A novel biomechanical model for reproducing and analysing the causal mechanisms of PFNA "cut-in" phenomenon.

Authors

Kumaran Rasappan, K Joshua, Siaw Meng Chou, Leanne Kayla Rebecca Mei-Yi Shaw, Daran Huang, Andy Yew, Ernest Beng Kee Kwek

Published in

Archives of orthopaedic and trauma surgery. Volume 146. Issue 1. Jul 09, 2026. Epub Jul 09, 2026.

Abstract

"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.
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.
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.
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.

PMID:
42426490
Bibliographic data and abstract were imported from PubMed on 10 Jul 2026.

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