Identifying the center of the femoral head using ultrasonography to assess the higher accuracy of femoral extramedullary guides in TKA.

To identify the center of the femoral head (FHC), which is critical for determining the mechanical axis of the femur in total knee arthroplasty (TKA), we used a high-resolution ultrasound technique in 200 hips in 128 patients with osteoarthritis of the knee. FHC was defined as the intersection of two lines: the maximum diameter of the femoral head between the superior and inferior edges of the head and the maximum diameter of the femoral head between the medial and lateral edges. Ultrasound identified the FHC within 5 mm in 56% of cases and within 10 mm in 89.5%. Ultrasound appears to be a highly reliable, noninvasive imaging modality for identifying the FHC preoperatively or even intraoperatively. Therefore, our procedure might ultimately be helpful in establishing the correct alignment of the prosthesis.