OTA 1996 Posters - Tibia Fractures

A Comparison of Posterior Cortical Strain During Insertion of Reamed and Unreamed Intramedullary Tibial Nails

David J. Hak, MD, David L. Boardman, MD, Gary L. Zohman, MD, Eric E. Johnson, MD, J. Michael Kabo, PhD

University of California at Los Angeles, Los Angeles, CA

Purpose: To compare the strain produced in the posterior tibial cortex during insertion of four different unreamed tibial nails to that produced during insertion of a reamed tibial nail in human cadaveric tibiae.

Materials and Methods: The unreamed nails tested differed in their composition (stainless steel versus titanium) and in their design characteristics (solid versus cannulated, location and angle of proximal bend, distal bend and tip tapering, and cross sectional design). Bone mineral density was determined for each specimen using a Dual Energy X-Ray Absorption scanner (DEXA). Nail insertion was performed with an instrumented hammer to record the impulse to the nail during insertion. Three rosette strain gauges were applied to the posterior cortex at 2 cm axial intervals in the vicinity where the nail first made contact with the posterior cortex. Data was collected at a rate of 83,333 samples per second.

Results: Peak strain values up to 16,400 microstrain were recorded for the maximum principle strain, and up to -11,100 for the minimum principle strain. The reamed nails required substantially lower impulse to achieve complete insertion than the unreamed nails (p = 0.0107). The solid unreamed nails required a higher total impulse for insertion than either the unreamed cannulated nails or the reamed nails but this was not statistically significant (p = 0.070). The maximum strain index was determined by dividing the maximum strain during unreamed nail insertion by the maximum strain during reamed nail insertion for each specimen. The average maximum strain index was lowest in the solid titanium nail and highest for the cannulated stainless steel nail. An adjusted average maximum strain index was calculated by dividing the average maximum strain index by the average bone mineral density. When normalizing for the bone mineral density, the solid titanium nail continued to have the lowest strain index and the cannulated stainless steel nail still had the highest strain index. Cannulated unreamed nails showed higher posterior cortical strains during insertion than solid unreamed nails made of the same material.

Conclusions: Microstrains in the range reported to occur with failure of cortical bone (7,000 to 31,000 microstrain) were achieved during the insertion of both reamed and unreamed nails. Cannulation of unreamed tibial nails is not associated with a lower posterior cortical strain during nail insertion. Surgical technique, especially the entrance site and angle of insertion, is critical to prevent intraoperative fracture of the posterior tibial cortex during nail insertion.