Prosthesis loosening

From the RSNA refresher courses. Total hip arthroplasty: radiographic evaluation

BJ Manaster
Department of Radiology, University of Utah Medical Center, Salt Lake City 84132, USA.

Expected appearances of total hip arthroplasty vary according to type of implant, its method of fixation (cemented, porous coating for bone ingrowth, press fit), and whether it is a revision. Cemented arthroplastic components normally may show 1-2-mm-wide radiolucent zones at cement interfaces. Definite loosening is diagnosed when progressive widening of the radiolucent zone, migration of a cemented component, or change in alignment is seen. In cementless arthroplasty, normal findings include calcar resorption, radiolucent zones up to 2 mm in width, cortical thickening, periosteal reaction, endosteal sclerosis, and even subsidence of the femoral component that stabilizes at less than 1 cm. The most reliable radiographic signs of loosening in cementless arthroplasty are progressive subsidence, migration, or tilt of the component. Because subsidence or change in alignment may be very subtle, serial radiography and measurement are often required for diagnosis. Other signs that indicate loosening include bead shedding (in porous-coated prostheses), extensive cortical hypertrophy, endosteal bone bridging at the stem tip, endosteal scalloping, and a radiolucent zone wider than 2 mm. In revision arthroplasty, wide radiolucent zones and subsidence are common. The diagnosis of revision failure is based on progressive widening of the radiolucent zones and change in component position after 12 months.

 

 

 

Aseptic loosening of prosthetic components is still the most common cause for revision surgery. The mechanism of loosening can be secondary to mechanical (stress) or biologic factors (degradation of the cement-bone or cementless interface resulting from the migration of wear particles). In both situations, the failure can occur at the prosthesis-bone interface, prosthesis-cement interface, or cement-bone interface. Progressive radiolucent areas greater than 1 mm at these interfaces are worrisome for prosthesis loosening. A radiolucent area greater than 2 mm in any of the three of acetabular zones (1 = superolateral, 2 = central, 3 = medial aspect of the bone–acetabular component interface), superior or medial migration of the cup, or change in inclination of the cup is indicative of loosening. The femoral component–bone interface is divided into zones 1–7 on the anteroposterior view (zones 1–3 at the lateral side proximal to distal, zone 4 at the tip of the femoral stem, zones 5–7 at the medial side distal to proximal) and into zones 1–7 on the lateral view (zones 1–3 at the anterior side proximal to distal, zone 4 at the tip of the femoral stem, and zones 5–7 at the posterior side distal to proximal) (Fig 18). The same rules regarding potential prosthesis loosening are used for both the prosthesis-bone interfaces of the femoral stem and those of the acetabular component (1–4,20,23,24).

View larger version (26K): [in this window] [in a new window] [Download PPT slide]   Figure 18.  Diagram illustrates the radiographic zones for acetabular and femoral prostheses. The seven zones are explained in text. (Reprinted, with permission, from reference 4.)

 
Periprosthetic bone remodeling can occur after total hip arthroplasty because of stress alterations in the proximal femur and pelvis after prosthesis implantation. Proximal medial cortical bone loss and distal cortical thickening are commonly seen in the femur. Bone remodeling changes seen around cementless acetabular components are most commonly due to resorption of the subchondral plate and relative osteoporosis in zone 2 (1–4,20,23).

Focal osteolysis about the prosthesis was first recognized by Charney in the 1960s and was thought to be related to the cement used to anchor the prostheses. It has subsequently been recognized that any small particles (metal, cement, or polyethylene) can play a role in initiating osteolysis. With the increased prevalence of cementless fixation, polyethylene wear debris is the most common cause for initiating osteolysis. Eccentric position of the femoral head component within the acetabular component leads to polyethylene wear (1–4,20,23,24). Small particle disease can take place with any joint arthroplasty.

Femoral stem fracture was seen more commonly in the past when alloys with relatively low fatigue strength (stainless steel) were used for prosthesis manufacturing. Prosthesis breakage has become rare with the introduction of high-strength metal alloys (forged cobalt-chromium alloy, titanium-6–aluminum-4–vanadium, and high-strength stainless steel) (20). Dislocation is a relatively uncommon complication (0.4%–0.8% of cases) in primary total hip arthroplasty; it is more common in revision hip arthroplasty (up to 16% of cases) (23).

In the patients with aseptic loosening of the acetabular component and significant bone loss, several types of reconstruction rings are available for management of the acetabular bone loss during revision hip surgery (Fig 19) (25). Custom endoprostheses are used as needed for limb salvage surgery and occasionally after failed primary joint replacement and chronic fracture nonunion (Fig 20) (7).

View larger version (109K): [in this window] [in a new window] [Download PPT slide]   Figure 19.  Anteroposterior radiograph of the left hip shows an acetabular reconstruction ring placed during total hip revision arthroplasty (Smith & Nephew). Note postoperative overlying drain.

 

View larger version (129K): [in this window] [in a new window] [Download PPT slide]   Figure 20.  Anteroposterior radiograph of the left hip shows a custom tumor prosthesis (saddle) (Waldemar link; Hamburg, Germany) in a patient with extensive multiple myeloma lesions involving the left acetabulum and other pelvic bones.