The demand for primary total hip arthroplasty (THA) is expected to increase during the next few decades, ensuring a concomitant increase in revision THAs. Revisions are projected to increase from 40,800 cases in 2005 to 96,700 cases in 2030. Revision of the acetabular component accounts for more than 50% of cases, and common reasons for these procedures include instability, infection, polyethylene wear, and aseptic loosening.
Severe bone loss can result from asymptomatic osteolysis, stress shielding, and component migration. One of the most challenging aspects of revision THA is the management of acetabular bone loss that compromises stability and fixation of the implant, a situation that will become more common in the near future.
Indications for Revision
Patients requiring revision of the acetabular component may present in two ways. One group can be asymptomatic despite radiographic evidence of substantial and progressive wear of the polyethylene liner with or without osteolysis. The amount of bone loss or potential bone loss in this group can threaten reconstruction options in the future. THA patients in the second group are symptomatic as a result of failure and migration of the components due to extensive bone loss.
Standard anteroposterior and lateral radiographs of the hip often underestimate the size of osteolytic lesions of the acetabulum. Multislice computed tomography with metal artifact minimization is more sensitive than plain radiographs for identifying osteolysis around the cup. This additional information can aid preoperative planning and quantify the amount of acetabular bone loss.
Acetabular Bone Loss Classification Systems
Many classification systems have been developed over the years to describe and organize acetabular bone loss associated with THA. The main functions of a classification system are to facilitate communication between surgeons, assist with surgical decision making, and provide a framework for research and education. There are three commonly used classification systems for acetabular bone loss in hip revision surgery.
The classification system developed by Paprosky and colleagues is based on the presence or absence of an intact acetabular rim and its ability to provide initial rigid support for an implanted acetabular component. The key point is determining the ability of the host acetabulum to provide initial stability for a hemispherical, cementless acetabular component until ingrowth can occur.
Intraoperative decisions are based on the stability of trial components. A trial component that has full stability does not change position when a trial reduction is performed or the surgeon applies a force on the rim. A partially stable component does not change position with the removal of the inserter, but it does not withstand the force of a trial reduction or application of a force on the rim. A trial component with no stability changes positions with the act of inserting or removing the inserter.
Defects are classified by type, indicating whether the remaining host acetabulum structures are completely supportive (type 1), partially supportive (type 2), or nonsupportive (type 3) of the implanted component ( Table 43.1 ). The extent of remaining host structural support determines the amount and type of allograft bone required (i.e., bulk or supportive grafts).
|Type of Defect||Rim||Wall or Dome||Column||Host Bone Bed for Component|
|I||Intact||Intact||Intact and supportive||>50%, cancellous|
|II||Distorted||Distorted||Intact and supportive||<50%, cancellous|
|III||Missing||Severely compromised||Nonsupportive||Tissue membrane or sclerotic bone|
Preoperative findings on the anteroposterior radiograph of the pelvis are used to calculate the type of defect and plan for the appropriate reconstructive technique. Four criteria are used to evaluate preoperative radiographs: (1) superior migration of the hip center, (2) ischial osteolysis, (3) teardrop osteolysis, and (4) position of the implant relative to the Kohler line. Superior migration is significant for column loss at the level of the dome, whereas superomedial migration indicates that anterior column loss is greater than posterior column loss, and superolateral migration indicates that posterior column loss is greater than anterior column loss. When medial migration occurs (i.e., cup medial to the Kohler line), anterior column deficiency, medial wall deficiency, or both exist. Radiographic loss of the teardrop indicates bone loss from the inferomedial aspect of the acetabulum, inferior aspect of the anterior column, and medial wall. Ischial osteolysis signifies that bone loss is from the inferior posterior column and posterior wall. Table 43.2 and Figure 43.1 summarize the radiographic and intraoperative findings for the Paprosky classification.
|Type of Defect||Radiographic Findings||Intraoperative Findings|
|Component Migration||Ischial Osteolysis||Teardrop Osteolysis||Kohler Line||Acetabular Status||Trial Stability|
|I||None||None||None||Intact||Rim and columns intact |
Small, focal area of bone loss
|IIA||<3 cm directly superior||None||None||Intact||Oval enlargement of bone loss and superior osteolysis with intact rim |
Host bone contact >50%
|IIB||<3 cm superior or superolateral||None||None||Intact||Uncontained superior rim defect <33% |
Intact anterior and posterior rim and column
Host bone contact >50%
|IIC||Cup medial to Kohler line||Mild||Obliterated||Disrupted||Medial wall defect |
Rim intact and supportive
|IIIA||<3 cm superolateral||Moderate||Moderate||Intact||Circumferential defect of 33%-50% between 10- and 2-o’clock positions |
Host bone contact 40-60%
|IIIB||>3 cm superior and medial||Severe||Obliterated||Disrupted||Host bone contact <40% |
Circumferential defect >50% between 9- and 5-o’clock positions