A 39-year-old West African man underwent his first total hip arthroplasty 13 years before presentation. His hospitalization was complicated by prolonged postoperative drainage and persistent pain. He presented for treatment with an 8-year history of hip pain with ambulation and night sweats. On physical examination, the patient was afebrile, had a shortened extremity, walked with a profound limp, and had groin pain with any passive rotation of the left hip. Radiographs of the pelvis and hip revealed grossly loose cemented components ( Fig. 57.1 ). Serology demonstrated a normal white blood cell count but an elevated C-reactive protein level and erythrocyte sedimentation rate. Aspirate cultures grew Staphylococcus aureus .
The patient was taken to surgery, where he underwent removal of his implants, followed by a radical soft tissue débridement and primary revision ( Fig. 57.2 ). The patient was treated with 6 weeks of intravenous antibiotics. His wounds healed uneventfully, and he remains pain and limp free 5 years after revision.
Difficulty in establishing the appropriate position for placement of the acetabular component in total hip arthroplasty remains a vexing problem for even the most experienced hip surgeons. In a review of the experience of surgeons at one of the best-known teaching centers in the United States, only 50% of components were found to be properly placed. This chapter reviews the goals of acetabular placement and identifies intraoperative landmarks that can facilitate proper cup placement and orientation.
Goals of Acetabular Placement
The techniques used to achieve optimal acetabular component placement are based on an understanding of hip kinematics and the relationship between component position and function and depend on relating this knowledge to bony landmarks that can be identified intraoperatively. The goals of acetabular placement include providing a stable range of motion and positioning the implant so that the forces across the joint are minimized, thereby improving gait and reducing wear. Optimal acetabular placement requires attention to the rotational alignment of the socket (i.e., abduction angle and anteversion) and the position of the hip center (i.e., medial-lateral and cephalad-caudad translation).
Achieving stability requires the surgeon to localize the appropriate rotational position of the acetabulum. Although the surgical approach (anterior or posterior) and femoral version have some bearing on ideal acetabular cup anteversion, the accepted safe range lies somewhere between 5 and 25 degrees, as espoused by Lewinnek and colleagues more than 30 years ago. The acetabular abduction angle has long been recognized as an important determinant of the wear rate, with higher abduction angles associated with increased wear, edge loading, and polyethylene fracture. In ceramic applications, high abduction angles have been associated with microseparation, wear, and squeaking. The optimal range of cup abduction remains between 30 to 50 degrees, regardless of the bearing material. With improved bearing surfaces, a more horizontal abduction angle may decrease the risk of catastrophic bearing failure.
The other two clinically important components of acetabular position are translational: hip center height and medialization. Reducing the body-weight lever arm by medializing the cup reduces the forces across the joint, reducing wear and improving gait and trunk stability. Similarly, the height of the hip center is an important determinant of abductor muscle strength and a stable gait. The surgeon should avoid raising the hip center to preserve abductor muscle efficiency.
Planning and Landmark Identification
Achieving the alignment goals previously described requires the surgeon to combine preoperative planning with identification of specific bony landmarks that can be reliably located during the procedure. The pelvis is quite mobile during operative manipulation, which can lead to errors in component placement. Because the pelvis cannot be well controlled, particularly in very large patients, my colleagues and I rely on the pelvis itself to guide us.
Templating can provide a landmark for the surgeon to evaluate abduction of the component intraoperatively. An acetate template can be positioned in the bony socket, resting on the floor of the fovea or along the ilioischial line at a desired abduction angle ( Fig. 57.3 ). The overhanging bone is marked, and its size can be approximated, providing the surgeon with an intraoperative guide to component abduction. If the surgeon does not see this protruding bone, the component has been placed in a more vertical or abducted position. The process also can be applied to the cephalad side. The preoperative template can predict whether there is overhanging bone or exposed prosthesis when the component is appropriately aligned as long as the component is seated on the medial wall of the socket or at the templated depth.