Total Hip Replacement (Arthroplasty)

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.

This is not an uncommon scenario in complex revision surgeries, traumas, or in cases where the patient has severe hip dysplasia and never formed a proper socket. In these cases, the socket is typically reconstructed with a metal cage that fits over the large hole and may be supplemented with bone graft to fill the defect. Multiple screws anchor the construct into the pelvis, and a liner is then cemented into the cage.

Next attention is returned to the femur. The femoral canal, or the softer marrow within the femur, is then reamed and/or rasped to prepare a slot for the stem. The hardest bone is the outer cortex, like a pipe. Ideally the prosthesis should rest as close as possible to that outer cortex, but if the slot is reamed too much, a fracture can result from the surrounding bone being too thin. Conversely, if not enough of the interior of the femur is reamed, the prosthesis will be sitting in soft bone and may sink (subside) over a period of time and fail.

As with the acetabulum, some femoral stems are designed to be cemented in place and others are designed with porous coating so that the bone grows into the stem. Increasingly, there has been a movement to use cement less and less and to use porous noncemented stems. These tend to last longer, as the cement surrounding the stem can loosen like grout around a tile over time, leading to failure and a need to revise the hip replacement. Most surgeons agree that noncemented stems last longer in younger, more active patients, and cemented stems have a less desirable track record for these patients. The cement (polymethylmethacrylate) also can have some dangerous effects on blood pressure during anesthesia, and although this side effect is uncommon, it can be serious in elderly patients. However, cemented stems are still used in some places, especially for patients who have very poor bone stock (e.g., very osteoporotic) that is unlikely to adequately grow into the prosthesis. Trial components are often used at this point during the surgery to determine first if the hip is stable without dislocating, and to determine second if the leg lengths are reapproximated.

In recent years, some newer hip replacements offer modular designs. These designs allow different sizes for the femoral stem, the neck, and the head. In this way, the patient's anatomy can be reapproximated more accurately than with a single piece (i.e., monoblock) prosthesis. This is discussed in further detail in the chapter on implants for those that are interested. Finally, the femoral head (ball) is selected and impacted onto the top of the femoral stem (trunion). The hip is relocated and carried through a range of motion to test how well it reapproximates normal motions and how stable it is to resisting dislocation. The incision is then closed in multiple layers, sometimes over a drain if the surgeon feels it is needed based on the degree of bleeding seen from the tissues and if there is a large amount of space (e.g., in an obese patient) that needs to be closed down under suction. The drain typically is removed in one to two days. Depending on the surgeon and the quality of the skin (i.e., healthy skin, very thin elderly skin, thin skin from being on prednisone for a long time, thick scar tissue from previous surgeries, etc.), various closures may be used for the skin. Staples are strong and are often used in high tension areas (like the knee). Absorbable sutures leave nothing behind that needs to be removed. Sometimes large nylon or prolene sutures are used for scarred skin that may be difficult to hold together otherwise. A sterile dressing is applied and the patient is then taken to the recovery room.

Hemiarthroplasty (Partial Hip Replacement)

A hemiarthroplasty is a partial hip replacement (just the stem and ball, without replacing the socket). In this type of surgery, the large metal ball fits within the patient's own natural socket. This surgery is most commonly performed on frail hip fracture patients with significant medical risk (because of severe cardiac or pulmonary disease, etc.). This is a faster, shorter surgery that is typically performed to stabilize the hip and allow them to walk, but the surgeon opts not to take the additional operative time (and thus anesthesia risk, bleeding time, etc.) to replace the socket. Although it is faster (typically by about 25-50%, depending on the surgeon) and has less blood loss, the partial hip will cause wear against the cartilage in the hip socket over time and may require conversion to a total hip replacement at a later date.

Across the country, there is a growing trend to perform total hip replacements instead of the quicker (and less expensive) partial replacements in hip fracture patients who have a significant life expectancy and are active, community ambulators (e.g., they get out and about and are still mobile). Outcomes have thus far been better than partial hip replacements for active patients, although some studies suggest that outcome depends on whether or not the surgeon doing the emergency total hip replacement does total hip replacements routinely. In our practice, if the hip fracture patient walks on his or her own and has his or her mental wits about them (e.g., is not demented and able to follow instructions to prevent dislocation), we will often perform a total hip replacement.

Please remember the information on this site is for educational purposes only and should not be used to make a decision on a condition or a procedure. All decisions should be made in conjunction with your surgeon and your primary care provider.