# Damage-Based Assessment of the Risk of Cut-Out in
## 1 Introduction  
Screw or blade cut-out is the most relevant mechanical complication following proximal femoral fracture fixation [1; 2; 3; 4; 5]. Amongst the factors that may affect the risk of cut-out, the blade position is reportedly one of the most relevant [4]. Many clinical studies have evaluated the blade position immediately after surgery and after a follow-up period to identify statistical relations between the post-surgical position and the risk of cut-out [6; 7; 8; 9; 10]. Computational studies using the finite element method have also been used to find whichblade positions lead to less bone damage [5; 11; 12; 13; 14; 15; 16]. Considering static analyses of stress or strain, and maximum principal stress or strain criteria as a measure of the risk of cut-out, [PERSON] et al. [5; 16], [PERSON] et al. [11], [PERSON] et al. [12], and [PERSON] et al. [13] investigated the influence of different anterior-posterior and superior-inferior blade positions on the risk of cut-out. Two primary factors set these studies apart: the implants considered, which included the Proximal Femoral Nail Anti-rotation (PFNA) implant by Synthes (Oberdof, Switzerland) [5; 16], the Omega3 Compression Hip Screw by Stryker Osteosynthesis (Schoenkirchen, Germany) [11], the Dynamic Hip Screw by Tipson Tibbi Aletler (Bornova, Turkey) [12], and the Asia Anatomic Anteversion Hip Nail by A Plus Biotechnology (New Taipei City, Taiwan) [13]; and the fractures considered, which included a stable fracture classified as 31-A1 in the Muller AO classification system [5; 13; 16] and an unstable 31-A2 fracture, with an intrusion distance of 30% [11; 12]. In addition to different superior-inferior positions, [PERSON] et al. [14] and [PERSON] et al. [15] also considered different medial-lateral positions, set by blade length, to investigate their influence on the risk of cut-out in the treatment of a stable 31-A1 fracture and an unstable 31-A2 fracture, with an intrusion distance of 30%, respectively, using the PFNA implant. While the central placement of the hip blade in the anterior-posterior direction is accepted to be less prone to cut-out in both clinical and computational studies, findings about the placement in the other directions are either limited or inconsistent. In the superior-inferior direction, literature conclusions are divided between central [5; 6; 16; 17] and inferior [12; 13; 14] positions. In the medial-lateral direction, a deep placement of the hip blade, with the tip positioned near the surface of the femoral head surface, is suggested to reduce the risk of cut-out [14; 15]; however, limited data exist on this direction when compared to the anterior-posterior and superior-inferior directions. Despite the debate in the literature, no clear consensus exists yet about the optimal position to reduce the risk of cut-out [3; 16].  
Among the described computational studies addressing the risk of cut-out [5; 11; 12; 13; 14; 15; 16], no study considered damage progression, which could also be an important contributing factor in its occurrence. Considering three-dimensional finite element models of two femora with unstable trochanteric fractures coupled with a bone damage model to simulate damage progression, the aim of this study was to advance the computational modelling of cut-out and investigate the risk of cut-out for different superior-inferior and medial-lateral positions of the PFNA implant. To simulate a severe scenario, an unstable 31-A2 fracture with an intrusion distance of 95% was modelled. The influence of the loading intensity on the risk of cut-out was also evaluated.