General Session: Biomechanics
Presented by: H.W.D. Hey - View Audio/Video Presentation (Members Only)
H.W.D. Hey(1), C.G. Wong(2), E.T.C. Lau(1), K.A. Tan(1), L.L. Lau(1), G.K.P. Liu(1), H.K. Wong(1)
(1) National University Health System, Orthopaedic Surgery, Singapore, Singapore
(2) National University of Singapore, Yong Loo Lin School of Medicine, Singapore, Singapore
Purpose of the Study: Surgical fusion of the lumbar spine involves the creation of an immobile segment in order to provide the stability necessary to maintain deformity correction or symptom relief. The final position in which the affected vertebrae are fixed, should provide maximum residual function. However, the ideal position for fusion is still poorly understood. The purpose of this study is to assess lumbar spine alignment in 6 common physiological human postures, and calculate the loss in range of motion using the standing posture as reference. Describe the methods used: This is a cross-sectional study of prospectively collected data. Seventy patients above the age of 45 years old, with low back pain were recruited from the spine specialist outpatient clinic over 1-year. All subjects had X-rays performed in four physiological postures; these were the forward bending, backward bending, slump sitting and half squatting positions. When clinically indicated, subjects also had X-rays performed in the erect standing and supine postures. Global and segmental lumbar angular alignments measured using the Cobb Method from L1 to S1. Using erect standing posture as a reference, loss in range of motion is calculated for each posture. ANOVA testing was used to identify differences in alignment between postures. Unpaired t-test was performed when comparing erect standing and supine postures. These postures were then ranked in order of global lordosis. ROM changes in each posture were studied. Summarize the
Findings: Slump sitting gives the greatest flexion (p< 0.001) followed by forward bending (p< 0.001), and supine (p< 0.001) postures. Backward bending gives significant extension (0=0.035) compared to standing. No significant differences were found between half-squatting and standing erect postures (p=0.938). Regardless of the posture, L4/L5 and L5/S1 levels remained in lordosis with L4/5 having a greater ROM compared to L5/S1. L1/2 turns kyphotic at supine, L2/3 at forward bending and L3/4 at slump sitting in the form of a kyphosing cascade. The degree or force equivalent at the proximal junction if fusion was performed at L5/S1, L4-S1 and L2-S1 is 8.7°, 20.8° and 40.8° respectively. Include statement of
Conclusions: Slump sitting gives the greatest lumbar flexion followed by forward bending and supine. Assuming that fusion recreates the lordotic profile of the standing spine based on current deformity correction standards, significant loss in flexion is anticipated with slump sitting likely to generate supraphysiological forces at the adjacent. This may be a reason for complications such as proximal junctional failure (PJF).