Mechanical effects of off-axis insertion of locking screws: should we do it?

OBJECTIVES: The purposes of this study were to evaluate the cantilevered bending strength and failure modes of locking screws inserted at various angles in a plate with fully circumferential threaded holes. As an additional measure, the amount of screw head prominence at these angles was also assessed. METHODS: Standard 3.5-mm locking screws were inserted into round fully circumferential threaded holes through a standard straight 3.5-mm locking plate at various angles. The achieved angle of insertion and its prominence protruding from the far-bone side of the plate was measured using an optical luminescence technique. Each screw was then loaded at a constant rate until failure in a cantilevered bending scenario. The maximum cantilevered bending strength was measured, and the moment at failure was calculated. RESULTS: There was a positive correlation between increasing insertion angle and increasing prominence; a higher screw insertion angle yielded greater prominence. Prominence values ranged from negligible to 2 mm. As screw insertion angle increased, the bending moment at failure decreased. Screws inserted to 3 degrees or below primarily failed through screw deformation at the minor diameter below the head, whereas screws inserted to greater than 3 degrees primarily failed through locking mechanism disengagement. CONCLUSIONS: These findings indicate that cross threading may not be biomechanically advantageous and may change screw mode of failure. Based on these findings, screws inserted to 3 degrees or higher would reduce the bending moment at failure to approximately 50% of an orthogonally inserted screw.

Can we trust ex vivo mechanical testing of fresh--frozen cadaveric specimens? The effect of postfreezing delays.

BACKGROUND: Because embalming has been demonstrated to decrease the mechanical integrity of bone, most investigators favor fresh-frozen specimens for biomechanical evaluation. However, little is known about how the integrity of fresh--frozen specimens may change during biomechanical testing or may be affected by standard practices in testing. OBJECTIVE: The purpose of this study was to evaluate how the time after removal from a freezer may affect the mechanical properties of fresh--frozen diaphyseal bone. METHODS: Matched pairs of nonosteoporotic fresh--frozen human cadaveric femora were thawed before instrumentation with bicortical screws. Matched femora were reserved for either control or delayed use. Each specimen received standard diaphyseal bicortical screws (six or more in each group). At specified time points, screws were axially pulled out following the guidelines of ASTM F543-07. Test groups were stored in air (21 ± 0.5°C) for 16, 50, or 90 hours. In the control group, screws were pulled out at 16 hours, which corresponds to the minimum elapsed time for specimen thawing, instrumentation, potting, and biomechanical test initiation. This represents the baseline mechanical properties of the fresh--frozen bone at the inception of any biomechanical test. The 90-hour group corresponds to the time needed to cycle a construct 300,000 times at a physiological test frequency of 1 Hz. This corresponds approximately to 2 to 4 months of in vivo loading. A midpoint of 50 hours was also tested, representing approximately 180,000 cycles. RESULTS: Failure for all specimens occurred as a result of bone failure at the screw-to-bone interface. There was a decrease in screw pullout strength as exposure time in air increased. The 50-hour test group showed a 9% decrease in screw pullout strength as compared with the 16-hour control group (P = 0.81). However, the 90-hour test group showed a 30% decrease in screw pullout strength as compared with the 16-hour control group (P = 0.04). CONCLUSION: This study indicates that when using fresh-frozen cadaveric bone in biomechanical tests to simulate the orthopaedic clinical setting, specimen exposure time should be considered. The timing of testing should be kept constant between specimens to allow for a proper comparison. Furthermore, for fresh--frozen cadavers, the physical properties of bone may be detrimentally affected in biomechanical testing that exceeds the 50-hour time point after removal from the freezer.

Stress corrosion cracking of an aluminum alloy used in external fixation devices.

Treatment for compound and/or comminuted fractures is frequently accomplished via external fixation. To achieve stability, the compositions of external fixators generally include aluminum alloy components due to their high strength-to-weight ratios. These alloys are particularly susceptible to corrosion in chloride environments. There have been several clinical cases of fixator failure in which corrosion was cited as a potential mechanism. The aim of this study was to evaluate the effects of physiological environments on the corrosion susceptibility of aluminum 7075-T6, since it is used in orthopedic external fixation devices. Electrochemical corrosion curves and alternate immersion stress corrosion cracking tests indicated aluminum 7075-T6 is susceptible to corrosive attack when placed in physiological environments. Pit initiated stress corrosion cracking was the primary form of alloy corrosion, and subsequent fracture, in this study. Anodization of the alloy provided a protective layer, but also caused a decrease in passivity ranges. These data suggest that once the anodization layer is disrupted, accelerated corrosion processes occur.