Classification of Bone Defects

Introduction

Revision total knee arthroplasty (TKA) is challenging in the presence of significant bone loss. In the revision setting, bone deficiencies can be secondary to removal of the implant, subsidence of a loose implant, periprosthetic osteolysis, osteonecrosis, stress shielding, or infection. Goals of revision TKA include preservation of host bone, restoration of flexion/extension balance, optimization of ligamentous stability, correction of sagittal and coronal alignment, and establishment of a stable bone–implant interface. There are numerous strategies for managing bone loss in the revision setting. A systematic assessment of bone defects can result in successful management. The reconstructive method selected is dependent on the location and quantity of the osseous deficit. This chapter evaluates the pros and cons of the various surgical approaches to manage bone loss in revision TKA.

Preoperative Assessment

Before any surgical intervention, the surgeon must determine whether the patient’s symptoms are consistent with a failed TKA. One must rule out conditions in which revision TKA may be contraindicated, such as infection, Charcot’s arthropathy, neuromuscular disease, or adverse medical conditions. Preoperative patient evaluation begins with a meticulous history and detailed clinical examination. Analysis of previous surgical procedures, including review of previous operative reports, is necessary to assess the surgical approach used, soft tissue releases performed, and size and type of prosthetic components implanted. To rule out an infection, blood studies such as a complete blood count (CBC) with differential, erythrocyte sedimentation rate (ESR), and C-reactive protein (CRP) level should be obtained. We also favor routine knee aspiration for culture and cell count with differential. Synovial fluid aspirates with leukocyte counts of 2500 cells/mm ³or greater in conjunction with a neutrophil percentage of 60% or greater are highly suggestive of infection.

Critical review of imaging studies directs the preoperative planning. Weight-bearing anteroposterior, lateral, and Merchant patellar views are required to evaluate femoral and tibial component size, to assess current bone stock and diaphyseal deformities, to review the position and fixation of present implants, and to critique patella height and coronal position. Full-length radiographs detail coronal alignment and the presence of diaphyseal deformities. Because preoperative radiographs often underestimate the true amount of bone loss, definitive evaluation and management of bone loss will take place intraoperatively. Computed tomography (CT) can aid in more accurately assessing bone loss and component rotation.

The goal at completion of the preoperative assessment is precise determination of the mechanism of failure so as to not repeat mistakes that led to the failure of the initial TKA. Results of revision TKA in cases of unexplained pain are often unsatisfactory. The surgeon needs to determine what is deficient and, subsequently, what is necessary to reconstruct both the bone and the soft tissue deficits.

Bone Loss Classification

Multiple classification systems have been designed to assess bone loss. Bone defects in TKA are classified based on size, symmetry, and extent. After the prosthetic components have been removed, bone defects are initially identified as contained or uncontained. A contained defect has an intact peripheral cortical rim, whereas an uncontained defect does not.

Rand developed a classification system describing the extent of bone loss. Bone loss was subdivided into minimal (type I), moderate (type II), extensive (type III), and massive cavitary (type IV) categories. Minimal defects comprise an area of less than 50% of a single condyle with a depth of less than 5 mm. Moderate defects comprise 50% to 70% of a single condyle with a depth of 5 to 10 mm. Extensive bone defects comprise more than 70% of a condyle and have a depth of 10 mm or greater. Massive cavitary defects are divided into those with an intact peripheral rim and those with a deficient peripheral rim.

The Anderson Orthopaedic Research Institute (AORI) system is currently the most accepted classification for categorizing bone loss encountered during revision TKA ( Fig. 20.1 ). It divides bone loss of the distal femur or proximal tibia into three types. Type I describes intact metaphyseal bone with minor defects. Type II describes damaged metaphyseal bone and is further subdivided into damage to one condyle (type IIA) or two condyles (type IIB). In type III defects, the majority of the metaphyseal bone is deficient. This classification system will be used here to present treatment options for dealing with bone deficiencies in revision TKA.

Bone Loss Management

AORI Type I Defects

AORI type I bone defects are minor osseous deficiencies that typically have an intact cortical rim, a near-normal joint line, and limited or no component subsidence. Treatment options include removing the bone defect with additional bone resection or shifting the component away from the defect. Also, the defect can be filled with morselized bone graft, cement, or cement and screws.

Increasing the bone resection from the tibia or femur can remove the bone deficiency. Unfortunately, the more aggressive the bone resection, the greater the reduction in bone support. Harada and colleagues found an abrupt decrease in tibial bone strength over the first 5 mm of resection. Taking a larger resection can also decrease the size of tibial component available. This leads to a decreased surface area and corresponding increase in the unit load across the tibial tray and fixation interface. It is important for the revision arthroplasty surgeon to realize that bone was already removed at the time of initial knee replacement, additional bone loss has occurred due to primary TKA failure, and further bone is typically lost during component removal. For these reasons, removal of substantial additional bone in revision TKA should be avoided. We recommend removal of no more than 1 to 2 mm from the most prominent femoral or tibial condyle. We then manage any remaining deficits of the contralateral condyle using the following methods.

Shifting the tibial component in the coronal plane away from an osseous defect to an area of greater osseous support is an option when dealing with small (<5 mm) defects. However, component shift can be detrimental if it requires decreasing the size of the tibial component because of the higher unit force transmission across the tray to the underlying bone. Because the tray is supported by more cancellous bone, the risk of subsidence increases. Ligament kinetics are also affected by shifting of the tray. For example, medial tray shift for a lateral tibial defect lateralizes the tibial tubercle and increases the risk of patellofemoral instability. If chosen, shifting of the tray should be limited to no more than 3 mm. Shifting of the femoral component is rarely indicated because of the risk of patella instability and collateral ligament irritation. Downsizing of the femoral component can cause enlargement of the flexion gap and possible elevation of the joint line.

Cement and screws are occasionally used for small defects in elderly patients. Screws within the cement act as internal girders to increase the strength of the defect construct. Filling a bone deficiency with cement is indicated for peripheral deficiencies less than 5 to 10 mm in depth and less than 10% in condylar area and for small central defects ( Fig. 20.2 ). Cement filling is economical, and the cement can easily be contoured to the osseous defect. Disadvantages of cement augmentation include the potential for osseous thermal necrosis secondary to the heat of polymerization of a large mass of cement. Also, pressurization of cement into the bone can be difficult when dealing with sclerotic uncontained defects. Lastly, bone cement loses 2% of it volume as it cures, which can result in decreased support. Although cement has demonstrated inferior load transfer compared with custom implants or metal augments, it has had favorable clinical results, at least in primary TKA. Ritter and associates followed forty-seven TKAs treated with cement and screws for an average of 6.1 years. They observed no prosthetic loosening at the bone–cement interface but did report that 27% of the knees exhibited radiolucent lines.

For cystic and small localized type I bone deficiencies, morselized allograft or local autograft may be used. The advantages of bone graft include the potential to restore bone stock for future revision and the ability to shape the graft to fit the host defect. Bone grafts are cost-effective (autologous), are useful in large defects, and have increased physiologic load transfer compared with cement. Options for autograft include the resected condyles, intercondylar notch, and iliac crest. Allograft can be harvested from numerous sources including the femoral head, distal femur, and proximal tibia. Allograft material can be fresh frozen, frozen with radiation, or freeze dried. The disadvantages of bone graft use include risk of nonunion, malunion, or late collapse. There is a minimal risk of disease transmission with allografts. There are also limitations on autograft amount and size based on donor site morbidity. The technical keys of bone grafting include developing a healthy, bleeding host recipient site, obtaining complete graft capture (morselized) or graft–host interlock (structural), and supplying rigid implant fixation with no instability or malalignment. Whiteside reported on the use of morselized femoral head allograft in fifty-six cementless revision TKAs. New bony trabeculation was seen in fifteen knees with femoral grafting and twenty-one knees with tibial grafting. After 2 years, 84% had mild to no pain, and 16% had moderate to severe pain. Lotke and colleagues reported on a prospective study of forty-eight consecutive patients treated with impaction allograft for substantial bone loss. All radiographs showed incorporation and remodeling of bone graft with no mechanical failures. Six complications were noted, including two infections and two periprosthetic fractures.

AORI Type II Defects

Type II defects are more extensive. The cortical rim may be intact or partially absent, and there is typically some loss of both central and peripheral metaphyseal bone. Loss of metaphyseal bone is often associated with joint line alteration or implant subsidence. Collateral ligament origins and insertions are preserved in type II defects. Many of the operative strategies discussed earlier for minor defects may also be used for lesser type II defects. Additional treatment options available for these defects include structural bone graft, prosthetic augments, and metaphyseal sleeves or cones to restore the joint line.

In our opinion, use of cement with or without screws should be limited to type II defects involving less than 50% of condylar width and less than 10 mm in depth. Morselized cancellous bone grafts are best used for contained type II defects in which the graft can be captured; when compacted, they provide some degree of structural support. Extensive type II contained defects can be managed with impaction bone graft ( Fig. 20.3 ). Mesh may be used to create a contained defect if the peripheral rim is not intact.