Archives of Orthopaedic and Trauma Surgery - 2026-07-09 - Journal Article
A novel biomechanical model for reproducing and analysing the causal mechanisms of PFNA "cut-in" phenomenon.
Rasappan K, Joshua K, Chou SM, Shaw LKRM, Huang D, Yew A, Kwek EBK
Topics
Key Takeaway
A bidirectional loading model combining axial compression (720 N) and superolateral tensile loading (120 N) reproduced full PFNA helical blade cut-in in 100% of osteoporotic synthetic femurs (n=5) with 18 mm canal diameter.
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Summary
This proof-of-concept study developed a bidirectional loading model to reproduce PFNA helical blade cut-in by simulating Trendelenburg gait mechanics via superolateral tensile loading (120 N) combined with axial compression (720 N) at 2 Hz in AO/OTA 31-A1.1 fractures fixed with PFNA-II. The calibration stage showed partial cut-in was more frequent with 18 mm versus 12 mm intramedullary canal diameter but produced no full cut-in. The final stage using osteoporotic Sawbones specimens with 18 mm canal diameter achieved full cortical perforation in all five specimens.
Key Limitation
The entire definitive conclusion rests on five synthetic specimens, and synthetic bone models do not replicate the heterogeneous trabecular architecture of osteoporotic human femora, limiting direct clinical translation.
Original Abstract
INTRODUCTION
"Cut-in" is a rare but serious complication associated with cephalomedullary nail fixation for intertrochanteric hip fractures, particularly with Proximal Femoral Nail Antirotation (PFNA). This phenomenon involves paradoxical superomedial migration of the helical blade through the femoral head. Although increasingly recognised, the underlying mechanisms remain incompletely defined. This study aims to describe a new superolateral tensile loading set up in a bidirectional loading model, with the aim of incorporating possible mechanical deviations introduced by abductor weakness.
MATERIALS AND METHODS
This study was conducted in two stages. An initial stage with synthetic femurs (SYNBONE ® 2420) was used to calibrate the set up. Within this stage, intramedullary canal diameter was compared (12 mm vs. 18 mm). In the final stage, 5 osteoporotic synthetic femurs (Sawbones ® 3503) were used. In both stages, standardised AO/OTA 31-A1.1 intertrochanteric fracture were created in the synthetic femurs and fixed with PFNA-II implants. All femurs were subjected to cyclical bidirectional loading with a universal testing machine. Each cycle consisted of axial compression (720 N) and superolateral tensile loading (120 N) at 2 Hz. This configuration was designed to simulate altered hip biomechanics postulated with the trendelenburg gait. Testing was continued until mechanical failure, construct deformation, or specimen fracture occurred. Full cut-in was defined as complete perforation of the blade through the femoral head cortex, while partial cut-in was defined as superomedial migration of the blade without perforation of the femoral head cortex.
RESULTS
The initial stage demonstrated partial cut-in more frequently in specimens with larger canal diameters (18 mm), but did not produce any full cut-in. The final stage with an 18 mm canal diameter achieved full cut-in in all five specimens.
CONCLUSIONS
Our bidirectional loading model with superolateral tensile loading reproduced full 'cut-in' in all 5 specimens of the final stage, serving as a proof-of-concept tool. Larger studies are warranted for validation.