How does the cartilage in the knee work?
The knee is a synovial joint that is lined with smooth articular (surface) cartilage. The joint is further lubricated with synovial fluid (joint fluid) creating a low friction environment for the femur to rotate and glide on top of the tibia. Depending on the type of activity, forces transmitted across the knee joint can be several times the entire body weight of a person, placing significant load on the cartilage – especially behind the knee-cap. Healthy cartilage is able to withstand these loads through numerous adaptations:
- Load Distribution
The knee joint is highly congruent (one side fits the other like a jig-saw puzzle), making for a large contact area over which the joint reactive forces can be dissipated.
- Joint Lubrication
The synovial fluid within the knee joint is trapped within the joint lining. This important labral function serves to evenly distribute the joint fluid allowing for more even load distribution.
- Cartilage Water Content
The surface cartilage is made of cells (1%) surrounded by a water-based matrix (99%), which contains collagen along with various other proteins that attract water. The water moves out of the matrix and into the synovial fluid when a load is placed across the joint, and returns when the load is released (like a sponge). This water action dissipates much of the load by conferring a spring like quality to the cartilage. As the cartilage becomes damaged and calcified (with progressive osteoarthritis) this function is compromised.
What causes cartilage damage in the knee?
There are two types of cartilage damage in the knee: focal and generalized. Focal cartilage damage typically occurs in the setting of an injury (knee contusion or sprain) or over time with long-standing knee instability – typically due to ACL tear. The area of damage (lesion) is typically a well-circumscribed defect with healthy neighboring cartilage. The severity of the defect ranges from mild softening (Grade I) to exposed underlying bone (Grade IV). Lesions that are Grades I – III are typically treated with shaving and debridement, while Grade IV lesions are addressed with various cartilage restoration and regeneration techniques. Generalized cartilage damage affects the majority of the surface cartilage and can come about over time with untreated focal lesions or with osteoarthritis. The goal is to address cartilage damage before it becomes generalized, at which point treatment options become limited. Unfortunately, minimally invasive knee arthroscopy is not a good option for patients with significant (Grades III & IV) generalized cartilage damage, as it is difficult to regenerate a large area of cartilage loss. Platelet-rich-plasma (PRP) injections may be of benefit for patients who want to maintain an active lifestyle and are not ready for the risks and limitations of a knee replacement. Otherwise, the only viable long-term treatment option is joint replacement surgery.
How is cartilage damage diagnosed?
Surface cartilage is best visualized with magnetic resonance imaging (MRI) utilizing specialized sequences (or ways of running the scan) to isolate the cartilage. A more comprehensive analysis of surface cartilage composition can be obtained with a delayed gadolinium enhanced-MRI of cartilage (dGEMRIC) scan, using contrast material injected through an IV.
What is cartilage restoration or regeneration?
When surface cartilage damage is restricted to a focal, well-circumscribed area with healthy neighboring cartilage there are several methods by which new cartilage growth can be achieved to “fill-in” the defect:
- Platelet-Rich-Plasma (PRP)
A great non-surgical treatment option to stimulate healing for small, focal cartilage defects or bone cysts is through an injection of platelet-rich-plasma (PRP). This is a relatively low cost option that utilizes your own body’s natural growth factors, concentrated through centrifugation and delivered directly at the site of injury. Please refer to the PRP section for more detail.
The first line surgical treatment for lesions that are < 2.5 cm2 is to stimulate bleeding from the underlying exposed bone by a minimally invasive arthroscopic procedure called a microfracture. The defect is first neatly debrided of loose or frayed cartilage and the calcified cartilage layer is scraped off of the underlying bone to expose the subchondral plate. Then a high-speed drill is utilized to drill several small holes to puncture the bone and allow for marrow elements to bleed into the defect, carrying stem cells and other beneficial growth factors. Over a period of 6 – 12 weeks, new cells proliferate and begin making new cartilage (called fibrocartilage) to fill in the defect and reconstitute a smooth surface. Outcomes vary depending on patient age, smoking status, and chronicity of the lesion, however excellent results are typical for this treatment option in appropriately selected patients.
- Osteochondral Allograft Implantation
When microfracture fails to achieve healing-in of the defect, or when the defect size is > 2.5 cm2, osteochondral allograft implantation may be considered. This is a more invasive procedure that typically requires a formal incision and surgical knee dislocation. The lesion is prepared as described above in the microfracture section and a tissue bank is utilized to obtain a size-matched bone/cartilage plug that is then used to fill in the defect. Healing rates vary depending on the location and size of the defect, however good to excellent results can be achieved in appropriately selected patients.
How can I prevent cartilage damage in my knee?
The integrity or health of the surface cartilage is by far the most important consideration when choosing between different treatment options for any knee disorder. You can take several steps to ensure that your knees remain healthy and youthful for years to come:
- Cyclic Exercise
Studies have shown that low-impact cyclic exercises (riding a bicycle, swimming, or using an elliptical machine) promote surface cartilage health and reduce the likelihood of developing osteoarthritis. Keeping a regular exercise program of 30 minutes of low to moderate intensity cyclic activities 4 – 5 days/week can keep your cartilage and your heart in good shape.
Obesity is a well-known risk factor for many diseases, but it is especially detrimental to cartilage health and can cause progressive osteoarthritis. Make an effort to maintain the recommended body weight for your build and reduce the stress placed on your knees to keep your joints healthy and youthful.
A balanced, nutritious diet is essential for your body to maintain robust and healthy cartilage. Calcium, Vitamin D, and essential amino acids promote self-healing qualities that native cartilage employs to maintain a functional matrix that can withstand normal physiologic loads. Glucosamine and chondroitin sulfate have also been shown to be beneficial for patients with mild osteoarthritis.