THE AUDIBLE POP FROM HIGH-VELOCITY THRUST MANIPULATION AND OUTCOME IN INDIVIDUALS WITH LOW BACK PAIN Timothy W. Flynn, PT, PhD, OCS,a John D. Childs, PT, PhD, OCS,b and Julie M. Fritz, PT, PhDc
ABSTRACT Objective: To determine the relationship between an audible pop with spinal manipulation and improvement in pain and
function in patients with low back pain. Methods: In this pragmatic study, 70 patients from a multicenter clinical trial were randomly assigned to receive high-
velocity thrust manipulation and included in this secondary analysis. Patients were managed in physical therapy twice the first week, then once a week for the next 3 weeks, for a total of 5 sessions. A single high-velocity thrust manipulative intervention purported to affect the lumbopelvic region was used during the first two sessions. Therapists recorded whether an audible pop was heard by the patient or therapist. Outcome was assessed with an 11-point pain rating scale, the Oswestry Disability Questionnaire, and measurement of lumbopelvic flexion range of motion. Repeated measures analyses of variance were used to examine whether achievement of a pop resulted in improved outcome. Results: An audible pop was perceived in 59 (84%) of the patients. No differences were detected at baseline or at any follow-up period in the level of pain, the Oswestry score, or lumbopelvic range of motion based on whether a pop was achieved ( P N .05). The odds ratios and 95% confidence intervals for achieving a successful outcome at each of the followup periods all approximated a value of 1, suggesting no improvement in the odds of successful outcome among patients in which an audible pop occurred. Conclusions: The results of this pragmatic study suggest that a perceived audible pop may not relate to improved outcomes from high-velocity thrust manipulation for patients with nonradicular low back pain at either an immediate or longer-term follow-up. (J Manipulative Physiol Ther 2006;29:40- 45) Key Indexing Terms: Manipulation; Spinal; Low Back Pain; Sacroiliac Joint; Audible Pop; Cavitation
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a Department of Physical Therapy, Regis University, Denver, Colo. b Assistant Professor, Director of Research, US Army–Baylor University Doctoral Program in Physical Therapy, San Antonio, Tex. c Department of Physical Therapy, University of Utah, Salt Lake City, Utah. Sources of support: Supported in part by a grant from the Foundation for Physical Therapy and the Wilford Hall Medical Center Commander’s Intramural Research Program. Disclaimer: The opinions and assertions contained herein are the private views of the author (J.D.C.) and are not to be construed as official or as reflecting the views of the Department of the Air Force or the Department of Defense. Submit requests for reprints to: Timothy W. Flynn, PT, PhD SHCP- Department of Physical Therapy, Regis University, 3333 Regis Blvd, G-4, Denver, CO 80221-1099 (e-mail: tflynn@regis.edu). Paper submitted September 13, 2004; in revised form May 13, 2005. 0161-4754/$32.00 Copyright D 2006 by National University of Health Sciences. doi:10.1016/j.jmpt.2005.11.005
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ow back pain (LBP) is the second most common reason individuals visit a physician’s office,1 and it affects 60% to 90% of individuals during their lifetime.2 Spinal manipulation is an intervention commonly used in the treatment of individuals with LBP and is reported to be more effective than placebo3 - 6 or other interventions.7 - 10 However, a recent meta-analysis concluded that there is no evidence that spinal manipulative therapy is superior to other standard treatments for patients with acute or chronic LBP.11 This continued discrepancy in the literature regarding the efficacy of spinal manipulation is likely due to the generally low quality of many of the previous randomized controlled trials as well as a failure to identify appropriate subgroups of patients likely to respond to manipulation. In fact, recent studies have shown that when patients with LBP are correctly selected, the treatment response from spinal manipulation is robust.12,13 One subgroup in which spinal manipulation is proposed to be effective is in individuals with sacroiliac (SI) region dysfunction.5,7,8,14 High-velocity low-amplitude thrust manipulation technique (HVT) is one of the most frequently used forms of spinal manipulation. A cracking sound or audible bpop Q often
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accompanies HVT manipulation of the lumbopelvic spine. A practitioner’s perception of the occurrence of an audible pop during spinal manipulation is reportedly very accurate,1 and many clinicians and patients consider an audible pop necessary for the manipulation to be successful.2,15 It is generally believed that the audible pop occurring with manipulation is caused by a cavitation mechanism that occurs with separation of the facet surfaces within the spinal zygapophyseal joint.16 Cavitation is a well-documented engineering phenomenon that describes the generation and collapse of gas or vapor bubbles in a liquid.17 Roston and Haines18 were the first to investigate the cavitation phenomenon in a synovial joint. After an HVT manipulation of the metacarpophalangeal (MCP) joint, the authors reported the presence of a radiolucent cavity and change in the mechanical behavior of the MCP joint.18 Subsequent cineradiographic studies have reported increased joint space and increased carbon dioxide gas within the MCP joint after an HVT manipulation.19,20 Carbon dioxide is reportedly the gas with the highest miscibility in synovial fluid and this has been suggested as a mechanism that would improve range of motion (ROM) of a joint after HVT manipulation.17 Furthermore, it has been hypothesized that the cavitation phenomenon may be the mechanism responsible for initiating certain reflex relaxation of the periarticular musculature, which purportedly occurs with HVT manipulation.17 However, despite the general acceptance that the audible pop is the outward manifestation of the occurrence of the cavitation phenomenon within a joint, this relationship in the spine has not been established. The literature is sparse regarding the audible pop during HVT manipulation in the spinal region.1,6,15 Previous reports have primarily reported on the forces required to achieve an audible sound. Beffa and Mathews15 noted that two different manipulation procedures were not able to consistently achieve a cavitation sound in the targeted spinal segment despite a proposed specificity of the techniques. Ross et al 21 determined the accuracy of audible cavitations of the thoracic and lumbar manipulation using accelerometers. These authors noted that lumbar spine manipulation was accurate only about 50% of the time and that frequently multiple cavitations occur.21 Unfortunately, these previous studies did not evaluate the relationship of the presence of an audible pop to changes in patient outcomes. Ultimately, practitioners of manipulation should base the significance of cavitation sounds on their relationship to clinical decision making and patient outcomes. However, there is a paucity of evidence describing the relationship between an audible pop and symptomatic improvement with manipulation, yet many practitioners continue to gauge the success or failure of the procedure based on presence or absence of a pop.6 Recently, we reported on a series of 71 patients with nonradicular LBP where HVT thrust manipulation was used in their treatment.22 In this cohort, there was no relationship between an audible pop during SI
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region manipulation and improvement in ROM, pain, or disability.22 In addition, the occurrence of a pop did not improve the odds of a dramatic improvement with manipulation treatment. This previous study was limited to only the short-term outcome (2 days) and did not attempt to measure the relationship over a longer period. Therefore, the purpose of this study was to investigate whether the occurrence of a manipulative pop during SI region manipulation is related to the outcome of the intervention over a 4-week period. Furthermore, we attempted to assess the robustness of the previous finding using a new sample of patients and clinicians from a variety of practice settings.
METHODS Seventy patients from a multicenter clinical trial comparing manipulation to exercise were randomly assigned to receive HVT manipulation and were used in this secondary analysis. The primary results of this study are reported elsewhere.13 Patients between the ages of 18 and 60 years with a primary complaint of LBP with or without referral into the lower extremity and a modified Oswestry Disability Questionnaire (ODQ) score of at least 30% were considered for inclusion. Patients with bred flagsQ for a serious spinal condition (eg, tumor, compression fracture, infection) were excluded as were individuals who were pregnant, had prior surgery to lumbar spine or buttock, or had any signs consistent with nerve root compression (ie, positive straight leg raise b 458 or diminished reflexes, sensation, or lower extremity strength). A total of 13 physical therapists at 8 clinics located in a variety of health care settings and geographical regions throughout the United States provided the interventions. These settings included two academic medical centers in addition to private and public outpatient practice settings, including several clinics in the US Air Force. Each site’s institutional review board approved the study before recruitment and data collection began. Patients completed several self-report measures and then received a standardized history and physical examination. Self-report measures included a body diagram to assess the distribution of symptoms.23 An 11-point pain rating scale ranging from 0 (no pain) to 10 (worst imaginable pain) was used to assess current pain intensity and the best and worst level of pain during the last 24 hours.24 The average of the three ratings was used. The modified ODQ is a regionspecific disability scale for patients with LBP.25 Previous research has shown this instrument to have high levels of reliability, validity, and responsiveness.26 Physical examination measures included lumbar spine active ROM 27 and a variety of tests purported to identify dysfunction in the lumbopelvic region.28 Lumbopelvic ROM was measured with an inclinometer in standing. The inclinometer was centered over the T12 spinous process and zeroed. The patient was instructed to bend forward as far as possible
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Table 1. Baseline demographic characteristics
Variable
All subjects
Age (y) 33.3 (11.2) Sex 43% Female Male 40 Female 30 Duration of 111 (366) symptoms (d) Body mass 27.7 (4.7) index
Audible pop present (n = 59)
Audible pop absent (n = 11)
33.2 (10.6) 37.6 (9.8) 36 25 117 (393) 27.6 (4.2)
Significance .22 .81
6 5 82 (169) .45 27.8 (4.8)
.91
Variable values represent mean (SD).
Fig 1. Sacroiliac region manipulation technique used in this study. Reprinted from Childs JD, Fritz JM, Piva SR, Erhard RE. Clinical decision making in the identification of patients likely to benefit from spinal manipulation: a traditional versus an evidence-based approach. J Orthop Sports Phys Ther 2003;33(5):259-72, with permission of the Orthopaedic and Sports Physical Sections of the American Physical Therapy Association.
without bending the knees and the inclinometer value was recorded. This method has been shown to have excellent reliability (ICC = 0.94).27 Outcome was assessed with the ODQ and measurement of lumbopelvic flexion ROM at baseline, 1-week, and 4-week follow-up.
Treatment Protocol Patients attended physical therapy twice during the first week and then once a week for the next 3 weeks, for a total of 5 sessions. During the first two sessions, patients received spinal HVT manipulation and a ROM exercise (tilting the pelvis while supine) only. The use of ROM exercise protocol was designed to be consistent with current best practice guidelines that emphasize the early and gradual activation of patients with acute LBP.29 The manipulation was performed first using a supine technique previously described in the literature.12 To perform the technique, the patient was supine with the physical therapist on the opposite the side to be manipulated. The patient was moved into side bending toward the side to be manipulated. The patient was asked to interlock the fingers behind the base of the neck. The physical therapist then rotated the patient and delivered a smooth thrust manipulation to the pelvis in a posterior and inferior direction (Fig 1). The side to be manipulated was the more symptomatic side, based on the patient’s report. If the patient was unable to specify a side, the physical therapist selected a side to be manipulated. If the patient or therapist heard a cavitation (ie, a pop), the physical therapist proceeded to instruct the patient in the
ROM exercise. If no cavitation was perceived, the patient was repositioned, and the manipulation was attempted again. If no cavitation was achieved on the second attempt, the physical therapist repeated the procedure on the other side. A maximum of two attempts per side was permitted. If no cavitation was perceived after the fourth attempt, the physical therapist proceeded to instruct the patient in the ROM exercise. Patients were instructed to perform 10 repetitions of the ROM exercise in the clinic and 10 repetitions 3 to 4 times daily on the days they did not attend physical therapy. Beginning on the third session, patients began a stabilization exercise program that included a low-stress aerobic and a lumbar spine strengthening program. The strengthening program was designed to target the trunk musculature identified as important stabilizers of the spine in the biomechanical literature.30 All patients were provided an exercise instruction booklet outlining the proper performance and frequency of each exercise and were instructed to perform their assigned exercise program once daily on the days they did not attend therapy. Based on the benefits associated with remaining active,31 patients were given advice to maintain usual activity within the limits of pain.
Data Analyses A 2-way, group (pop vs no pop) time (baseline, 1 week, and 4 weeks) repeated measures analysis of variance was used to examine whether achievement of a pop resulted in improved disability with the group time interaction serving as the primary hypothesis of interest. The a level was established a priori to be .05. Separate repeated measures analyses of variance were performed to examine whether achievement of a pop resulted in improved pain and ROM. At each follow-up period, patients who achieved at least a 50% improvement in their ODQ score were classified as a success. v 2 Tests and odds ratios with associated 95% confidence intervals were calculated to determine to determine if the odds of a
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those patients in which an audible pop occurred and in those in which it did not occur in terms of age, sex, duration of symptoms, or body mass index ( P N .05) (Table 1). No differences in pain, disability, or lumbar ROM existed at baseline, 1 week, or 4 weeks between the patients who experienced an audible pop and those who did not (Fig 2). The statistical power to detect a clinically meaningful difference of 6 or 8 points on the ODQ was 57% and 78%, respectively. The statistical power to detect a clinically meaningful difference of 2 points on the pain rating scale was 91%. The statistical power to detect a clinically meaningful difference of 108 of lumbar spine flexion ROM was 37%. Finally, the occurrence of an audible pop did not improve the odds of achieving a successful outcome (z50% reduction in the ODQ) at the 1- or 4-week period based on the inclusion of 1 in the confidence interval. The odds ratio was 1.1 (95% CI, 0.29-3.86) at 1 week and 1.7 (95% CI, 0.41-7.1) at 4 weeks.
DISCUSSION
Fig 2. Comparison of outcomes between the group where an audible pop occurred and those that did not for ODQ (A), pain intensity (B), and lumbar flexion ROM (C). successful outcome improved among patients in whom a pop was achieved.
Clinicians who use HVT spinal manipulation often believe that an audible pop is necessary if a manipulation technique is to result in improvements in ROM, pain, and function. This belief persists despite a paucity of evidence regarding this relationship.16,17 Consistent with our previous report using short-term outcomes,22 this study using a new sample of patients and a different sample of physical therapist clinicians also failed to show statistical or clinically important differences between the patients who experienced an audible pop and those who did not when using a specified manipulation technique. Our results show that an audible pop during a spinal manipulation occurs frequently (84%) yet is not related to the outcome of the intervention. The actual phenomenon that the audible pop during spinal manipulation represents remains speculative. There is little evidence to suggest that the cavitation phenomenon thought to be responsible for the audible pop in the MCP joint is the same mechanism responsible for the audible pop produced during spinal manipulation.16 Alternate theories of the biomechanical effects of spinal manipulation have been proposed, which incorporate the bsnapping backQ of distended capsular ligaments or the movements of fat pads in and out of the zygapophyseal joint.17,32
RESULTS Seventy patients received a manipulation and exercise intervention. All were available for 1-week follow-up and 68 were available for 4-week follow-up (one discontinued due to a family emergency and one due to employer time constraints). An audible pop occurred in 54 of the patients in session 1 and 51 of the patients in session 2. Therefore, an audible pop occurred in 59 (84%) of the patients and was absent in 11 (16%) of the patients in at least one of the treatment sessions. No differences were observed between
Limitations In the present study, there was no mechanism available to independently verify where in the lumbopelvic region the sound emanated from or if an audible pop occurred but was not perceived by either the patient or therapist. The therapists attempted to specifically localize to the SI region and insure that the SI region was where the noise occurred. However, this is very difficult to verify in clinical practice without the use of accelerometry. Anecdotally, the authors
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and others have described the sound from an SI region manipulation to be more of a bclunkQ rather than a bpop.Q However, any joint sound that was present was considered a positive audible pop regardless of the tone of the sound. Although the treating therapists documented whether or not an audible pop occurred or was felt by the patient, the therapists did not consider different popping sounds to be associated with different outcomes. There was no independent verification of this sound via microphone or accelerometers in this study. However, Ross et al21 has clearly shown that manipulation of the lumbar spine is not very accurate. The manipulative providers in the study of Ross et al21 were chiropractic faculty members with a large quantity of clinical experience. Most manual practitioners do not attach accelerometers to their patients before a manipulative intervention. However, there is documentation of manipulative practitioners bconsideringQ the relevance of this noise in terms of success or nonsuccess of the treatment. As experienced providers of manipulation, we are aware of the bpalpableQ release that sometimes occurs; however, this was not examined in the present study. We focused on the baudibleQ release that is often associated with manipulative interventions. Therefore, this study provides information to those providers that use an audible sound in their clinical decision making yet do not have access to accelerometers in everyday practice. In this study, we only used one spinal manipulation technique owing to the treatment standardization required in the larger controlled clinical trial. It is possible that different manipulation techniques or removing other patient interactions or co-interventions (eg, ROM mobilization) could result in different responses or relationships to the audible pop. A recent report15 using two seemingly distinct manipulation procedures showed that the location of the cavitation sounds were neither consistent nor related to which technique was performed. So despite the accuracy of the practitioner’s perception of the occurrence of an audible cavitation during spinal manipulation,1 the precise spinal location and anatomical tissues responsible for the audible pop and the meaning that practitioners should place on these sounds remain illusive. Further research should replicate this study using accelerometer technology, in attempt to more precisely define the various audible and palpatory phenomenon that occur during manipulation procedures. However, these types of studies should ultimately focus on the relevance to clinical decision making and patient outcomes.
CONCLUSION That a large percentage of our subjects perceived a pop (84%), and there appeared to be no relationship with the patient outcome, which suggests that the audible pop should not be the primary focus of the clinician or the patient
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during spinal manipulation. The present study conducted using a new sample of patients and clinicians is consistent with our previous report22 and suggests that the opinion of Bourdillon, who stated that the belief shared by some patients and operators that if there is no joint noise associated with manipulative procedure, then nothing happened, may be incorrect.33 It appears that there may be little to no relationship between the audible pop frequently noted during spinal manipulation and improvement in pain, lumbar ROM, and disability in individuals with nonradicular LBP. Furthermore, the occurrence of a pop did not improve the odds of improvement after spinal manipulation. Therefore, practitioners who use these techniques should be cautious in attributing therapeutic benefit to the audible pop for patients with nonradicular LBP.
ACKNOWLEDGMENT The authors would like to acknowledge the following physical therapists at the University of Pittsburgh Medical Center Health System’s Center for Rehab Services and at a variety of physical therapy clinics in the US Air Force for their assistance with data collection: Maria West, Evan Kelley, David Browder, Mike Blowers, Sherri Morrow, Brian Langford, Jeff McGuire, Cory Middel, and Trevor Petrou.
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