Does spinal kyphotic deformity influence the biomechanical characteristicsof the adjacent motion segments? An in vivo animal model

Study Design. In an in vivo sheep model, the effects of spinal fusion and k yphotic deformity on the neighboring motion segments were analyzed. Objectives. To investigate the effects of spinal fusion and kyphotic deform ity on the adjacent motion segment. Summary of Background Data. The in vivo effects of kyphotic deformity on th e neighboring motion segments have not been investigated in any studies. Methods. Eighteen sheep were equally randomized into three groups based on surgical procedure: L3-L5 in situ posterolateral fusion (n = 6) L3-L5 kypho tic posterolateral fusion (n = 6), and surgical exposure alone (n = 6). Aft er a Is-week survival period, the adjacent motion segment changes were anal yzed radiographically, biomechanically, and histologically. Results. The kyphosis group showed 5.0 degrees +/- 2.6 degrees and 1.7 degr ees +/- 1.8 degrees compensatory hyperlordosis at L2-L3 and L5-L6, respecti vely. compared with surgical exposure and in situ posterolateral fusion, th e kyphotic posterolateral fusion significantly influenced cranial adjacent motion segment biomechanics by inducing more stiffness in the posterior lig amentous complex (P < 0.05) and increasing lamina strain under flexion-exte nsion loading (P < 0.05). Results of histologic analysis showed significant degenerative changes of the L2-L3 facet joints in the kyphosis group. Conclusions. It is inferred that in the kyphosis group, compensatory hyperl ordosis at the cranial adjacent level leads to lordotic contracture of the posterior ligamentous complex. The increased lamina strain, exhibited by th e in situ group under flexion-extension, was further increased in the kypho sis group, indicating higher load transmission through the posterior column . Significant degenerative changes of the cephalad adjacent facet joints ob served in the kyphosis group served to corroborate the biomechanical data. These results indicate that a kyphotic deformity may lead to facet joint co ntracture and facet arthritis and may serve as the origin of low back pain at the cranial adjacent level.