A local reference frame for describing the proximal human femur: application in clinical settings

The conventional reference frame for the femur has limited relevance for the planning of hip surgery as the femoral neck axis, a crucial reference for surgeons, has to be independently derived. The purpose of this study is to develop and validate a reliable frame of reference for the proximal femur that can be applied in clinical settings.

Ten three-dimensional models of femurs were obtained. An iterative method was developed to find the femoral neck axis (X-axis). A second axis was also created from the lesser trochanter to the piriformis fossa (LTPF). The origin was defined as the femoral head centre. The cross product of the neck and LTPF axes provided the Z-axis and the third axis (Y-axis) was perpendicular to the other two. Intra-/inter-investigator reliability was assessed on the ten femur models; ten times by one investigator and twice by three investigators respectively. The results were then compared with the conventional reference frame using landmarks on the distal femur.

The femoral neck and LTPF axes had mean intra-/inter-investigator angle differences of 0.5° (SD 0.4°) and 0.7° (SD 0.5°), and 0.8° (SD 0.5°) and 0.9° (SD 0.6°) respectively while the variations of the X-, Y- and Z- axes were SD 0.6°, 0.7° and 0.5°.

A reliable method of obtaining the three-dimensional proximal femoral frame was developed, using the femoral neck axis, with greater relevance to clinical settings, preoperative planning and accurate assessment of procedures post-operatively.