The institutional review board for human research of all participating institutions (National University of Health Sciences, Canadian Memorial Chiropractic College, Auburn University) approved this project. All subjects provided consent to participate. Forty healthy subjects between the ages of 18–30 years were recruited for this study. Because this was the first study assessing the specific location of cavitations, healthy subjects were sought in order to introduce as few confounding variables (e.g., Z joint degeneration) as possible.
Subjects responding to recruitment materials went through phone screening and a history and physical examination that applied rigorous exclusion criteria (). The inclusion and exclusion criteria were similar to those previously used in a case series (5 subjects) designed to assess the general feasibility of the methods used in this study.21
Subjects eligible after initial screening, including history and physical examination, were scheduled for an appointment that included an MRI scan and the accelerometry procedures.
Inclusion and Exclusion Criteria.1
Identification of Landmarks to Assure Proper Placement of Accelerometers
The L4 spinous process (SP) was used as the primary landmark from which all other landmarks were identified. Subjects were placed in the prone position on the MRI gantry table, the L4 SP was identified via palpation and marked with a grease pencil, a high signal MRI marker was then placed over the marked L4 SP (), and the subject was MRI scanned in the neutral supine position (). The high signal marker was readily visible on the mid-sagittal scout MRI scan (). The location of the high signal marker was carefully noted and used for proper placement of 9 accelerometers (see below).
Figure 1 Methods used in this study to accurately identify proper vertebral levels; a = high signal intensity MRI marker, which when taped to the subject (b) was used to verify palpated L4 spinous process (L4 spinous process was used as the major landmark for (more ...)
Placement of 9 Accelerometers
This project used accelerometers to assess cavitations. An accelerometer is an electromechanical device () that measures static or dynamic acceleration. In this study, dynamic vibration acceleration originating from the Z joints during cavitation was measured. More specifically, piezoelectric accelerometers
were used in this project. Piezoelectricity is the ability of some materials (notably crystals and certain ceramics) to generate an electric field or electric potential (voltage) if the piezoelectric material undergoes mechanical strain (e.g., vibration). Consequently, the output (signal) of the piezoelectric accelerometers is voltage generated by the piezoelectric effect. This voltage is amplified and the signal is then read from an oscilloscope.22
Figure 2 Placement of accelerometers and spinal manipulation; a = illustration showing placement of 9 accelerometers; b = close-up of accelerometer (actual size ~1.0 cm3); c = subject with the 9 accelerometers placed for recording (white dots placed over each (more ...)
The same 2 research assistants placed the accelerometers and collected the accelerometry data throughout the study. To standardize accelerometer placement and data collection, two months of training preceded the study launch.
Following MRI scanning, subjects were placed in the prone position. The high signal marker over the L4 SP was removed. The skin indentation left by the high signal marker was used along with the scout MRI scan to accurately identify the L4 SP. Once this landmark was positively identified the accelerometers were positioned. Tape with strong adhesive characteristics was used to affix seven 1.0-cm3
accelerometers (DataTron 4507, Bruel & Kjaer, Naerum, Denmark; detailed specifications of the accelerometry data acquisition system are presented elsewhere21
) to the spinous processes of L1–L5 and the S1 and S2 sacral tubercles. Two additional accelerometers were then positioned 3 cm left and right lateral to the L4/L5 inter-spinous space (). The sample rate for recording from the accelerometers was 320,000 Hz, which gave an actual sample rate of 317,460 Hz per channel. Therefore a sample was taken every 1/317,460 sec = 3.15 ×10−6
seconds. The system allowed the origin of a cavitation to be determined within 0.4 cm, more than ample discrimination to determine the Z joint segmental level of origin (e.g., left or right L5/S1) without frequency aliasing.21
The arrangement of accelerometers and sampling methods allowed for assessment of cavitations from all lumbar Z joints (i.e., left and right L1/L2 – L5/S1) and also the left and right SIJs.
Randomization of Subjects
Cavitation during side-posture positioning alone was noted in a previous feasibility study.21
Consequently, a side-posture only (no SMT) group was included to control for the effects of the force of side-posture positioning and to determine if side-posture positioning alone would result in cavitations. The total number of subjects and the number of subjects randomized into each group were based on a power analysis using the effect sizes and data variability of the previously completed feasibility study.21
Subjects were randomized into 1 of 2 groups: Group 1 – side-posture SMT (n=30), and Group 2 – control group, side-posture positioning without SMT (n=10). A predetermined randomization scheme (blocked randomization) was performed prior to study initiation and randomization allocation was sealed in sequentially numbered envelopes for both males and females. All personnel were blinded to each subject’s random group assignment until after the first MRI scan. When the first MRI was completed, the research assistant opened the next envelope to inform the clinician of the group into which the subject was randomized.
Subject Positioning, Spinal Manipulation, and Recording from Accelerometers
Subjects were placed on their right sides (i.e., the left side was always the up-side). This allowed for consistency in positioning for SMT and for the second MRI scan, as well as consistency in assessing cavitations from the up-side vs. down-side during SMT and side-posture positioning. When ready to initiate the final positioning and SMT, the treating doctor of chiropractic would say “start” and the accelerometry research assistant would commence a 4-second recording from the accelerometers. The recordings included the final rotation phase of side-posture positioning (for both Groups 1 and 2) and the SMT (for Group 1 only). A LabView-based program (LabView 2009 Platform, National Instruments, Austin, Texas) tailored for this study was used to make the recordings.
A standard general manipulation, the hypothenar ilium technique,23
was used in this study (). This technique positioned the patient’s up-side thigh and leg in a flexed position. The clinician palpated the interspinous spaces to determine segmental motion during thigh flexion. The amount of thigh flexion in this technique is related to the Z joint segmental level targeted for gapping, with more thigh flexion (segmental motion palpated at higher lumbar levels) being used to target higher lumbar Z joints and less flexion for lower Z joints. Thigh flexion was modulated to target the L3/L4 – L5/S1 Z joints in this study. The clinician’s “contact hand” (thrusting hand) is usually on the up-side posterior superior iliac spine; however, to avoid physical contact with accelerometers, contact in this study was on the infero-lateral (left lateral) sacrum. The clinician’s “non-contact hand” (indifferent hand) stabilized the patient’s up-side shoulder and/or chest area. The thrust through the contact was posterior-to-anterior, following the modest amount of up-side to down-side rotation induced by moving the pelvis forward while holding the shoulders stable. The intent of the procedure is to open (gap) the up-side targeted joints. The SMT was delivered by a chiropractic physician with 19 years of practice experience (SS) and included 2 high-velocity, low-amplitude thrusts delivered in rapid succession. The clinician targeted the L3/L4 – L5/S1 Z joints (subsequently referred to as the “target area”). The procedures were the same for the side-posture positioning without SMT (control) group (Group 2), except no manipulative thrust was delivered.
Following these procedures, the subjects and the SMT clinician (blinded to each other) were asked if they heard or felt a cavitation during SMT. This information was used in 1 of the 2 reliability studies (see “Reliability Studies” section, below).
Assessment of Cavitations
The presence of cavitations was assessed using a LabView program tailored to analyze the accelerometer recordings of this project. Cavitations were identified from the computer oscilloscope as shifts from the baseline of several accelerometer recordings within a short (<0.0001 sec) timeframe (). The specific level of cavitation (e.g., left L4/L5 or right L1/L2) was identified by the order in which the recording line for each accelerometer deviated from the baseline ().
Figure 3 Recordings from accelerometers; a = oscilloscope recording from accelerometers of a cavitation; b = the same recording shown in “a” with the timeline expanded to show the order in which the accelerometers recorded a vibration. Notice that (more ...)
Two types of reliability studies were conducted. The first was to determine the reliability of identifying cavitations from the accelerometer recordings as displayed on a computer oscilloscope, and the second was to determine the agreement between subject report, clinician report, and computer oscilloscope recordings in determining if any cavitation occurred during SMT or side-posture positioning.
Reliability Study 1: Identifying Cavitations from the Accelerometer Recordings as Displayed on the Computer Oscilloscope
Ten (10) cavitations from the project were selected using a random number generator. Two trained observers then independently viewed the LabView oscilloscope recording of each cavitation. The observers reviewed the oscilloscope’s 9 lead recordings, identified the joint from which the cavitation originated, and logged the joint onto a data sheet. The observers had 12 options for their responses for each recording (left and right L1/L2 – L5/S1 and left and right SIJs). Each of the 2 observers (blinded to the results of one another) gave their completed data sheet to a third investigator for analysis of agreement. Weighted Kappa Coefficients (Kw) were used to assess agreement.
Reliability Study 2: Agreement Between Subject Report, Clinician Report, and Accelerometer Recordings
During the MRI and accelerometry appointment the clinician left the room immediately following the SMT (Group 1) or side-posture positioning alone (Group 2, control group) and was out of the hearing of the subject. A research assistant then separately asked both the clinician and subject if they heard and/or felt a cavitation. The clinician and subject were blinded to the answer of one another. After all subjects had completed the study, kappa coefficients (non-weighted, K) were used to assess the agreement between 1) clinician and subject, 2) clinician and interpretation of LabView accelerometer recordings, and 3) subject and interpretation of LabView accelerometer recordings.
Assessment of Cavitations
Data analyses for the 40 subjects began with identification of the locations of cavitations (i.e., identification of specific Z joints and SIJs from which cavitations originated). This information was summarized descriptively and inferential statistics were used to compare the presence/absence of cavitations, based on the oscilloscope recordings, between the following: Group 1 vs. Group 2, left side (up-side) vs. right side (down-side), target area (L3/L4, L4/L5, L5/S1) vs. non-target area (L1/L2, L2/L3, SIJ). Group 1 and Group 2 were compared using a chi-square test. Because the independence of individual joint cavitations was unknown, McNemar’s test was used in conjunction with the binomial distribution to assess statistical significance for cavitations of left side vs. right side and of target vs. non-target joints.