Modelling the Influence of Pelvic Anatomy on the Pelvic Rotation Required to Compensate for Loss of Lumbar Lordosis

Conflicts of interest: No conflicts of interest Sources of funding: No funding obtained Purpose of the study This study explored the influence of pelvic anatomy on the amount of pelvic tilt required to rebalance the lumbar spine when lordosis is reduced due to degeneration. Background Posterior pelvic rotation (tucking the tailbone under) is a compensatory mechanism for the loss of lumbar lordosis associated with disc degeneration and muscle atrophy. The ability of the pelvis to compensate is thought to depend on pelvic incidence (the angle defining the orientation of the sacral (S1) endplate within the pelvis), but sagittal pelvic thickness (the distance of S1 from the hips) may also be important. Methods A geometrical model of the lumbar spine, sacrum and hip joints was created in MATLAB software. The model was defined with an initial lumbar lordosis of 60°, and the uppermost lumbar vertebra (L1) was positioned vertically above the hips. Lumbar lordosis was reduced to 40°, displacing L1 anteriorly. The pelvic rotation required to reposition L1 above the hips was determined for nine combinations of pelvic incidence (40°, 50°, 60°) and pelvic thickness (90 mm, 100 mm, 110 mm). Results The required pelvic rotation was greatest for large pelvic incidence and small pelvic thickness. The effect of pelvic incidence, however, was around half that of pelvic thickness. Conclusion Pelvic incidence is considered important for influencing the ability of the pelvis to compensate for loss of lumbar lordosis. Our model suggests that pelvic thickness is also relevant; however, further investigation is required to establish the effects in real-life data.