Biomechanical strain across rat sutures with fusion inhibited by Tgf-β3

Objectives: Occlusal force generation in rats presumably produces significant sutural strains that are altered following fusion. Using Tgf-β3 to delay fusion in interfrontal sutures (IFS), we examined relationships between growth, bite force, and sutural strain in IFS and sagittal sutures (SGS) in a rat model. Methods: 198 nine day-old (P9) rats were divided into four groups: (1) unoperated (2) sham operated, (3) collagen gel only, and (4) collagen gel with Tgf-β3. The latter two groups had the gel placed above IFS and underneath the periosteum. Group 1 animals were sacrificed at P9, and animals from all groups were sacrificed at 15 and 61 days (P24, P70). Immediately prior to sacrifice, masticatory muscles in anesthetized rats were bilaterally stimulated and strain measured with 3-element rosette strain gages affixed across either IFS or SGS, or on parietal bones (PB). Incisor biting forces were measured with a force transducer. Suture patency was analyzed by histology. Results: Control groups 1-3 showed <30% IFS fusion at P24 and <70% fusion at P70. Group 4 averaged <10% fusion at P24 and <30% fusion at P70. Biting forces increased with age in all groups: < 100gf at P9, 450-800gf at P24, and >1700gf at P70. Tensile strains across unfused SGS increased with age, corresponding to changes in bite force. Strains on PB were low (>200με) but also increased with age. Strains across IFS were compressive, and low (>200με). Compressive strain parallel to the suture increased with age in the control groups but not in group 4. Conclusion: Normal fusion in rat IFS occurs in the absence of tensile strain across the suture during occlusal force generation. Delay of IFS fusion does not alter strains across IFS, SGS, or on PB during growth, but may increase compressive strain or shear parallel to IFS.