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Articular Cartilage Injury of the Knee
Overview
Articular cartilage is the smooth, gliding, cushioning tissue that lines the ends of bones where they contact one another in a joint. Cartilage is quite remarkable tissue due to its low-friction surface and compressibility. And while it is quite durable considering the repetitive stresses it endures daily, it is also, at the same time, delicate and susceptible to injury. Cartilage or “chondral” injury can occur due to direct impact, a twisting injury, or repetitive wear to the joint surface. Chondral lesions may occur with or without other injuries in the knee, such as ligament tears.
Cartilage has minimal inherent healing capacity on its own. Once the cartilage is damaged, it typically does not repair itself. Surgeons can, however, surgically restore cartilage within lesions that are bordered by healthy tissue. Symptoms that may prompt treatment are joint pain, swelling, catching or locking. Cartilage restoration procedures include marrow stimulation, autogenous osteochondral graft transfer, autologous chondrocyte reimplantation and osteochondral allograft implantation. Since these procedures are indicated for isolated chondral lesions and not for diffuse degenerative cartilage disease throughout the joint, it is vital to perform a complete preoperative evaluation to ensure the patient is a suitable candidate for this approach versus orthobiologic injection therapy or joint replacement. A proper assessment usually includes weight-bearing X-rays and MRI. Sometimes the evaluation and treatment regimen is staged, especially with more complicated cases. The surgeon performs a diagnostic arthroscopy first to assess the chondral damage with direct visualization. Initial debridement can be performed, the exact dimensions of the chondral lesions documented, and the condition of the other cartilage surfaces inspected. Then, after formulating a comprehensive surgical plan, the surgeon can perform the definitive reconstructive procedure.
When performing cartilage restoration, it is critical to consider all other pathologic issues in the joint aside from just the cartilage lesion to ensure the best possible result. If there is lower limb malalignment, the surgeon may need to add a corrective osteotomy to the procedure. Suppose there is abnormal joint laxity due to one or more injured ligaments. In that case, the surgeon will need to add ligament reconstruction to stabilize the knee. When necessary, incorporating these concomitant procedures will protect the cartilage repair and maximize the likelihood of a successful outcome.
Cartilage restoration can be an effective therapy in appropriately selected patients to relieve symptoms, restore function, and possibly reduce the risk and severity of cartilage disease progression.
There are four main approaches to cartilage restoration. Each has its strengths and weaknesses, so it is crucial to understand when selecting the best method for a particular application.
Marrow stimulation techniques involve drilling small holes into the bone at the base of a cartilage lesion. This allows blood and bone marrow cells to seep out of the bone, creating a blood clot that forms repair tissue. Marrow stimulation methods include microfracture, abrasion arthroplasty, and subchondral drilling. The downside to marrow stimulation is that it results in fibrocartilage, a type of scar tissue inferior to the native hyaline tissue, making up normal articular cartilage. Fibrocartilage is not nearly as durable as normal hyaline cartilage and tends to degrade prematurely.
Autogenous osteochondral graft transfer involves harvesting healthy bone and cartilage plugs from one part of the knee and transferring them into sockets placed in an area of cartilage injury. The surgeon will take these plugs from “lower contact” areas of the articular surface where, in theory, the joint can spare them to resurface a “higher contact” area of injury where healthy tissue is needed. Advantages to this technique are that normal hyaline cartilage is restored, and bone-to-bone healing occurs relatively rapidly. The downsides are that the healthy area from where the plugs are harvested may be compromised, which could have adverse consequences later, leading to degenerative joint disease. There is also a limit to the chondral lesion size that can be treated since the amount of donor tissue is finite. This procedure is best suited for small to medium size lesions only.
Autologous chondrocyte reimplantation (ACI) is a cartilage regeneration process where the surgeon harvests a small amount of healthy cartilage tissue from a patient’s knee during an arthroscopic procedure. This cartilage biopsy is then sent to a laboratory where the cartilage-producing cells (chondrocytes) are extracted and then cultured to produce many clones of those cells. The cloned cells are then implanted back into the patient’s knee during a second surgery and sealed off within the chondral lesion under a watertight membrane. The original ACI procedure has now been supplanted by the next-generation matrix-induced autologous chondrocyte reimplantation (MACI), which involves using autologous cultured chondrocytes embedded into a collagen membrane. Instead of injecting a fluid suspension of the cultured chondrocytes underneath a thin collagen membrane as with ACI, MACI involves implanting a collagen matrix impregnated with the patient’s expanded chondrocytes into the base of the chondral defect. The patient’s cloned chondrocytes then produce new cartilage that fills the defect. The advantages of MACI are that it results in a true hyaline cartilage repair, and it is well-suited to treat large and multiple defects. The downsides are that it always requires two surgeries, laboratory costs are expensive, and obtaining insurance coverage can be challenging in some cases.
Allograft osteochondral implantation is a method of restoring cartilage and bone using material from a tissue donor. Surgeons can use both fresh and cryopreserved osteochondral allografts for cartilage restoration and regeneration. The advantages of using allograft tissue are that they can address virtually any size lesion. This is helpful particularly for extensive defects and for cartilage lesions associated with significant bone loss. Fresh allografts are ideal for complex structural defects, but the timing of surgery depends on the availability of suitable donor tissue. For more moderate-sized lesions, the surgeon may opt for cryopreserved allograft tissue that they can use without any timing restrictions. In the past, osteochondral allografts did not perform as well as patient’s own autogenous tissue, but newer processing techniques utilized nowadays have improved the reliability of allografts considerably. A significant upside to using osteochondral allografts is that it restores actual hyaline cartilage already anchored to its underlying bone.
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