A biomechanical evaluation of cortical onlay allograft struts in the treatment of periprosthetic femoral fractures

Periprosthetic femoral fractures are increasingly addressed through the use of cortical onlay allografting. This study was designed to determine the effect of allograft cortical strut length, configuration, cable number, cable tension and the use of wire or cable on the fixation of periprosthetic femoral fractures. Ten cadaveric femora-strut constructs were tested using anteroposterior and axial loads to simulate the forces at the hip during gait. A transverse fracture at the level of the tip of the femoral stem was simulated. A biaxial servohydraulic testing machine was used to apply one hundred cycles of craniocau-dal load of 1.53 × bodyweight at a frequency of one Hz, along with an anteroposterior load of 0.15 × bodyweight at one half Hz. Variables for different constructs included the strut length (twelve cm, sixteen cm, or twenty cm), the number of cables (two, three, or four above and below the fracture site), cable tension, strut configurations and orientation (single strut or two struts, adjacent or opposite), and the use of wires instead of cables. Cable tension was measured using a calibrated tensioner. Movement at the fracture site was measured using a precision optoelectronic camera system. There was significantly less motion when cables were used rather than wires (p<0.05). Increasing the number of cables decreased fracture motion in some directions (p<0.05) and increasing cable tension showed a trend towards decreased fracture motion. We observed strut fractures in four cases when a single strut alone was used to stabilise the fracture. There was a significant decrease in fracture motion if two struts were used rather than one (p<0.01), but there was no significant difference between the anterior and lateral, and the medial and lateral strut configurations. Decreasing the strut length from twenty cm to twelve cm led to a significant decrease in axial rotation (p<0.05). Our data strongly favour the use of two struts, rather than a single strut alone. Cables enhance fracture stability compared to wires, presumably due to increased tension and to different surface characteristics. Increasing the cable tension gave greater stability although this may not fully translate to the clinical situation because the cable may garrotte or fracture the strut. Increased cable number and decreased strut length also enhanced fracture stability. The cortical struts essentially represent biological bone plates. If appropriately selected and prepared they can be customised to fit any femur. Our improved understanding of this technique should contribute to high rates of fracture union with an increase in bone stock and overall bone strength.