Experimental study on stability of locked nailing in humoral shafts

Purpose of the study A clinical trial on the treatment of humeral shaft nonunions with locked na iling evidenced 5 failures among 13 cases. The circumstances leading to the nonunion, the patient's condition, and the nailing method were not found t o have a predominant effect explaining this outcome. Inversely, clinical da ta suggested that abnormal mobility of the nonunion appeared to result from play in the assembly. To check this hypothesis, we measured primary stabil ity in three nailing models using cadaver bones. Materials and methods Three nailing models, Seidel (S), Russel-Taylor (RT) and ACE were tested, e ach on 5 cadaver specimens. A 1 cm segmental resection was made in the mid third of the humerus to simulate an unstable nonunion. The nailing was perf ormed in accordance with the instructions furnished by the manufacturers. T he nailed specimens were placed in a testing device which alternatively app lied a rotation force around the longitudinal axis (+/- 0.5 Nm), an axial c ompression-traction force (+/- 20 N) and a transverse shear force applied a t the level of the osteotomy (+/- 20 N). Results This study demonstrated an instability of the three nails when submitted to a rotation force or a shear force: 14 to 28 degrees and 1.6 to 3.4 mm resp ectively for the RT nail; 8 to 20 degrees and 1 to 3 mm for the S nail; 5 t o 15 degrees and 1.7 to 3.2 mm for the ACE nail. The ACE nail appeared to b e more stable when submitted to compression-traction force; the S nail acce pted a 0.05 to 0.65 mm play which reached 9.7 mm for the RT nail. This inst ability appeared to result from play in the locking systems. Discussion These findings would demonstrate that these nailing systems cannot, in them selves, provide satisfactory primary stability. The experimentally evidence d instability would contribute, probably in association with locally unfavo rable physiological or biological conditions, to the failure rate observed when nailing is used alone. Conclusion The locking system for tested nails would have to be modified to eliminate play in the assembly before continuing their use for the treatment nonunion of the humeral shaft.