Acetabular Bone Loss Classification

Wayne G. Paprosky
Karl Dermingian
Brett Levine

INTRODUCTION

The most challenging aspect of acetabular revision surgery relates to the management of bone loss encountered intraoperatively. Severe pelvic bone loss can compromise implant fixation and may result in poor clinical outcomes. The revision of cemented acetabular components is most commonly related to aseptic loosening with associated migration of the acetabular component. The extent of migration of a cemented implant can predict the degree of bone loss at the time of surgery. Cementless acetabular fixation can present with severe pelvic bone loss due to asymptomatic osteolysis and pelvic stress shielding. Therefore, the amount of bone loss encountered at the time of revision surgery with a well-fixed cementless implant can be pronounced prior to migration of the acetabular component. More extensive bone loss at the time of revision acetabular surgery is expected to become more common as cementless acetabular fixation becomes increasingly popular and hip replacement is being performed in younger and more active patients.

Adequate preoperative planning is critical for surgeons who perform hip revisions. Acetabular reconstruction options will ultimately be based upon the remaining host bone and the ability to obtain stable fixation. An acetabular defect classification can aid in the preoperative evaluation of bone loss. A useful classification system should not only accurately predict the severity and location of bone loss but also guide treatment options and allow surgeons to compare treatment results among patients. Therefore, a useful classification should remain detailed enough to alert surgeons when alternative reconstruction methods are required (allograft bone, structural augments, cages, etc.), yet remain simple enough to ensure a high degree of intraobserver and interobserver reliability.

CLASSIFICATION AND DECISION MAKING

The two most common acetabular defect classification systems are from The American Academy of Orthopaedic Surgeons (AAOS) and that described by Paprosky. The acetabular defects are described based upon intraoperative findings. However, a useful classification system will allow preoperative radiographic evaluation to anticipate the observed intraoperative findings. As a result, the preoperative acetabular defect may be “upgraded” or “downgraded” based upon intraoperative findings.

AAOS AAOS classification of bone defects, described by D’Antonio et al.¹ identifies the pattern and location of bone loss but does not quantify the defect. The bone loss is classified as contained, segmental, combined contained/segmental, pelvic discontinuity, and ankylosis. This is the most commonly sited classification in the literature.

Paprosky The classification system of Paprosky is based on the severity of bone loss and the ability to obtain cementless fixation for a given bone loss pattern.² The key to the classification is the ability of the remaining host bone to provide initial stability to a hemispherical cementless acetabular component until ingrowth occurs. Intraoperative decisions are based on the use of the trial components. The amount of rim remaining determines the stability of the trial implant and is one variable determining the type of acetabular defect. A Type I defect has an undistorted rim, a Type II defect has a distorted intact rim with adequate remaining bone to support a hemispherical cementless implant, and a Type III defect has a nonsupportive rim.

RADIOGRAPHIC CORRELATION

Preoperative radiographic findings on the AP radiograph of the pelvis are used to predict the type of defect present and will allow surgeons to plan for the acetabular reconstruction accordingly. The four criteria on the preoperative radiograph that are important to assess include (i) superior migration of the hip center, (ii) ischial osteolysis, (iii) teardrop osteolysis, and (iv) position of the implant relative to Kohler’s line.

Superior migration of the hip center represents bone loss in the acetabular dome involving the anterior and posterior columns. Superior and medial migration indicates a greater involvement of the anterior column. Superior and lateral migration indicates a greater involvement of the posterior column. The amount of superior migration is measured as the distance in millimeters (adjusting for magnification) relative to the superior obturator line.

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