INTRODUCTION: While sagittal realignment using long posterior spinal fixation is the treatment of choice for global sagittal imbalance, patients often develop proximal junctional kyphosis (PJK) through failure of the uppermost instrumented or adjacent vertebra. The effects of surgical and patient variables on PJK have not been studied biomechanically.
OBJECTIVES: To develop a 2D equilibrium model of sagittally imbalanced adult spines to predict intervertebral loading pre‐ and post‐op and to predict the effect of osteotomy location and extensor muscle function on the occurrence of PJK.
METHODS: Vertebral positions from T1 to S1 were digitized from lateral radiographs of seven patients with sagittal imbalance. Using static equilibrium, extensor muscle and vertebral body loads were calculated for each pre‐ and post‐op spine curvature. From pre‐op curves, the changes in loading behaviour due to simulated osteotomies at L2, L3, or L4, and decreasing levels of extensor muscle function were assessed.
RESULTS: Compared to normal spines,1 those with sagittal imbalance developed pre‐op vertebral compressive loads nearly double in magnitude but with similar loading patterns from T1 to S1 (see one patient's data in Fig.). These loads were decreased after surgical correction but then increased by up to 20% after the development of PJK when compared to pre‐op loads. The chosen level of the osteotomy did not have a notable effect on predicted loads. Our data suggests that the inherent loss of extensor muscle function following surgery results in substantially increased compressive loads to the spine.
DISCUSSION: Spines with global sagittal malalignment experience abnormally high compressive loads, predisposing them to unanticipated failure. After surgical correction to a neutral alignment, compensatory mechanisms to maintain sagittal balance through the vertebral column may increase vertebral body loads and lead to the development of PJK. 1Harrison DE et al. ESJ 14: 234‐42, 2005. (cont'd)