A biomechanical comparison of two screw fixation methods in a Letenneur type I Hoffa fracture

The treatment of Hoffa fractures is challenging, for which the ideal fixation and approach are still controversial. Osteosynthesis with plate or screws fixation in different trajectories have been described in previous literature. The purpose of this study was to compare the biomechanical strength and stability of two types of screw trajectories used to stabilize displaced coronal fractures of the lateral femoral condyle.

Sixteen synthetic femurs (Sawbones Pacific Research Laboratories, Vashon, WA) were divided into two groups. A vertical osteotomy was performed to mimic a Letenneur type I Hoffa fracture. Group A (n = 8) was fixed with screw in anteroposterior direction (A-P) screws. Group B (n = 8) was fixed with crossed screws. Both groups were tested with a nondestructive axial compression aligned with the femur axis. After that, 10,000 cyclic loading tests were applied to the specimen with a force ranging between 200 to 600 N, and the interfragmental displacement was recorded, respectively, after 10, 100, 1000 and 10,000 cycles. Finally, a destructive axial compression test was applied until catastrophic failure.

There were no statistical between-group differences in regard to the average axial stiffness, interfragmental displacement, and ultimate failure load. The average axial stiffness of the A-P screw was comparable to that of the crossed screw (361 ± 113 N/mm vs. 379 ± 65 N/mm, p = 0.753). All specimens completed the entire cyclic loading test without catastrophic failure, and the interfragmental displacement after loading for 10,000 cycles was 1.36 ± 0.40 mm for the A-P screw and 1.29 ± 0.61 mm for the crossed screw, there were no statistical differences between the groups (p = 0.823). The average ultimate failure loads for the A-P and crossed screws were 1214 ± 127 N and 1109 ± 156 N, respectively (p = 0.172).

Based on our in vitro study, the crossed screws can provide comparable mechanical performance as traditional A-P screws in Hoffa fracture fixation. Considering the screws trajectories are commonly determined by the choice of surgical approach, the current study provides support from a biomechanical perspective for the application of crossed screws in direct lateral approach.