The Brtl/+ mouse is a heterozygous knock-in model for OI type-IV with a Gly349Cys substitution in one COL1A1 allele. While mechanical alteration at whole bone level in these mice is established, little is known about the contribution of bone material properties to OI bone fragility and brittleness. Transverse femoral midshaft sections (30 μm thickness) of 2-month old mice (wild-type n=6; Brtl/+n=6) were studied. Elastic properties were assessed using time-of-flight scanning acoustic microscopy in combination with quantitative backscattered electron imaging (qBEI). Sound velocity (2 μm pixel, 0.125ns time-resolution) and calcium content-derived material density maps were combined to extract dynamic elastic moduli (E-modulus) maps. The measurements were focused on relatively homogenous bone avoiding the third trochanter and excluding regions rich in vascular canals. We focused on bone matrix deposited from endosteal and periosteal surfaces, respectively designated as endosteal region (ER) and periosteal region (PR), and further excluded regions with residual cartilage and woven bone. The analysis revealed that wild-type material density (+2.1%), sound velocity (+6%) and E-modulus (+15%) were all significantly greater in PR than in ER. In contrast, Brtl/+ mice had significantly increased ER sound velocity (+9.4%) and E-modulus (+22%) compared with wild-type ER. This change in ER properties of Brtl/+ bone abolished the normal differences between E-modulus in periosteal and endosteal bone, despite the lower material density (-1.8%) of Brtl/+ ER compared with PR. Our findings indicate that the bone matrix formed from Brtl/+ endosteal osteoblasts has a higher E-modulus than expected despite material density values similar to wild-type. Elastic properties of mineralised collagen are known to depend strongly on fiber orientation, and we found that the lamellar organisation of Brtl/+ bone appeared consistently less distinct under polarized light than in WT femora. Therefore, the unexpected mechanical properties of Brtl/+ bone are most likely linked to alterations in collagenous fibril arrangement.
Disclosure: The authors declared no competing interests.