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The Swiss Cheese Effect: A Biomechanical Study of 5th Metacarpal Structural Integrity after Multiple K-wire Passes
Ryan James Bickley, MD
1; Gabriel Yohe, MS
2; Pooyan Abbasi, MS
3; Aviram M. Giladi, MD, MS
31Tripler Army Medical Center, Tripler AMC, HI; 2The Curtis National Hand Center, MedStar Union Memorial Hospital, Baltimore, MD; 3The Curtis National Hand Center, Baltimore, MD
IntroductionClosed pinning is a common surgery that requires practice. Fewer passes through a bone are likely superior because theoretically each pass weakens the bone. However, there is no defined threshold for how many extra passes creates a clinically vulnerable hand at risk for fracture. Our study examined how many extra K-wire passes significantly weakens a bone compared to one extra pass. Secondarily, we investigated if a construct with extra passes could withstand early motion.
Materials and MethodsThis biomechanical study with paired cadaveric hands modeled a 5
th carpometacarpal dislocation to test the effect of extra K-wire passes in the 5
th metacarpal. Specimens were prepared with a control group of 1 extra pass and experimental groups with contralateral paired 5
th metacarpals with either 3 parallel or 8 crossing extra passes. All passes were bicortical and located just proximal to the distal metaphyseal-diaphyseal junction. Specimens were prepared with the metacarpals still in the hands and the hands were connected to a jig that pulled the flexor and extensor tendons, opening and closing the hands 2000 times to simulate early motion. Load-to-failure was then measured by cantilever bending with the 5
th metacarpal removed from the hand, potted at its base, and a piston was applied from dorsal to volar centrally in the metacarpal head until fracture occurred. Comparative statistics were performed between groups.
ResultsTwelve pairs were tested. Six pairs had 8 crossing extra passes and six had three parallel extra passes. Specimen ages ranged from 35-88 (average 68). Five of 12 pairs were non-osteopenic/osteoporotic based on second metacarpal cortical percentage. All specimens that underwent cyclic opening and closing survived without fracture, regardless of extra pass count. Load-to-failure testing showed no statistically significant differences between paired specimens with 1 and 3 (97 vs 105 N) and paired specimens with 1 and 8 (212 vs 196 N) extra passes.
ConclusionsWe performed a biomechanical study to identify how many extra K-wire passes in a 5
th metacarpal can occur before the bone is at increased risk of fracture. Even with 8 extra passes, load-to-failure was high and survived 2000 cycles of gripping. A construct with 3 extra passes similarly had a high load-to-failure. Finally, both extra pass groups were not statistically different from their paired control group with 1 extra pass. These results suggest both constructs are likely safe for allow early motion and light gripping.
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