The Swiss Cheese Effect: A Biomechanical Study of Fifth Metacarpal Structural Integrity After Multiple Kirschner Wire Passes.

Purpose: Percutaneous pinning is common in hand surgery. There is no defined threshold for how many extra passes create a clinically vulnerable bone at risk for fracture. Our study examined whether extra Kirschner wire (K-wire) passes meaningfully weaken the fifth metacarpal. Methods: Paired cadaveric hands modeling a fifth carpometacarpal dislocation were used. Specimens were prepared with a control group of one extra pass and experimental groups consisting of the contralateral paired fifth metacarpals with either three parallel or eight crossing extra passes. The ability to withstand simulated grip cycles and load-to-failure was measured. Comparative statistics were performed between groups. Results: Twelve pairs were tested. Six experimental limbs had eight crossing extra passes, and six had three parallel extra passes. Specimen age ranged from 35 to 88 (mean 68) years. Five of 12 pairs were nonosteopenic/osteoporotic based on the 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 or clinically significant differences between paired specimens with one versus three (97 vs 105 N) or paired specimens with one versus eight (212 vs 196 N) extra passes (all Conclusions: We performed a biomechanical study to identify how many extra K-wire passes in a fifth metacarpal can occur before the bone is weakened to the point of potential fracture. Load-to-failure was high with eight extra passes also withstanding 2000 cycles of simulated gripping. A construct with three extra passes similarly had high load-to-failure. Finally, both were not meaningfully different from their paired control group with one extra pass. These results suggest both situations are likely safe for early motion and light gripping. Clinical relevance: This study shows evidence that multiple K-wire passes during the reduction of a carpometacarpal joint dislocation may not lead to increased hardware failure or future fracture.