Fracture fixation has advanced significantly with the introduction of locked plating and minimally invasive surgical techniques. However, healing complications occur in up to 10% of cases, of which a significant portion may be attributed to unfavorable mechanical conditions at the fracture. Moreover, state-of-the-art plates are prone to failure from excessive loading or fatigue. A novel biphasic plating concept has been developed to create reliable mechanical conditions for timely bone healing and simultaneously improve implant strength. The goal of this study was to test the feasibility and investigate the robustness of fracture healing with a biphasic plate in a large animal experiment. Twenty-four sheep underwent a 2mm mid-diaphyseal tibia osteotomy stabilized with either the novel biphasic plate or a control locking plate.

Different fracture patterns in terms of defect location and orientation were investigated. Animals were free to fully bear weight during the post-operative period. After 12 weeks, the healing fractures were evaluated for callus formation using micro-computer tomography and strength and stiffness using biomechanical testing. No plate deformation or failures were observed under full weight bearing with the biphasic plate. Osteotomies stabilized with the biphasic plate demonstrated robust callus formation. Torsion tests after plate removal revealed no statistical difference in peak torsion to failure and stiffness for the different fracture patterns stabilized with the biphasic plate. However, the biphasic plate group specimens were 45% stronger (p=0.002) and 48% stiffer (p=0.007) than the controls.

The results of this large animal study demonstrate the clinical potential of this novel stabilization concept.

Background

Osteoporotic fracture fixation in the proximal humerus remains a critical challenge. While the biomechanical benefits of screw augmentation with bone cement are established, minimising the cement volume may help control any risk of extravasation and reduce surgical procedure time. Previous experimental studies suggest that it may be sufficient to only augment the screws at the sites of the lowest bone quality. However, adequately testing this hypothesis in vitro is not feasible.

Hip fractures constitute the most debilitating complication of osteoporosis with a steadily increasing incidence in an aging population. Intramedullary nailing of osteoporotic proximal femoral fractures can be challenging because of poor implant anchorage in the femoral head. Recently, cement augmentation of PFNA blades with Polymethylmethycrylate (PMMA) has shown promising results by enhancing the cutout resistance in proximal femoral fractures. The aim of this biomechanical study was to assess the impact of cement augmentation on the fixation strength of TFNA blades and screws within the femoral head, and compare its effect with head elements placed in a center or antero–posterior off–center positions.

Eight groups were formed out of 96 polyurethane foam specimens with low density, simulating isolated femoral heads with severe osteoporotic bone. The specimens in each group were implanted with either non–augmented or PMMA–augmented TFNA blades or screws in a center or antero–posterior off–center position, 7 mm anterior or 7 mm posterior. They were mechanically tested in a setup simulating an unstable pertrochanteric fracture with lack of postero–medial support and load sharing at the fracture gap. All specimens underwent progressively increasing cyclic loading until catastrophic construct failure. Varus–valgus and head rotation angles were monitored by an inclinometer mounted on the head. A varus collapse of 5° or a 10° head rotation were defined as the clinically relevant failure criterion.

Load at failure for specimens with augmented TFNA head elements (screw center: 3799 N ± 326 (mean ± SD); blade center: 3228 N ± 478; screw off–center: 2680 N ± 182; blade off–center: 2591 N ± 244) was significantly higher compared to the respective non–augmented specimens (blade center: 1489 N ± 41; screw center: 1593 N ± 120; blade off–center: 1018 N ± 48; screw off–center: 515 N ± 73), p<0.001. In both non–augmented and augmented specimens, the failure load in center position was significantly higher compared to the respective off–center position, regardless of head element, p<0.001. Non–augmented TFNA blades in off–center position revealed significantly higher load at failure versus non–augmented screws in off–center position, p<0.001.

Cement augmentation clearly enhances fixation stability of TFNA blades and screws. Non–augmented blades outperformed screws in antero–posterior off–center position. Positioning of TFNA blades in the femoral head is more forgiving than TFNA screws in terms of failure load. Augmentation with TFNA has not been approved by FDA.