A Study of Retrieved Allografts Used for ACL Surgery
A Study of Retrieved Allografts Used for ACL Surgery
Abstract & Commentary
Synopsis: Analysis of retrieved ACL allografts indicates that remodeling of ACL allografts is a slow process requiring years, not months, for complete remodeling with cellular replacement.
Source: Malinin TI, et al. A study of retrieved allografts used to replace anterior cruciate ligaments. Arthroscopy. 2002;18: 163-170.
Malinin and colleagues report on their experience of grossly and histologically analyzing 10 human ACL grafts (1 autograft and 9 allografts) retrieved from 20 days to 10 years following implantation. Two of the specimens were osteochrondral allografts used to replace bone, cartilage, and ligaments following wide resection of the lower femur for tumor. Two specimens were cryofrozen Achilles allografts, and the remainder were cryofrozen bone-patellar tendon-bone allografts (BPTB). Four specimens were obtained at the time of total joint replacement, 2 were obtained at the time of revision surgery for bone tumors, 2 were obtained at autopsy, 1 specimen was harvested following graft ligament rupture secondary to trauma, and the tenth specimen, the autograft, was obtained at the time the patient underwent wide local resection for a malignant tumor.
A 20-day-old cryofrozen BPTB autograft was 1 of 2 autopsy specimens obtained. It was retrieved from a 37-year-old male who died of a pulmonary embolus. On staining, the graft contained "eosinophilic connective tissue strands devoid of cellular detail." The margins of the graft in the bone tunnels "had embedded in them numerous, nonviable bone fragments" surrounded by hemorrhage and granulation tissue with some fibroblastic ingrowth.
The second autopsy specimen was a BPTB allograft from a 31-year-old male who died of a gunshot wound 2 years following ACL reconstruction. On physical examination at 14 months following the reconstruction, this patient was reported to have had a negative pivot and a negative Lachman examination. In cross-sectional cuts of this man’s knee, the transplanted ACL compared to the patient’s adjacent normal PCL, showed fibroblast-like cells penetrating the graft to approximately one third the graft diameter from the surrounding vascularized tissue and synovium. "However, in the central portion, the allograft remained largely acellular. . . ." In tibial and femoral bone tunnels where the graft had been fixed with interference screws, the allograft bone was not distinguishable from surrounding recipient cancellous bone. The central portion of the allograft tendon found in bone tunnels consisted of acellular collagen bundles with only spotty vascularization in the periphery, and sparse ingrowth of fibroblasts.
A BPTB autograft, obtained from a 29-year-old man who underwent wide extra-articular excision for a periosteal osteosarcoma 2 years following ACL reconstruction, was reported to be "similar in appearance" to the allografts with a large portion of the graft remaining acellular.
Comment by Letha Y. Griffin, MD, PhD
Although the histology of transplanted grafts has been detailed for animal models, this study illustrates the difficulty in trying to define the histology of transplanted ligaments in humans. This series consists of varieties of graft types (Achilles tendon allograft, BPTB allograft, and BPTB autograft), preserved in several ways (frozen, freeze-dried, and cryofrozen) with little information on the status of the knee’s stability prior to obtaining the specimen or for that matter, on the operative procedures at the time of specimen implantation including the appropriateness of tunnel placement. Moreover, in this particular series, 4 patients whose specimens were obtained at time of joint replacement were approximately 40 years or older at the time of their ACL reconstruction. What type of rehabilitation was used following reconstruction and how compliant the patients were with this is not known. Such variables can influence graft incorporation and remodeling.
Malinin et al do emphasize that cryofrozen grafts are different than cryopreserved grafts. Cryopreservation preserves viable cells in the freezing process and, therefore, those grafts are more antigenic. The difference in immune response may play a role in graft incorporation. All of the specimens in this study were cryofrozen so that immune rejection was not an issue. Despite the many limitations of a study such as this, the outstanding message is that the time for ACL graft remodeling is measured in years, not months. Perhaps, in the future, growth factors to augment the rate of graft incorporation and remodeling will be available.
Dr. Griffin, Adjunct and Clinical Faculty, Department of Kinesiology and Health, Georgia State University, Atlanta, GA, is Associate Editor of Sports Medicine Reports.
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