Influence of plate working length on fatigue life in load bearing osteosynthesis constructs: Experimental insights...

Fatigue failure of osteosynthesis plates in load bearing constructs remains a significant clinical challenge, with plate working length (PWL) influencing stress distribution and implant life span. Despite conflicting evidence on PWL's impact, finite element (FE) models offer potential for predicting fatigue life, yet their application to PWL-specific fatigue in bone-plate constructs is limited.

This study investigated the effect of PWL on fatigue life in load bearing constructs using experimental cyclic testing and FE modeling. Synthetic bone models with a 10 mm osteotomy gap were stabilized with 3.5 mm stainless steel locking compression plates, testing short (1 empty hole), medium (3 empty holes), and long (5 empty holes) PWL configurations (N = 6 per group) under sinusoidal loading (260 N peak, 3 Hz). A second sub-study assessed the medium PWL across nine load levels (220-380 N). FE models, validated against experimental force-displacement curves, predicted cycles to failure using Basquin's stress-based criteria. Statistical analyses compared experimental and FE-predicted cycles.

Shorter PWL significantly increased fatigue life (short: 1.19 × 10⁶ ± 0.28 × 10⁶ cycles; medium: 0.35 × 10⁶ ± 0.07 × 10⁶; long: 0.20 × 10⁶ ± 0.04 × 10⁶; p < 0.003). FE predictions closely matched experimental cycles for medium and long PWL (p > 0.05) but underpredicted for short PWL (p = 0.03), likely due to tied interface assumptions. Most short PWL constructs survived beyond 10⁶ cycles, reaching up to 1.5 million cycles in the very high-cycle fatigue regime without failing, where Basquin's accuracy may decrease. Sub-study 2 showed a strong load-life correlation (R² = 0.96), with FE predictions achieving high accuracy (CCC = 0.972, REE = 6.3 %).

Shorter PWL enhances fatigue life in load bearing constructs by reducing plate stress, challenging traditional beliefs favoring longer PWL. FE models effectively predict fatigue life for medium and long PWL, supporting preoperative optimization, but require refinement for short PWL, including frictional contact modeling and alternative fatigue models for very high-cycle fatigue. Validation in physiological conditions is needed to enhance clinical applicability.