Objective: To test the a priori hypothesis that one of the positive mechanisms of action of chiropractic side-posture manipulation (adjusting) of the lumbar spine is to separate, or gap, the zygapophysial (Z) joints.
Design: Before and after study with randomization.
Setting: Chiropractic college clinic and magnetic resonance imaging (MRI) facility.
Participants: Sixteen healthy student volunteers (8 men and 8 women) ages 22 to 29 years with no history of significant low back pain. Nineteen volunteers were screened, with 3 disqualified from the study. Subjects were randomized into 4 groups, each with 2 men and 2 women.
Interventions: Lumbar side-posture spinal adjusting (manipulation) and side-posture positioning.
Main outcome measures: Comparison of anterior to posterior measurements of the Z joints from MRI scans taken before and after side-posture spinal adjusting and before and after side-posture positioning, and a rigorous subjective evaluation protocol of the Z joints by 3 radiologists blinded to the randomized groups.
Main results: Observers making measurements were blinded to what group subjects were placed in and whether they were measuring first or second scans; radiologists were blinded to what group subjects were assigned. Differences were found between the groups. Those receiving side-posture spinal adjusting and remaining in side posture showed the greatest increase in gapping (0.7 mm vs 0.0 mm for controls).
Conclusions: Lumbar side-posture spinal adjusting produced increased separation (gapping) of the zygapophysial joints. Side-posture positioning also produced gapping, but less than that seen with lumbar side-posture adjusting. A larger clinical trial should be performed to further define the results of this study.