The knee is composed of three compartments, the (1) medial compartment, (2) lateral compartment, and (3) patellofemoral compartment. Osteoarthritis is a degenerative condition affecting articular cartilage. In this article, we will discuss osteoarthritis that is limited to a single compartment of the knee, known as unicompartmental arthritis. We will limit the discussion to arthritis of the medial and lateral compartments. Patellofemoral arthritis and pain syndrome are discussed here.
Anatomy and Function
The knee represents the articulation, or the joint, between the femur (thigh bone), and the tibia (shin bone). It is responsible for bending the leg while bearing the weight of the body. Components of the knee joint include the meniscus, the anterior cruciate ligament (ACL), the posterior cruciate ligament (PCL), the medial collateral ligament (MCL), lateral collateral ligament (LCL), and the articular cartilage. The articular cartilage is a protective covering of the ends of bone within a joint. In the knee, articular cartilage covers the ends of the femur and tibia, as well as the undersurface of the patella.
Osteoarthritis is a painful condition that results in the destruction of articular cartilage. Because articular cartilage covers the bones within a joint, its destruction results in exposure of bone below the articular cartilage. This exposure can lead to the pain characteristic of osteoarthritis. Additional symptoms of osteoarthritis include stiffness and swelling, often worse at the end of the day.
Unicompartmental arthritis is an example of osteoarthritis affecting either the medial or lateral compartment of the knee. While osteoarthritis most commonly affects both compartments, certain factors may predispose to the uneven development of osteoarthritis. These factors include:
(1) Malalignment: Patients with malaligned knees are commonly known as being “bowlegged” (varus malformation) or “knock-kneed” (valgus malformation). Malaligned knees are predisposed to the development and progression of unicompartmental arthritis because of the excessive load placed on one compartment, or side, of the knee.
(2) Articular cartilage injury: Direct injury to the articular cartilage during a traumatic event is a frequent cause of unicompartmental arthritis. This occurs because articular cartilage heals very poorly on its own after injury. If left untreated, the damaged cartilage alters the normal smooth gliding of the femur on the tibia and the injury worsens, eventually leading to painful osteoarthritis.
(3) Meniscus injury or debridement. Because one of the roles of the meniscus is to function as a shock-absorber between the femur and the tibia, removal of or injury to the meniscus results in an increased force on the articular cartilage below. Over time, this increased force wears away articular cartilage, resulting in osteoarthritis. Injury to the meniscus commonly occurs as a result of a traumatic event. Damaged meniscus can be repaired under certain circumstances but often times must be debrided, or trimmed, during arthroscopic surgery to treat the pain caused by the damage. Unfortunately, removing pieces of the meniscus does result in less shock absorption and an increased risk for unicompartmental arthritis.
Diagnosis of unicompartmental arthritis is made through a combination of history and physical exam as well as imaging. A history of pain isolated to the medial or lateral portions of the knee associated with joint swelling and stiffness is suggestive of unicompartmental arthritis. On physical exam, orthopedic surgeons and sports medicine specialists will evaluate the alignment of the knee to assess for joint malalignment indicative of uneven load distribution. X-rays of the knee will be taken and may demonstrate signs of osteoarthritis limited to one compartment of the knee. These signs include joint space narrowing, bone spur formation, and increased density of the bone adjacent to the overloaded compartment.
Treatment for unicompartmental arthritis can be divided into non-surgical and surgical treatment options. Treatment choice depends on many factors considered by the orthopedic surgeon or sports medicine specialist. Such factors include age, activity level, degree of symptoms, and the underlying cause of unicompartmental arthritis.
(1) Weight loss
Weight loss is the single best option for non-surgical management of unicompartmental arthritis. By decreasing the force transmitted through the knee joint with each step, the progression of osteoarthritis is delayed and pain diminished. It is recommended that all patients suffering from unicompartmental arthritis initiate treatment with exercise and weight reduction if necessary.
Multiple injections are available to help relieve the pain of unicompartmental osteoarthritis. Corticosteroid injections are commonly used to decrease the inflammation of an osteoarthritic knee and relieve associated pain. Pain relief typically last from 4-6 months. Hyaluron is a natural component of joint fluid that helps to lubricate and absorb shock. Hyaluron injections are thought to help restore the shock-absorbing functions of an osteoarthritic knee and protect damaged cartilage.
If the underlying problem of osteoarthritis is malalignment of the knee, a special brace, called an offloader brace, may be worn by the patient. The offloader brace attempts to correct the malalignment of the knee and distribute the patient’s weight evenly across the knee joint. The offloader knee brace has success on its own to relieve the pain of osteoarthritis and also may serve as an indicator of how well a patient will respond to surgical intervention to correct limb alignment (see below).
(1) Articular Cartilage Restoration Procedures
Because articular cartilage does not heal well on its own, surgical techniques have been developed to restore damaged cartilage. These options are typically most appropriate for smaller cartilage defects in younger patients and include:
- Osteochondral allograft transplantation
- Autologous chondrocyte implantation
For a complete discussion of treatment options for cartilage injury, please click here.
If the cause of unicompartmental arthritis is malalignment of the knee, a surgical procedure may be performed to correct the underlying malalignment. In this procedure, a wedge of bone is cut out from either the tibia (a high tibial osteotomy) or the femur (a distal femoral osteotomy). The removal of this wedge allows the surgeon to adjust the angulation of the tibia or femur so that the knee can be properly straightened. The goal of this procedure is to relieve the uneven distribution of weight across the knee and prevent progression of osteoarthritis. Patients are typically not allowed to bear weight on the leg for 6 weeks and aren’t able to return to full activity until 6 months.
- Advantages: retain native knee joint, allow for continued normal activity level.
- Disadvantages: long recovery period, painful recovery period.
The optimal candidate for osteotomy is a highly active, highly motivated patient willing to deal with the long and painful recovery process. Additionally, there must be clear demonstration of knee malalignment as well as truly isolated unicompartmental osteoarthritis.
(3) Unicompartmental Knee Replacement
In certain situations, the best treatment for unicompartmental osteoarthritis may be replacing the damaged portion of the joint. This is done by removing the damaged cartilage from the femur and tibia in either the medial or lateral compartment of the knee. The femur and tibia are then covered with a metal surface and a piece of plastic placed between them to simulate a normal joint space. This procedure is a highly successful option when performed in the correct patient population.
- Advantages: quicker recovery than total knee replacement, less pain than total knee replacement, may be a more “natural” feel than total knee replacement
- Disadvantages: less predictable pain relief than total knee replacement, may not last as long as total knee replacement
(4) Total Knee Replacement
Similar to a unicompartmental knee replacement, a total knee replacement involves the surgeon removing damaged cartilage from the surface of the femur and tibia. In a total knee replacement, however, this is done to both the medial and lateral joint surfaces, as well as the undersurface of the patella, in order to effectively remove all of the articular cartilage from the joint and replace it with metal and plastic.
- Advantages: removal of all osteoarthritic cartilage, more predictable pain relief.
- Disadvantages: may be a less “natural” feel than alternative options