Types of Hip Replacement Surgeries

Over the past 40 years, there have been many improvements in both the materials and the methods used to hold the femoral and acetabular components in place. Today, the most commonly used bone cement is an acrylic polymer called polymethylmethacrylate (PMMA).

A patient with a cemented total hip replacement can put full weight on the limb and walk without support almost immediately after surgery, resulting in a faster rehabilitation. Although cemented implants have a long and distinguished track record of success, they are not ideal for everyone.

Cemented fixation relies on a stable interface between the prosthesis and the cement and a solid mechanical bond between the cement and the bone. Today's metal alloy stems rarely break, but they can occasionally loosen. Two processes, one mechanical and one biological, can contribute to loosening.

• In the femoral component, cracks (fatigue fractures) in the cement that occur over time can cause the prosthetic stem to loosen and become unstable. This occurs more often with patients who are very active or very heavy. The action of the metal ball against the polyethylene cup of the acetabular component creates polyethylene wear debris. The cement or polyethylene debris particles generated can then trigger a biologic response that further contributes to loosening of the implant and sometime to loss of bone around the implant.
• The microscopic debris particles are absorbed by cells around the joint and initiate an inflammatory response from the body, which tries to remove them. This inflammatory response can also cause cells to remove bits of bone around the implant, a condition called osteolysis. As the bone weakens, the instability increases. Bone loss can occur around both the acetabulum and the femur, progressing from the edges of the implant.

Despite these recognized failure mechanisms, the bond between cement and bone is generally very durable and reliable. Cemented total hip replacement is more commonly recommended for older patients, for patients with conditions such as rheumatoid arthritis, and for younger patients with compromised health or poor bone quality and density. These patients are less likely to put stresses on the cement that could lead to fatigue fractures.

In the 1980s, new implant designs were introduced to attach directly to bone without the use of cement. In general, these designs are larger and longer than those used with cement.

They also have a surface topography that is conducive to attracting new bone growth. Most are textured or have a surface coating around much of the implant so that the new bone actually grows into the surface of the implant. Because they depend on new bone growth for stability, porous implants require a longer healing time than cemented replacements.

The orthopedic surgeon must be very precise in preparing the femur for a porous impact. The implant channel must match the shape of the implant itself very closely. New bone growth cannot bridge gaps larger than 1 mm to 2 mm.

Your surgeon may recommend a period of protected weight-bearing (using crutches or a walker) to give the bone time to attach itself to the implant. This protected weight bearing helps to ensure there is no movement between the implant and bone so a durable connection can be established.

Porous femoral components tend to be much larger at the top, with more of a wedge shape. This design enables the strong surface (cortex) of the bone and the dense, hard spongy (cancellous) bone just below it to provide support.

The acetabular component of a porous total hip replacement also has a coated or textured surface to encourage bone growth into the surface. Depending on the design, these components may also use screws through the cup or spikes, pegs, or fins around the rim to help hold the implant in place until the new bone forms. Usually these components have a metal outer shell and a polyethylene liner.

The pelvis is prepared for a porous acetabular component using a process similar to that used in a cemented total hip replacement procedure. The intimate contact between the component and bone is crucial to permit bone ingrowth.

Initially, it was hoped that porous total hip replacement would eliminate the problem of bone resorption or stem loosening caused by cement failure. Although certain porous stem designs have excellent long-term outcomes, porous stems can loosen if a strong bond between bone and stem is not achieved.

Patients with large porous stems may also experience a higher incidence of mild thigh pain. Likewise, polyethylene wear, particulate debris, and the resulting osteolysis (dissolution of bone) remain problems in both cemented and uncemented designs. Improvements in the wear characteristics of newer polyethylene, and research into newer bearing surfaces may help resolve some of these problems in the future.

Although some orthopedic surgeons are now using porous devices for all patients, porous total hip replacement is most often recommended for younger, more active patients and patients with good bone quality where bone ingrowth into the components can be predictably achieved. Individuals with juvenile inflammatory arthritis may also be candidates, even though the disease may restrict their activities.

Hybrid Total Hip Replacement

A hybrid total hip replacement has one component, usually the acetabular socket, inserted without cement, and the other component, usually the femoral stem, inserted with cement. This technique was introduced in the early 1980s, so long-term results are just now being measured. A hybrid hip takes advantage of the excellent track records of porous hip sockets and cemented stems.

Partial Hip Replacement

In a partial hip replacement surgery, the socket is usually left intact. The head of the femur is replaced with an artificial ball shaped component similar to that used in a total hip replacement. Sometimes a device is fitted over the bone which means the top of the femur does not have to be cut.

For younger patients, a total hip replacement may not be the best solution for their hip pain because it can mean difficult and numerous revisions later in life. Hip resurfacing, however, leaves more of the bone in place, giving these patients more time before a total hip replacement becomes necessary.

Partial hip resurfacing, or hemi-resurfacing, is the most bone-conserving approach to hip surgery. During this procedure, only the femoral head (where the leg joins the hip) is reshaped and resurfaced. The hip socket (acetabulum) is left completely intact. The obvious benefit with partial resurfacing is that the patient keeps most of his or her own bone, which allows for easier revisions in the future (if one becomes necessary).

Possible surgical complications of Partial hip replacement may include:

• Bleeding
• Infection
• Adverse reaction to anesthesia
• Little to no improvement in mobility
• Adherence to physical therapy and rehabilitation
• Inflammation around artificial joint
• Absorption of bone around artificial hip joint
• Artificial hip dislocation
• Debris from artificial components