Quantification of In Situ Ulnar Nerve Strain and Constraint Tension in the Forearm

Surgical interventions for ulnar neuropathy are typically identified as releases or decompressions. With the expectation that ulnar nerve strain contributes to cubital tunnel syndrome and with the basic hypothesis that nerve tension will most significantly increase in positions of highest elbow flexion, wrist extension, and classic sites of compression, we initiated a new experimental technique to quantify strain and tension in the ulnar nerve.

In five fresh-frozen cadaveric upper extremities from the spine to the wrist, we percutaneously placed small radiopaque spheres into the ulnar nerve using ultrasound guidance to allow tracking of ulnar nerve motion and elongation under fluoroscopy. This technique caused minimal disruption to the soft tissues tethering the nerve. In a custom jig, the forearm was fixed in space while varying elbow and wrist range of motion. After removal of the hand, we measured nerve motion and the tensions constraining the nerve using proportional techniques with application of loads at four standardized locations between the radiocarpal joint and elbow.

Means of all in situ tensions varied from 0.54 to 4.28 N. There were significant differences in these constraints tensions among the different elbow flexion angles (p = 0.020). There were differences in in situ tension at the sites of constraint but consistent strains across all four sites.

The fluoroscopic technique with percutaneous marker placement successfully allowed strain and tension measurements. The differences in tensions but consistent strains suggest that surgical repair with attention to the attachment of the nerve might restore native nerve gliding and mechanical behavior.