BASIC SCIENCE

Although the efficacy of osteochondral allografts has been shown for many years, disease transmission continues to be the area of greatest concern.³ Unfortunately, methods used to sterilize tissue have significant detrimental effects on osteochondral allografts.⁴ Sterilization methods not only devitalize all the chondrocytes, but also have negative effects on the material properties of the graft. Fortunately, the risk of disease transmission is slight if the tissue is retrieved, handled, and processed in strict accordance with standardized guidelines of the American Association of Tissue Banks (AATB).⁵

Grafts are harvested within 24 hours of the donor’s death to minimize contamination and maintain chondrocyte viability. The grafts are then processed in a clean room environment and thoroughly lavaged to remove blood components, which are the main source of pathogens and immunologic sensitization. After cultures are obtained, the grafts are treated with several antimicrobials and subsequently stored at 4° C until used.

The initial clinical series reported using grafts within 1 week from procurement because it was found that the sooner the graft is implanted, the greater the chance of chondrocyte survival.⁶^,⁷ Although the minimum chondrocyte viability for graft success is unknown, it is clear that this play a vital role in graft integrity.⁸ However, disease testing and safety precautions have resulted in tissue banks generally not releasing grafts for use until about 3 weeks. Fortunately, current storage methods are able to maintain 80% cell viability at 4 weeks.⁹^,¹⁰ In addition, the biomechanical properties of the graft are not statistically affected at that time.¹¹^,¹² However, more recent testing has shown that in commercially available grafts, a large percentage of the viable cells do not exhibit full function.¹³

Because of the limitations of fresh grafts with regard to storage and assurance of sterility, other methods of preservation, including freeze drying and fresh-frozen methods, have been evaluated.¹⁴ Unfortunately, fresh-frozen grafts have no viable chondrocytes. Freeze drying not only destroys all cells but also alters the graft’s material properties.¹⁵

Host immune response is another area of concern with allograft transplantation. It is well known that musculoskeletal allografts are capable of inducing cell-mediated and humeral immune responses in the host.¹⁶^,¹⁷ The predominant mechanism is cell-mediated, and by reducing the number of allogenic cells, the immune response would therefore be reduced. The primary source of allograft cells is the blood and bone marrow elements. The immune load is significantly reduced by removing them during graft processing,.

Chondrocytes can also evoke an immune response and matching the surface antigens has been presented as one method to reduce the load further. However, the limited number of osteochondral allografts available would make it extremely difficult to match the donor to the recipient. Fortunately, any host sensitization has not precluded favorable results. It has been suggested that because chondrocytes are embedded in the dense matrix, which acts as a barrier, intact grafts are considered immunologically privileged. However, there is a consensus that patients with autoimmune disease or inflammatory arthropathies are not appropriate candidates for osteochondral allografts. Although it may be implemented, the morbidity associated with immunosuppression does not justify its use.

PATIENT EVALUATION

History and Physical Examination

Candidates for osteochondral allografts for focal defects typically fall into three categories—osteochondritis dissecans (OCD), post-traumatic lesions, and revision surgery.¹⁸^–²⁰

Making the diagnosis of OCD by history can be difficult. Most patients with OCD have nonspecific joint pain that is insidious in onset. It is usually aggravated with activity and resolves with rest. In the pediatric population, it can often be overlooked and considered to be growing pains. Intermittent swelling may also occur if early symptoms are ignored. As the disease progresses, it may be associated with mechanical symptoms, such as catching and locking and subsequent feeling a loose body. Post-traumatic lesions and revision surgery have an obvious cause to raise awareness of articular cartilage injury. However, the symptoms are typically the same as those of OCD.

Physical examination findings may very widely. In early stages of OCD, the examination may be unremarkable. With any lesion, swelling may be present, ranging from a subtle swelling to a frank effusion. One must remember that any knee swelling in the younger patient without obvious cause should raise the suspicion of OCD, especially in a very active individual.

Pain on deep palpation of the involved area may be present. If the lesion has become partially detached, pain with clicking or popping can be found at a distinct range of pain. If the lesion has become completely separated, mechanical symptoms of a loose body may be seen. Quadriceps atrophy may be present; knowing the degree of atrophy may be helpful if the duration of symptoms is unclear.

Diagnostic Imaging

Image techniques are important not only for diagnosis, but also for treatment planning. Most osteochondral lesions can be seen on plain radiographs and should be the starting point. The initial series should include standing anteroposterior with the leg in full extension, 45-degree weight-bearing posteroanterior (PA; tunnel), lateral, and patellar sunrise views. Typically, a magnification marker is placed on the tunnel and lateral views and used as a reference if graft size matching is needed. Most OCD lesions occur on the lateral aspect of the medial femoral condyle and are most clearly seen on the tunnel view. Lateral femoral OCD lesions tend to be more posterior and are thus not always as obvious.

Plain radiographs are also used for comparison and serial following of the lesions during healing and deterioration. Standing full length films should be obtained once the diagnosis of a chondral lesion is confirmed to evaluate the mechanical axis.

Magnetic resonance imaging (MRI) is the gold standard for evaluating osteochondral and chondral lesions. If the diagnosis is in question, MRI is very sensitive for excluding or confirming pathology. If a defect is seen on plain films, MRI is recommended to determine the extent of the lesion and integrity of the articular cartilage. For example, if fluid is present behind the lesion, this indicates an unstable fragment and the need for surgery.

Bone scans (technetium bone scintigraphy) have been used to determine the extent of activity within the lesion and monitor progress of healing. However, their use has declined because of the advantages of MRI.

TREATMENT

Indications and Contraindications

Because of the risk of infection, limited availability, and cost, as well as consideration of other available treatment options, osteochondral allografts are typically reserved for chondral lesions that are 2 cm² or larger. The cause of the defect, patient age and activity level, concurrent pathology, and rehabilitation requirements are other variables that need to be considered when evaluating a potential candidate.

With regard to cause, isolated OCD and traumatic lesions have the best outcomes. Defects in the presence of diffuse degenerative changes or inflammatory arthropathies are contraindicated. Treatment of lesions secondary to avascular necrosis (AVN) may be appropriate, but only if the involvement is not progressive and the cause is understood. In addition, lesions limited to one joint surface (unipolar) fare much better than those on opposing surfaces (bipolar or kissing lesions).

When evaluating the knee, one must look beyond just the lesion. Ligament instability, absence of the meniscus, and limb malalignment have negative effects on outcome. Ligament reconstruction and meniscal allograft transplantation need to be performed concurrently or in a staged capacity. With respect to the mechanical axis, there is general agreement that if it passes through the involved compartment, realignment should be performed. However, there it is also general agreed that the degree of correction does not need to be as much as when performing an osteotomy for an arthritic knee. Typically, the aim is to pass the mechanical axis through the opposite tibial spine.

Patient age is of some debate, with most reporting an upper age limit of 40 to 45 years; it is thought that these lesions are associated with degenerative changes. However, others have implanted grafts in patients 60 years of age or older. The cause of the lesion is vital when dealing with the age factor. In the uncommon case of a mature patient with an isolated lesion and no other pathology, age alone should not be considered a contraindication. Older patients with low activity demands may be better suited for joint arthroplasty. Other relative contraindications include obesity (body mass index [BMI] >30 kg/m²), smoking, and chronic steroid use. Although sometimes taken for granted, patient expectations also need to be strongly considered. For example, it is not uncommon for patients with large lesions to regard the procedure as one that will enable them to return to unrestricted activities. Patients need to understand that high-impact activities such as distance running and contact sports should be avoided.

Conservative Management

Focal chondral defects are a common finding and can have a significant effect on limiting a patient’s activities and quality of life.²¹^,²² Symptomatic lesions can be treated nonoperatively with modification of activities, anti-inflammatories, viscosupplementation, and rehabilitation. Although these can be of benefit, many patients continue to be symptomatic.

Surgical Treatment

Several surgical options are available in addition to osteochondral allografts, but each has its advantages and disadvantages. The simplest is débridement-chondroplasty, but the benefits are commonly short-lived. Microfracture is technically easy to perform, with limited morbidity, but results in fibrocartilage filling the defect.²³^,²⁴ As a result, the outcomes deteriorate after a few years, and lesions larger than a few centimeters do poorly. Autogenous osteochondral transfer has the benefit of improving normal articular cartilage and being able to fill bone deficiencies.²⁵^,²⁶ However, donor availability is limited and it is recommended for lesions smaller than 2 to 2.5 cm². Autologous chondrocyte implantation (ACI) is often discussed as an alternative to osteochondral allografts. It can also treat large lesions and is recommended for lesions up to 16 cm².²⁷ However, it results in hyaline-like cartilage and should not be used by itself for defects more than 6 to 8 mm deep. In addition, it is technically challenging, with many published studies having a reoperation rate of 30%.