Session IX - Tibia

Saturday, October 10, 1998 Session IX, 11:54 a.m.

The Fatigue Life of Tibia Nail Distal Interlocking Screws: An Analysis of Six Manufacturers

Daniel Segina, MD; Edward Humbert, DO; Robert Ostrum, MD; Alan Litsky, MD, ScD; Ajay Seth, BS, Ohio State University Hospitals, Columbus, OH

The indications for the use of intramedullary nails in the management of open, unstable fractures of the tibia have been significantly expanded recently with the introduction of nonreamed nails. The absence of reaming has necessitated a decrease in nail diameter and subsequent loss of mechanical strength, especially with the interlocking screws. The distal interlocking screws for both large-(10-11mm) and small-(8-8.25mm) diameter tibial nails from six different manufacturers were tested in a simple 3­point bending model with each nail loaded through its corresponding nail. Fatigue curves were generated for each screw tested, and the fatigue life for each screw was determined at one million cycles. Statistical comparisons were made between the large- and small-diameter nail interlocking screws for a single manufacturer and between each manufacturer. Significant differences in fatigue life were seen when small- and large-diameter interlocking screws, for an individual manufacturer, were compared; Synthes (p=0.001), DePuy/Ace (p=0.041), Howmedica (p=0.005), Richards (p=0.023). A statistical difference was not seen in the Zimmer (p=0.748) or Biomet (p=0.116) screws. Analysis of the smaller interlocking screws among all six manufacturers, showed no statistically significant difference in fatigue life. Analysis of the larger interlocking screws from all six manufacturers showed only Howmedica interlocking screws, designed with a smooth shaft at the nail / screw interface, had significantly greater fatigue life (p < 0.05). For the smaller diameter nails, there appears to be no significant advantage, based on screw fatigue life, in choosing one manufacturer over another. Based on our data, significant differences exist between smaller and larger interlocking screws in four out of six manufacturers. Tibia fractures prone to nonunion that have been treated with smaller diameter nails should have a protocol that addresses the potential for screw failure as well as the biology of the injury. In addition, screw geometry can significantly affect fatigue life.