There are many types of arthroplasty. Some require prostheses (artificial body parts), and others do not. When prostheses are required, there are many options available.

Joint replacements are the most common kind of arthroplasty and are among the most common procedures performed by orthopedic surgeons. Many joints may be totally or partially replaced, including the knees, hips, ankles, shoulders, elbows, wrists, fingers and toes.

Total joint arthroplasty may be used to repair acute conditions, such as a hip fracture, or to treat chronic conditions if the patient is experiencing disabling daytime and nighttime pain that prevents everyday activities. There also needs to be x-ray evidence of joint damage, including:

• Obvious narrowing of the joint space
• Osteophytes (bone spurs)
• Subchondral (below the cartilage) cysts
• Subchondral sclerosis (thickening or hardening of the bone)

With chronic disorders, joint replacement is typically used only when conservative therapies such as bedrest or physical therapyphysical therapy have failed. Total joint arthroplasty is typically reserved for patients over age 60, sometimes 55. Sometimes younger patients  are candidates if they are experiencing moderate arthritis and increasing deformities that seem likely to jeopardize future results from an arthroplasty.

Variations on joint replacements include:

• Single-stage bilateral total joint arthroplasty. Both corresponding joints (e.g., both hips) are replaced at the same time. This is most common with the knees.
  
  
• Unilateral total joint arthroplasty. One joint is replaced. An option recently approved by the U.S. Food and Drug Administration for less-invasive hip replacement is hip resurfacing, in which a small amount of bone is removed from the ball-like head of the femur (thighbone) and metallic parts are implanted into the femur and the socket-like acetabulum of the pelvis.
  
  
• Hemiarthroplasty. Only one side of the joint is replaced. For example, only the head of the femur in the hip joint may be replaced, leaving the natural acetabulum as is.
  
  
• Unicompartmental joint arthroplasty. Only a single piece of the joint is replaced. This is an even more limited replacement than hemiarthroplasty.
  
  
• Minimally invasive surgery. Works with smaller incisions. This may be employed in total joint arthroplasty. Some centers can even perform this variation as an outpatient procedure. The soft tissues around the joint suffer less trauma, and the patient recovers much more quickly. However, this technique has only recently been developed, and long-term results are not yet available.
  
  
• Replacement arthroplasty. When a joint prosthesis wears out or becomes infected, another arthroplasty is required to replace it. This is most common in patients younger than 60 because of their more active lifestyles.

Other types of arthroplasty include:

• Interposition arthroplasty. The affected joint is resurfaced with a piece of the patient’s own body tissue. Either a piece of the patient’s skin (usually from the abdomen) or tendon (usually from the Achilles tendon) is stitched in place between the bones of the joint. This keeps the bones from rubbing together and reduces pain. This option is generally best for patients under age 60.
  
  
• Osteocapsular arthroplasty. This involves the removal of osteophytes, loose bone, loose cartilage and bits of inflamed synovium (fluid sac that lubricates between joints) and the reshaping of deformed or deteriorated bones. Osteocapsular arthroplasty can be very successful, but it is not widely performed. It is a relatively new surgery that employs a very difficult technique and requires substantial expertise.

With joint replacements, there is a wide range of options available for prostheses. The many materials that may be used include:

• Polyethylene. This has been the most common material used in prostheses. However, it has been heavily scrutinized because of its high production of wear debris, which leads to osteolysis (enzymes break down and reabsorb the bone tissue in an attempt to get rid or foreign particles).

In the past, this wear rate has been very high, but because of improvements in sterilization and development, new polyethylene designs have become much more resistant to wear. Still newer developmental designs demonstrate even less wear but require further long–term study. Polyethylene is the least expensive material for prostheses.

• Metal alloys. Chromium cobalt is often used, but stainless steel or titanium may also be employed. These metals produce very small amounts of wear debris and the wear rate actually improves once the prosthesis has been broken in. Metal alloys may still cause osteolysis, but to a smaller degree than polyethylene. Any metal surface, including the metal alloys used in prostheses, may generate corrosion, but this is typically reduced, especially in newer designs.

There is some concern over the absorption of metal ions in the body. These ions may be deposited throughout the body and metallic levels in the blood and urine may be many times higher than normal. No adverse effects have been reported over 20 years of use in any individual, but this issue requires further study.

• Ceramics. Alumina is typically used. Ceramics are more brittle than other materials used for prostheses but have lower wear rates than metal. However, though they do generate less debris, they may still cause osteolysis. Little is known about the response of the body to ceramic wear particles.
  
  
• Carbon. Pyrolytic carbon and graphite have been used successfully in small joint implants and can be incorporated into composites of larger implants.

A prosthesis may incorporate any of a number of designs, including:

• Metal-on-polyethylene. This is the most commonly used prosthesic design. It has a low coefficient of friction close to that of normal joints and typically lasts for about 20 years.
  
  
• Metal-on-metal. The earliest prosthesis designs were metal-on-metal. These early designs showed many failures, generally due to design flaws and the misunderstanding of how specific joints work. The designs were nearly replaced by metal-on-polyethylene designs as a result. However, metal-on-metal regained popularity when polyethylene particles were linked to osteolysis. These designs typically last for about 20 years.
  
  
• Ceramic-on-ceramic. These designs were initially investigated for high scratch resistance and wearability. Early designs had many problems with cracking, but this fracture problem has been nearly eliminated in current designs. Ceramic-on-ceramic prostheses typically last for about 15 years but have not yet been approved for widespread use in the United States.
  
  
• Silicone spacers. These are typically reserved for very small joints, such as the knuckles. This is a one-piece design rather than a hinge. Small silicone discs are placed between the bones of the joint to cushion them and keep them from rubbing together. Over time, the soft tissues and ligaments around the joint heal and strengthen and joint movement and function is partially regained. Silicone spacers typically last about 10 years.

A final option in the choice of prosthesis lies in the method used to adhere the prosthesis to the bone. These options include:

• Bone cement. Polymethylmethacrylate (PMMA) is used to secure the prostheses to the bone. It may wear and cause osteolysis and has higher rates of loosening than with press fit. The decision on whether to use bone cement is often dependent on the clinical situation and the surgeon’s preference. Though cemented components are currently the most popular method of fixation, cementless components are becoming increasingly popular.
  
  
• Press fit. This cementless approach encourages bone to grow directly into the prosthesis. The prosthetic surface is rough, to increase surface area, and coated in an osteoconductive substance, such as hydroxyapatite, that encourages the bone to grow and adhere to the surface.
  
  
• Hybrid approach. Bone cement is used on part of the joint and press fit on the other.