Novel Findings in Rodent Pelvic Limb Anatomy

Rodent models are used for a variety of orthopedic research applications; however, rodent anatomy references are mostly artistic drawings of observed anatomy, which are subject to artistic interpretation and variation. While these drawings are useful, new imaging techniques, such as micro‐computed tomography (μCT), can provide more accurate representations of subtle anatomical characteristics. The purpose of this study was to evaluate differences in the hindlimb musculoskeletal anatomy of mice and rats, and to compare the findings to other rodent species. We hypothesized there would be no difference between mouse and rat anatomy, and the boney anatomy would be similar to other domestic species. The tarsi and femora from two adult C57BL/6 mice and two adult Sprague‐Dawley rats were scanned with μCT at 55kV, 145 μA using 6 μm or 16 μm isotropic voxels for mice and rats, respectively. In addition, an intact mouse hindlimb was stained with Lugol's iodine solution and scanned at the aforementioned parameters for visualization of musculature. Three‐dimensional reconstructions were created and boney anatomy was evaluated. General bone anatomy was similar between mice and rats; however, the mouse was identified to have a fused central and second tarsal bone. This fusion is not represented in conventional anatomy manuals of the mouse and was not observed in the rats. Both species have a medial tarsal bone, which to our knowledge is only present in rodents. Preliminary evaluation of the musculature within the mouse shows similarities to structures found in the domestic species, however further characterization of the tendinous structures crossing and attaching within the tarsal region will be completed based on the aforementioned boney findings. Current knowledge suggests that there is no intra‐species variation of tarsal bone anatomy within the mouse, so it is unclear whether the fusion observed is individual or strain variation, or an unidentified anatomical characteristic. The salamander Plethodon cinereus has been reported to have 5 tarsal bone configurations; therefore, variation within the rodent species is possible; however, no tarsal bone variations are identified within the domestic species. Further research is needed to determine if other inbred mouse strains exhibit this tarsal fusion, as well as to compare results to other rodent species, including the New Zealand white rabbit ( Oryctolagus cuniculus ), guinea pig, ( Cavia porcellus ) and nutria ( Myocastor coypus ). Tarsal bone configurations will influence overall joint kinematics, and fusion within the joint may be explained by evolutionary locomotion behaviors for each species. Differences in joint kinematics may impact applicability of various orthopedic research outcomes for translational studies.