Modern total hip replacement was first pioneered by Sir John Charnley in England in the early 1960's. Although previous attempts in the early 20th century included ivory prostheses and other materials, Sir Charnley was the first to essentially develop the modern design that has been the basis for subsequent variations and to produce successful results.
Over the past half century, hip replacement has become one of the most successful interventions not just in orthopaedic surgery, but in all of modern medicine. Over 95% of patients have good results (probably closer to 98% in large centers). The outcomes have been steadily improving and the life of the implants increasing over the past several decades. The basic concept of a total hip replacement (also known as total hip arthroplasty) is to replace the ball and socket joint with an artificial ball and socket. After the joint is replaced, there is no longer any arthritis in the joint, because the joint is entirely artificial. At the time of surgery, the ball (femoral head) and socket (acetabulum) are typically quite worn out. Frequently, the femoral head looks very similar to a head of cauliflower in a very worn out hip, covered with lumpy and bumpy osteophytes and areas of exposed bone where the cartilage has worn away.
Nuts & Bolts: The Replacement Procedure
Regardless of the surgical approach used, the same steps have to be performed during the surgery. After exposing the hip joint, the femoral head (the ball of the thigh bone) is cut and removed. The femoral head is usually sent to pathology in most hospitals for routine evaluation, although it is usually kept on the surgical field until the end of surgery in case bone graft is needed.
Next the hip socket (acetabulum) is debrided and scraped clean. Hemispherical reamers are then used to ream the hard, sclerotic arthritic surface of the acetabulum until a bowl-shaped area (similar to the shape the socket is naturally supposed to have) has been reamed out. It is important for the surgeon to ream and prepare this socket at the proper angles; if the cup is placed too steep (vertical pelvic tilt), the hip will have a tendency to dislocate and pop out of the socket. If the cup is placed too flat, the femur will impinge against it when the patient tries to lift the hip out to the side. The optimum pelvic tilt angle is typically about 45 degrees.
It is also important to pay attention to whether the cup faces forward (anterior) or backward (posterior). This is called anteversion or retroversion. Depending on the surgical approach, somewhere between 0 and 15 degrees of anteversion usually is desirable. Too much in either direction, and the hip will dislocate as the leg is rotated inward or outward. Essentially, the acetabular cup (artificial socket) has to be positioned correctly in 2 planes to prevent dislocation or impingement.
At this point, a hemispherical shell (artificial socket) is then installed in the pelvis. In the early days of hip replacement, this usually entailed cementing a plastic socket into the bone, and this is still done for some special circumstances (such as performing a hip replacement for an elderly patient with a hip fracture, who has very soft bone that may break while impacting a press fit socket into place). More commonly today, however, a porous coated metal shell is impacted into place. The tight fit usually is adequate to hold the shell in place, although sometimes screws are used if supplemental fixation is needed. The back of the metal shell is often coated with a porous metal surface that allows the bone to grow into the prosthesis.
A liner is then inserted into the socket shell. Traditionally, this has often been a plastic liner (polyethylene), and this is still the most commonly used material because of its lower cost. However, in an active and/or young patient, a ceramic liner or even a metal-on-metal liner may be inserted. The choice of bearing surfaces is discussed later in a another chapter. Sometimes there is not enough of a socket in the pelvis to support the metal shell.
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