Cartilage Defects of the Knee
Articular cartilage is the smooth surface that covers the end of the bones. Its function is to help with a smooth gliding surface but also to cushion the ends of the bone with weight-bearing. Articular cartilage defects may be the result of degeneration from age, repetitive wear and tear, or a traumatic event such as an injury from falling on the knee, jumping down, or twisting the knee. Sometimes, these injuries also occur in relation to a fracture or ligamentous injury in the knee. Sometimes, these injuries do not always produce immediate pain secondary to the fact that there are no nerve endings in the cartilage. However, patients may experience swelling, stiffness, and mechanical type symptoms such as locking and catching. Pain when it occurs may be related to other injuries that occur such as fracture, ligamentous injury, bone bruise or in severe articular cartilage injury where the injury is full-thickness and down to the nerve endings in the bone.
Degrees of articular cartilage injury can range from softening of the superficial layer to a full thickness defect down to the bone. In addition, the type of injury is also measured by the size of the defect (area). As the size and thickness of the defect increase, there is a larger propensity for the defect to increase in size and depth with time. The articular cartilage has no blood supply and therefore is unable to heal itself. More significant injuries in terms of size and depth of the defect may require operative treatment to replace or repair the defect.
Treatment of Cartilage Defects
The treatment depends on the size and degree of involvement of the articular cartilage. In the case of global thinning of the cartilage such as seen with osteoarthritis, the treatment is typically conservative including rest, ice, anti-inflammatory medication, cortisone injection, and physical therapy or a home exercise program. If this treatment does not improve the situation, then some form of replacement arthroplasty is performed, either unicompartmental or total knee arthroplasty. For other more focal defects the treatment options include:
- Arthroscopic debridement
- Arthroscopic microfracture
- Osteochondral autograft transplantation (OATS)
- Autologous chondrocyte implantation
- Osteochondral allograft
This is generally reserved for smaller partial thickness articular cartilage defects with loose articular cartilage fragments. This includes cases of arthritis. In this case, the loose articular fragments are removed with a shaver so as to help to smooth the articular defect to allow for an improved but not normal gliding surface. This can also help with alleviating swelling in the knee since swelling in the knee can be related to loose articular fragments. After this type of surgery, patients are able to move the knee without restriction as well as bear weight without restriction immediately.
Microfracture is performed for smaller articular cartilage defects which are near full thickness or full thickness. In this case, small perforations are made in the underlying bone. This allows some blood supply into the defect. This blood supply allows reparative cells to access the defect so as to form a new fibrocartilage filler in the defect. This new fibrocartilage is not the same as articular cartilage and is not as durable as articular cartilage, but it is helpful to stabilize the defect. Again, microfracture is reserved for smaller and more focal articular cartilage lesions. Depending on where the microfracture is performed in the knee, patients may require to be non-weight-bearing for a period of 6-8 weeks. They also may be required to wear a brace for this period of time so as to protect the articular cartilage during the repair and healing process.
Osteochondral Autograft Transplantation (OATS)
This procedure is reserved for smaller and medium-sized near full-thickness or full-thickness defects. In this process, a core of articular cartilage and bone is removed from another site in the knee and transferred into the articular cartilage defect that is to be repaired. Generally, these cores are taken near the trochlear groove which is the groove that the kneecap rides in. After surgery, the rehabilitation again is the same as for microfracture. This includes the potential use of a brace and limited weight-bearing for a period of 6-8 weeks.
Autologous Chondrocyte Implantation (ACI) or De novo Procedure
This procedure is reserved for medium sized to a larger sized near full thickness or full thickness defects. In the case of ACI, patients will have a small piece of their articular cartilage harvested arthroscopically. This piece of tissue is sent to a lab where the cells in the cartilage tissues are isolated and cultured to allow the cells to multiply. At a later date, these cells are re-implanted into the knee joint at the site of the articular cartilage defect. This is typically done through an open procedure. After surgery, there is a period of time that may require the use of a brace and/or non-weight-bearing. This same procedure can be done with the use of allograft (cadaver) cartilage (de novo). The use of allograft tissue does not require the arthroscopy procedure for harvest.
This procedure is reserved for large full thickness articular cartilage defects that also include underlying bone defects. In this case, a piece of cartilage and bone is harvested from a cadaver knee joint. This piece of cartilage and bone is then implanted through an open procedure in the knee into the cartilage and bone defect. Again, after this procedure a period of non-weight-bearing or use of a brace is required. Issues with this procedure include obtaining a suitable cadaver knee in a timely manner. Sizing the defect appropriately and matching this defect with that of the cadaver knee can sometimes be a time-consuming and expensive process. Nonetheless, for large full-thickness articular cartilage defects with associated bone loss, osteochondral allograft may be the only option.