Evaluating Quality of Tissue After Cartilage Repair of the KneeThere are different ways to surgically repair damaged knee cartilage. The type of cartilage being considered here is the hyaline cartilage that lines the joint and covers the bone. Damage to this layer of cartilage can result in full-thickness lesions that are like small potholes in the surface of the joint. The defect goes all the way down to the bone. The person with this type of injury experiences knee pain and loss of knee motion and function.
One method of repair that has been successful for large, full thickness, painful cartilage lesions is called autologous chondrocyte implantation (ACI). In this technique, a number of healthy chondrocytes (cartilage cells) are harvested (removed) from a nonweight-bearing area of the joint. These chondrocytes are then taken to the lab where they can be stimulated to grow even more healthy cells. When there are enough intact and healthy chondrocytes to patch up the hole, surgery is done to implant them in and around the lesion.
Studies have been done to show how well patients fare after autologous chondrocyte implantation (ACI) for full-thickness articular cartilage defects. In this study, scientists take a look at how well the actual repair tissue holds up over time. Evaluating the quality of tissue after ACI isn't easy. The surgeon can go back in with an arthroscope and take a look but this requires another surgery and is invasive. During the arthroscopic procedure, the surgeon takes a small piece of the healing tissue to examine it under a microscope. No one knows if this procedure (which disturbs the healing or healed tissue) might cause problems later.
MRIs have been used because it is a noninvasive way to look inside the joint. But standard MRIs don't show the cellular structure of the cartilage in enough detail to really assess the repair tissue. So the authors of this article conducted a study using a newer MRI technique called dGEMRIC. dGEMRIC stands for delayed gadolinium-enhanced MRI of cartilage. How does it work?
Well, the radiologist puts a dye into the knee joint -- that's the gadolinium compound (short for gadolinium diethylene triamine pentaacetic acid or Gd-DPTA). The gadolinium spreads throughout the joint and into the cartilage. The gadolinium seeps in the best wherever there is healthy cartilage tissue with plenty of glycosaminoglycans (GAG). GAGs form an important component of connective tissues. They attract water, which helps keep the cartilage healthy and slippery for joint motion.
A normal level (concentration) of gadolinium is around 65 mg/mL in normal cartilage tissue. By comparison, severely damaged cartilage from osteoarthritis only absorbs about 10 mg/mL. After the dye is injected into the joint, the patient walks around for about 15 minutes to help spread the gadolinium into the cartilage. Then MRIs are taken to look at the filling of the defect, the smoothness of the surface, and the presence of bone edema. The dGEMRIC technique has the specific ability to measure GAG concentration and give surgeons an idea of how much degeneration is present in the autologous chondrocyte implantation (ACI) repair.
In this study, testing was done anywhere from nine to 18 years after the original autologous chondrocyte implantation (ACI). What they found was good quality of repair tissue -- very similar to the surrounding normal cartilage. But there were osteophytes (bone spurs), bone cysts, and bone edema (fluid) also present in, under, or around the repair tissue. These new lesions did not seem to bother the patients or affect knee function.
Age was a factor in that older patients had more cysts and younger patients had more bone spurs. Given the older age of patients with subchondral cysts, the presence of these cysts under the implanted cartilage is probably a sign of osteoarthritis developing. And the presence of osteophytes (bone spurs) seemed to be linked with patients who had a condition called osteochondritis dissecans or who had a different cartilage repair technique called marrow-stimulation. Areas of damage with an irregular surface had better outcomes than defects with a smooth surface. The reason for this is unknown but will be studied further.
The authors concluded that assessing and predicting the long-term durability of autologous chondrocyte implantation (ACI) can be difficult. But studying the results of ACI (and other ways of repairing cartilage defects) is important in order to find the best way to treat the problem. As this study showed, the dGEMRIC technique can be used as a noninvasive alternative to arthroscopic exam and tissue biopsy.
This newer MRI method provides valuable information about the composition of repair tissue. This is a different way to judge the results of autologous chondrocyte implantation from the usual clinical tests (motion, strength, pain, joint stability) performed on the patient. dGEMRIC may become a universally accepted noninvasive way to evaluate the results of all methods of cartilage lesion repair.
Haris S. Vasiliadis, MD, PhD, et al. Autologous Chondrocyte Implantation in Cartilage Lesions of the Knee. In The American Journal of Sports Medicine. May 2010. Vol. 38. No. 5. Pp. 943-949.
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