Effect of force tightening on cable tension and displacement in greater trochanter reattachment

The purpose of this study was to evaluate cable tension during installation, and during loading similar to walking in a cable grip type greater trochanter (GT), reattachment system. A 4th generation Sawbones composite femur with osteotomised GT was reattached with four Cable-Ready^® systems (Zimmer, Warsaw, IN). Cables were tightened at 3 different target installation forces (178, 356 and 534 N) and retightened once as recommended by the manufacturer. Cables tension was continuously monitored using in-situ load cells. To simulate walking, a custom frame was used to apply quasi static load on the head of a femoral stem implant (2340 N) and abductor pull (667 N) on the GT. GT displacement (gap and sliding) relative to the femur was measured using a 3D camera system. During installation, a drop in cable tension was observed when tightening subsequent cables: an average 40+12.2% and 11±5.9% tension loss was measured in the first and second cable. Therefore, retightening the cables, as recommended by the manufacturer, is important. During simulated walking, the second cable additionally lost up to 12.2+3.6% of tension. No difference was observed between the GT-femur gaps measured with cables tightened at different installation forces (p=0.32). The GT sliding however was significantly greater (0.9±0.3 mm) when target installation force was set to only 178 N compared to 356 N (0.2±0.1 mm); p<;0.001. There were no significant changes when initial tightening force was increased to 534 N (0.3±0.1 mm); p=0.11. In conclusion, the cable tightening force should be as close as possible to that recommended by the manufacturer, because reducing it compromises the stability of the GT fragment, whereas increasing it does not improve this stability, but could lead to cable breakage.