Acetabular bone loss classification

Introduction

As with many clinical scenarios encountered in orthopaedics, classification systems have been developed to concisely communicate radiographic and/or intraoperative findings among providers and researchers and, perhaps most importantly, provide guidelines for clinical management. Acetabular bone loss in the setting of primary or revision total hip arthroplasty (THA) can result from osteolysis, component migration, infection, iatrogenic bone removal during component extraction, and many other causes. With the projected increase in the number of revision THA procedures being performed annually, familiarity with the classification systems used to describe acetabular bone loss is important as it is often the first step in planning for definitive management. In this chapter, we will explore the Paprosky bone loss classification system in depth and highlight how key radiographic and intraoperative findings within each subclassification can guide treatment in the setting of primary or revision THA.

Anatomy and biomechanical stability of primary press-fit shells

To better understand pathological states, we will first review the normal anatomy of the acetabulum and its relevance to acetabular shell stability in primary THA. The acetabulum is formed by the fusion of the ischium, ilium, and pubis, which constitute the pelvis. Typical anteversion is 19.9° (± 6.6°) with males averaging less anteversion (18.5°) than females (21.3°). Studies have also suggested that female acetabula have greater inclination angles than males, with a mean of 56.5° ± 4.3° across both sexes. The ilioischial line (also known as Köhler’s line) and the iliopectineal line represent the posterior and anterior columns of the acetabulum, respectively, and are key radiographic landmarks that should be recognized.

The success of press-fit acetabular components relies on initial stability to promote osseointegration. A recent systematic review calculated that less than 112 µm (± 176 µm) of micromotion is necessary for reliable osseointegration. In contrast, micromotion equal to or greater than 349 µm (± 231 µm) will lead to fibrous tissue formation. In a typical, healthy acetabulum, the endoprosthetic cup is supported by three pillars of bone at the periphery of the shell: the iliac or superior region, ischial or posteroinferior region, and anteroinferior or pubic region. In the cases of compromised bone stock, particularly in the superior dome region, an acetabular shell can be “pinched” between the anterosuperior column of bone and the posteroinferior column of bone. As expected, the highest initial stability of press-fit shells is produced by a completely supportive circumferential acetabular socket and a large surface area of contact at the implant-bone interface. As the discussion shifts to considering varying degrees of acetabular bone loss, it is important to remember that the stability of a cementless acetabular shell is critical to osseointegration, long-term clinical success, and implant longevity.

Alternative classification systems

The American Academy of Orthopaedic Surgeons (AAOS) classification developed by D’Antonio is a commonly used classification system based on intraoperative assessment that describes the pattern and location of bone loss ( Table 2.1 ). ^, ^, Acetabula are categorized first by pattern as segmental, cavitary, combined deficiency, pelvic discontinuity, or arthrodesis. Within segmental, cavitary, and combined deficiencies, the pattern can be further subclassified by location as superior, anterior, posterior, or central. A segmental deficiency involves bone loss in the supporting peripheral hemisphere of the acetabulum or centrally, including the medial wall. In contrast, cavitary defects are those in which the acetabular columns are intact, but there is volumetric loss of the bony substance. Patients with combined deficiencies present with both segmental and cavitary defects which may be in the same or different locations. A limitation of this system is that it does not quantify the bony loss or guide clinical treatment.

TABLE 2.1

American Academy of Orthopaedic Surgeons (AAOS) Classification of Acetabular Deficiencies

From D’Antonio JA, Capello WN, Borden LS, et al. Classification and management of acetabular abnormalities in total hip arthroplasty. Clin Orthop Relat Res . 1989;243:126-137. Adapted from Garbedian S, Backstein D, Gross AE et al. Acetabular bone loss classification. In: Scuderi G, ed. Techniques in revision hip and knee arthroplasty . 1st ed. Philadelphia: Elsevier; 2015:349-353.

Type	Description
I	Segmental Peripheral Superior Anterior Posterior Central (medial wall absent)
II	Cavitary Peripheral Superior Anterior Posterior Central (medial wall intact)
III	Combined Deficiencies
IV	Pelvic Discontinuity
V	Arthrodesis

An alternative system developed by Saleh et al. categorizes deficiencies based on the amount of bone loss appreciated on preoperative anteroposterior (AP) and lateral hip radiographs. Bone loss is classified as contained, uncontained involving less than 50% of the acetabulum, uncontained involving more than 50% of the acetabulum, and pelvic discontinuity with uncontained loss of bone stock. While this classification system does guide treatment based on the type of bone loss encountered, it is limited in that it does not describe the location of the defects.

Paprosky classification system

The Paprosky classification system was developed through the preoperative and intraoperative evaluation of 147 failed THAs. The system identifies which acetabular supporting structures are deficient for the purposes of predicting the biologic and metallic augments that may be needed during reconstruction. Patterns are classified by the degree of superior migration of the hip center compared to the superior obturator line and the integrity of key anatomic landmarks, including the teardrop, the ischium, and Köhler’s line as noted on preoperative radiographs. ^, ^, These parameters are utilized to classify host acetabular columns as completely supportive (type I), partially supportive (type II), and non-supportive (type III) ( Table 2.2 ). It was developed based on AP radiographs to allow for widespread use by the orthopaedic community. This classification system remains the most widely used and has demonstrated good inter- and intra-observer reliability. ^, Furthermore, treatment recommendations are based on the remaining, intact supportive acetabular columns. Of note, revision surgeons should be familiar with various treatment options as the defects should be reclassified after all components have been removed because of potential iatrogenic bone loss resulting in a different pattern of bone loss.

TABLE 2.2

Paprosky Definitions of Acetabular Defects

From Paprosky WG, Perona PG, Lawrence JM. Acetabular defect classification and surgical reconstruction in revision arthroplasty: a 6-year follow-up evaluation. J Arthroplasty . 1994;9:34. Adopted from Garbedian S, Backstein D, Gross AE et al. Acetabular bone loss classification. In: Scuderi G, ed. Techniques in revision hip and knee arthroplasty . 1st ed. Philadelphia: Elsevier; 2015:349-353.

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	Non-supportive	Tissue membrane or sclerotic bone

Bone loss in the acetabular dome, which can involve the anterior or posterior columns, is indicated by superior migration of the hip center on the radiograph. Superior and lateral migration indicates greater involvement of the posterior column, while more superior and medial migration raises concern for the involvement of the anterior column. Similarly, migration medial to Köhler’s line is reflective of anterior column deficiency. With respect to ischial osteolysis, this is indicative of bone loss in the inferior aspect of the posterior column and the posterior wall. Lastly, osteolysis in the area of the teardrop is indicative of bone loss in the inferior aspect of the anterior column and lateral aspect of the pubis.

Table 2.3 summarizes the radiographic and intraoperative findings by defect type. For the reasons mentioned above, it is the preferred classification system of the authors and will be discussed in greater detail here.

TABLE 2.3

Paprosky Classification System for Acetabular Defects

Data from Paprosky WG, Perona PG, Lawrence JM. Acetabular defect classification and surgical reconstruction in revision arthroplasty: a 6-year follow-up evaluation. J Arthroplasty . 1994;9:33-44; Sporer SM, Paprosky WG, O’Rourke M. Managing bone loss in acetabular revision. J Bone Joint Surg Am . 2005;87:1620-1630. Adopted from Garbedian S, Backstein D, Gross AE et al. Acetabular bone loss classification. In: Scuderi G, ed. Techniques in revision hip and knee arthroplasty . 1st ed. Philadelphia: Elsevier; 2015:349-353.

	Radiographic Findings				Intraoperative Findings
Type of Defect	Component Migration	Ischial Osteolysis	Teardrop Osteolysis	Köhler’s line	Acetabular Status	Trial Stability
I	None	None	None	Intact	Rim and columns intact Small, focal area of bone loss	Stable
IIA	<3 cm directly superiorly	None	None	Intact	Oval enlargement of bone loss and superior osteolysis with an intact rim Host bone contact >50%	Stable
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%	Stable
IIC	Cup medial to Köhler’s line	Mild	Obliterated	Disrupted	Medial wall defect Rim intact and supportive	Stable
IIIA	>3 cm superolateral	Moderate	Moderate	Intact	Circumferential defect between 10 and 2 o’clock positions Host bone contact 40%-60%	Partial
IIIB	>3 cm superior and medial	Severe	Obliterated	Disrupted	Host bone contact <40% Circumferential defect between 9 and 5 o’clock positions	None

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