Reduction of cortical bone mass and mechanical properties in Col12a1^−/− long bones. (A) Col12a1^−/− mice have smaller diameters and less/thinner cortical bone compared with wild-type controls. 3D micro-CT images from the midshaft of a femur from P30 Col12a1^+/+ and Col12a1^−/− mice. Bars, 0.5 mm. (B) Relative cortical area is significantly decreased in Col12a1^−/− femurs. The relative cortical area was measured by 3D micro-CT analysis as the percentage of mineralized cortical bone area/mineralized tissue bone area normalized to body weight in P30 Col12a1^+/+ (n = 9) and Col12a1^−/− (n = 10) mice. **, P < 0.01. Error bars are mean ± SD. (C–E) The biomechanical properties of the Col12a1^−/− femurs were significantly reduced when compared with wild-type controls. Maximum to load failure (C), bone robustness (D), and bone stiffness (E) were measured in femurs of both Col12a1^+/+ (n = 9) and Col12a1^−/− (n = 10) P30 mice by loading to failure in four-point bending at 0.05 mm/s and were normalized to body weight. Maximum load to failure, bone robustness, and bone stiffness are significantly decreased in Col12a1^−/− mice. TtAr/Le, total cross-sectional area/bone length; N/mm, Newtons per millimeter. P < 0.01.