Biomechanical comparison of polyaxial-type locking plates and a fixed-angle locking plate for internal fixation of distal femur fractures.

Objectives: To test the stability to axial loading of 2 new polyaxial locking screw-plate designs and analyze different angles of screw insertion. The noncontact bridging (NCB) polyaxial locking plate (Zimmer) and the POLYAX plate (DePuy) were compared with a fixed-angle less invasive stabilization system (LISS; Synthes).

Methods: Twenty-five synthetic femurs were divided into 5 groups and assigned fixation with the LISS plate (group I), POLYAX plate (groups IIA and IIB), or NCB plate (groups IIIA and IIIB). The polyaxial constructs were divided into parallel and crossed distal condylar screw configurations. Each construct was tested under axial loading and stressed to failure at a displacement rate of 5 mm/min with a preload of 100 N. Outcome measurements included stiffness, load to failure, peak force, and mode of failure.

Results: All LISS and POLYAX constructs failed by plastic deformation of the plate, whereas 9 of 10 NCB constructs failed by an intra-articular lateral condyle fracture. No failures occurred at the screw-plate interface in either polyaxial constructs. Load to failure of the LISS was 33% greater than the parallel POLYAX (P < 0.01) and 24% greater than the crossed POLYAX (P < 0.01). Load to failure of NCB (parallel and crossed) were 24% greater than the parallel POLYAX (P < 0.01 and P < 0.01, respectively) and 15% greater than the crossed POLYAX (P < 0.01 and P = 0.02, respectively). The POLYAX also had significantly lower stiffness and peak force compared with the LISS and NCB. There was no difference between the LISS and NCB with regard to stiffness, load to failure, and peak force. Parallel and crossed polyaxial constructs showed no difference in stiffness or failure loads.

Conclusions: There were no failures of either polyaxial screw-plate interface despite large forces and screw angle did not affect the overall strength of these constructs, supporting the biomechanical soundness of both polyaxial device designs under axial loading. However, the POLYAX supported smaller loads compared with the LISS and NCB while under axial loading. In addition, the mode of failure of the NCB plate, creating an intra-articular fracture propagating from the distal posterior screw hole, may be of some concern. Additional testing is needed to determine the clinical importance of the demonstrated differences among these plate designs.