A Comparison of the Wear Resistance of Normal, Degenerate, and Repaired Human Articular Cartilage

In our aging population, arthritis is becoming an increasingly common problem. Pain, loss of joint function and other negative affects make arthritis a major health problem. The most common form of arthritis, osteoarthritis, is caused by the "wear and tear" of articular cartilage on the surface of bones in synovial joints. It is a chronic problem that is slowed with different types of therapies, including pharmaceutical, nutritional and surgical, but to date the wearing down of the cartilage cannot be stopped or reversed.

Normal, mature, articular cartilage does not spontaneously repair itself after an injury. In light of this, several surgical techniques are being developed to repair degenerate and/or osteoarthritic cartilage. One such approach uses Autologous Chondrocyte Implantation (ACI). Dr. Mats Brittberg, and associates at Goteborg University in Sweden began using this cartilage repair procedure in 1987. Other techniques attempt to stimulate the subchondral bone to generate cartilage, such as Abrasion Arthroplasty. Still others use tissue grafts to attempt to repair lesions in cartilage. The surface biomechanics of these repaired tissues have not yet been studied. How well does the repaired cartilage resist wear? How long will it last? How does the repaired cartilage compare to "normal" cartilage in terms of wear-resistance? It is the goal of this research to gain initial knowledge to help answer these questions. Dr. Brittberg has provided 17 sample of cartilage, from 9 Swedish patients, including repaired and normal pairs using the aforementioned repair techniques and others, as well as a degenerate and normal cartilage pair. The intention of this paper is to report the findings of experiments performed using these samples, and compare the wear-resistance of repaired and degenerate cartilage to that of normal cartilage.

Wear and friction tests were carried out on 2 mm diameter specimens using a biotribology device and a new, modified technique developed specifically for these small samples. The cartilage samples were mounted, using specially designed adapters, in our biotribology device for oscillating contact against polished stainless steel disks at a constant applied normal load, oscillating frequency, and test time. A buffered saline solution was used as the lubricant. Cartilage wear was determined from hydroxyproline analysis of the test fluid and washings from the wear test. Thin layers of transferred cartilage-like films to the stainless steel disks were also analyzed. Also, friction data was recorded throughout the tests.

The results of these experiments show that:

1. For the two pairs of ACI repaired cartilage, the repaired cartilage gave substantially less wear than that of normal cartilage.
2. For all other repair techniques tested, the repaired cartilage produced more wear than normal cartilage.
3. The single osteoarthritic cartilage tested produced similar wear to that of normal cartilage. This is surprising since the current thought is osteoarthritic cartilage is more susceptible to wear.
4. The hydroxyproline concentration, by weight, of cartilage increases after the wear test.
5. Friction levels were in the boundary lubrication regime, and had no correlation with the amount of wear.

To our knowledge, this research represents the first controlled "in vitro" study of an important unknown in cartilage repair, i.e., the wear-resistance of the repaired cartilage. It shows that ACI produces a cartilage with very good wear-resistance, better than that of other repair techniques, and possibly better than normal, healthy cartilage. ACI and its applications to the treatment of degenerate and osteoarthritic joints are promising, and studies will continue to investigate this and other types of cartilage repair.