Figure 5-2 A, Diagram of acetabulum on anteroposterior radiograph. In a normal acetabulum, the socket is anteverted. On the anteroposterior radiograph, the anterior acetabular rim is above posterior rim. Three dimensionally, the acetabular socket is open and faces in an anterior direction. B, Diagram of acetabular retroversion. On the anteroposterior radiograph, the anterior rim is in a lower position, whereas the posterior rim is higher. Radiographically, the two rim lines cross, creating the crossover sign. Frequently with acetabular retroversion, the ischial spine is prominently seen. Three dimensionally, the acetabular socket is closed and faces in a posterior direction. Ant., anterior; AP, anteroposterior; Post., posterior.

Figure 5-4 A, Anteroposterior radiograph measuring α-angle. The α-angle is a method to evaluate head-neck dysplasia. The α-angle is formed between the lines of femoral head center and neck midline, and femoral head center and head-neck junction. Normal α-angle typically is less than or equal to 40 degrees. B, Anteroposterior radiograph measuring α-angle in a dysplastic proximal femur. α-Angle is increased.

Figure 5-5 Pincer type of femoral acetabular impingement. A, Diagram showing normal acetabulum and proximal femur. B, Diagram depicting acetabular retroversion. Retroverted acetabulum will limit functional hip flexion. C, Mechanics of pincer impingement. In hip flexion, the femoral neck abuts acetabular rim. Typically, this results in localized damage to the labrum. D, Ex vivo demonstration of pincer impingement. In this case, repetitive pincer impingement created a bony indentation trough on the femoral head at the site of flexion impingement.

Figure 5-6 Cam type of femoral acetabular impingement. A, Diagram depicting proximal femoral neck dysplasia with reduced head-neck offset. The “fat” femoral neck still is able to travel into acetabular socket a short distance. B, Mechanics of cam impingement. In hip flexion, the fat femoral neck acts as a cam (a raised area) that abrades the anterior-superior region of the acetabulum. Typically this results in shear delamination of the chondral surfaces of the acetabulum. C, Ex vivo demonstration of cam impingement. Notice how superior femoral neck is almost up to the level of the superior-lateral femoral head. This region (the cam) is what impinges upon the articular surfaces of the acetabulum in flexion. D, Magnetic resonance image of femoral acetabular impingement. In this case, there is a combination of cam and pincer impingement. There is an enlarged femoral neck (reduced head-neck offset), but clinically there is also pincer impingement upon the anterior acetabulum.

Figure 5-12 Ex vivo retrieval of bone ingrowth femoral stem. Notice metallic pores (circle) that allow bone to grow into prosthesis and osteointegrate.

Figure 5-13 Diagram of press fit technique for cementless hip fixation. A, The canal of the femur is prepared with serial reaming and/or broaching to achieve desired implant size (remember that cortical contact is preferred). An implant of slightly larger size is impacted into position. The stem usually has a gradual wedge taper. The wedging effect generates compression hoop stresses that hold the implant still to allow bone to grow into implant interface. B, The acetabulum is prepared with serial hemispheric reamers to achieve desired implant size (ream to cortical margin of acetabular rim). An implant of slightly larger size is impacted into position. The wedge effect occurs at the rim of the acetabulum. The compression hoop stress generated at the rim holds the implant still to allow bone to grow into the implant interface.

Figure 5-15 Photograph of bone ongrowth stem retrieval. The design of the stem has a sharp angle wedge design in two planes. The edges of the implant are sharp to provide rotational stability. Notice the rather scant amount of bone on the surface of this prosthesis. Implant was found biologically stable at the time of removal.

Figure 5-18 Diagram and radiograph of spot weld. A spot weld indicates stable osteointegration of an extensively porous-coated implant. In contrast, a bony pedestal is bone accumulation within the medullary canal below the tip of a mechanically loose stem. The bony pedestal seeks to keep the stem from sinking further down the medullary canal.

Figure 5-21 Reconstruction cage for segmental acetabular deficiency. Left, Displaced acetabular cup. Note segmental bone loss of posterior acetabulum. Center, Pelvic reconstruction with a triflange cage. The cage is secured to bone with screws. Acetabular cup is cemented into the cage. Right, Triflange cage before insertion.

Figure 5-22 Diagram showing four quadrants for acetabular screw placement. Line A is formed by drawing a line from the anterior superior iliac spine (ASIS) to the center of the acetabular socket. Line B is then drawn perpendicular to line A, also passing through center of socket.

Figure 5-30 Diagram demonstrating primary arc range in total hip arthroplasty. At the end of hip range, neck impingement occurs, limiting motion. Primary arc range is the arc of motion allowed between the two ends of impingement.

Figure 5-31 Diagram demonstrating lever range and excursion distance. A, Diagram showing lever range. When the femoral neck impinges on the acetabular cup, it begins to lever out of socket. The range of motion allowed before the hip dislocates is termed the lever range. B, Excursion distance. As the hip begins to lever, the femoral head is lifted out of socket. The excursion distance is the distance the head must travel to dislocate. The excursion distance is equal to the radius of the femoral head

Figure 5-33 Photographs showing how increasing head/neck ratio increases primary arc range. In this demonstration, the neck diameter remains constant. As the head diameter increases, the head/neck ratio increases, and primary arc range increases as well.

Figure 5-38 Diagram of acetabular cup version. A, In this figure, cup anteversion is referenced relative to the ilioischial line. The cup should be placed at 20 to 30 degrees of anteversion. B, With a retroverted cup, the femur impinges and levers in flexion, which can result in posterior hip dislocation.

Figure 5-39 Diagram of theta (θ)-angle. In the coronal plane, the cup should be placed at a θ-angle of 35 to 40 degrees. A low θ-angle cup is a risk for abduction impingement. A high θ-angle cup increases risk for posterior-superior dislocation.

Figure 5-40 Diagram of stem anteversion. A, Normal stem anteversion of 10 to 15 degrees. This allows functional hip flexion without impingement. B, Stem retroversion. With a retroverted stem, the proximal femur impinges early against pelvis. This can result in levering and posterior dislocation.

Figure 5-42 Diagram showing effect of decreased neck length. A decreased neck length brings the proximal femur closer to the pelvis. As the hip is ranged, the greater trochanter is more likely to abut the pelvis. This causes hip levering and increases risk for hip instability.

Figure 5-43 Diagram demonstrating the problem of using a long femoral head. A low neck cut is compensated by employing a long femoral neck (typically 9 mm or longer). When using a long head, a metal skirt is added to improve bending strength (to prevent metal fatigue and failure). The thick metal skirt can significantly reduce primary arc range, resulting in an increased risk for dislocation.

Figure 5-46 Example of greater trochanteric advancement. Diagram on the left shows implants in good alignment, but the abductor sleeve is lax in tension. When the greater trochanter is distally advanced (right), the abductor tension is improved, resulting in increased hip compression forces. This improves hip stability.