Fossa trochanterica of the proximal femur in rabbits: An anatomic structure for potential misinterpretation on magnetic resonance images

Background: Magnetic resonance imaging (MRI) has been increasingly used as an investigational tool for assessing the structure and function of animal skeletal disease models. However, to date, MRI tomographic knowledge of laboratory animal skeletal anatomy and pathology remains limited. Purpose: To demonstrate that fossa trochanterica of the proximal femur in rabbits can potentially be misinterpreted as a bone lesion on MR images. Material and Methods: Twenty 28-30-week-old male New Zealand white rabbits were used in the study. Corticosteroid-induced osteonecrosis induction was carried out with a combined administration of lipopolysaccharide and methylprednisolone. In this animal model, it is known that osteonecrosis commonly occurs in the proximal femur. MRI at 1.5T was performed before osteonecrosis induction, and at 1 and 2 weeks post-osteonecrosis induction. For anatomical imaging of the proximal femur, coronal sectional images were obtained to align the femoral head, neck, and proximal shaft in a single plane. Volumetric computed tomography (CT) comprised continuous axial acquisitions through the bilateral hip joints and femurs and was performed on one age- and sex-matched New Zealand white rabbit to correlate MRI anatomical findings. Results: In 30% of the acquired normal data sets, a low-signal area surrounded by high-signal bone marrow was observed slightly distal to the femoral head and medial to the third trochanter. This dark, low-signal area resembled an osteonecrosis lesion on MR images. Volumetric CT clarified that the low MR signal was due to the deep fossa trochanterica structure in the rabbit proximal femur. Conclusion: Improved understanding of the cross-sectional anatomy of the fossa trochanterica in the rabbit proximal femur will prevent misinterpretation as a bone lesion on MR images.