The internal calcar septum (femoral thigh spur) in computed tomography andconventional radiography

Objective. The femoral "thigh spur", a cortical septum in the region of the lesser trochanter of the human femur, was first described and named by the German anatomist Merkel in 1874, but it was never examined in detail. To e valuate the frequency and the shape of this structure, a combined anatomica l and radiological study was performed using saw-cuts from specimens, high- resolution CT and conventional radiography. Design. Thirty human cadaveric femora of central European origin were analy zed by high-precision computed tomography (CT) using thin slices and high-r esolution imaging. The CT data were image processed with thresholding to ob tain a reconstruction of high-density bone formations and for three-dimensi onal imaging, Additionally three macerated femur specimens were cut exactly corresponding to the CT slices. The computed images were validated with th e anatomical saw-cuts. Results. A dense trabecular ridge protruding endosteally from the posterome dial cortex was found in all femora. This cortical septum reaching from the femoral neck to the distal part of the lesser trochanter separated the fem oral cavity from the cancellous bone inside the lesser trochanter. On conve ntional radiography the femoral thigh spur could be visualized best in the frog-lateral view of the hip. Conclusion. The internal calcar septum is a constant cortical structure. It should be recognized when radiographs or CT images of the proximal femur a re interpreted. It could be of importance for metaphyseal fitting of an end oprosthetic stem.