The purpose of this study was to assess, morphologically and biochemically, the effect of hyaluronan (HA) on the early repair process of the anterior cruciate ligament (ACL). Following partial bilateral laceration in the midsubstance of the cruciate ligament, a single dose of HA (MW of 3.6 x 10(6] was injected in one knee and saline in the contralateral knee. Postsurgery, the rabbits were allowed normal (nonimmobilized) cage activity, and were killed after 4 (n = 11) and 12 (n = 10) weeks. The ligaments were evaluated by gross morphology and graded according to the degree of repair. We used grades 1,2, and 3 for uncovered, partially covered, and totally covered lacerations, respectively. Five of the HA-treated ligaments at each time studied were completely covered, compared to 0 at 4 weeks, and 1 at 12 weeks in the saline group. Paired evaluations of the lacerated ACLs showed that the HA-treated ligaments received a healing grade higher than the ligaments exposed to saline in 14 of the 21 animals. In the remaining animals, there was no difference between the sides. The repaired tissue of the ACLs was also examined by light and electron microscopy. When compared qualitatively with saline controls, HA-treated ligaments exhibited a more pronounced repair, with an increased angiogenesis and less inflammatory response. Biochemical analysis demonstrated a mean higher value of type III collagen in the HA-treated injured ACL than in saline-treated injured ACL (13.4 +/- 1.1% and 11.0 +/- 0.8%, respectively). This increased synthesis of type III collagen in the HA-treated injured ACL was statistically higher (p less than 0.05) when compared to the saline-treated injured ACL.

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The purpose of this study was to compare the levels of procollagen type I messenger RNA (mRNA) in normal and healing medial collateral ligament (MCL) and anterior cruciate ligament (ACL) in a rabbit model. Our method of injury involved a surgical model with identical partial lacerations in the midsubstance of the MCL and ACL. Paraffin sections of normal ligaments, and ligaments 3, 7, 14, and 28 days postlaceration were studied by in situ hybridization to compare and follow the level of type I procollagen mRNA in the two ligaments. A complementary DNA (cDNA) probe corresponding to alpha 1(I) procollagen mRNA was labeled with [32P]d-CTP. After hybridization, autoradiography, and staining of the sections, the level of procollagen mRNA was assessed by microscopic examination. A higher level of procollagen mRNA was consistently detected in normal MCL than in normal ACL, suggesting higher collagen synthetic activity in the MCL. At the injury sites of the MCL and ACL, the levels of type I procollagen mRNA increased at all post-laceration periods, reaching its highest level at 14 days postsurgery. The MCL healing site had a considerably higher level of procollagen mRNA than the ACL healing site (i.e., injury site) at all postoperative intervals. The results demonstrate that procollagen mRNA levels in MCL tissue are higher than those in ACL tissue under normal conditions, as well as in response to injury. The differences in the procollagen mRNA levels of MCL and ACL may reflect the synthesis of collagen in these tissues, and may contribute to the differences in their healing capacities.

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