Biomechanics of interlocked nailing in humeral shaft fractures. Comparison of 2 nail systems and the effect of interfragmentary compression with the unreamed humeral nail

Abstract In this biomechanical study the implanted Unreamed Humeral Nail (UHN) has been tested concerning bending and torsional stiffnesses. In literature other intramedullary implants have been criticized for insufficient rotatory stability especially in transverse and short oblique fractures of the humeral shaft. This study examined, whether the implanted UHN, as well as the UHN implanted with interfragmentary compression through a specific compression device, is able to augment torsional stiffness significantly. To evaluate bending and torsional stiffnesses, the UHN has been compared biomechanically to the Russell-Taylor humeral nail (RT) in paired mid-shaft osteotomized cadaveric humeri. Identic paired comparison has been performed with the UHN without and UHN with interfragmentary compression. In anterior-posterior, as well as medio-lateral direction stiffness under four-point-bending is significantly higher in stabilizing with the RT. Under torsional loading with moments of 4 Nm, 6 Nm and 8 Nm the UHN reached more than the double torsional stiffness. The RT, which is only dynamically interlocked, owns a high initial "play" between bolts and nail itself. Through additional interfragmentary compression stiffness of the UHN under four-point-bending in anterior-posterior, as well as medio-lateral direction augments significantly. Also under torsional loading with moments of 4 Nm, 6 Nm und 8 Nm torsional stiffness increases with interfragmentary compression significantly. In comparison to other biomechanical studies of different authorship it is clear, that this statically interlocked intramedullary nailing of the humeral shaft is superior to non-statically interlocked types of nailing concerning their stabilizing potency in torsion and serves especially for fracture types, which are critically under rotation, as transverse or short oblique humeral shaft fractures.