Aims

Asphericity of the femoral head-neck junction is common in cam-type femoroacetabular impingement (FAI) and usually quantified using the alpha angle on radiographs or MRI. The aim of this study was to determine the natural alpha angle in a large cohort of patients by continuous circumferential analysis with CT.

Introduction Besides great advances in hip-alloarthroplasty there are still numerous indications for joint saving procedures as correction osteotomies. Often these procedures include complex 3D rearrangements of the proximal femur, which are for the surgeon technically very demanding because of the (1) complexity of a precise 3D planning of the fragment position and as a second step (2) the exact operative realization of the plan.

The project aim was to minimize these two major problems by using computer assisted techniques for exact intraoperative virtual 3D planning including a detailed biomechanical analysis (as change of head offset, torsion, leg length etc.).

Methods A new key feature is that we extended our former developed geometry based approach using 2 fluoro frames from different angles of the proximal femur to inversely reconstruct the femoral head sphere and additionally mark the head-neck axis and the anatomical femur axis. For navigation a passive infrared based optical system was used with a Polaris-camera, a C-arm calibration kit and PC-based developed software. For in vitro evaluation complex osteotomies were performed on 8 femora under simulated OR conditions.

Results The evaluation showed that the difference between the planning and real surgical outcome for the wedge size was less then 3 and for the femur head center position less then 4.1 mm. No implant penetrated the femur neck isthmus, but in 2 femora the position of the plate resulted in a lateral space of maximal 2 mm between the OT-planes, which was by higher plate tensioning completely compensable. The planning process as well as the operative execution was practicable and time efficient.

Discussion The used method demonstrated from a clinical view point a high accuracy. With the described approach it is for the surgeon directly visible during the planning process what biomechanical impacts his planned procedure will have on the femur head offset, torsion, leg length etc.

So without changing the standard operative procedure the method can be of high clinical importance to improve the accuracy of the planning and the consecutive operative realization for a precise fragment positioning and the plate location without penetrating the isthmus of the femoral neck. So it can potentially help to reduce intraoperative complications and the use of the fluoroscope to minimally 4 frames for the whole procedure.