The main goal of flexor tendon surgery is to restore digital motion by providing tendon healing and preserving tendon gliding. The formation of peritendinous adhesion around the repair site is one of the several adverse events that may prevent the achievement of this goal. The purpose of this study was to investigate the effects of application of hyaluronic acid and amniotic membrane combination on peritendinous adhesion formation and tendon healing after flexor tendon repair in an adult chicken model. Seventy-two adult Leghorn white chickens were used. The deep flexor tendons of the third toes were transected partially in Zone II and immediately repaired with a modified Kessler technique in both legs. The tendons were divided into 4 groups of 36 each according to the type of repair used. In Group 1; simple tendon repair was performed, in Group 2; the repair site was wrapped with amniotic membrane, in Group 3; hyaluronic acid was injected around the repair site, in Group 4; the repair site was wrapped with amniotic membrane and hyaluronic acid was injected inside. The extent and the severity of adhesion formation in the peritendinous region and the healing status of the tendons were macroscopically and histologically evaluated according to the grading scale of “Tang” at 3 and 6 weeks. The least adhesion was observed in tendons treated with the combination of amniotic membrane and hyaluronic acid (p<.05, Mann-Whitney U test). No statistically significant difference was found between the groups in the healing status of the tendons. At 20 weeks, the peritendinous adhesions and the tendon healing were evaluated biomechanically. There was no significant difference in the tensile load required to rupture the repaired tendon between the groups. Peritendinous adhesions were evaluated by measuring the active and passive flexion of the distal interphalangeal, proximal interphalangeal and metacarpophalangeal joints with a goniometer. The range of toe motion at 20 weeks was the smallest in group 1, statistically the larger in groups 2 and 3, and statistically the largest in group 4 (p<.05, Mann-Whitney U test). The range of toe motion was of no statistical difference between group 2 and group 3 (p>.05, Mann-Whitney U test). Various treatment strategies have been used in order to prevent peritendinous adhesions after flexor tendon surgery. One of these methods was to encircle the tendon with some substance, but regardless of the material used this method has failed, for scars forms at the end of these materials. In this study, in order to prevent this scar formation, hyaluronic acid was injected inside after the tendon was wrapped by amniotic membrane. Preliminary investigations suggest that the application of high molecular weight hyaluronic acid in high concentrations around the repaired tendons promotes tendon healing and decreases adhesion formation. It is known that hyaluronic acid reduces scar formation by inhibiting lymphocyte migration, proliferation and chemotaxis, granulocyte phagocytosis and degranulation, and macrophage motility. Furthermore, amniotic membrane prevents the peritendinous adhesions by reducing the effect of beta transforming growth factor in the surgical area. In conclusion, this experimental study shows that the application of hyaluronic acid and amniotic membrane combination around the repair site significantly prevents the peritendinous adhesion without impairment of the tendon healing in the chicken
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