Biomechanical strain and morphologic changes with age in rat calvarial bone and sutures

Background: The role of calvarial sutures in transmitting biomechanical forces within the head is unclear. Methods: To examine the biomechanical characteristics of sutures, the authors measured bite force changes in rats and tested for alterations in strain across intrafrontal and sagittal sutures and within parietal bone with age. To understand the effects of suture fusion on strain distribution in the head, the authors measured percentage fusion of the intrafrontal sutures with age (n = 6 per age group). The masticatory muscles in anesthetized 9-, 24-, and 70-day old rats (n = 15 per group) were bilaterally stimulated. Stacked delta rosette gauges were fixed across the intrafrontal sutures and sagittal suture, or on the parietal bone. Strain and bite force were measured with a bite force transducer positioned at the incisors. Results: Bite force increased significantly (p < 0.05) with age (9-day old rats, 72.6 +/- 20 gf; 24-day old rats, 707.3 +/- 150 gf; 70-day old rats, 2425.6 +/- 255 gf). Some significant differences were found between the volume and direction of strain among sites and age groups. Compressive strains of 230 mu epsilon on average were found across the intrafrontal sutures at all ages. In contrast, tensile strains less than 180 mu epsilon on average were found across the sagittal sutures of 9- and 24-day old rats, increasing to 940 mu epsilon on average at day 70. Tensile strains in parietal bone tended to be less than 150 mu epsilon. Conclusions: The timing of sutural closure and patterns of transsutural strain do not suggest that strain patterns contribute to initial fusion in the intrafrontal sutures. Differences in strain are likely related to changes in rat skull kinetics with growth, perhaps resulting from fusion of the intrafrontal sutures.