EXPERIMENTAL STUDY OF STABILITY OF LOCKED NAILING IN HUMERAL SHAFTS

Abstract

Purpose of the study: A clinical trial on the treatment of humeral shaft nonunions with locked nailing evidenced 5 failures among 13 cases. The circumstances leading to the nonunion, the patient’s condition, and the nailing method were not found to have a predominant effect explaining this outcome. Inversely, clinical data suggested that abnormal mobility of the nonunion appeared to result from play in the assembly. To check this hypothesis, we measured primary stability in three nailing models using cadaver bones.

Materials and methods: Three nailing models, Seidel (S), Russel-Taylor (RT) and ACE were tested, each 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 performed in accordance with the instructions furnished by the manufacturers. The nailed specimens were placed in a testing device which alternatively applied a rotation force around the longitudinal axis (± 0.5 Nm), an axial compression-traction force (± 20 N) and a transverse shear force applied at 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° and 1.6 to 3.4 mm respectively for the RT nail; 8 to 20° and 1 to 3 mm for the S nail; 5 to 15° and 1.7 to 3.2 mm for the ACE nail. The ACE nail appeared to be more stable when submitted to compression-traction force; the S nail accepted a 0.05 to 0.65 mm play which reached 9.7 mm for the RT nail. This instability appeared to result from play in the locking systems.

Discussion: These findings would demonstrate that these nailing systems cannot, in themselves, provide satisfactory primary stability. The experimentally evidenced instability would contribute, probably in association with locally unfavorable 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.