TOTAL HIP REPLACEMENT: PROSTHESES
Arthroplasty, or surgical reconstruction of the joints, has revolutionized the treatment of crippling diseases such as osteoarthritis and rheumatoid arthritis, which destroy the joint’s smooth cartilage surfaces and lead to painful, decreased motion. Relief of pain and improved hip function are dramatic advantages of reconstruction procedures. Hip arthroplasty not only benefits the older patient, but total hip replacement and other procedures using prostheses also now permit young and middle-aged patients with congenital, developmental, arthritic, traumatic, malignant, or metabolic hip disorders to lead active and productive lives.
Treatment with a total hip prosthesis must always be weighed against nonsurgical treatment and other more conservative surgical procedures that do not sacrifice as much bone. Appropriate selection of patients is essential.
Hip prostheses must function under high mechanical loads for many years, and the strength of materials used is critical. The technique of total hip replacement began as an improvement on the placement of molds or films between degenerated joint surfaces (interpositional arthroplasty). In 1923, Smith-Petersen used a Pyrex cup to cover and reshape an arthritic femoral head in a technique called mold arthroplasty. This brittle cup broke under stress, but the technique led to the development of interpositional molds made of a stronger material, Vitallium, a noncorrosive and relatively inert cobalt-chromium alloy.
In the early 1960s, Sir John Charnley developed the technique of low-friction total hip arthroplasty, which is still the standard against which all newer variations must be measured. From his work, two important principles have stood the test of time and govern all subsequent modifications. The first is the principle of low friction, that is, a bearing of a highly polished metal alloy against ultrahigh-molecular-weight polyethylene. The second is the principle of rigid fixation of components to bone. For the first, he advocated a small- diameter (22.25 mm) bearing and, for the second, the use of methyl methacrylate (acrylic) cement to act as a grouting material by forming an interlocking mechanical bond with the trabecular bone.
Femoral and acetabular components are made in a variety of sizes, and it is possible to mix and match femoral stems with acetabular cups of different systems. All new implants have detachable heads that allow adjustment of the neck length of the prosthesis, making it much easier to correct and equalize leg length.
New designs in total hip prostheses include implants that do not require acrylic cement for fixation to bone. The metal backing of the acetabular cup and the sides of the femoral stem have pores allowing the ingrowth of bone trabeculae to produce a “biologic fixation” of prosthesis to bone. However, because good bone quality is needed for implant stability and maximal bone ingrowth, this type of prosthesis is not indicated for all patients.
Newer prostheses are currently in use that allow for faster and more complete bony ingrowth. The surface has a higher porosity and pore sizes that are more similar to normal trabecular bone. These devices have the potential to increase the longevity of total hip replacement; however, long-term data are not yet available.
TRABECULAR BONE AND TRABECULAR METAL SLIDE
In the United States, porous ingrowth prostheticimplant total hip arthroplasty is the gold standard. Cemented prostheses are currently used on y in the elderly or in patients with poor bone quality.