OTA 1998 Posters

Modified Tibial Nails for Treating Distal Tibia Fractures

James McKale, BS; John T. Gorczyca, MD; Kevin Pugh, MD; David Pienkowski, PhD, University of Kentucky Medical Center, Lexington, KY

Purpose: To determine if cutting 1 cm off the tip of a tibial nail will extend the indications for intramedullary nailing to tibia fractures located 4 cm from the tibiotalar joint.

Methods: Five matched pairs of fresh-frozen cadaveric tibia were randomized to have osteotomies made either 4 cm or 5 cm from the tibiotalar articular surface in order to resemble OTA Type 43 -A1 fractures. Standard 9 mm diameter intramedullary nailing was performed to stabilize the 5 cm distal tibias. Intramedullary nailing with a 9 mm diameter nail that had been modified by removing the distal 1 cm was performed to stabilize the 4 cm distal tibias. Two distal interlocking screws were used in all specimens.

The distal tibias and the proximal end of the nails were then potted separately, inserted in a servohydraulic materials testing machine (Instron 8521) and tested in compression, torsion, and compression-bending. Compression to 1000 N at a rate of 100 N/sec was performed five times for each tibia. Torsional load was applied at a rate of 1 N-m/sec to a maximum of 10 N-m five times for each tibia. Compression-bending testing was performed by repeating compression testing with the load offset 4.0 cm medial to the long axis of the nail. Calculation of stiffness was performed by determining the slope of the load-deformation curve.

Results: Compression stiffness was 4159 N/mm with the modified tibial nails and 3997 N/mm with the standard nails (NS). Torsional stiffness was 2.207 N-m/deg with the modified nails and 2.221 N-m/deg with the standard nail (NS). After the first compression bending test, it became evident that the stability of fixation had decreased and gross loosening of the nail in the coronal plane had occurred. Thus, a single test to failure was performed for each specimen. Compression-bending stiffness was 81.8 N/mm with the modified nails and 86.9 N/mm with the standard nails (NS).

We were unable to determine a failure point with compression-bending as the bending stiffness gradually decreased without a clear point of failure. Analysis of the specimens after testing revealed that the failure occurred by movement of the nail in the cancellous bone, by movement of the nail on the interlocking screws, and by movement of the interlocking screws through the bone.

Discussion and Conclusions: Intramedullary nailing is an accepted treatment for tibia fractures located 5 cm from the tibiotalar joint. Some surgeons cut the distal 1 cm off the nail in order to allow placement of two distal interlocking screws in fractures located 4 cm from the tibiotalar joint. The results of this study show that 4 cm distal tibia fragments stabilized with modified nails have comparable stiffness in compression and in torsion to 5 cm distal tibia fragments stabilized with standard tibial nails. Moreover, the stiffness in compression-bending was surprisingly low in both groups and only differed by 5.8%. Sample-size calculations (a = 0.05, b = 0.80) have shown that it would take 152 cadaveric tibias to demonstrate that this difference is statistically significant.

Failure in compression-bending occurred surprisingly quickly and resulted in movement and loosening of the nails and screws in the distal tibias.

Thus, removal of 1 cm from the tip of a tibial nail will allow placement of two distal interlocking screws in tibial fractures located 4 cm from the tibiotalar joint. It should be recognized, however, that intramedullary nailing of tibia fractures located 4 or 5 cm from the tibiotalar joint provides inadequate stability to compression-bending loads. Unrestricted weight-bearing before significant fracture healing occurs may therefore cause coronal plane malalignment of the fracture.