Low force direct anterior hip arthroplasty

The trend towards more minimal access has led to a series of instruments being developed to enable adequate access for Direct Anterior Approach (DAA) for hip arthroplasty. These include longer levers, hooks attached to the operating table and a series of special attachments to the operating table to position the leg and apply traction where necessary. The forces applied in this way may be transmitted locally, damaging muscle used as a fulcrum, or the knee and ankle joints when torque has to be applied to the femur through a boot. The arthroplasty surgeon's aim is to minimise the forces applied to both bone and soft tissue during surgery. We surmised that the forces needed for adequate access were related to the extent of the capsular and soft tissue releases, and that they could be measured and optimised. with the aim of minimising the forces applied to the tissues around the hip. Eight fresh frozen specimens from pelvis to mid tibia from four cadavers were approached using the DAA. A 6-axis force/torque sensor and 6-axis motion tracking sensor were attached to a threaded rod securely fastened to the tibial and femoral diaphysis. The torque needed to provide first extension, then external rotation, adequate for hip arthroplasty were measured as the capsular structures were divided sequentially. The Zona Orbicularis (ZO) and Ischiofemoral Ligament(IFL) contributed most of the resistance to both extension (4.0 and 3.1Nm) and external rotation torque (5.8 and 3.9Nm). The contributions of the conjoint tendon (1.5 and 2.4Nm) and piriformis (1.2 and 2.3Nm) were substantially smaller. By releasing the Zona Orbicularis and Ischiofemoral Ligament, the torque needed to deliver the femur for hip arthroplasty could be reduced to less than the torque needed to open a jar (2.9–5.5Nm).