The Effects of Balanced Seating Pressure on Pelvic Asymmetry: A Pilot Study

Trial Design This study is designed as a randomized, double-blind, placebo-controlled, two-period crossover trial to be conducted at the Department of Rehabilitation, Taoyuan Chang Gung Memorial Hospital. A total of 24 participants will be screened, with an estimated 20 completing the full protocol. Participants will be randomly assigned to one of two treatment sequences: Group A will receive the DaSuit® cushion followed by the placebo cushion; Group B will receive the placebo cushion followed by the DaSuit® cushion. Each intervention period will last approximately 10 minutes, separated by a 10-minute washout interval to minimize potential carry-over effects. The crossover design is chosen to reduce between-subject variability, as each participant will serve as their own control.

Participants The study will not involve patients or the public in the design, conduct, reporting, or dissemination.

Inclusion criteria are as follows: (1) adults aged 20 to 50 years; (2) experiencing non-specific low back pain exacerbated by sitting, with a VAS score between 3 and 7; (3) height between 150-180 cm; (4) BMI between 18.5-24; and (5) radiographically verified hemi-pelvic height discrepancy ≥ 0.5 cm in a seated posture.

Exclusion criteria include: (1) inability to sit for 20 minutes; (2) MMSE < 24; (3) significant spinal or pelvic disorders (e.g., fractures, prior surgeries, herniated discs with nerve compression); (4) diagnosed scoliosis with a Cobb angle ≥ 20°; (5) recent use of analgesics or muscle relaxants; (6) pregnancy; and (7) skin conditions affecting sitting tolerance. All participants will provide written informed consent following clinical pelvic examination confirming asymmetry.

Interventions Each participant will undergo both intervention conditions in a randomized order. In each session, the participant will sit for 10 minutes on either the DaSuit® cushion or the placebo cushion, placed on a standardized 40×40×40 cm wooden chair. A pressure sensor mat will be placed on the cushion surface to measure under-buttock pressure. Following the sitting period, pain intensity will be assessed using the VAS, and an anterior-posterior full-spine X-ray will be taken. After a 10-minute washout period-during which the participant will be instructed to walk-the alternate cushion condition will be administered.

Outcome Measures Adverse events will be monitored throughout the study. Participants will be asked to report any discomfort or pain during or after each session. No adverse events are expected.

Primary outcomes include:

Under-buttock pressure:

• X and Y coordinates of center of force
• Total force applied to the cushion
• Area of force distribution
• Maximum pressure (mmHg)
• Area exceeding a defined percentage of maximum pressure
• Right/Left peak pressures
• Distance from center of force to Right/Left peak pressure points

Spinal alignment parameters:

• Spinal alignment deviation (SAD; C7-S1) Participants will again sit upright on the test chair without a cushion, and a frontal full-spine radiograph will be taken. The deviation will be calculated as the horizontal distance between the vertical lines passing through the center of the seventh cervical vertebra (C7 plumb line) and the center of the first sacral vertebra (S1 plumb line). This measurement reflects the lateral displacement of the spine and serves as an indicator of coronal imbalance.
• Pelvic height discrepancy (PHD) Pelvic height discrepancy (PHD) will be determined by measuring the vertical distance between the highest point of the iliac crest and the lowest point of the ischial tuberosity on both sides. A side-to-side difference of ≥0.5 cm will be used as the diagnostic threshold for pelvic asymmetry.
• Shoulder height discrepancy (SHD) While shoulder height discrepancy (SHD) is listed as a secondary outcome, its assessment will be based on the vertical difference between the acromion landmarks on both sides, if visible within the imaging field.

Pain intensity, measured via the visual analog scale (VAS)

Instrumentation and Procedures Pressure Mapping and Sitting Protocol: The cushion will be positioned on a 40×40×40 cm wooden box serving as a chair, with a pressure mat placed on top. Participants will sit with hips, knees, and ankles at approximately 90° angles. A tablet will be mounted at 30-40 cm distance and a downward viewing angle of 5-10° to maintain visual focus and reduce head movement. Participants will sit for one minute to acclimate, after which under-buttock pressure will be recorded continuously for 10 minutes.

Radiographic Procedure: For spinal imaging, the chair will be placed on a 20 cm X-ray platform. A 40×40×20 cm footrest will ensure consistent lower limb positioning during imaging. The imaging field will extend from the external auditory meatus to the seat base. Participants will maintain a natural sitting posture for 10 seconds prior to image capture.

Randomization and Blinding Randomization will be performed using a computer-generated sequence with a 1:1 allocation ratio. The sequence will be prepared by an independent statistician not involved in recruitment or data collection. Allocation concealment will be achieved through the use of sequentially numbered, opaque, sealed envelopes (SNOSE), managed by staff independent of the enrollment process. Both the DaSuit® and placebo cushions are designed to be visually indistinguishable. Participants will be blinded to the cushion type. However, due to logistical constraints, outcome assessors will not be blinded.

Statistical Analysis As a pilot study, the primary objective is to assess feasibility and obtain preliminary effect estimates for planning future trials. The sample size is not based on formal power calculations and results should be interpreted with caution regarding external validity.

Within-subject comparisons between the DaSuit and placebo conditions will be analyzed using paired t-tests. To examine potential sequence effects, between-group comparisons (Group A vs. Group B) will be conducted using independent t-tests. A two-tailed p-value of < 0.05 will be considered statistically significant.

All analyses will use complete case data; missing data are not anticipated. If missingness exceeds 10% in future trials, per-protocol analysis will serve as the primary strategy, supplemented by sensitivity analyses as appropriate.