Summary Segmental fixation of the spine by means of rods or plates and pedicle screws posteriorly usually results in a stable and rigid construct. The magnitude of the stresses on the instrumentation particularly at the bone-screw and rod (or plate) interfaces will depend on the load distribution between the bone and the implant as well as the number of sites of bone purchase of the implant. If a fusion is to be obtained in the case of a degenerative spine, the role of the instrumentation is to prevent translation of the motion segments, thereby allowing the compressive loads to be transmitted through the vertebral bodies and the degenerated discs. In the case of a fracture, the instrumentation is made to withstand the bulk of the loads since the structural integrity of a motion segment may have been lost. This study was undertaken to evaluate the effect of different constructs on the stresses in Cotrel-Duboussett (C-D) pedicle (tulip) screws close to their junction with the rod bridging the motion segments to be immobilized. In order to minimize the effect of anatomical and material property variation between spines, adult porcine spines were used, thus providing a reproducible experimental model. Fresh 3-year-old porcine spines were potted in holders after the soft tissues were removed (with the exception of the ligamentous structures). The potted spine was then fixed into the loading frame of a materials testing machine (MTS 858 Bionix Test System) and cycled 500 times with an axial load of 380 N. Following this pre-cycling, 6.5-mm diameter C-D tulip screws mounted with strain gauges (Micro-Measurements CEA-06–125UN350) were inserted and aligned. The constructs tested were an intact spine with an eight-screw assembly, a spine with an L4 corpectomy with an eight-screw assembly and a spine with an L4 corpectomy with a four-screw assembly. Each of these constructs was loaded with and without the presence of cross-linking devices. For each construct, five spines were tested. A computerized data acquisition system converted the strain gauge data to a bending moment for each screw. Statistical tests at the 95% level of significance show that the presence of cross links did not affect the mean bending moments within a given construct. Although the bending moments were greater in the distal screws than in the proximal screws in the four-screw corpectomy model, the overall bending moments of the corpectomy models were greater than those encountered in the intact models by a factor of 3. In the four-screw corpectomy model, the bending moments on distal screws were larger than on the proximal screws. The proximal screws accounted for ∼ 12% of the total bending moment, whereas the corresponding fractions for the distal screws was ∼75%. In the corpectomy models, the loads of the intermediate screws were distributed proportionate to the number of distal screws; thus failure is less likely to occur in vivo with four distal screws than with two screws since the proportionate load increased from 60 to 190 Nm, respectively.
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Kostuik et al. (1994) studied this question.