Intramedullary stabilization of extraarticular proximal tibial fractures: a biomechanical comparison of intramedullary and extramedullary implants including a new proximal tibia nail (PTN).

Objectives: To determine in the laboratory whether there are or are not differences between individual geometrical designs of intramedullary and extramedullary devices used for the fixation of extraarticular proximal tibial fractures.

Methods: Five devices were tested: a newly developed Proximal Tibia Nail (PTN), conventional double-plate osteosynthesis (DPO), the Less Invasive Stabilization System (LISS), an augmented Unreamed Tibial Nail with a T-stabilization-plate (UTN + TSP), and an external fixator (ExFix). A 10-mm defect osteotomy was performed on paired human tibiae, and the proximal and distal ends were potted in polymethylmethacrylate cement (PMMA). Each pair of bones was randomly stabilized with the new PTN in 1 tibia (Groups PTN1 through PTN4) and in 1 of the 4 comparative implants in the corresponding contralateral bone. A biomechanical test of the bone implant construct was then performed with a vertical axial force of 350, 600, and 900 N, a bending moment of 6 Nm and a bidirectional rotational strain of 8 Nm. Displacement of bone fragments was measured and depicted as a force-displacement diagram.

Results: For axial loading, significant differences were seen between the PTN 2 group compared to the LISS group (P = 0.016) and the PTN 4 group compared to the ExFix group (P = 0.016). No statistically significant differences were seen for the PTN 1 group compared to the DPO group (P = 0.125) and the PTN 3 group compared to the UTN + TSP group (P = 0.453). The bending stiffness of the PTN 1-4 groups was not significantly different from any of the 4 alternative implants. There was comparable torsional stiffness in all implant groups except for the UTN + TSP group, which was less stable and significantly different from the PTN 3 group (P = 0.016).

Conclusions: Given the parameters of this investigation, the new PTN would theoretically provide the same mechanical stability as the DPO in axial loading. Higher stability in axial loading may be present when compared to the LISS or the ExFix. Further clinical investigation of this implant will determine its usefulness among proximal tibial fixation devices.