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.

Aim

The aim of this FE study was to analyse the comparative behaviour of cement and metal based augments in TKR and quantify the stresses within these different augments and underlying cancellous bone.

Introduction

In complex primary and revision total knee replacement (TKR) the operating surgeon may encounter proximal tibial bone defects. The correct management of such defects is fundamental to both the initial stability and long-term survival of the prosthesis. Block or wedge-shaped metal augments are used to address some such type II unconstrained defects.