Strain and excursion of the sciatic nerve in the dog: biomechanical considerations in the development of a clinical test for increased neural mechanosensitivity.

Analysis of human nerve biomechanics has contributed to the validation of clinical tests to diagnose back pain of neural origin. Equivalent clinical tests for dogs would be valuable to differentially diagnose lumbosacral pain. To develop such a test, the increase in strain in the sciatic nerve of dogs and the longitudinal excursion of the nerve in relation to its surrounding structures during hind limb movements were evaluated. A miniature strain gauge was inserted into the sciatic nerve in seven canine cadavers and excursion was measured using a digital calliper. A cumulative increase in strain of 7.2% (+/-2.8%) was observed for the combined movements of hip flexion, stifle extension, hock flexion and digit extension (P<0.0001). Although all components contributed significantly to the increase (P=0.03), the sciatic nerve demonstrated a curvilinear response to increased loading. A 10.0+/-1.0mm excursion was recorded with stifle extension (P=0.002). It was concluded that, the sciatic nerve in dogs accommodates to joint movements by stretching and gliding. This biomechanical analysis has contributed to the development of a clinical test equivalent to the human straight leg raise to evaluate increased mechanosensitivity of the lumbosacral plexus.