Articular cartilage is the smooth surface covering (or gristle) that coats the ends the bones that make up the knee. It is a complex layer of cells that cushions the joint. Once it is damaged it can cause pain, grating, catching and swelling. It can be damaged through injury, overuse or as part of wear-and-tear osteoarthritis.
If a piece of surface is chipped off with a fragment of bone attached, this can sometimes be fixed back in place if diagnosed and treated promptly. Damage to the cartilage can be described as ‘partial’ or ‘full thickness’.
Partial thickness defects do not heal, but removing loose flaps and smoothing and stabilising the remaining surface can improve the symptoms. This is called ‘debridement’ or ‘chondroplasty’ and can be performed with mechanical shavers or radiofrequency probes during a knee arthroscopy. Improvement is good in injury cases but modest in arthritic cases.
Full thickness defects require measures to ‘fill the divot’. Treatment options include:
- Microfracture: This procedure is usually carried out as part of a simple arthroscopy. The edges of the defect are stabilised and the area of bare bone is perforated with a series of small holes placed a few millimetres apart. This allows blood and growth factors from the bone to fill the divot. In about 70% of patients, a solid plug of scar tissue fills the defect and acts like a new surface. This technique is usually the first option for full thickness defects, but is not appropriate for treating all defects or for treating arthritis. The technique requires restriction of weight bearing for 6 weeks post-operatively.
- Osteochondral plugs: This technique (often called mosaicplasty) involves taking a plug of healthy surface cartilage, with its attached underlying bone, from an area of the knee that is not important for bearing load. This plug is placed to fill the defect. For larger defects, a few plugs are needed - giving the appearance of a ‘mosaic’. This technique is limited to small defects because there are limited areas from which plugs can be harvested. For massive defects, donor plugs can be used. Synthetic plugs (TruFit) are a new technology that can be considered. Plug techniques usually involve open surgery on the knee and a number of months recovery time.
- Cartilage cell grafting (ACI/MACI): This technique involves an arthroscopy to harvest a small amount of surface cartilage from your knee. This sample of tissue is sent to a special laboratory where millions of your cartilage cells are grown over a 4–6 week period. An open operation is then required to implant the cells into the defect. Significant recovery time is required, as the cells need to attach to the bone and then reform the complex structure of the surface layers. Return to full weight bearing is possible within 6 weeks but return to unrestricted sport can take over a year.
The relative benefits and suitability of each of the techniques will vary depending on the size and position of the defect, the severity of symptoms, and other factors such as the stability and alignment of the knee. Your specialist knee surgeon can discuss all these options with you.