Extract
Osteolytic lesions may develop after total hip arthroplasty from a biologic reaction to particulate debris. Loss of bone results from osteoclastic resorption and can be seen on radiographs as cystic lesions or radiolucent regions in proximity to the femoral and acetabular components. Osteolysis may be associated with pain, particularly if bone loss results in decreased mechanical support for the prosthetic components and implant loosening. However, osteolysis may also be asymptomatic and only detected with radiographic or other imaging modalities.
Osteolytic lesions may develop after total hip arthroplasty from a biologic reaction to particulate debris. Loss of bone results from osteoclastic resorption and can be seen on radiographs as cystic lesions or radiolucent regions in proximity to the femoral and acetabular components. Osteolysis may be associated with pain, particularly if bone loss results in decreased mechanical support for the prosthetic components and implant loosening. However, osteolysis may also be asymptomatic and only detected with radiographic or other imaging modalities.
Osteolytic lesions usually appear on radiographs as well-demarcated, scalloped areas of bone loss. Osteolysis can be differentiated from bone loss resulting from stress-shielding, which causes more diffuse trabecular thinning. Stress-shielding is also typically associated with an area of sclerosis and cortical thickening below the area of osteopenia or trabecular thinning. Osteolytic lesions may mimic bone loss due to infection, which should be considered in the differential diagnosis.
Since radiographs show a two-dimensional image of a three-dimensional structure, anteroposterior and lateral views should be made to provide the most accurate assessment of the lesion size and location. Radiographs are useful as a screening tool to detect osteolysis, but may not accurately delineate the lesion size. Shon et al., using radiographs to detect periacetabular osteolysis in comparison with computed tomography (CT), found a sensitivity of only 57.6% but a specificity of 92.9%1. The use of oblique radiographs increased the sensitivity to 64% without changing the specificity. Other authors have similarly observed a higher specificity than sensitivity with use of radiographs to assess osteolysis2,3.
CT or magnetic resonance imaging (MRI) can be used to supplement the information obtained from radiographs. These imaging modalities can provide cross-sectional images of the osteolytic lesions, and are indicated when radiographs do not provide adequate visualization of the lesion size, location, or progression for clinical decision making. MRI and CT images can be distorted by metal artifact from the adjacent prosthetic components. However, the use of metal artifact reduction protocols permits better visualization of the periprosthetic bone and soft tissues than with conventional CT or MRI protocols. CT is faster, and acceptable visualization of osteolytic lesions is achieved by modifying standard CT protocols. MRI requires more challenging protocol changes or use of new sequences, which are not uniformly available. In general, if we are more concerned with an osteolytic lesion without soft-tissue extension, we make a CT scan; if the lesion is primarily in soft tissue or involves soft-tissue extension of an osteolytic lesion, then we prefer MRI.
Acetabular osteolytic lesions typically develop around the dome or screw-holes of the acetabular component and in proximity to the cup rim, where particulate debris from the bearing surface tends to migrate4,5 (Fig. 1). The location and size of the defects affects treatment. Medial acetabular defects, which do not compromise the osseous support around the periphery of the cup, can be effectively treated with revision to a cementless hemispheric component with screw fixation and medial acetabular bone-grafting (Fig. 2). Although the cup shown in Figure 2 was well fixed, symptoms of hip pain occurred in association with osteolysis. This is likely related to synovitis and effusion that can develop in response to ultra-high molecular-weight polyethylene (UHMWPE) wear debris. If the shell is well fixed and in good position, liner exchange and bone-grafting of the osteolytic lesions is an attractive alternative to cup revision since this does not risk further bone loss during acetabular component removal6.
Superior or posterior acetabular defects that may compromise mechanical support for the acetabular component in weight-bearing regions should be treated surgically (Fig. 3). In comparison with the osteolytic lesion shown in Figure 2, which is a large medial acetabular defect with relative preservation of superior supporting bone, the lesion shown in Figure 3 illustrates marked narrowing of the superior supporting bone stock. The lesion shown in Figure 3 should be treated urgently, whereas surgical treatment of the lesion shown in Figure 2 could be delayed if necessary. Osteolysis leading to further loss of superior or posterior bone stock can result in massive segmental bone loss and requires more extensive revision procedures with use of metal augments or structural bone grafts to restore mechanical support of the acetabular component.
The decision to treat osteolytic lesions around well-fixed acetabular components surgically or to observe them is made on the basis of the presence or absence of symptoms, as well as the size, location, and rate of progression of the defect. However, the relative urgency of surgical treatment is based on the potential adverse consequences of nonoperative treatment. Two types of catastrophic clinical problems can be encountered with prolonged observation of osteolytic periacetabular defects. These are (1) loss of superior supporting bone resulting in a segmental acetabular bone defect, which converts a contained or cavitary bone defect into a more severe, uncontained segmental defect, and (2) loss of anterior and posterior column support, which results in a pelvic discontinuity (Figs. 4-A and 4-B).
Superior supporting bone stock can be visualized on an anteroposterior radiograph (Figs. 2 and 3), while visualization of the osseous support of the posterior and anterior columns can be obscured by the metallic acetabular shell and may not be visualized well on an anteroposterior radiograph. Use of Judet radiographs and CT or MRI should be considered to assess the integrity of the posterior and anterior columns. For patients at risk of developing loss of superior or posterior osseous support of the acetabular cup or development of pelvic discontinuity, surgical treatment is indicated.
Femoral lesions typically occur in proximity to the bearing surface along the calcar or greater trochanter. Trochanteric lesions may compromise the mechanical integrity of the greater trochanter, leading to fracture and loss of hip abductor muscle function (Fig. 5). Progressive osteolysis of the greater trochanter, which weakens the bone and may lead to a fracture, is an indication for femoral head and acetabular liner exchange with bone-grafting of the osteolytic lesion. If a fracture of the greater trochanter occurs and it is nondisplaced, nonoperative treatment can result in healing.
Wear debris that reaches the distal femoral component along the bone-implant interface or the so-called effective joint space may lead to the development of periprosthetic femoral fracture7. Osteolysis that compromises the structural integrity of the femoral cortex in proximity to the stem tip, where stress transfer from the implant to bone is high, is a particular risk factor for fracture at or near the stem tip. We consider osteolysis that results in narrowing the thickness of one femoral cortex to one-half or less than its normal width in proximity to the stem tip to be a substantial risk for periprosthetic femoral fracture and an indication for surgery (Fig. 6).
The rate of development and progression of osteolytic lesions is variable. The relative rate of osteolytic progression is evaluated most effectively when viewed on serial radiographic examinations (Figs. 7-A and 7-B). When an osteolytic lesion that may increase in size and could lead to periprosthetic fracture or loss of structural support for the prosthetic components is detected radiographically, we make a follow-up radiograph four months later and a subsequent radiograph four to six months after the second radiograph to determine the relative rate of progression or stability of the lesion.
Most CT scanners are now multidetector CT scanners and provide three-dimensional volumetric datasets that can be reconstructed in any imaging plane; coronal and sagittal reformations are standard, but three-dimensional reformations, maximum intensity projections, and other, more sophisticated image reconstructions may be made. CT images are grossly distorted around cobalt-chromium and stainless-steel implants, whereas artifacts around titanium implants are relatively mild.
Various techniques are available to suppress metal artifacts with use of CT imaging. In a standard clinical setting, use of a multidetector CT scanner and an increase in exposure dose (milliampere-seconds [mAs]) have been advocated8. In addition, higher peak voltage (kilovolt peak [kVp]) and narrower collimation have been used with smooth or standard reconstruction filters and thicker reconstructed sections to reduce metal artifacts8-10. Also, an extended CT scale, which allows an expansion of the Hounsfield scale from a standard maximum window of 4000 HU to 40,000 HU, is available on some scanners. This technique makes use of the fact that metals have high linear attenuation coefficients that lie outside the normal range of reconstructed CT numbers; most metallic implants are in the range of 8000 to 20,000 HU, whereas the standard upper limit of CT scanners is 4096 HU11. More advanced techniques use complicated image data processing algorithms by ignoring or interpolating the metallic objects in the raw data12. These techniques, however, are research applications and not established in clinical routine.
CT scans with metal artifact reduction techniques have been used successfully to quantitate the size of periacetabular osteolytic lesions13-16. Howie et al. used CT to assess osteolytic lesions after total hip arthroplasty and found considerable variation in the rate of progression of osteolysis17. Factors associated with progression of the lesions included high wear rate, high patient activity level, large-diameter heads, and a lesion size of >10 cm3. However, in comparison with radiographs, the use of CT is associated with increased radiation exposure and cost. Therefore, CT scanning should generally be used in addition to radiographs when indicated to better delineate the extent and location of bone loss.
We use CT to better determine if or when surgery is indicated in the treatment of periprosthetic osteolysis. For acetabular lesions, CT is used to assess the integrity of the anterior and posterior columns and the posterior acetabular wall since this area is not well visualized on radiographs. For femoral lesions, CT is helpful in determining the structural integrity of the greater trochanter and femoral diaphysis. CT scanning is also helpful in delineating areas of remaining bone stock when planning surgical reconstruction. CT permits three-dimensional reconstructions, which are typically utilized in planning for the use of custom triflange acetabular components to salvage massive acetabular bone loss and pelvic discontinuity (Figs. 4-A and 4-B).
MRI is an attractive alternative to CT since there is no ionizing radiation exposure with MRI. However, metal, particularly cobalt chromium and stainless steel, substantially impacts image quality of MRI scans because of susceptibility artifacts. Factors that affect artifacts on MRI scans include the composition of the metallic implant, the orientation of the implants in relation to the direction of the main magnetic field, the strength of the magnetic field, the pulse sequence type, and other imaging parameters (mainly, voxel size, which is determined by the field of view, image matrix, section thickness, and echo train length)8. To reduce metal artifacts, the use of lower field strength has been recommended as higher field strength increases susceptibility artifacts. Studies have used 0.2 to 0.3-T systems; this, however, impacts image quality because of a low signal-to-noise-ratio, which may produce blurry images, providing limited anatomic detail18. With use of high-field systems, improvement of image quality may be achieved by increasing bandwidth. Also, a small field of view with a high-resolution matrix, thin sections, and high gradient strength can help to reduce metal-related artifacts8. Instead of frequency-selective fat saturation, other techniques of fat saturation such as use of short tau inversion recovery sequences have been recommended. Recently, specific metal suppression sequences have been developed; among these, multi-acquisition with variable resonance image combination (MAVRIC) hybrid sequences have shown promising results in reducing artifacts and providing high-quality images near total joint replacements in a clinical setting19,20 (Figs. 8-A and 8-B).
Potter et al. utilized metal artifact reduction protocols with MRI to assess bone and soft-tissue lesions after total hip arthroplasty21. Osteolysis, synovitis, trochanteric bursitis, and loosening have been effectively visualized on MRI with these techniques (Figs. 8-A and 8-B)22. Walde et al. compared the accuracy of radiographs, CT, and MRI in assessing periacetabular osteolytic lesions using a cadaver model23. The sensitivity for detecting lesions was 51.7% for radiography, 74.7% for CT, and 95.4% for MRI. The sensitivity increased with increasing lesion size for all three methods, and MRI was the most effective in detecting small lesions.
Metal-on-metal resurfacing or total hip arthroplasty can produce both osteolytic lesions in bone and so-called soft-tissue pseudotumors. Pseudotumors, which have been associated with an adverse local tissue reaction, can develop in soft tissue and are not well visualized on radiographs. The pseudotumors may be filled with fluid and are effectively visualized on MRI24 (Fig. 9). Ultrasound is also useful for detecting soft-tissue masses and can help to delineate soft-tissue pseudotumors after metal-on-metal total hip arthroplasty25. However, ultrasound is not effective for detecting osteolytic bone lesions.
Osteolysis is related to many factors, but primarily it is affected by wear volume, which increases with use and patient activity. As expected, younger and more active male patients have a greater risk of developing osteolysis26. It takes time to produce the wear debris, and osteolysis is uncommon before five years after arthroplasty, whereas the risk increases after ten years27. Wear is also affected by the bearing surface materials used. Highly cross-linked UHMWPE wears less, and the risk of osteolysis is less compared with conventional UHMWPE27. UHMWPE that was gamma-irradiated in air was discontinued by most manufacturers in the mid-1990s because of oxidation and increased particle debris generation, and it was replaced with non-gamma-irradiated in air sterilization methods. Many of these implants continue to be in use and may generate higher rates of wear and osteolysis than those with highly cross-linked UHMWPE.
Highly cross-linked UHMWPE has been used in large numbers in total hip arthroplasty for over ten years with excellent clinical results. However, since the long-term results with use of highly cross-linked UHMWPE have not been established, routine monitoring of this patient population for wear and osteolysis is appropriate. Patients at higher risk of developing osteolysis, such as those with non-cross-linked UHMWPE and young, active patients, should be monitored more closely. We recommend that patients have a radiograph of the hip made every two to three years, beginning five years following total hip arthroplasty. If osteolysis is detected, then repeat radiographs at four to six months are helpful to determine the rate of progression of the lesion and if surgical intervention is necessary. If the lesion size and location seen on radiographs suggest that clinical failure of the implant may develop or additional quantitative measurements of the lesion are needed, then CT or MRI scans with metal artifact suppression should be acquired. Once a lesion develops, the rate of progression is best determined with serial radiographs and may require serial CT scans15.
Osteolysis after total hip arthroplasty develops in response to particulate wear debris and may not be associated with clinical symptoms. Osteolysis is associated with more particulate wear debris and greater wear volume. Wear increases with use and activity of the joint, so patients having longer in vivo use of their total hip replacement are at increased risk of developing osteolysis. Patients with non-cross-linked UHMWPE and younger, more active patients are at greater risk of developing osteolysis.
We recommend routine monitoring for osteolysis at five years after total hip arthroplasty, with a radiograph made every two to three years thereafter. Patients at greater risk of developing osteolysis should be monitored more closely. Once a lesion is seen radiographically, serial radiographs help to determine the relative rate of progression of the lesion. CT with metal artifact reduction can be used effectively to quantitate the lesion size and location. MRI can be used to visualize osteolytic areas as well as soft-tissue pathology. Both MRI and CT with metal artifact reduction protocols have been developed to effectively visualize osteolytic lesions in proximity to total hip arthroplasty implants and to provide supplemental information to radiographs.
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