ADVANCES IN MEDICINE ARTICULAR CARTILAGE REPAIR By Karen Hambly BSc MCSP Dip. Sp. Med.

INTRODUCTION Articular cartilage is found in synovial joints where it functions to separate artic- ulating bone surfaces by means of a pro- tective, wear-resistent surface that serves to increase contact area whilst concurrent- ly reducing contact pressure. To further facilitate smooth joint articulation hyaline cartilage has one of the lowest known coefficients of friction (just 1/15th that of a skate on ice).

Damage to articular cartilage is common and thousands of people each year experi- ence symptoms related to chrondral and osteochondral defects with the knee being the most prevalent joint (1). Studies have shown chondral defects in up to 63% of knee arthroscopies (1-4) with between 4% and 11% of patients showing defects that may be suitable for cartilage repair proce- dures (3,4).

Articular cartilage damage can occur as a result of acute trauma, chronic repetitive trauma or osteochondritis dissecans (OCD). Participation in high impact sports can increase the risk of developing articular cartilage damage as can habitual repetitive stress with or without associated biome- chanical malalignment (5).

TO REPAIR OR NOT TO REPAIR? Designed to last a lifetime, articular carti- lage is both avascular and aneural and as a result has a limited capacity for self-repair. Although articular cartilage is aneural, sub- chondral bone is abundantly supplied with nerve endings which when exposed, as in full thickness defects, frequently generates pain that can often be severe in larger sized lesions. Chondral lesions can also result in swelling, reduced mobility and additional mechanical symptoms. Even though articular cartilage damage isn’t life

BOX 1. KEY FEATURES OF CARTILAGE

■ Function of articular cartilage - shock absorption, optimising load transmission and facilitating smooth articulation

■ Articular cartilage damage is common and has a poor capacity for healing ■ Not all chondral defects are appropriate for cartilage repair ■ Hyaline cartilage is more durable than fibrocartilage in the long term ■ Rehabilitation is long and demanding but critical to the outcome

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threatening it can, and does, threaten quality of life especially in an active popu- lation where athletic ability can be limited (6,7). The limited capacity for regenera- tion, coupled with the prevalence of lesions and the potential lifestyle impact has meant that management has presented a significant medical challenge, especially for the young athlete (8).

A range of surgical repair techniques have been developed on the grounds that the natural history of chondral defects is even- tual progression to arthrosis (9) and that full-thickness articular cartilage defects rarely heal (10). The evidence base to sup- port the progression of chondral defects to osteoarthritis is increasing but is not yet conclusive, so the current focus is on pro- viding symptom relief and improved func- tion whilst supplying a durable structural repair that may slow down the progression of a defect to osteoarthritis later in life.

When articular cartilage defects are found by chance during surgery for other knee problems opinion varies as to whether there is any benefit for the patient to undergo cartilage repair if the defect is not sympto- matic (11). It is important to point out that not all chondral defects will be symp- tomatic and with such a high prevalence of articular cartilage defects there is a real challenge in determining which patients would benefit from repair techniques.

Total and partial knee joint replacements are, and continue to be, extremely effec- tive in partially restoring the joint surface but leave a lot to be desired for younger or more active patients. With the current focus on the health benefits of maintaining an active lifestyle, long-term restricted mobility and function is increasingly unac- ceptable. In recent years new techniques have emerged that offer younger, active patients a reasonable alternative.

WHAT ARE THE OPTIONS? The ultimate goal of any cartilage repair procedure is to restore the surface to ‘nor- mal’ biological hyaline cartilage. This quest has led to the development of a variety of surgical procedures including arthroscopic debridement and lavage; perichondrial arthroblasty; marrow stimulation tech- niques such as Pridie drilling and microfracture; and osteochondral auto- grafting and allografting (9).

It is critical to match the cartilage repair procedure to individual patient demands and several algorthims have been devel- oped to provide a clearer basis for treat- ment selection in the management of artic- ular cartilage defects. The basis of such parameters include age, activity level (work, lifestyle and sports), size and loca- tion of defect, prior surgical procedures and duration of symptoms (7,9).

Clinical research studies report that many of these cartilage repair procedures do provide a reduction in symptoms and restoration of function but as the replace- ment tissue is commonly fibrocartilage the long-term durability of the repair is ques- tionable, especially in the high demand population (12,13).

AUTOLOGOUS CHONDROCYTE IMPLANTATION The emergence and development of thera- peutic tissue engineering techniques over the last two decades has provided the opportunity for a viable cell-based cartilage repair known as autologous chondrocyte implantation (ACI). ACI is one of the first clinically available orthopaedic biotech- nologies. Introduced by Professor Lars Peterson in the 1980s with the first results published in 1994, ACI has become increas- ingly popular with over 12,000 procedures to date. It is currently the most widely researched cartilage repair technique (15).

The first generation of ACI procedures utilised a periosteal flap that acted as a membrane under which chondrocytes are implanted (14). These periosteal ACIs are still used clinically with good results but excision of the periosteum from the tibia is an extra invasive procedure. Second gener- ation ACI procedures still necessitate two surgical stages but uses a collagen mem-

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