DYSPLASTIC ACETABULUM AND PROTRUSIO ACETABULI
Reconstruction of the dysplastic or deficient acetabulum presents a particularly difficult surgical challenge, because the anatomic landmarks commonly used as reference points may not be in their normal positions. Portions of the bony circumference of the acetabulum may be deficient as a result of old fractures or congenital dysplasia. For example, in a long-standing posterior fracture dislocation of the hip, the posterior wall of the acetabulum is usually severely deficient; in congenital dislocation of the hip, the acetabulum is shallow and poorly developed. If the femoral head has been dislocated for many years, it articulates with the iliac wing in a pseudoacetabulum. The true acetabulum is stunted, small, and shallow, but its anatomic configuration is usually preserved and identifiable once the contracted overlying inferior capsule is reflected.
Dysplasia of the acetabulum is often seen without actual dislocation of the femoral head. Usually, the acetabulum is shallow and the femur is displaced laterally. The superolateral wall, or roof, of the acetabulum is deficient and must be reconstructed with a bone graft before it can support an acetabular prosthetic component.
Total hip replacement in the patient with congenital hip dislocation is extremely difficult. Because the acetabulum may be deficient in bone mass and the proximal femur malformed, as is common, modular prostheses are sometimes required.
Treatment. A bone graft is often used to reinforce the superior acetabulum; it can be fashioned from the resected femoral head. The bone graft is fixed to the ilium with screws and then reamed to receive a small acetabular component, with the pubic bone and ischium used as anterior and posterior landmarks to avoid excessive reaming. Modular femoral components with varying angles and neck lengths can be used to achieve a near-normal abductor lever arm.
After a failed total hip replacement, bone autografts or allografts may also be used to reconstruct an acetabulum that is deficient in bone mass. After thorough removal of the loose acetabular component, bone mass deficiencies are repaired. A new acetabular com- ponent, often made with trabecular metal, is then impacted and secured with screws. Segmental defects are preferably reconstructed with trabecular metal augments. Cavitational defects, on the other hand, may be filled with allografts, usually best impacted into the cavity in the form of chips. A smooth surface can be created by reaming in reverse.
MEDIAL WALL DEFECT
Another common site of bone mass deficiency is the medial wall, or floor, of the acetabulum. Often discovered during revision surgery, this problem is frequently related to the first procedure, during which the medial wall was perforated. This condition is also seen in inflammatory arthritis.
Treatment. Medial wall deficits have been classified by size into three types: minor (diameter < 1 cm); intermediate (diameter < 3 cm); and major (diameter > 3 cm). All deficits are repaired surgically with one of three types of bone grafts. Bulk bone is used to recon- struct major deficits. For example, a large plug is fashioned from a resected femoral head and used to fill a medial wall defect. Screws may be added to further stabilize the graft, and a hemispheric depression can be reamed in the graft. Chips of trabecular bone are harvested and used as packing material to fill small deficits, cysts, and cracks. Pulverized bone (from reaming or a bone mill) can be made into a soft, pastelike consistency and finger packed into small deficits.
Bulky pieces of bone allograft maintain their structural integrity and may be variously replaced by host living bone through a process called creeping substitution; the trabeculae in the bone allograft act as a “trellis” for the ingrowth of live bone. Cementing into a bone graft is possible as long as the host bone–bone graft interface is adequate and intimate.
As a result of any disease that causes bone resorption, the pelvic bones become osteoporotic and soft and the medial wall of the acetabulum may be gradually displaced medially. Bone remodeling in response to applied load causes varying degrees of superomedial protrusion of the femoral head (see Plate 2-50). Protrusio acetabuli occurs when the femoral head is displaced past the ilioischial line in the pelvis. Other conditions that can cause progressive protrusion of the acetabulum are osteomalacia, rickets, Paget disease of bone, and infections. Central dislocation of the femur due to trauma can also heal with a protrusion deformity. Arthrokatadysis (Otto pelvis) is a rare idiopathic form of severe bilateral protrusio acetabuli most often seen in adolescent females. Metastatic carcinoma to the pelvis can lead to pathologic fractures, which result in a protrusion deformity.
Primary manifestations of protrusio acetabuli are thigh pain and decreased range of motion. Lateral and rotatory movements are particularly inhibited by the impingement of the femoral neck against the acetabular labrum, or rim. A classification of protrusion deformities distinguishes cases with an intact medial wall from those with a perforated medial wall. Third-degree protrusio acetabuli is the most severe, occurring when the medial displacement is greater than 5 mm, and is coupled with penetration of the medial wall. Protrusio acetabuli prosthetica occurs when a hip prosthesis is gradually displaced through the soft bone of the medial wall.
Treatment. Like medial wall defects, protrusio acetabuli is treated with various types of bone grafts to reinforce areas deficient in bone mass to ensure that prosthetic components remain in the correct anatomic positions.
If there is enough anterior and posterior wall remaining, a cementless socket is impacted in place with cancellous bone graft behind it. If severe deficiencies are present, an antiprotrusio cage may be placed over the top of the cementless socket. This cage rests on the ilium and ischium, thereby transferring the load from the hip to the intact pelvis.