy of each orthotic was evaluated by noting reductions in plantar fascia strain rates as the limb was loaded. Regardless of materials used, orthotics made from neutral position semi-weight-bearing foam step-in techniques all significantly reduced strain inside the plantar fascia during full axial load. Note that because the arches of the positive models were not lowered prior to manufacturing the orthotic shells, and because the foam step-in technique captures an elevated picture of arch
Figure 6.7. Actual tracings of arch shape when the negative foot impression is taken with neutral position semi-weight-bearing foam (A) and off weight-bearing plaster cast (B). To obtain these tracings, the patient’s foot was marked at the center of the first metatarsal and the medial condyle prior to taking each impression. After the positive model was obtained, a carpenter’s contour gauge was placed along a line connecting these points, duplicating the shape of the medial longitudinal arch. For comparison, C represents the shape of the arch during double-limb relaxed calcaneal stance. Note the resultant arch height captured in each cast does not represent arch height present in the finished orthotic, as the laboratory adds 4-6 mm of plaster to the positive model to allow for arch deflection during stance phase.
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Chapter Six Foot Orthotics height (which the authors were unaware of as research showing this technique captures an inaccurate picture of arch height had not been published), the resultant orthotics were so distorted that their elevated arch apex prevented displacement of the plantar fascia. Conversely, the orthotic made from the neutral position off weight-bearing plaster cast (the positive model had the medial arch lowered prior to molding the orthotic shell) did not alter tension in the plantar fascia, in spite of the fact that it was made from a rigid graphite material, as the arch support was too low. The findings of this study are consistent with the notion that orthotic materials are not as important as the type of negative impression utilized. Although excessive elevation of the arch reduces plantar fascial tension in a cadaver, there would most likely be a range of side effects associated with use of this technique in vivo. Specifically, an elevated orthotic shell may damage the tissues directly beneath the medial longitudinal arch, as it may contuse the abductor hallucis muscle and create a bowstring effect that increases tensile strain placed on the plantar fascia (31). Excessive arch support may also produce a neuropraxia of the medial and lateral plantar nerves that can take months to heal, even after the arch support has been re moved. As noted by Robbins et al. (32,33), excessive arch support in an orthotic may also produce proprioceptive deficits (potentially producing a neurotrophic arthropathy) and, by stimulating cutaneous receptors under the medial longitudinal arch, may eventually lead to injury of the metatarsal heads secondary to impaired muscular stabilization. Glancy (34) states that because an exaggerated medial arch support will block the storage and eventual return of elastic energy during the contact period, the long and short digital flexors may be chronically strained as they attempt to compensate for the weakened calf musculature by firing vigorously during midstance and propulsion (i.e., tibialis posterior is especially weakened by the complete blockage of rearfoot eversion). For the aforementioned reasons, nonspecific buildup of the medial longitudinal arch should always be avoided. Although an elevated arch will initially reduce symptoms because it reduces tension in the plantar ligaments (31), excessive buttressing of the arch may eventually lead to proprioceptive deficits, decreased storage and return of energy and various compensatory soft tissue injuries. Although not included in the study by Laughton et al. (29), impressions made using contact digitized scanning would most likely produce reproducible and valid measurements for everything except the forefoot/ rearfoot relationship and the degree of a plantarflexed first ray. This is typically not a concern as an experienced practitioner can modify the digital image of the foot to the desired shape. The primary disadvantage is the depth of the scan is typically too shallow to fabricate an orthotic with a medial flange. In contrast, because the pressure mat technique does not capture a true 3-dimensional image of
foot morphology, the height of the arch must be estimated from the available pressure data. Additionally, because Redmond et al. (18) confirm that even custom orthotics made with large varus posts do not alter the pathway of force transfer, dynamic information obtained from pressure mats should not be used when evaluating orthotic efficacy. This finding is in agreement with research by Miller et al. (82), who question the belief that orthotics produce a shift in the pathway of ground-reactive forces. Despite their inability to detect shifts in ground-reactive forces, the static and dynamic information obtained from pressure mats allows for the production of very effective orthotics, since the pressure data allows for precise placement of various accommodations, arch fill-ins, and posts. The clinical efficacy of orthotics made from plantar pressure data was demonstrated in a study by Franklyn-Miller et al. (30). These researchers performed a randomized control trial of 400 military recruits during basic training and determined that the trainees wearing orthotics made from plantar pressure data were 49% less likely to develop overuse injuries compared to the control group that did not wear orthotics. Last, in spite of the poor performance of laser scanning in the study by Laughton et al. (29), laser imaging produces exceptionally accurate 3-dimensional images in other industries (such as dentistry) and as technologies improve, image distortion associated with laser scanning will lessen, making it an accurate and highly reliable method for capturing negative impressions of 3-dimensional foot shape. In fact, because they can be performed in seconds and require significantly less skill than plaster casting (which can take years to perfect), it is expected that when the technical difficulties are corrected, digital scans will become the preferred technique for capturing 3-dimensional images of foot morphology. Laboratory Preparation and Orthotic Fabrication Although the practitioner need not be familiar with all stages of orthotic fabrication, a cursory understanding of the various materials, posting techniques, and orthotic additions is necessary for the clinician to prescribe the most appropriate orthotic. This section reviews each step in the process of manufacturing an orthotic. Modification of the Positive Model To begin with, a positive model must be obtained from the negative impression. This is accomplished by po sitioning the negative impression so the bisection of the rearfoot is vertical and then pouring a mixture of plaster and water into the negative impression. After the plaster has hardened, the negative slipper is torn off and the plantar surface of the positive model is smoothed by wet rubbing it with a wire mesh or wet sandpaper. The positive cast is then placed upright and gently rubbed in 309
Human Locomotion: The Conservative Management of Gait-Related Disorders
Figure 6.8. Modification of the positive model (see text).
a cirÂcular motion against the table. This determines the weight-bearing contact points of the first metatarsal head and the plantar heel (Fig. 6.8A and B). The weight-bearing point beneath the first metatarsal head is used to determine the outline of the orthotic shell, as a mark is placed 1 cm proximal to this point indicating the medial distal border of the shell (Fig. 6.8C). A metatarsal platform is then made by adding plaster to the plantar forefoot (Fig. 6.8D and E). To allow for soft tissue displacement upon weight-bearing, additional plaster is placed along the entire lateral aspect of the positive model (this is referred to as the lateral expansion) (Fig. 6.8F). The lateral aspect of the cast is also modified by grinding down a small area just below the fifth metatarsal (Fig. 6.8G). This modification allows the orthotic to lock the lateral column during propulsion by lifting upwardly on the distal aspect of the fifth metatarsal. In order to allow adequate deflection of the midtarsal joint during early stance phase, plaster is applied beneath the medial longitudinal arch (Fig. 6.8H). The addition of plaster beneath the arch is the most clinically significant modification of the positive model, because it determines how much the resultant orthotic will support the medial arch during stance phase. The precise degree of arch support can be achieved by instructing the orthotic laboratory to add or remove specific amounts of plaster. After the arch plaster has been added, the positive model is completed by blending the arch plaster into the metatarsal platform (Fig. 6.8I). The proximal aspect of the platform represents the point where the finished orthotic will end and creates a smooth drop off that evenly distributes pressure. When deciding upon the degree of arch support to add, the navicular drop measurement is often useful, and individuals with large degrees of navicular drop often require excessive buttressing of the medial longitudinal arch, while individuals with low degrees of navicular drop rarely require arch support. In situations where excessive buttressing of the medial arch is appropriate, the
laboratory can be instructed to apply only 3 mm of plaster to the positive model of an off weight-bearing cast. In rare situations, the shell can be molded directly to the unaltered positive model. Although this technique is occasionally used to treat functional hallux limitus, it should be used cautiously because it may result in impaired storage of energy and contusion of the soft tissues beneath the arch. In the presence of a cavovarus foot structure, the lab should be told to add up to 9 mm of plaster to the positive, since support of the arch in this situation is rarely necessary. It is also customary to lower the arch height of the positive model when making orthotics for individuals involved in multidirectional sports (e.g., soccer and tennis), as these athletes require greater amounts of arch deflection to allow for the various cuts, pivots and lateral movements necessary for these athletes to “feel� the playing surface. In addition to deciding upon the precise degree of arch support, the positive model can be modified to allow the apex of the arch to be positioned in a specific location (Fig. 6.9). As demonstrated by Nigg et al. (35), placing the apex of the arch beneath the sustentaculum tali provides effective control of pronation during the early portions of stance phase. While many laboratories prefer this arch
Figure 6.9. Apex locations for medial longitudinal arch support. The partial circle indicates the most common areas for arch apex. Redrawn from Kogler et al. (31).
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Chapter Six Foot Orthotics the medial column causes the foot to collapse medially during propulsion, a painful blister often forms where the orthotic contacts the shaft of the first metatarsal (Fig. 6.10). To lessen the possibility iatrogenic injury, in addition to lowering the shell beneath the proximal first metatarsal, it is often necessary to treat a hypermobile first ray by instructing the laboratory to maintain an elevated arch height beneath the navicular, as this improves efficiency of the peroneus longus and tibialis anterior muscles (37) and transfers pressure away from the shaft of the unstable first ray. Orthotic Posting Posting refers to the process of tilting or angling specific aspects of an orthotic shell or insole to alter motion and/or joint forces (Fig. 6.11). In the past, the degree of posting used was determined by the bony alignment of the rearfoot/lower leg. For example, Blake and Ferguson (36) suggest using progressively larger posts as the degree of static calcaneal eversion increases while Weed et al. (38) suggest posting to the measured degree of rearfoot varus. Although logical, the method of posting to alignment needs to be reconsidered, since 3-dimensional research confirms that posts produce inconsistent changes in frontal plane position of the heel (39), particularly in individuals presenting with excessive pronation (40). Because several studies suggest post efficacy may be related it its ability to improve ankle inversion moments (13,15,19), an alternate technique for determining post angles is to base the degree of posting on the length of the ankle moment arms. Since medial drift of the navicular
Figure 6.10. Blister location with a hypermobile first ray.
placement, the apex of an arch support is more appropriately placed beneath the central point of the navicular (31) or beneath the navicular/cuneiform articulation (36). Although the posterior support recommended by Nigg (35) may be useful when treating excessive contact period pronation, incorporating a posterior arch apex frequently causes contusion of the soft tissues near the calcaneal incline angle (the most proximal portion of the arch beneath the sustentaculum tali) and should therefore be used cautiously, particularly in individuals possessing large degrees of navicular drop. In cases of first ray hypermobility, the orthotic lab should be instructed to add plaster beneath the proximal aspect of the first metatarsal: Because hypermobility of
Figure 6.11. Methods of extrinsically posting the rearfoot and forefoot. A rearfoot varus post is created by grinding material placed on the heel of an orthotic shell (A) into a varus post (B). Conversely, a forefoot post is created by grinding material placed on the forefoot of an orthotic shell (C) into an angled wedge (in this case a forefoot valgus post) (D).
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Human Locomotion: The Conservative Management of Gait-Related Disorders and/or medial malleolus provides information related to ankle moment arms, the degree of posting may be determined by the degree of medial drift: Large degrees of medial drift should be treated with large varus posts, while small degrees of medial drift should be either not posted or posted with valgus posts. A common practice is to prescribe 2° to 4° varus posting for medial drift measurements from 10 to 15 mm, 4° to 6° varus posting for medial drift measurements greater than 15 mm, and 0° or valgus rearfoot posting for medial drift measurements less than 3 mm. It is also possible to use measurements from a supination resistance device. Another option when deciding upon a specific post angle is to ignore bony alignment and post the orthotic to treat a specific symptom or movement pattern. The efficacy of this treatment approach was demonstrated by Joseph et al. (24), who used varus posts to alter valgus collapse of the knee regardless of foot type. Using this approach, an individual with a neutral foot and chronic tibialis posterior tendonitis could be treated with a 4° varus post. If it is determined that orthotic posting is necessary, it is possible to apply the post using either intrinsic or extrinsic techniques. As the name implies, intrinsic posting requires modifying the positive model so the final post angle is built directly into the shell, while extrinsic posting is accomplished by adding an angled material to the shell itself. The simplest method of intrinsically posting an orthotic is by grinding plaster off the medial aspect of the heel (Fig. 6.12). Referred to as a Kirby skive (41), an orthotic made from this impression would have a flattened and slightly inverted medial heel, allowing the heel of the finished orthotic to function as a rearfoot varus post. According to Blake and Ferguson (36), the flattened and inverted medial heel allows for greater inversion force production by the orthotic, because a flat heel creates a more vertically directed force than a rounded heel. In cases of extreme pronation, the Kirby skive can be extended distally to the talonavicular joint. It can also be incorporated into the lateral aspect of the heel to encourage pronation in high-arched feet. An intrinsic technique that protects against excessive supination is the cuboid notch or Feehery modification (36). This intrinsic addition requires grinding a 3 mm deep skive directly beneath the cuboid (Fig. 6.13) and is useful in the management of high-arched foot types and in individuals presenting with recurrent ankle sprains as a slight elevation of the orthotic shell beneath the cuboid may serve as a proprioceptive tool to improve balance. As described by Lundeen (42), a true intrinsic rearfoot post requires sectioning the positive model to the axis of the subtalar joint and rotating the rearfoot section into the desired degree of valgus or varus (Fig. 6.14). The crease in the positive model is then blended smoothly with additional plaster to prevent the formation of a potentially painful edge that would otherwise be molded into the shell.
Figure 6.12. The Kirby skive. This intrinsic post is manufactured by removing a small section of plaster from the heel of the positive model. Because the plane of the heel skive angles 15° to the transverse plane, it allows the heel of the finished orthotic to function like a varus post. To prevent discomfort, it is important that the medial skive is not perfectly flat or concave, since this would cause the orthotic to dig into the medial aspect of the heel.
Figure 6.13. The cuboid notch. By removing plaster from the section of the positive model abutting the cuboid, the finished orthotic shell will have a slight elevation supporting the cuboid.
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