MATURATION OF THE POSTEROLATERAL SPINAL-FUSION AND ITS EFFECT ON LOAD-SHARING OF SPINAL INSTRUMENTATION - AN IN-VIVO SHEEP MODEL

We investigated the temporal relationship among the biomechanical, rad iographic, and histological properties of a posterolateral spinal fusi on mass to elucidate the changes in load-sharing of the spinal instrum entation and that of the fusion mass throughout the healing process. D estabilization of the posterior spinal column and transpedicular screw fixation at the segments between the third and fourth and the fifth a nd sixth lumbar vertebrae was performed in twenty-four sheep. A poster olateral spinal arthrodesis with use of autologous corticocancellous b one graft was done randomly at one of the two segments; the other segm ent (without bone graft) served as the instrumented control. Six anima ls each were killed at four, eight, twelve, and sixteen weeks postoper atively. Biomechanical testing showed that the posterolateral fusion m ass had increased mechanical stiffness after the fourth week. The stra in on tile hardware, measured with use of rods instrumented with strai n-gauges, decreased significantly (p < 0.01) beginning at eight weeks. Radiographically three independent observations of each of the six an imals at each time-period showed that, although all of the fusion mass es mere considered solid unions al-sixteen weeks, bridging of trabecul ar bone was noted during only ten of eighteen observations at twelve w eeks, three of eighteen observations at eight weeks, and none of eight een observations at four weeks. Computerized tomography and histomorph ometric analyses demonstrated that mineralization in the fusion mass i ncreased in a linear fashion even after eight weeks. Histologically, t he fusion mass consisted predominantly of woven bone at eight weeks; t hereafter, it was gradually trabeculated. CLINICAL RELEVANCE: We found a great discrepancy between biomechanical stability and histological maturation of the posterolateral fusion mass. The biomechanical proper ties of a stable spinal fusion preceded the radiographic appearance of a solid fusion by at least eight weeks, suggesting that immature wove n bone provided substantial stiffness to the fusion mass. The spinal i nstrumentation eras subjected predominantly to bending stress rather t han to axial stress, and the load-sharing of the spinal instrumentatio n decreased concurrently with file development of the spinal fusion.