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Cement coating of osteoarticular allografts in rats to prevent the immune response

Departments of Orthopaedic Surgery, the University of California, Davis

H. Reynolds, MD

University of California, San Francisco, California

E. Thorson, MD

Departments of Orthopaedic Surgery, the University of California, Davis

J. Gray

University of California, San Francisco, California

E. Thompson, MS

University of California, San Francisco, California

D. Heitter

Departments of Orthopaedic Surgery, the University of California, Davis

Recently, more attention is being paid to the treatment of the articular surface defect in the young, active individual. Resurfacing or "shell" allografts are being considered as one of the alternatives for treatment. The immune response by the host may be the chief obstacle to the successful use of fresh osteoarticular allografts. Since proteins on the surfaces of allograft bone cells are considered to be the principal antigens eliciting the immune response, we hypothesized that mechanical masking of these cells by a biodegradeable blocking agent might prevent the immune response. Osteoarti cular allografts of the distal femur were performed in rats and the cut surfaces of the graft were coated with cement before implantation. In a preliminary experi ment, six biodegradeable cement materials were tried (bone wax, zinc polycarboxylate, collodion, fibrin ad hesive, zinc oxide, and isobutyl-cyanoacrylate). Of these, the zinc oxide cement and isobutyl-cyanoacry late were effective in inhibiting the immune response observed in control rat distal femur allografts.

A second experiment was performed using nine rats with zinc oxide and nine rats with isobutyl-cyanoacry late cements. These were compared with 20 control rats that had allografts without cement coating. Eight een of the control rats (90%) developed an immune response, while only 22% (2/9) of the rats with zinc oxide coating and 33% (3/9) of the rats with isobutyl- cyanoacrylate developed an immune response.

In a third experiment distal femur allografts were performed in 13 rats using only the liquid component of the zinc oxide cement. Forty percent (5/13) of the rats developed an immune response, suggesting that the effect of the zinc oxide preparation (and perhaps also of the isobutyl-cyanoacrylate) might better be ex plained by a chemical alteration of the antigens of the allograft bone cells rather than by a mechanical blocking effect. It might also be explained by a systemic inhibition of the immune response by chemicals in the cement.

These observations in a rat model demonstrate that the immune response can be inhibited by local treat ment of the allograft. The theory needs now to be tested in a larger animal model with an articular surface defect where adequate surgery can be done. Then conclusions can be drawn for the possible practical application to the young, active individual with an artic ular surface defect.