Imaging in the Failed Total Knee Arthroplasty

Introduction

In the patient with a failed total knee replacement (TKA), imaging modalities represent an integral part of diagnostic evaluation. As joint replacement arthroplasty becomes more common with the aging population and implant longevity improves, the number of patients requiring imaging evaluation for their TKA will continue to increase. This chapter discusses diagnostic imaging of the failed TKA, including the roles of radiographs, radionuclide scans, computed tomography (CT), and magnetic resonance imaging (MRI).

Radiographic Assessment

Standard radiographs used in the initial evaluation include weight-bearing anteroposterior (AP), lateral, tangential axial (Merchant), and long leg alignment views of the knee ( Fig. 4.1 ). The radiographs are evaluated for component alignment, size, and any overhang, stress fracture, component loosening, periprosthetic lucencies, osteolysis, wear of the polyethylene insert with loss of joint space or asymmetric joint space, and heterotopic ossification. The femoral component should be in 4 to 7 degrees of anatomic valgus, and the anterior flange should be in contact with the anterior cortex without notching. Any overhang should be noted. Femoral component overhang of greater than 3 mm doubles the risk of clinically important knee pain 2 years after TKA. The tibial component should be perpendicular to the long axis of the tibia on the AP radiograph and perpendicular or posteriorly sloped, appropriate for the implant design, on the lateral radiograph. The likelihood of tibial tray loosening is increased when it is placed in greater than 5 degrees of varus. The tibial tray should cover at least 85% of the tibial surface without overhang. Medial overhang of the tibial component can be a source of pain due to impingement on the pes anserinus or medial collateral ligament.

Preoperative and postoperative films are compared for size of the femoral component relative to the original anatomy, posterior condylar offset, patella height, and position of the joint line in relation to the patella. It is important to distinguish true patella baja, caused by a short patellar tendon, from iatrogenic patella baja, which results from a raised joint line, because the treatments of these conditions differ.

The value of oblique radiographs in accurately delineating the peripheral margins of osteolytic lesions is uncertain. Previous studies have suggested that plain radiographs, despite multiangle and multiprojection approaches, often underestimate the size of the osteolytic lesion. However, a study by Miura and colleagues demonstrated that radiographic analysis using the oblique posterior condylar view is reproducible and is significantly more accurate than standard radiographs for the detection of radiolucencies or osteolysis of the posterior aspects of the femoral condyles.

Loosening

Serial radiographs are an essential component in the evaluation of the painful TKA ( Fig. 4.2 ). Radiographic signs of loosening include progressive increase in radiolucent lines, changes in component position, subsidence, fracture of the cement mantle, and reaction about the tip of stemmed components. The cement–bone or component–bone interface is closely evaluated; loosening is suspected with all radiolucencies greater than 2 mm. Ritter and co-workers found that proper preparation of the cancellous bone and pressurization of the cement can reduce the initial occurrence of a radiolucent line. A loose tibial component frequently shifts into varus alignment, whereas a loose femoral component typically shifts into flexion. Visualization of osteolysis located in the posterior condyles may be obscured by the femoral component.

Fluoroscopically positioned radiographs can assist in ensuring an optimal view of the interface, particularly in uncemented prostheses ( Fig. 4.3 ). Although arthrography is rarely necessary, loosening is suggested by any flow of iodinated contrast material into the area of lucency. However, the absence of contrast flow does not exclude the diagnosis. For unicompartmental knee arthroplasty, lucency at the tip of the femoral peg is most associated with loosening of the femoral component.