Osteoarthritis is the end result of progressive cartilage damage. Bone on bone degeneration is the end result of osteoarthritis. A joint in this condition is unlikely to respond to cartilage restoration procedures, and the “replacement” of the joint surfaces is indicated. A knee has 3 compartments and cartilage degeneration can occur in one, two or all 3 of these compartments. Similarly, replacement procedures can address one, two or all 3 compartments. A replacement that replaces all 3 compartments is a Total Knee Replacement (TKR). A replacement procedure that replaces one or 2 of the knee compartments is called a “Partial Knee Replacement.” Partial Knee Replacements come in several varieties, which are expanded upon in the sections below.

Total Knee Replacement

Unicompartmental Knee Arthroplasty (UKA) or Partial Knee Replacement

Arthrosurface ("mini" replacement)

Conformis Custom Replacement

Partial Knee/Patellofemoral Replacement

The first knee replacements were performed in the 1950’s, much of the credit given to LPG Shiers’ publication in 1954 in which a constrained prosthesis with abundant bone removal was proposed. The next generation of designs emerged in the 1970’s, including the “non-constrained models,” such as the Geomedic and Total Condylar designs. A popular “re-surfacing design” introduced in this period was called the Marmor modular knee, The condylar knee concept of replacing the tibiofemoral condylar surfaces with cemented fixation, along with preservation of the cruciate ligaments, was refined. To correct severe knee deformities, the condylar knee with posterior cruciate-sacrificing design was introduced. By 1974, replacing the patellofemoral joint and either preserving or sacrificing the cruciate ligaments had become standard practice. Subsequently, condylar knee designs were improved to include modularity and noncemented fixation, with use of universal instrumentation. Partial knee replacements have been available since the 1970’s.

The design and materials of TKR’s continues to improve. Today, over 19 companies in the United States distribute total knee implants. The intent is to “resurface” the diseased knee, i.e., replace only the damaged surface of the knee with durable materials. The wear characteristics of these materials must withstand years of compression and shear without failing. The movement and mechanics of the artificial joint must have stability yet minimize contact stress, which leads to premature polyethylene wear. Polyethylene wear particles are deposited into the adjacent soft tissue and initiate an inflammatory response, which may cause failure of the components, particularly at the bone-prosthesis interface. Future directions of knee replacements include:

• The continued development of wear-resistant surfaces
• Improved kinematic function
• Fixation techniques
• Implantation techniques that are minimally disruptive
• Greater longevity
• Minimize complication rates
• Shorten hospital stays and recovery

The best candidate for a knee replacement is a person who has:

• Daily pain that is significant and disabling and interferes with activities of daily living. The pain is typically worsened by any weight-bearing activity.
• Significant stiffness in the knee. Morning stiffness is often present but flexibility improves somewhat over the course of the day. Swelling is often a component of stiffness.
• Significant knee deformity (knock-knees or bowlegs) caused by the arthritis.
• Non-operative treatments have been tried unsuccessfully.
• Standing X-ray evaluation reveals advanced osteoarthritis with accompanying deformity, abnormal narrowing of the joint space, sclerosis and spurs or cysts in the bone.

Techniques generally employ the basic principle of minimizing incisions and tissue disruption without compromising the accuracy of the operation. Traditional implantation techniques continue to be the standard in the industry. Techniques are outlined according to the specific procedure in the sections below.

The recovery varies according to the magnitude of the procedure. Most joint replacement procedures are significantly invasive, requiring open incisions and bone cuts. Though outpatient scenarios are possible with new technologies, these situations are currently infrequent. Most patients stay from 1 to 3 nights in the hospital. Immediate weight-bearing is generally permitted. The first month after surgery might be considered “down time.” Light activity is conducted. Patients are 80% recovered at 2 months post operatively. The full effect of the surgery may take 6 to 12 months in some cases.

Several studies have shown that the overall success rate of joint replacements is higher among institutions and surgeons who do high volumes of these procedures.