Background and Rationale

The fundamental concept governing fresh osteochondral allografting is the transplantation of architecturally mature hyaline cartilage, with living chondrocytes that survive transplantation and are thus capable of supporting the cartilage matrix.¹

Hyaline cartilage possesses characteristics that make it attractive for transplantation. It is an avascular tissue and therefore does not require a blood supply, meeting its metabolic needs through diffusion from synovial fluid. Second, it is an aneural structure and does not require innervation for function. Third, articular cartilage is relatively immunoprivileged, as the chondrocytes are imbedded within a matrix and are relatively protected from host immune surveillance.

The second component of the osteochondral allograft is the osseous portion. This functions generally as a support for the articular cartilage, as well as a vehicle to allow attachment and fixation of the graft to the host. The osseous portion of the graft is quite different from the hyaline portion, as it is a vascularized tissue, and cells are not thought to survive transplantation; rather, the osseous structure functions as a scaffold for healing to the host by creeping substitution (similar to other types of bone graft).

Generally, the osseous portion of the graft is limited to a few millimeters. It is helpful to consider a fresh osteochondral allograft as a composite graft of both bone and cartilage, with a living mature hyaline cartilage portion and a nonliving subchondral bone portion. It is also helpful to understand the allografting procedure in the context of a tissue or organ transplantation, as the graft essentially is transplanted as an intact structural and functional unit replacing a diseased or absent component in the recipient joint.

The transplantation of mature hyaline cartilage obviates the need to rely on techniques that induce cells to form cartilage tissue, which are central to other restorative procedures

Indications

Fresh osteochondral allografts possess the ability to restore a wide spectrum of chondral and osteochondral pathology. As a result, the clinical indications cover a broad range of pathology. In addition to evaluating the particular lesion, the careful assessment of the entire joint and limb is important.

Many proposed treatment algorithms suggest the use of allografts for large lesions (>2 or 3 cm) or for salvage in difficult reconstructive situations. In our experience, allografts can be considered as a primary treatment option for osteochondral lesions >2 cm in diameter, as is typically seen in osteochondritis dissecans and osteonecrosis (Fig. 7-1).

FIGURE 7-1 Preoperative anteroposterior radiograph demonstrating osteochondral lesion of the medial femoral condyle.

Allografts are useful as a revision cartilage restoration procedure when other cartilage treatments, such as microfracture, osteochondral autologous transfer, or autologous chondrocyte implantation, have been unsuccessful. Allografts are also indicated for salvage reconstruction of posttraumatic defects of the tibial plateau, patella, or the femoral condyle.

In selected cases, allografts can be used to treat more severe disease situations such as unicompartmental arthrosis.

Graft Sizing

The surgical technique for fresh osteochondral allografting depends on the joint and surface to be grafted. Common to all fresh allografting procedures is matching the donor with recipient.^2,3. This is done on the basis of size.

In the knee, an anteroposterior (AP) radiograph with a magnification marker is used, and a measurement of the medial-lateral dimension of the tibia is made and corrected for magnification. Some surgeons may prefer to use measurements based on magnetic resonance imaging (MRI) or computed tomography (CT) images, but we have not found this to be more useful than plain radiographs.

The tissue bank makes a direct measurement on the donor tibial plateau. Alternatively, a measurement of the affected femoral condyle can be performed. A match is considered acceptable at ±2 mm; however, it should be noted that there is a significant variability in anatomy, which is not reflected in size measurements. In particular, in treating osteochondritis dissecans, the pathological condyle typically is larger, wider, and flatter; therefore, a larger donor generally should be used.

Decision Making for Dowel or Shell Technique

The two commonly used techniques for the preparation and implantation of osteochondral allografts include the press-fit plug technique and the shell graft technique. Each technique has advantages and disadvantages.

The press-fit plug technique is a similar in principle to autologous osteochondral transfer (OATS). A number of commercially available instruments are available (Fig. 7-2).

FIGURE 7-2 Preoperative anteroposterior radiograph demonstrating osteochondral lesion of the medial femoral condyle.

This technique is optimal for contained condylar lesions between 15 and 35 millimeters in diameter. Fixation is generally not required because of the stability achieved with the press-fit. Disadvantages include the fact that very posterior femoral condyle and tibial plateau lesions are not conducive to the use of a circular coring system and may be more amenable to shell allografts. Additionally, the more ovoid or elongated a lesion is in shape, the more normal cartilage needs to be sacrificed at the recipient site to accommodate the circular donor plug.

Shell grafts are technically more difficult to perform and typically require fixation. However, depending on the technique employed, less normal cartilage may need to be sacrificed.

Surgical Approach

The surgical approach for osteochondral allografting involves an arthrotomy of variable size (depending on the position and dimension of the lesion). Usually patients have been previously operated or are at least fully imaged and the size and location of the lesion(s) are known; otherwise, a diagnostic arthroscopy can be performed before the allografting procedure to confirm adequacy of the available graft or to treat coexisting pathology. It is the responsibility of the surgeon to inspect the graft and to confirm the adequacy of the size match and quality of the allograft tissue before surgery.

The patient is positioned supine with a proximal thigh tourniquet. A leg or foot holder is extremely helpful to position and maintain the knee in between 70° and 120° of flexion. For most femoral condyle lesions, eversion of the patella is not necessary.

A standard midline incision is made and elevated subcutaneously, depending on the location of the lesion (either medial or lateral) and the joint entered by incising the fat pad and retinaculum without disrupting the anterior horn of the meniscus or damaging the articular surface.

In some cases, where the lesion is posterior or very large, the meniscus must be detached and reflected; and generally, this can be done safely, leaving a small cuff of tissue adjacent to the anterior attachment of the meniscus.

Once the joint capsule and synovium have been incised and retractors carefully placed, the knee is brought to a degree of flexion that presents the lesion into the arthrotomy site.

Extending the arthrotomy proximal or distal may be necessary to mobilize the extensor mechanism.

Once the joint capsule and synovium have been incised and the joint has been entered, retractors are placed medially and laterally to expose the condyle. Care is taken for the positioning of the retractor within the notch to protect the cruciate ligaments and articular cartilage.

The knee is then flexed or extended until the proper degree of flexion is noted that presents the lesion into the arthrotomy site (Fig. 7-3).

FIGURE 7-3 Intraoperative view of the exposed osteochondritis dissecans lesion. Note the patella retractor within the femoral notch.

Lesion Inspection and Preparation

The lesion is inspected and palpated with a probe to determine the extent, margins, and maximum size. The size of the proposed graft is then determined, utilizing sizing dowels (Fig. 7-4). If the lesion falls between two sizes it is generally preferred to start with the smaller size. At this point the surgeon should also determine if the allograft tissue is adequate in dimension (usually diameter) to harvest the proposed allograft plug (this becomes critical in grafts 25 mm or greater).

FIGURE 7-4 Sizing of the lesion and determination of the location of guide pin placement.