Oh, your aching knees. People of all ages and all levels of physical activity experience knee pain. When pain is widespread and diffuse in one or both knees, the problem is often arthritis. But when the problem comes from one place in one knee, with concentrated pain, swelling that doesn't go away, or a sense that the knee is 'clicking' when it is moved back and forth, injured cartilage may be the culprit.

We asked John A. Grant, Ph.D., M.D., F.R.C.S.C., Dip. Sports Med., to provide an overview of cartilage-related knee pain and what can be done about it. Grant is an orthopaedic surgeon at Michigan Medicine's MedSport, who has a focus in knee cartilage treatment and research.

What is cartilage and what does it do?

Cartilage is the slippery tissue on the ends of bones, in between joints, and elsewhere in the body. There are two kinds of cartilage in the knee: Articular cartilage sits at the end of a bone to provide a slippery surface that allows the two bones of the joint to glide easily against one another. The meniscus are two pieces of cartilage between the bones in the knee joint that distribute impact and act as shock absorbers (like the shocks in your car).  For the purposes of this discussion, we'll focus on how we address damage to articular cartilage.

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Compared with other body tissues, cartilage has a poor blood supply, and that limits its potential to heal itself when injured. That's why, when cartilage is torn or damaged by a sports-related injury or other trauma to the knee, fixing it can be especially tricky.

What options exist for repairing or replacing damaged cartilage?

When we evaluate patients for surgical treatment, the first thing we do is determine into which of two categories the injury falls. The first is surface damage only – the cartilage is damaged but the bone beneath it is healthy. The second involves damage to both the cartilage and underlying bone.

When only the cartilage needs repair, it's something like fixing a pothole, and there are a number of techniques to consider.

According to the FDA, the current "gold standard" of care is a procedure called microfracture. During microfracture, a surgeon cleans up the area of injury, removing the damaged cartilage and exposing healthy edges of the surrounding cartilage. The surgeon then drills small holes in the bone at the base of the defect. Over time, bone marrow cells will rise up through the holes and form a clot. After surgery, the patient must participate in a course of specific exercises and therapy. This specific rehabilitation program helps to ensure that the clotted cells will turn into cartilage, rather than becoming stiff like bone or scar tissue.

Patients with small areas of damage who commit to the necessary rehabilitation have good success with microfracture, although the benefits tend to decrease over time, with symptoms returning after a few years.

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Another way to 'fill the pothole' is to implant cartilage derived from the patient's own cartilage cells. One approved procedure is called matrix-induced autologous chondrocyte implantation (MACI).

Two surgeries are involved. The first, performed arthroscopically (using a small telescopic camera and instruments inserted through a small hole), removes a small amount of healthy cartilage cells from the effected knee. Those cells are sent to a commercial lab where they are grown and multiplied many times, then placed onto a scaffold-like collagen structure made to fit the shape and size of the damaged area. In about three weeks, the cells have generated a new piece of cartilage on the scaffold. At that point, during a second surgery, the damaged area of knee cartilage is cleaned and the scaffold containing the new cartilage is implanted.

A similar approach, called NOVOCART® 3D, is currently being evaluated in a clinical trial. Michigan Medicine is the only site in the state participating in the trial.

NOVOCART® 3D uses a similar technique as MACI, replacing damaged cartilage with new cartilage cultured in a lab using the patient's own cells. But this option uses a new, three-dimensional collagen scaffold structure. Collagen is one of the main building blocks of human tissue. It's an exciting advance we're hoping will help many patients.

To learn more about the NOVOCART®3D trial, visit

https://umhealthresearch.org/#studies/HUM00119336

What about when both the cartilage and the bone need repair?

That can happen as a result of an injury, when a segment of bone is missing due to a disease, when there are cysts in the bone, or when it is not healthy enough for new cartilage cells to stick to it and grow. In such a case, we would replace both bone and cartilage with a procedure called osteochondral transplantation.

In this procedure, the surgeon opens the knee and cleans out the unhealthy bone and cartilage, making a socket in the bone. Into that socket, the surgeon transplants a dowel or cylinder of healthy bone and cartilage. The transplanted bone heals to the patient's healthy bone at the edges of the socket, similar to a broken bone or fracture, and the area benefits from the implantation of mature, healthy cartilage.

When the injured area is small, surgeons may be able to transplant a bone and cartilage dowel from a minimal-weight-bearing area of the patient's own knee (called an autograft). But for larger repairs, one or more bone and cartilage dowels from an organ and tissue donor body (called an allograft) are used.

What can patients expect before and after surgery?

During an initial consult in our clinic, we review the patient's symptoms, what treatments he or she has received to date – including injections, braces, physical therapy, and previous surgeries – and do a physical exam to assess joint swelling, instability, and alignment. We also review X-rays and MRIs to determine the location and size of damage. All of that helps determine which option is best.

The majority of cartilage surgeries are performed on an outpatient basis. Depending on the procedure, patients can expect to use crutches for six to eight weeks, and to begin physical therapy and home exercise right away, continuing for six months or longer. While each patient's healing timeline and treatment goals are different, on average patients return to full activity in six to 18 months.

Surgery is a big step. Can anything be done earlier to prevent or minimize damage to knee cartilage?

Of course, non-operative options for managing cartilage health are always our first recommendation. At MedSport, we encourage patients to exercise to keep their legs strong, especially the quadriceps muscles on the front of the thigh. And keeping one's weight under control is so important, especially if there is already some damage to the knee. The knees bear two to four times the body's weight with each step –and up to ten times that much when running or climbing. That means a ten-pound weight loss takes 40 pounds of pressure off the knee with every step taken through out the day (calculate that with your step counter!).  

Make an Appointment

To make an appointment with a Michigan Medicine MedSport orthopaedic specialist call 734-936-7400