Osteochondral Allografts

Chapter 7 Osteochondral Allografts




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






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).



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Jul 12, 2016 | Posted by in RHEUMATOLOGY | Comments Off on Osteochondral Allografts

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