Christopher Wybo

Biomechanical Comparison of Bicortical Versus Unicortical Screw Placement of Proximal Tibia Locking Plates: A Cadaveric Model

Objective: The purpose of this study was to compare the biomechanical properties of bicortical with unicortical screws in a proximal tibial fracture cadaveric model. Setting: Biomechanics laboratory at a Level 1 trauma center. Patients/Participants: Eight pairs (4 male and 4 female) of elderly (average age, 79 years; range, 63 to 104 years) cadaveric tibiae. Intervention: Osteotomies were performed in the proximal tibia to reproduce a 41-C2 bicondylar fracture pattern. The 4.5-mm proximal tibial periarticular locking plates (Smith-Nephew, Memphis, TN) were applied to the tibiae with 4 proximal bicortical or unicortical locking screws and 3 screws distal to the fracture site. The fixed tibiae were tested by using a materials testing machine (Instron, Canton, MA) with the axial load on the medial condyle. Outcome Measurements: The bicortical and unicortical constructs were compared for stiffness, yield load and displacement, and maximum load and displacement to failure. Results: Bicortical screw placement significantly outperformed unicortical screw placement in stiffness (53.1 ± 6.7 N/mm versus 35.6 ± 7.2 N/mm, P < 0.002) and maximum load (476.5 ± 83.8 N versus 258.9 ± 62.1 N, P < 0.001) but the yield properties and the ultimate displacement were not significantly different. Conclusion: Bicortical screw placement may provide a biomechanically superior construct than unicortical screw placement for the stabilization of unstable proximal tibia fractures.

Biomechanical analysis of different techniques in revision spinal instrumentation: larger diameter screws versus cement augmentation.

Study Design. Biomechanical analysis. Objective. To determine the relative strengths of 2 different forms of revision spinal instrumentation using a validated, constant load, cyclic testing mechanism. Summary of Background Data. Spinal fusion with instrumentation procedures are on the rise. As such, so are revision procedures. A few studies have looked at revision instrumentation techniques. Both increased pedicle screw diameter as well as cement augmentation of pedicle screw fixation have been proposed, used clinically and tested biomechanically. To our knowledge, no comparative study exists between these techniques. Methods. Using an instron servohydraulic loading machine, we tested pedicle screws inserted in both the anatomic (angled) and Roy-Camille (straight) insertion technique with both larger diameter (8 mm) pedicle screws, as well as standard diameter (6 mm) pedicle screws augmented with polymethylmethacrylate bone cement. Each of these techniques was subjected to constant load under cyclic conditions for 2000 cycles at 2 Hz. Computerized data collection was used at all time points. Comparisons were made between primary instrumentation data (previously published) and large diameter screws for revision. Further comparisons were made between large diameter screws and cement augmented screws. Results. The larger diameter screws compared with the cement augmented screws showed significant differences in: initial stiffness with straight insertion technique (P < 0.01), stiffness damage with straight insertion technique (P < 0.01), and creep damage with straight insertion technique (P = 0.01). There was also a significant difference between large diameter and primary instrumentation technique all calculated values (P < / = 0.05). Conclusion. The larger diameter screws were equivocal or significantly more resilient than the cement augmented standard diameter screws at the strongest of the insertion angles for all values. Since rigidity of the instrumentation construct is one of the very few factors that is surgeon controlled, this could influence the choice of instrumentation in revision spinal arthrodesis.