Chronic and subacute slipped capital femoral epiphysis with slip angles of 30–60° on a lateral radiograph.

 Avascular necrosis affecting a focal segment of the femoral head. The goal of a intertrochanteric osteotomy in this clinical scenario is to move an unaffected portion of the femoral head into the weight-bearing area.

 The hip is examined for range of motion. It is especially important to document deficiency of internal rotation and flexion as these may be corrected with rotation and flexion through the osteotomy. Extension, abduction, and adduction should also be recorded.

 Any leg length discrepancy should be noted as this may be corrected with the osteotomy.

 Anteroposterior (AP) (Fig. 1) and lateral (Fig. 2) radiographs of the affected hip must be obtained. The slip angle should be determined from the lateral radiograph.

 A computed tomography (CT) scan of the proximal femur with three-dimensional rendering may provide the surgeon with a better understanding of the deformity (Fig. 3A and 3B).

 Contrast magnetic resonance imaging (MRI) of the hip may be used to identify intra-articular pathology, such as labral tears and chondral flaps, arising from mechanical damage due the presence of the chronic slip.

 MRI or CT may be used to assess overall femoral version and determine the degree of internal rotation of the distal fragment required to correct the external rotation deformity.

 Superficial anatomy includes the gluteus maximus muscle, tensor fascia lata muscle, and iliotibial band (Fig. 4).

 Deep anatomy includes the gluteus medius and vastus lateralis muscles.

 Perforating vessels may be found entering the vastus lateralis through the intermuscular septum (see Fig. 4). These vessels should be identified and coagulated as the vastus lateralis is reflected off the intermuscular septum and lateral aspect of the femur.

 The vastus ridge is the site of insertion of both the gluteus medius and vastus lateralis muscles (Fig. 5). This bony landmark is useful for planning the osteotomy.

 It is important to understand the blood supply to the femoral head.