The effect of screw orientation on internal fixation of Letenneur type II Hoffa fractures: a biomechanics study

Background: To investigate the biomechanical effects of screw orientation and fracture block size on the internal fixation system for Letenneur type II Hoffa fractures.

Methods: The fracture models were randomly divided into six groups according to the fracture subtypes and the direction of nail placement, and a plumb line of the posterior condylar tangent was made across the base of the posterior femoral condyle. The fracture blocks of the three types of fracture were calculated and recorded in the sagittal position, and the biomechanical performance of the six groups was evaluated by biomechanical tests. The axial load on the fracture block at a displacement of 2 mm was set as the failure load, a gradually increasing axial load was applied to each fracture model using a customized indenter at a load of 250-750 N, and the displacements and failure loads of the six groups were recorded at different axial loads.

Results: Biomechanical test results showed that the larger the fracture block, the greater was the stability when nailing from front to back, and the smaller the fracture block, the greater was the strength when nailing from back to front (p < 0.001). As the fracture block became larger, the biomechanical advantage of nailing from posterior to anterior decreased.The displacement under 250 N load were 1.351 ± 0.113 mm, 1.465 ± 0.073 mm for Group IIa AP and Group IIa PA. The displacement under 500 N load were 2.596 ± 0.125 mm, 2.344 ± 0.099 mm for Group IIa AP and Group IIa PA. The displacement under 750 N load were 3.997 ± 0.164, 3.386 ± 0.125 mm for Group IIa AP and Group IIa PA. The failure loads were 384 ± 14 N, 415 ± 19 N for Group IIa AP and Group IIa PA. In the type IIa fracture group, the difference was no longer significant (p > 0.001). Therefore, there is a mechanical threshold that ranges from 38.36 to 52.33% between type IIa and type IIb fractures.

Conclusions: The effect of the nailing direction on the strength of fixation has a fracture-block critical point, which is consistent overall with the trend that the larger the fracture block is, the greater the stability when nailing from anterior to posterior, and the smaller the fracture block is, the greater the strength when nailing from posterior to anterior.