One of the diseases carrying major 3D problems is slipped capital femoral epiphysis (SCFE), defined as the slippage of the femoral head relative to the femoral neck along the proximal femoral growth plate. This disease affects particularly the proximal femur of adolescents whose growth plate is not yet calcified and therefore is soft enough to give way for a slippage under certain circumstances. During a slippage the femoral head shifts and rotates along the proximal end of the femoral neck, usually posteriorly and inferiorly (Fig. 1). Such a slippage changes the geometry of the hip joint, leading to an incorrect position of the femoral head relative to its joint partner, the acetabulum. This misalignment is the cause of a dysfunction of the hip joint. Initially the loss of motion may be tolerated, but eventually pain and stiffness of the joint will result from early arthritic degeneration.
In general, the planning of a surgical treatment in these cases is based on measurements made on anterio-posterior and lateral plain radiographs (Figs 2 and 3). Currently, slippage angles measured on such images are essential for the planning of correctional surgeries [9,14]. However, it is obvious that in most SCFE cases it is impossible to position the hip in a reproducible and accurate manner for plain radiographs.
This chapter describes a new concept for angular measurement based on 3D computer models and illustrates its use for the cases of SCFE. A 3D computer graphics environment provides user-friendly interaction that allows measuring angles directly on virtual 3D structures, and therefore without projection errors or positioning problems.
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