X-ray plays vital role in hip replacement surgery

The first hip replacements were performed about 50 years ago. As surgical techniques subsequently evolved, radiological criteria for evaluating prosthesis components and their complications adapted accordingly. Our hospital has performed more than 200 hip replacements annually over the past 14 years. Surgeons have used a variety of materials to replace the hip joint, including metal, polyethylene, and ceramics. All patients undergo imaging examinations immediately after surgery, at six months, and then yearly or when necessary. This follow-up is most frequently done using plain-film x-rays, occasionally with ultrasound or CT, and, rarely, with MRI. A retrospective review of 1854 patients (36 to 91 years old; male/female ratio 1:3) who underwent hip arthroplasty at our hospital in an eight-year period revealed that 5.2% experienced acute complications, and 9.2% developed complications later. The acute complications included infection, cement extrusion, periprosthetic fracture, and immediate dislocation. Late complications, some of which occurred as long as 15 years after surgery, have included aseptic loosening, aggressive granulomatosis, stress shielding, heterotopic ossification, luxation, bone fractures, and prosthetic fracture. Hip replacements can be cemented at the acetabular and/or femoral stem. Polymethylmethacrylate cement, used for filling the space between the bone and the prosthesis, provides the necessary fixation and force distribution. Cement-free prostheses are coated to provide a porous surface that enables fixation by bone ingrowth. Acetabular fixation may be completed with spikes or screws. Our standard follow-up imaging evaluation of hip arthroplasty begins with routine plain-film radiography. It is important that the entire prosthesis is included on two orthogonal x-rays. Leg length, acetabular inclination, and varus or valgus stem position should be evaluated and compared with the immediately postoperative study. Radiography of cemented prostheses will generally show a thin lucent line, less than 2 mm wide, along the cement-bone interface. This is a normal finding that represents fibrous tissue and is stable over time. The line is delimited from adjacent bone by a thin line of sclerosis, the demarcation line.1 Lucency of 2 mm or more at the bone-ingrowth surface with cement-free arthroplasty is considered abnormal. Lucency that is less than 2 mm in width should be followed on consecutive plain films for progression. Well-defined areas of lucency around the prosthesis usually indicate bone resorption destruction (histiocytic response), but aseptic loosening and infection can demonstrate a similar appearance.2 COMMON COMPLICATIONS Infection, which occurs in 1% of artificial joint replacements, can be a serious early complication, or it may appear months or even years after surgery. Radiological findings in patients with indolent infection can be unremarkable or show minimal changes,3,4 and a normal-appearing radiograph does not exclude infection.5 Sinus tract formation or bone destruction may occur with more aggressive infectious agents. Infection may also be present when postoperative imaging shows lytic lesions, progressive interfacial widening, periosteal reaction, or smooth endosteal scalloping with cement lucency, as is typical in aseptic loosening or aggressive granulomatosis disease. Extensive bone destruction, air in the soft tissue and/or joint, extensive or aggressive periosteal