Comparative Anatomy of Rabbit and Human Achilles Tendons with Magnetic Resonance and Ultrasound Imaging

We sought to describe the comparative anatomy of the Achilles tendon in rabbits and humans by using macroscopic observation, magnetic resonance imaging, and ultrasonography. The calcaneus–Achilles tendon–gastrocnemius–soleus complexes from 18 New Zealand white rabbits underwent ultrasound (US) and magnetic resonance (MR) imaging and gross anatomic sectioning; these results were compared with those from a cadaveric gastrocnemius–soleus–Achilles tendon–calcaneus specimen from a 68-y-old human male. The medial and lateral gastrocnemius muscle tendons merged 5.2 ± 0.6 mm proximal to the calcaneal insertion macroscopically, at 93% of their course, different from the gastrocnemius human tendons, which merged at 23% of their overall course. The rabbit flexor digitorum superficialis tendon, corresponding to the flexor digitorum longus tendon in human and comparable in size with the gastrocnemius tendons, was located medial and anterior to the medial gastrocnemius tendon proximally and rotated dorsally and laterally to run posterior to the Achilles tendon–calcaneus insertion. In humans, the flexor digitorum longus tendon tracks posteriorly to the medial malleolus. The soleus muscle and tendon are negligible in the rabbit; these particular comparative anatomic features in the rabbit were confirmed on the MR images. Therefore the rabbit Achilles tendon shows distinctive gross anatomical and MR imaging features that must be considered when using the rabbit as a research model, especially for mechanical testing, or when generalizing results from rabbits to humans.