October 2011 Newsletter

Archived Newsletters

Articular Cartilage Injuries of the Knee
By Dr. Stephen A. Hunt

"I have a torn cartilage in my knee."

To some this may sound like an obvious statement, but for me, this statement always begs many more questions. Many people have a history of having a cartilage tear in their knee or know someone else who has had such a tear. However, there are different types of cartilage and each injury may be treated in a different manner.

What are the different types of knee cartilage?
Articular cartilage is the shiny white cartilage on the end of the bones. In the knee, it lines the femur (thigh bone), the top of the tibia (shin bone), as well as the undersurface of the patella (kneecap). It is also known as hyaline cartilage. It is a very complex living tissue and provides a low-friction surface to allow the joint to withstand weightbearing through a wide range of motion. Essentially, it functions as a very thin shock absorber. However, its structural integrity is very complex, containing five distinct layers between its bony attachment and its outer surface.

Meniscus cartilage is also found in the knee. This is a fibrocartilage with slightly different mechanical properties. There are a medial, or inner, meniscus and a lateral, or outer, meniscus in our knees. The meniscus cartilage, in addition to acting as an additional shock absorber in the knee, increases the contact area between the femur and tibia and provides some stability during knee movement. It also helps with nutrition of the articular cartilage by facilitating the diffusion of joint fluid throughout the knee.

What causes an articular cartilage injury?
Articular cartilage injuries can occur as a result of a traumatic event or can be part of a progressive wear-and-tear degenerative process such as arthritis. During a traumatic event, a direct blow can cause damage to the articular cartilage cushion. Because the articular cartilage has no direct blood supply, there is little capacity for these injuries to heal. If they are deep enough to go into the bone, then the blood coming from the bone may form a type of fibrocartilage cap. Mechanical degeneration is a much more common way to have an articular cartilage injury. The initial loss begins with cartilage softening and then progresses to fragmentation and eventually full thickness loss of cartilage, leading to the process of osteoarthritis.

What causes a meniscus cartilage tear?
An injury to the meniscus often occurs from a traumatic event. It is generally the result of a simultaneous injury to the ligaments, in particular the ACL of the knee. Occasionally, the quality of the meniscus tissue changes as we age. Athletes can develop degenerative tears as well.

How do these injuries present?
An articular cartilage injury often causes knee swelling and vague pain that sometimes can be catching or locking if a fragment of cartilage breaks free and acts like a loose body. If it is more of a degenerative type injury to the articular cartilage, patients will often experience stiffness, episodic swelling, decreased range of motion, and generalized joint pain and swelling. Meniscus tears often have an acute injury associated with them and can cause a sense of swelling or discomfort with the knee in a flexed position, or occasionally they will cause a catching or locking sensation. They tend to be very specific in terms of location on the inner or outer portion of the knee.

How is an articular cartilage injury diagnosed?
Both articular and meniscus cartilage injuries may present with similar localized pain and acute events. Meniscus injuries almost always cause discomfort right along the joint line, whereas articular cartilage injures can sometimes present underneath the kneecap or in the front part of the knee joint at the end of the thigh bone. Additionally, meniscus cartilage injuries may have significant swelling in the back of the knee called a Baker’s cyst. Examinations may otherwise be very similar and x-rays often will not show much change unless there is significant arthritis. MRIs are often ordered to evaluate the integrity of the articular cartilage as well as the meniscus cartilage.

How do you treat these injuries?
Articular cartilage lesions that are small and do not go down to the bone often can be observed and monitored. If they are large defects that have a loose body, surgery is often recommended to remove the loose body to try to enhance the potential healing of the defect. If it is more a process of osetoarthritic change and diffuse cartilage damage, there are multiple treatments aimed at decreasing some of the associated pain; ultimately, replacement type procedures are the definitive surgical treatment. With meniscus tears we often recommend surgery following acute traumatic events, or certainly recommend addressing them in conjunction with ligamentous injuries such as ACL reconstruction. If they are degenerative in nature, they sometimes may be a source of mechanical symptoms and pain that do not respond to nonoperative treatments and we would consider an arthroscopic procedure to address them. However, a meniscus tear in the setting of underlying osteoarthritis is a common finding that does not always need to be addressed, as it may be more part of the process of osteoarthritis as opposed to the definitive pain generator in that situation.

REFERENCES:

1. AOSSM Sports Medicine Media Guide.