Conventional radiography is the primary imaging modality to evaluate the condition of hip resurfacing implants and the preferred method of assessing implant stability over time. Radiographs assess the angle of inclination of the femoral and acetabular components, implant stability, and femoral neck narrowing. Ultrasonography detects solid or soft tissue masses adjacent to the implant. Magnetic resonance imaging (MRI) detects osteolysis and complications in the periprosthetic soft tissues such as wear-induced synovitis, periprosthetic collections, neurovascular compression, and quality of the muscle and tendons of the rotator cuff of the hip. For pain after hip resurfacing, early use of optimized MRI is recommended.
Imaging of the painful surface replacement requires correlation between imaging and clinical data, including a comprehensive physical examination and careful patient history. The first mainstay of imaging evaluation is an appropriately performed radiograph to assess for radiographic findings that obviate more advanced imaging. In the setting of radiographs with equivocal or negative results, more advanced imaging may be necessary, and both magnetic resonance imaging (MRI) and ultrasonography have shown to be effective in the evaluation of these joints. Ultrasonography is helpful in detecting periprosthetic fluid collections, which may then undergo ultrasound-guided percutaneous aspiration. Computed tomography (CT) can detect osseous complications such as osteolysis and allow more accurate assessment of component alignment. Optimized MRI can not only identify fluid collections but also assess for any adverse synovial response that may indicate wear-induced synovitis or infection as well as identify periprosthetic osteolysis. Further modification of pulse sequences relevant to MRI is currently underway, resulting in marked improvement in the degree of artifacts generated by metal-on-metal (MOM) prostheses, allowing for superior characterization of the type of synovial reaction.
Radiographic assessment
Because of its widespread availability, conventional radiography continues to be the primary imaging modality to evaluate the condition of hip resurfacing implants. With the ability to visualize a large field of view, conventional radiographs are able to assess implant position relative to the native anatomy of the hip, thus allowing the measurement of femoral stem-shaft and cup abduction angles. Sequential radiographs remain the preferred method of assessing implant stability over time, such as loosening. With proper radiographic technique, one can assess a hip resurfacing implant for position, heterotopic bone formation, periprosthetic lucency, impingement, osteolysis, migration, and neck narrowing.
Technique
Conventional radiographs should be taken in 2 planes to allow for biplanar evaluation of the implant and surrounding bone. Most commonly, an anteroposterior view of the pelvis is taken in the frontal plane; visualization of both hips allows for a side-to-side comparison of the biomechanical axes of the hips. In the lateral plane, there is a wide variation in the preferred technique. At the authors’ institution, the preferred view is the cross-table (shoot-through or Johnson ) lateral view. This radiograph is taken with the patient supine and the nonoperated hip flexed to 90° so that it does not obstruct the field of view. The beam is aimed from the midline, and the cassette is positioned lateral to the hip. This type of imaging allows visualization of the lateral plane of the cup position and assessment of the amount of anteversion and the presence of anterior cup overhang. Studies have debated the accuracy of the cross-table lateral view with regard to anteversion because of the variability in native pelvic tilt, but this view still allows one to see the anterior aspect of the native acetabulum relative to the prosthetic edge ( Fig. 1 ) .
Interpretation
Femoral implant position
Conventional radiographs should be inspected for implant position. Commonly, the femoral stem-femoral shaft angle in both planes is of interest. This angle is assessed by measuring the angle created by a line drawn along the femoral stem and a line drawn down the femoral shaft ( Fig. 2 A). In the frontal plane, this angle typically approximates the native femoral neck-shaft angle of 135°, but several surgeons have advocated a more valgus orientation (higher stem-shaft angle) in order to improve the biomechanical forces on the neck. In the lateral plane, the angle created by the femoral stem and femoral shaft should approximate the native version of the femoral neck; however, the angle may be directed from more posterior to anterior to improve bone contact with the implant. Thus, there may be relative retroversion of the implant relative to the native femoral neck.
Acetabular implant position
Cup position is assessed by measuring the lateral edge of the acetabular component relative to a horizontal reference line in the frontal plane (see Fig. 2 B). This abduction angle indicates the amount of lateral opening; typical values are 35° to 50°. Several studies have examined the effect of cup position on wear, and the steeper the cup angle (higher values) the greater the wear of an MOM implant. Abduction angles greater than 55° are considered “steep,” thus the preferred angle of an MOM surface arthroplasty is approximately 40° in the frontal plane.
In the lateral plane, anteversion of the socket is measured by the angle created from a vertical line perpendicular to the horizontal plane and the edge of the acetabular component (see Fig. 2 C). Because of variation in technique, the true amount of anteversion may be obtained more accurately by Einzel-Bild-Roentgen-Analysis. Typical values for anteversion are similar to those of the native hip, measuring between 5° to 20°. Visualization of the anterior edge of the acetabular component is necessary to assess for protrusion of the component beyond the native bone, such as overhang of the component, which may lead to iliopsoas impingement and groin pain.
Implant stability
Sequential radiographs allow for the assessment of stability, over time, of an implant relative to bone. Implant migration, that is, change in position over time, is the sine qua non of a loose implant. Commonly, migration occurs on the femoral side and may be a result of osteonecrosis of the retained femoral head.
Most femoral resurfacing implants are cemented onto the retained femoral head and neck; the short metaphyseal stem is not commonly cemented or designed for fixation. It is present merely to guide the placement of the femoral implant during the surgical procedure. However, radiographically, the stem is an extremely useful tool in assessing implant stability. The stem has been likened to an “antennae” broadcasting the condition of the femoral head because of its extension. In examining the interface of the femoral stem and the bone of the native femoral neck, it is helpful to have a convention to describe the regions of interest. The most widely used convention is that used by Amstutz and colleagues. In this methodology, the bone around the femoral stem is divided into 3 regions in the frontal plane: medial, lateral, and at the tip, the so-called peg zones. There may be a thin sclerotic line in all 3 peg zones that indicates a bony reaction to the metal stem, which does not indicate loosening. However, asymmetry along one of the peg zones is a sign of implant migration ( Fig. 3 ).
The socket must also be carefully inspected for bone integration. Because all hip resurfacing acetabular components used today are designed for osseous integration, the presence of spot welds and bone trabeculae streaming to the implant interface are welcome signs. Radiolucencies around the socket may be thin and faint but are worrisome for failure to integrate ( Fig. 4 ).
Impingement
Impingement, or abnormal contact of the bone on the arthroplasty, is another finding of conventional radiography. As opposed to total hip replacement, in which the neck is replaced by a prosthetic, hip resurfacing retains the native femoral neck, which can abut the acetabular component. In certain patients, this contact between the neck and rim of the socket can be seen as an indentation along the femoral neck; however, these patients typically are not symptomatic ( Fig. 5 ).
Neck narrowing
Neck narrowing, or neck thinning, is a phenomenon noted only in hip resurfacing. It is defined as a decrease in the femoral neck diameter measured at the implant-neck junction. In some studies, neck narrowing has been detected in 77% to 90% of resurfaced hips. Neck narrowing is thought to be a benign process, indicating remodeling because of alterations in bone loading (ie, stress shielding), and should be nonprogressive after 3 years ( Fig. 6 ).
Heterotopic bone formation
Hip resurfacing may be more prone to forming heterotopic bone because of the larger surgical dissection necessary to perform the procedure. In some studies, significant heterotopic ossification was noted in up to 7% of patients undergoing the procedure. The most common methodology used in describing heterotopic ossification is the same as with total hip replacements, the classification by Brooker and colleagues.
Stress fracture of the femoral neck
Femoral neck fracture is the most common reason for failure of hip resurfacing. Typically, it occurs within the first 3 months after operation and is evidenced by complete displacement at the implant-neck junction. Occasionally, if identified early, a fracture line can be detected, beginning from the tension side of the femoral neck at the head-neck junction. In this situation, immediate protected weight bearing may be able to prevent this type of fracture from displacement ( Fig. 7 ).
Ultrasonography and CT
Although conventional radiographs are useful for assessing the angle of inclination of the components, implant stability, and femoral neck narrowing, all of which may indicate potential implant failure, radiographs may fail to detect or may grossly underestimate the presence of osteolysis. Radiographs are also unable to detect soft tissue complications, including the presence of solid or cystic masses adjacent to the implant, which have been described as part of the spectrum of metal hypersensitivity.
Adverse local tissue reaction (ALTR) is a relatively unique complication of MOM implants and may manifest as solid or cystic groin lesions, which have been termed pseudotumors, as well as synovitis, bursitis, or periprosthetic osteolysis. On biopsy or at revision surgery, the periprosthetic tissues are characterized by the presence of perivascular or diffuse infiltrates of B and T lymphocytes. The incidence of such reactions after MOM hip resurfacing is unknown, but in larger retrospective studies, pseudotumors have been reported in up to 1% of patients at revision surgery and are likely associated with high wear. The incidence may in fact be higher with certain designs and component malorientation. In addition, this reaction can occur in asymptomatic patients.
Rarely, ALTR manifests as an allergic type of delayed hypersensitivity and is accompanied by the histologic diagnosis of aseptic lymphocytic vasculitis associated lesions (ALVALs), often with excessive necrosis but without significant wear-induced debris.
Ultrasonography is a useful screening tool to detect the presence of a soft tissue mass adjacent to an implant, which may indicate metal hypersensitivity. Ultrasonography has also been used as a screening tool in asymptomatic patients to monitor for the presence of pseudotumors, which if present, may alert the clinician as to the need for closer monitoring for implant-related failure. However, this technique is operator dependent and may not show deeper lesions optimally because of limited transmission.
CT can delineate solid or cystic masses in proximity to the implant and has the additional benefit of providing bony detail to better delineate osseous complications such as osteolysis. The use of CT with 3-dimensional (3D) measurement software has been suggested to allow better assessment of component alignment, which may be difficult to evaluate on axial CT or plain radiography because of obscuration of the acetabular component by the large diameter metal head.
Related posts:
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Imaging of Metal-On-Metal Hip Resurfacing
Survivorship of Conserve® Plus Monoblock Metal-on-Metal Hip Resurfacing Sockets: Radiographic Midterm Results of 580 Patients
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A Prospective Metal Ion Study of Large-Head Metal-on-Metal Bearing: A Matched-Pair Analysis of Hip Resurfacing Versus Total Hip Replacement
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