Rotational stability of Seidel nail distal locking mechanism.

The intramedullary distal locking mechanism in the Seidel nail uses interference fit, avoiding the need of a screw traversing soft tissue and the risk of neurovascular injury. However, there is concern for its rotational stability. This study was designed to test the rotational stability of the distal locking mechanism under the least stable conditions. A transverse osteotomy was performed 10 cm proximal to the trochlea in 10 cadaveric humeri, followed by insertion of a 9 mm diameter Seidel nail and distally locked. The proximal humeral segments were removed. Rotational test to failure was performed with a material testing machine. The initial medullary diameter and the amount of flange spread measured 9.95 mm and 11.28 mm, respectively. Most of the flanges were found to be incompletely deployed. Failure occurred by slippage of the flanges. The average failure torque and torsional stiffness of the nail bone construct tested were 0.262 N m and 0.070 N m/ degrees . The Seidel nail distal locking mechanism has low rotational stability on mechanical testing. However, the low failure torque tested in vitro does not always preclude successful fracture healing and a good clinical outcome. Whether the presence of rotational micro motion would be beneficial to bone healing would need further investigation.