Biomechanical Analysis of Single and Double Anterior Vertebral Body Screw Fixation and the Effect of Increasing Screw Diameters

• a, e - DePuy Spine

Introduction: Anterior thoracic scoliosis correction is dependent on the security of the bone-implant interface. The purpose of this study was to determine biomechanical differences in the failure loads of the vertebral body screws of varying diameter, as well as the effect of single versus double screw fixation subjected to cantilever loading.

Methods: Ten cadaveric human spines were harvested and the vertebral bodies from T3 to T12 were sectioned. The vertebral bodies were randomly assigned to constructs with single or dual screws of either 5, 6, or 7 mm diameters. All screws were placed with bicortical purchase. Each instrumented vertebra was potted and mounted in an MTS machine, loading the screws at 90° to their long axis, simulating cantilever loading. The ultimate load to failure was analyzed with a two-way ANOVA with the dependent variables being screw diameter and number of screws. The thoracic spine was sub grouped into proximal (T3-T5), middle (T6-T8) and distal (T9-T12) regions.

Results: When combining all the levels of the thoracic spine, there was a significant increase in failure load with both increasing screw diameter (p <0 .05) as well increasing number of screws (p < 0.05). However, when the different sub regions thoracic spine were evaluated, this trend only remained for T9-T12 levels. Over levels T6-T8, there was no effect screw diameter; however, two constructs had a greater failure load. This is in contrast to upper region (T3-T5) where dual failed at lower loads than single constructs. The load proximal roughly half value distal region.

Conclusion: Proximal thoracic anterior screw fixation remains a challenge and is substantially less secure than in distal levels. Although at distal thoracic levels fixation may be improved by using dual screw constructs, this approach is less productive in the smaller proximal thoracic segments. Double screw fixation was superior to single screw fixation at levels distal to T6. The challenge of increasing proximal screw fixation, however, requires a solution other than doubling the number of standard vertebral body screws utilized.

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