The quality of articular cartilage is fundamental to knee health. Sam Oussedik and Fares Haddad explain.
Articular cartilage covers the ends of long bones. This highly specialised tissue is principally made up of hyaline cartilage secreted by chondrocytes. Because of the poor blood supply at the ends of the bones, these cells work in a low oxygen environment and are vulnerable to injury.
Hyaline cartilage has a complex structure arranged in layers. This allows it to carry out its functions of load-bearing and reducing friction, but it means the task of restoring this structure after injury is particularly difficult.
Damage to articular cartilage has long been linked to a range of symptoms which vary according to the size and depth of the lesion. Articular cartilage has very little capacity for spontaneous repair, although fortunately partial thickness defects are rarely associated with significant symptoms. However, because even the slightest change to the shape of a structure alters the way it transmits forces, damage to any part of the articular cartilage can change its biomechanical properties, leading to further degeneration. Ultimately such injuries may herald post-traumatic arthritis.
The incidence of chondral injuries is difficult to assess. Curl et al (1997) studied the results of a large number of knee arthroscopies (1) and found that 63% of patients had an average of 2.7 articular cartilage injuries per knee examined. The knee is not the only joint to be at risk, with the dome of the talus, the hip and the capitellum at the elbow joint also being common sites of injury.
Articular cartilage may be damaged acutely by excessive shearing forces. Even minor, low-energy trauma can lead to a disruption of the superficial layers of cartilage, which may then initiate a cascade of changes leading to degeneration. Higher energy trauma can lead to fissuring or partial thickness loss of cartilage, and full thickness injuries may result in osteochondral fractures: breaks that propagate through the whole thickness of cartilage to damage the underlying bone.
Patient factors which may predispose to injury include previous trauma, varus or valgus alignment, ligamentous insufficiency and meniscal injury in the knee.
High impact combined with a twisting injury can generate a shearing force at the chondral surface, as one surface impacts against the other.
Patients may present with apparent soft tissue injuries, such as minor ligament sprains. Chondral injury should be suspected in any patient who fails to recover and continues to complain of pain and stiffness in the affected joint for a longer time than would be expected with a simple strain or sprain. Injuries which may be associated with chondral damage, such as meniscal tears or anterior cruciate injuries in the knee, should be ruled out.
X-rays of the joint involved often fail to show any abnormality. Magnetic resonance imaging (MRI) has revolutionised the investigation of suspected chondral injuries, allowing detailed imaging of the articular surface along with the underlying bone. Chondral lesions can be clearly visualised together with injuries to the subchondral bone, such as ‘bone bruising’ – an area of inflammation within the bone itself.
Arthroscopy allows for direct examination of the surface of the lesion, and enables therapeutic work to be carried out.
The severity of damage to the articular surface is commonly graded using the Outerbridge classification (1961)(2), in which defects are rated on a scale of I to IV depending on the surface area involved and the depth of lesion. A grade I lesion denotes superficial injury, while grade IV describes damage that extends down to the underlying bone. There are also more complex modern grading systems such as that introduced by the International Cartilage Research Society.
Physiotherapy plays an important role in both prevention and treatment. Correcting muscular imbalances, such as strengthening vastus medialis obliquus in cases of patellar maltracking, offloads the affected cartilage (helping with pain), and prevents further injury by restoring correct tracking. Post-surgical physiotherapy is also vital to regain optimal joint function.
There are several surgical procedures to treat chondral injuries, ranging from palliation to repair and restoration (3). To date most research has concentrated on lesions within the knee, although techniques may well be transferable to other joints.
The first line of operative treatment for many patients is arthroscopy, during which any loose edges are removed from the chondral lesion, and the joint is thoroughly washed out. This leads to a decrease in mechanical irritation from the loose flaps, and washes out any harmful inflammatory mediators (chemicals released by damaged tissue which increase inflammation). This can provide good relief of symptoms.
For lesions that involve the full thickness of the articular cartilage and the bone beneath it, there are techniques to repair the damage. In addition to trimming back the defect’s loose edges, new repair cells and growth factors can be recruited to the area by the technique of microfracture.
A small pick is used to create tiny fractures in the base of the subchondral bone. This allows blood to enter the defect, and a clot to form. Over time this can organise itself into scar tissue, helping to restore some of the function to the deficient area. As this scar tissue does not share the same highly specialised structure of hyaline cartilage, full function is not restored. But it is often enough to significantly improve symptoms.
Research is focused on attempting to fully restore the missing articular cartilage, in the hope of returning normal function. This involves transplanting the patient’s own articular cartilage to fill the defect, either from a non-articulating part of the joint or grown in the laboratory after harvest of a suitable sample. The resulting tissue can then be implanted into the site of the defect.
Autologous cartilage implantation (ACI) is now widely used in the treatment of full thickness chondral injuries. This technique involves harvesting chondrocytes from the patient, culturing these in the laboratory, and introducing them into the defect.
Initially implants were secured in place by fixing a layer of periosteum over the top. More recent work has explored using synthetic materials to keep implants in place, such as animal-derived collagen. But this approach can create an uneven distribution of chondrocytes, so further work has led to the development of matrix-induced autologous chondrocyte implantation (MACI). Here, the chondrocytes are cultured on a collagen membrane, where they can be evenly distributed, and then the collagen/chondrocyte construct is implanted whole into the defect.
Although MACI would appear to have advantages over the ACI technique, no significant difference has yet been shown (4).
Stem cell research also offers potential treatment options. A sample of the patient’s bone marrow is taken, and the stem cells are isolated. These can then be cultured to expand their numbers. One of the properties of stem cells that makes them attractive is their ability to multiply almost without limit. Thus from a very small sample a large number of chondrocytes can be produced. This expansion allows potentially large defects to be treated. Once cultured, the cells can be delivered in much the same way as discussed above. Early results for this type of treatment are encouraging, although a great deal more work is required before this becomes a viable treatment option.
Chondral injuries are relatively common, and as treatment options improve diagnosis becomes more important. Most partial-thickness injuries are asymptomatic. Referral to an experienced clinician when this type of injury is suspected is vital. MRI scanning will help with diagnosis. Only those injuries which affect the full thickness of cartilage and extend to the underlying bone are suitable for cartilage implantation.
Sam Oussedik is clinical and research fellow in orthopaedics at University College London. His primary interest is football
Fares Haddad BSc MCh (Orth) FRCS (Orth) is a consultant orthopaedic surgeon at University College London Hospital and editorial consultant to Sports Injury Bulletin