Cementless acetabular revision for the distorted acetabulum with intact columns (type IIA, B, and C defects)

Background

The Paprosky classification of acetabular bone loss was established in 1994 to define the severity of bone loss and to suggest a revision algorithm to obtain a stable acetabular reconstruction. ^, The treatment is based on the location and degree of bone loss. Four variables need to be considered: (1) the location and migration of the hip center of rotation, (2) the degree of teardrop and (3) ischial osteolysis, and (4) the integrity of Köhler’s line. Thus, the superior dome, the medial acetabular wall, and the posterior column all need to be assessed. Please see Chapter 2 for further detail.

In Paprosky type II defects, there is moderate bone loss, and the hip center has migrated less than 3 cm superior to the native hip center. ^, There is no substantial osteolysis of the ischium and the teardrop, meaning that the ischial osteolysis is smaller than 7 mm distal to the obturator line. The anterosuperior and posteroinferior columns of the acetabulum remain intact and supportive.

In type IIA defects, there is anterosuperior bone loss with superomedial hip center migration, but Köhler’s line remains intact ( Fig. 6.1 ). There may be minimal osteolysis of the teardrop and the ischium. The remaining anterosuperior column can be used as a buttress for an allograft or a metal wedge if needed for additional support. ^,

In type IIB defects, the hip center of rotation migrates less than 3 cm superiorly or superolaterally ( Fig. 6.2 ). The dome stays partially supportive, with the remaining anterosuperior and posteroinferior columns being able to support the acetabular implant. ^,

In type IIC defects, there is a violation of Köhler’s line, and more severe osteolysis is encountered at the teardrop, which correlates with damage to the anterior column and the medial wall ( Fig. 6.3 ). The migration of the hip center is less than 3 cm superior with mild ischial osteolysis. Type IIC defects are treated similarly to protrusio acetabuli as both the anterosuperior and posteroinferior columns are intact and can support an implant. As there is migration of the cup medially, either a structural bone graft or disc-shaped metal augment is used to lateralize the hip center of rotation to restore its anatomic position. ^, In severe type IIC defects, a chronic pelvic discontinuity may be encountered and must be identified intraoperatively based on a high preoperative index of suspicion.

Cementless reconstruction with hemispheric acetabular cups is the recommended treatment for Paprosky type IIA-C defects. Several long-term studies show a 90% survivorship and an aseptic loosening rate of 0%-11%. ^, Host anterosuperior and posteroinferior column support with more than 50% of contact between the implant and host bone is required. If there is inadequate acetabular host bone, structural bone grafts or modular metal augments are indicated to make up for bone defects and provide stability. ^{, ,} To prevent abduction failure of the implant, screws into the ischium or superior pubic ramus are recommended.

Premise: The goals of acetabular revision surgery in Paprosky type II defects are implant stability and reconstruction of the hip center of rotation to restore native hip biomechanics. It is critical to achieving stable fixation of the implant to the acetabulum to prevent migration or loosening.

Preoperative planning

Preoperative planning requires a thorough patient history, including a history of previous hip surgeries, periprosthetic joint infections, as well as a history of poor bone quality because of radiation and steroid treatment. Women, especially postmenopausal women, are also more likely to have poor bone quality, and this should be expected. Physical examination assessing local tissue status, muscle atrophy, gait pattern, and leg length inequality is essential.

Radiographic images and computed tomography (CT) scans are necessary to plan the procedure. Please see Chapter 3 for further detail. A critical part of acetabular revision planning is to understand the extent of bony defects. It is also important to note that women are more likely to have a smaller diameter of the acetabulum and a smaller pelvis which may compromise the cup diameter and subsequently limit the use of larger femoral heads. Other than for defect classification, a standing Anteroposterior (AP) pelvic x-ray can also be used to evaluate leg length discrepancy. Cross table views allow for evaluation of the posteroinferior column and note the presence of ischial osteolysis. Judet views are used to evaluate both the anterosuperior and posteroinferior columns, as well as the direction of hip center migration.

CT scans with 3D reconstruction give further insights on the extent, location, and the pattern of bone loss. Furthermore, the AP diameter of the acetabulum can be accurately measured. In patients with poor bone quality, host bone-implant contact needs to be maximized.

Patient positioning and surgical approach

In acetabular revision surgery, the surgical approach needs to be extensile and allow for visualization of the entire acetabulum, specifically the anterosuperior and posteroinferior columns.

The surgical approach depends on prior surgical approaches but most importantly, on the surgeon’s preference and experience. While not used frequently today, a trochanteric osteotomy can be considered to visualize the acetabular socket when the femoral component is well fixed. A standard trochanteric osteotomy provides exposure of both columns. The posterolateral approach, the author’s preference, is the most frequently used surgical approach as it allows for excellent acetabular exposure and access to the posterior ilium and posteroinferior column. An extensile anterior or a modified Hardinge approach can also be used.

Surgical technique

An extensile exposure is needed to properly assess the pattern of acetabular bone loss. When removing the acetabular component, it should be removed with the least amount of iatrogenic bone loss to preserve the bone stock. Following component removal, all fibrous and granulation tissue should be debrided. To determine the inferior margin of the acetabulum intraoperatively, a blunt retractor is placed into the obturator foramen.

In type IIA and IIB defects, the anterosuperior and posteroinferior columns must be assessed; reaming with sequentially larger reamers is employed to enlarge and mill the bony cavity to a hemispheric socket. Cavitary defects require particulate allograft, which is impacted into the defect and then the acetabulum is once again reamed on reverse with a reamer 2 mm smaller than the largest previously used reamer. The implant should be 2 mm larger than the largest reamer to allow for press-fit fixation. Cancellous screws must be used for supplemental fixation to enable initial stability. ^,

Type IIC defects demonstrate medial migration of the hip center of rotation. Therefore, lateralization of the hip center is required. A structural allograft such as a disc of a femoral head can be inserted into the defect flowed by reaming in reverse. More recently, porous metal discs have been used for defects of the medial wall. The diameter of the graft or metal disc needs to be bigger than the defect to use it as a buttress.