Biomechanical study of patient positioning: influence of lower limb positioning on spinal geometry.

Objective: The objective of this study was to use finite element model (FEM) simulations and experimental testing to study the relationship between lower limb positioning for surgeries of the spine and changes in sagittal curves.

Methods: Four volunteers underwent lower limb flexibility and range of motion testing before being placed prone on a new surgical frame where lateral radiographs of their spines were taken in positions of hip flexion (average 48 degrees) and extension (average 13 degrees). Personalized FEMs were created representing each volunteer's spine, rib cage, pelvis, and lower limbs. Optimization of model behavior was performed by adjustment of lower limb muscle initial strains. The FEMs were exploited to examine the impact of more extreme and intermediate lower limb positions; 30 degrees of hip extension to 90 degrees of flexion at intervals of 20 degrees.

Results: With increased hip flexion, lordosis and kyphosis decreased to an average of 52% (35 degrees) and 16% (6 degrees), respectively. Personalization of the 4 FEMs allowed reproduction of the experimental results within 5 degrees and their subsequent exploitation showed the linear changes in lordosis and kyphosis between extreme positions decreasing an average of 84% (59 degrees) and 34% (13 degrees) with increased hip flexion. A strong correlation was found between experimental change in lordosis and individual hamstring flexibilities (R=-0.93) which allowed for the development of a predictive equation for lordosis in terms of hip flexion which factors straight leg raise test results.

Conclusions: Knowledge gained through this study can be used to improve intraoperative control of sagittal curves through lower limb positioning.