FROM: J Manipulative Physiol Ther. 2017 (Mar); 40 (3): 187200 ~ FULL TEXT
Gregory D. Cramer, DC, PhD, Matthew Budavich, DC,
Preetam Bora, MSME, Kim Ross, DC, PhD
Department of Research,
National University of Health Sciences,
OBJECTIVE: This feasibility study used novel accelerometry (vibration) and microphone (sound) methods to assess crepitus originating from the lumbar spine before and after side-posture spinal manipulative therapy (SMT).
METHODS: This study included 5 healthy and 5 low back pain (LBP) participants. Nine accelerometers and 1 specialized directional microphone were applied to the lumbar region, allowing assessment of crepitus. Each participant underwent full lumbar ranges of motion (ROM), bilateral lumbar SMT, and repeated full ROM. After full ROMs the participants received side-posture lumbar SMT on both sides by a licensed doctor of chiropractic. Accelerometer and microphone recordings were made during all pre- and post-SMT ROMs. Primary outcome was a descriptive report of crepitus prevalence (average number of crepitus events/participant). Participants were also divided into 3 age groups for comparisons (18-25, 26-45, and 46-65 years).
RESULTS: Overall, crepitus prevalence decreased pre-post SMT (average pre = 1.4 crepitus/participant vs post = 0.9). Prevalence progressively increased from the youngest to oldest age groups (pre-SMT = 0.0, 1.67, and 2.0, respectively; and post-SMT = 0.5, 0.83, and 1.5). Prevalence was higher in LBP participants compared with healthy (pre-SMT-LBP = 2.0, vs pre-SMT-healthy = 0.8; post-SMT-LBP = 1.0 vs post-SMT-healthy = 0.8), even though healthy participants were older than LBP participants (40.8 years vs 27.8 years); accounting for age: pre-SMT-LBP = 2.0 vs pre-SMT-healthy = 0.0; post-SMT-LBP = 1.0 vs post-SMT-healthy = 0.3.
CONCLUSIONS: Our findings indicated that a larger study is feasible. Other findings included that crepitus prevalence increased with age, was higher in participants with LBP than in healthy participants, and overall decreased after SMT. This study indicated that crepitus assessment using accelerometers has the potential of being an outcome measure or biomarker for assessing spinal joint (facet/zygapophyseal joint) function during movement and the effects of LBP treatments (eg, SMT) on zygapophyseal joint function.
KEYWORDS: Facet Joint; Spinal Manipulation; Zygapophyseal Joint
From the FULL TEXT Article:
Zygapophyseal Joint Vibrations and Sounds
Crepitus (audible sound arising from tissues during normal motion) originating from the vertebral column has been associated with degeneration of the posterior joints of the spine (facet joints/zygapophyseal [Z] joints).  More subtle crepitus (fine crepitus) may possibly be related to joint hypomobility (increased joint stiffness) or loss of joint lubrication.  Protapapas and Cymet  made an interesting observation: The noises of normal and abnormal [Z] joints are built into the nature of the structures. To ignore these noises would be foolish, as they might be used to help determine the effectiveness of treatment.  Cavitations are audible sounds commonly associated with spinal manipulative therapy (SMT)  and are theoretically associated with gas (probably carbon dioxide) [11, 12] entering joints as they gap during manipulation. [6, 7, 10, 13]
One theory of the mechanism of the anatomic and biomechanical beneficial effects of SMT is depicted in Figure 1. Briefly, connective tissue adhesions develop in hypomobile Z joints,  which would be accompanied by increase joint sounds and vibrations during motion (eg, crepitus); the Z joint facet surfaces separate (gap) during SMT,  which is sometimes accompanied by cavitation ; and the gapping is thought to break up the connective tissue adhesions, allowing restoration of joint mobility. [15, 16] Over time (weeks to months) this would lead to decreased joint vibrations and sounds during motion (decreased crepitus).
In addition, we hypothesize that LBP participants would have more crepitus than healthy participants because of increased muscle tightness of LBP participants, leading to relative compression of the articular surfaces making up the Z joints. Such compression could conceivably interrupt the normal smooth gliding motion of the Z joints, resulting in crepitus vibrations. We also hypothesize that older participants would have more crepitus than younger ones because of decreased joint lubrication and increased joint degeneration in older Z joints, resulting in more crepitus vibrations/sounds.
A few studies have been conducted to explore the mechanisms and consequences of cavitation during SMT. [9, 10, 1821] However, there have been no studies assessing lumbar crepitus. In addition, the peer-reviewed literature has no record of a study assessing tissue characteristics of the spine using acoustics or a study comparing accelerometers with microphones in assessing acoustic data originating from the spine (including data associated with Z joint crepitus).
Such sounds have been studied in the knee.  In addition, analysis of vibration and sound waves recorded from accelerometers and microphones have been used rather extensively to assess sounds as a type of biomarker associated with temporomandibular joint dysfunction,  as compared with temporomandibular joints functioning normally. [33, 34] Interestingly, Brown even identified unique sound patterns in muscles of patients with Parkinsons disease. 
Our previous work used accelerometers to assess Z joint cavitations during SMT.  This feasibility study (N = 10) was designed to continue the process of addressing the current gaps of knowledge by further developing previous methods and evaluating additional methods to assess and localize Z joint crepitus recorded during lumbar motion both before and after SMT. More specifically, this study evaluated recordings made simultaneously from accelerometers and a specialized directional microphone. Recordings were made from both healthy (n = 5) and low back pain (LBP, n = 5) participants of different age groups. The reason for using both accelerometers and a microphone is that each may produce unique and complementary information. 
Summary of Main Findings
This feasibility study successfully collected crepitus data from the lumbar region of 10 participants (5 healthy, 5 LBP) from 3 different age ranges. Overall, crepitus prevalence decreased after SMT. In addition, prevalence progressively increased from the youngest to oldest age groups and was higher in LBP compared with healthy participants. The majority of crepitus was recorded during flexion and participants generally did not hear or feel the recorded crepitus.
The research extended previous work on cavitation during SMT  by evaluating recordings of crepitus made from both accelerometers and from a specialized directional microphone during full ROMs. These methods are novel to the assessment of spinal crepitus, and the peer-reviewed literature has no record of a study assessing spinal crepitus by any means.
Accelerometers vs Microphone
Even though the amplitude of crepitus was approximately one-tenth that of cavitation, the accelerometers performed well in this study and crepitus was identified as distinct events of multiple accelerometers responding in such a way that the specific Z joints of origin could be identified.
Previous reports using both accelerometers and a microphone indicated that each produced unique and complementary information in the assessment of other tissues [2529, 36] and in the assessment of cavitation of the lumbar region associated with SMT.  However, in this study, even though the microphone recordings did move from the baseline during crepitus, the response was erratic. The amplitude of the microphone signal was close to baseline in all artifacts, which contrasted with the movement from baseline identified in crepitus. The artifacts in this study were very similar to those identified in previous research assessing cavitation and were usually the result of a participants gown or shorts bumping 1 or more accelerometers. This created a distinct appearance on the accelerometer recordings (Fig 5C).  As mentioned, the artifacts could have been identified without the use of a microphone. The microphone may not have been useful in this crepitus study because the amplitude of crepitus was markedly less than that previously recorded for cavitations.  The low amplitude, combined with the location of the microphone a considerable distance from most of the recorded crepitus, likely precluded adequate assessment of crepitus by the microphone. The microphone was positioned at T12 to avoid contact with any of the 9 accelerometers. Future studies of crepitus by our group will probably not include microphone recordings.
Z Joints as the Origin of Crepitus
Although other anatomic structures (eg, muscles, ligaments) cannot be completely ruled out as the possible source of the recorded vibrations (crepitus), the Z joints are the most likely source of the recorded crepitus for 2 primary reasons. First, similar waveforms were identified from artificially produced crepitus conducted during validity testing of the methods  (unpublished data). These experiments were conducted on a spine embedded in silicone and vibrations originating directly from the Z joints were produced and recorded. Second, recordings not matching the types of waveforms previously reported from cavitations (only with lower amplitudes) [5, 1921, 43] and artificial crepitus (unpublished data), produced in reliability testing, were not included as crepitus in this study. Consequently, crepitus may have potentially been underreported in this study; however, the methods increased the confidence that reported crepitus was of Z joint origin.
Crepitus increased with age. This is consistent with the theory that crepitus increases with arthritic changes [2, 3] and Z joint arthritic changes increase with age.  Although 8 participants were enrolled from the older 2 age groups, as was the goal, no LBP participants were enrolled from the oldest age group.
Crepitus in Healthy and LBP Participants Pre- and Post-SMT
The finding that more crepitus was recorded in pre-SMT LBP than in healthy participants in spite of the younger age of the LBP participants was interesting. After SMT (post-SMT), the number of crepitus events (both total crepitus and joints producing crepitus) in LBP and healthy participants was almost identical (Figs 6C and 6D). One hypothesis to explain the difference in pre-SMT crepitus is that increased muscle tightness of the LBP participants led to relative compression of the articular surfaces making up the Z joints. Such compression could conceivably cause the normal smooth gliding motion of the Z joints to change to a series of small rapid motions, which could result in crepitus vibrations. The results of 1 LBP participant in the study may provide an example that supports this hypothesis. In this participant, crepitus was recorded from the same joint (right L4/5) 6 times throughout the 4 seconds of pre-SMT flexion. The accelerometer recordings went completely to baseline between each instance of crepitus. After SMT there was only 1 crepitus recorded from this joint during flexion. The average amplitude of the 6 pre-SMT recordings was 0.10 mV and the amplitude of the single post-SMT R L4/5 recording was 0.04 mV. This would suggest the joint was moving in a more regular (smooth) pattern after SMT. The small number of participants in this feasibility study prevents further speculation on this issue.
Participant Report of Crepitus vs Recorded Crepitus
Overall, participant report of crepitus was not an indicator of whether or not a crepitus was recorded. This finding is quite different from previous studies on cavitation during SMT, where participant (and clinician) report of cavitation was closely related to recorded cavitation. [10, 19, 21] As discussed previously, the wave magnitude of crepitus was markedly less than that of a typical cavitation. The more subtle crepitus vibrations detected by the accelerometers likely went undetected by the participants in this study. There were also 12 instances when the participant heard or felt a sound or vibration during a motion, but none was recorded. At first this seemed more difficult to explain. However, in a previous study there was an instance where a participant responded that she or he heard a cavitation but none was recorded.  We hypothesized that the cavitation in this instance occurred above the lumbar level and therefore was not recorded by the lumbar accelerometers. The same phenomenon may very well be occurring in this study, one difference being that during the full ROM recordings, the thoracic region was going through full ROMs along with the lumbar region, whereas during the SMT cavitation study, the SMT force was directed to the lumbar region, resulting primarily in lumbar segmental motion. [1921, 41] Consequently, much more motion occurred in the thoracic region in this study than in previous ones, and we hypothesize that some of the crepitus heard and felt by the participants was originating from the thoracic region and the lumbar-placed accelerometers did not respond to the subtle thoracic crepitus. Participants were asked about sounds and vibrations in the spine and most likely responded yes if any sound or vibration was heard or felt in the thoracic region. To further assess this issue, future studies should use procedures to minimize thoracic motion during ROM recordings and ask specific questions to verify the location of the reported sound or vibration.
Ranges of Motion
Flexion was without question the motion that produced the most crepitus (60.9% overall). This result probably is due to the lumbar region having considerably more motion in flexion (60°) than any other motion.  Future studies using crepitus as a secondary outcome measure could perhaps be limited to recordings taken during flexion only in order to use time most efficiently. However, because crepitus was recorded during all 6 motions (Fig 7), research using crepitus as a primary outcome should continue to record during all ROMs.
SMT and Crepitus
Spinal manipulative therapy had an effect on crepitus in this study, both the Z joints producing crepitus and the amplitude of crepitus (overall less amplitude post-SMT). However, the results indicate that the relationship between crepitus and SMT may be complex. Total crepitus was reduced after SMT in this study; however, 9 instances of crepitus from 8 Z joints were recorded post-SMT (Fig 6). Interestingly, with only 2 exceptions, the Z joints producing crepitus post-SMT were different from those that produced crepitus pre-SMT. This would seem to indicate a change beyond that of mere chance.
There may be several reasons for the prepost SMT differences. The Z joint movement (gapping) produced by SMT [17, 37] may have added sufficient mobility to the pre-SMT crepitus-producing joints that they moved more freely and did not produce crepitus post-SMT. On the other hand, the SMT force on pre-SMT noncrepitus-producing joints that were somewhat hypomobile may have induced enough motion that they began to move into a range that had not been used immediately pre-SMT; consequently, the joint surfaces may have been contacting in facetal areas that were not as well lubricated (ie, less synovial fluid in these regions) as the more frequently used facetal regions, thus creating crepitus as the joints entered the previously hypomobile zones. Much more work is needed to assess the veracity of this speculation.
Future research on larger numbers of healthy and LBP participants from different ages would help to clarify the effects of SMT on crepitus. Combining studies assessing crepitus with imaging of the Z joints could potentially help to study the relationship between Z joint degeneration and crepitus that has been alluded to by other authors.  Diagnostic ultrasound, which is a cost-effective, low-risk (ie, nonionizing radiation) imaging modality,  would be an appealing candidate for complementary use with the accelerometry outcomes in future clinical mechanistic research. We are currently assessing the feasibility of adding ultrasound to future studies.
Limitations and Future Research
This was a feasibility study on a small number of participants. Therefore, the results cannot be generalized to a larger population but may be used to inform future studies. In addition, largely because of the convenience sample of faculty, staff, and students from a complementary and integrative health university, there were no LBP participants in the 46 to 65 age group; consequently, the LBP participants were younger than the healthy participants. In addition, they had only mild LBP (3 of 10). Even with these limitations, differences in crepitus were found for age, LBP vs healthy participants, the effects of SMT on crepitus, and the motions that produced crepitus.
Although reliability and validity studies have been completed for the phenomenon of cavitation during SMT, [1921, 43] further validity studies of the subtler phenomenon of crepitus are currently underway.
The findings of this small feasibility study indicate that further research in this area is merited. Crepitus in healthy and LBP participants and in participants from different age ranges and with Z joints of different levels of degeneration  should be evaluated in studies fully powered to allow for generalizability of data to larger populations. Such studies should recruit from a larger population, as has been done in previous, larger clinical trials conducted by our research group. [37, 5255] The broader recruiting methods will likely lead to more LBP participants from the older age group. In addition, the larger studies would allow for further assessment of SMT on crepitus, possibly shedding further light on the complex relationships between these 2 variables.
A power analysis was conducted using PROC POWER in SAS 9.4 (SAS Institute Inc., Cary, NC) to determine the number of participants needed for future clinical investigation. The sample size estimates were based on means and standard deviations from the descriptive statistics of this feasibility study. All power analyses conducted assumed 2-sided tests, β = 0.80, α = 0.05, and a normal distribution of the data. Power analysis to compare pre- and post-SMT measures of crepitus, which would be the primary study objective, that was done using a paired t test (SD = 1.8; assuming r = 0.5) indicated that 28 participants would be needed to complete the study to detect a mean difference of 1, which is hypothesized to identify a meaningful change.
Secondary objectives would be to determine if there are differences in mean crepitus between healthy and LBP participants, as well as in younger participants and older participants (2-sample t test assuming equal variances; mean difference = 1 crepitus/participant large effect). To compare crepitus pre-SMT in healthy vs LBP participants (SD = 2.5), 100 participants would need to be in each group. To compare pre-SMT crepitus in adults aged 26 to 45 years with adults aged 46 to 65 years (SD = 2.7), 116 participants would be needed in each group. Consequently, the ideal study would be 232 participants with 116 LBP and 116 healthy participants and the same numbers (ie, 116) between the ages of 26 to 45 and between the ages of 46 and 65. Studies assessing purely the effects of SMT on crepitus could be conducted on many fewer (ie, 28) total participants.
In addition, the accelerometry methods could be further enhanced through the process of automation. The methods for analyzing each recording were extremely time consuming. Each recording took approximately 1 hour for a well-trained researcher to assess, and there were 12 recordings per participant, 1 for each of 6 motions both pre- and post-SMT. In addition, to increase reliability, 2 observers assessed each recording independently and then came to consensus. Therefore, approximately 240 hours were required to assess the recordings. Automation of the methods is feasible (personal communications with P. K. Raju, PhD, and D. Marghitu, PhD, professors of acoustics at Auburn University),  and accomplishing such automation would allow for more rapid assessment and quantification of the data. Increasing the speed of crepitus analysis would aid in future clinical studies.
Biomarkers are essential to the advancement of research into complementary and integrative health therapies, including manual therapies.  If the accelerometry analysis automation and studies outlined earlier are successful, joint vibrations (crepitus) identified during lumbar motion could eventually be used as an outcome measure (biomarker) to assess Z joint function and to assess the effects of LBP treatments, including SMT, on the Z joints. Such new information could potentially help with the important efforts of identifying subpopulations of LBP patients who respond best to various types of treatments. [57, 58]
In this feasibility study, data collected from accelerometers provided useful information indicating the prevalence of crepitus increased with age, was higher in LBP than healthy participants, and decreased overall after SMT. Consequently, crepitus assessment using accelerometers has the potential of being an outcome measure or biomarker for providing meaningful information on Z joint function during movement and for providing information on the effects of LBP treatments (eg, SMT) on Z joint function. Further research on larger numbers of participants is warranted to assess crepitus in healthy and LBP participants of different age ranges and different severities of degeneration. In addition, assessment of the effects of SMT on crepitus is also warranted.
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