**1. Introduction**

Femoral neck fractures (FNF) in young adults are uncommon, accounting for 3% of all hip fractures, and usually result from high-energy trauma [1,2]. Treatment is focused on preserving the proximal extremity of the femur through anatomical reduction and stable internal fixation [3,4]. Despite their infrequency in the younger population, there is higher rate of femoral head osteonecrosis and nonunion, which directly contribute to a poor

**Citation:** Giordano, V.; Freitas, A.; Pires, R.E.; Battaglion, L.R.; Lobo, M.d.O.; Belangero, W.D. Evaluation of a Locking Autocompression Screw Model in Pauwels Type-3 Femoral Neck Fracture: In Vitro Analysis. *Bioengineering* **2022**, *9*, 464. https://doi.org/10.3390/ bioengineering9090464

Academic Editors: Christina Zong-Hao Ma, Zhengrong Li and Chen He

Received: 31 August 2022 Accepted: 6 September 2022 Published: 12 September 2022

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outcome and uneventfully are associated with reoperations and salvage procedures [2]. Although many factors have been shown to play a significant role in preventing these devastating complications, the quality of the reduction and its maintenance are the main recognized factors in reducing the risk of avascular necrosis of the head and nonunion of the femoral neck [2,4]. Epidemiological studies have shown up to a 59% nonunion incidence in FNF and from 12–86% avascular necrosis incidence in young patients after femoral neck fracture [2,5–7], with implant failure occurring in approximately 10% of cases in young patients [8].

The implant type is of paramount importance for maintaining anatomical reduction, since it must resist the deforming forces that act on the fracture focus [9]. The more unstable the fracture plane, the more critical this becomes, e.g., in Pauwels type-3 (P3) fractures, in which a dominant shear force is inherent to the fracture pattern, resulting in a higher rate of failure and nonunion [2,8–10]. The deformities often seen are varus angulation and inferior translation of the proximal femoral neck/head fragment, with failure often resulting after a non-anatomic reduction and inadequate fixation [2]. Thus, the search for effective methods of internal fixation has become the focus of scientific research over the years, resulting in the development of numerous implants that combine intra- and extramedullary characteristics [10–14].

Currently, the sliding hip screw, combined or not with an anti-rotation (or erotational) screw, is considered the standard implant in P3 FNF [7,10]. Several authors have shown that the sliding hip screw has less inferior femoral head displacement, less shearing displacement, and a greater load to failure when compared to multiple cannulated cancellous screws [9–13]. Bonnaire and Weber [15] observed that the sliding hip screw with the derotational screw presents the best mechanical environment for this challenging fracture pattern. Although the sliding hip screw has been found to be very effective in treating Pauwels type-3 femoral neck fractures, care should be taken in significantly comminuted fractures in a vertical orientation [16,17].

Despite the sliding hip screw's superior mechanical strength to other extramedullary implants, problems related mainly to its inability to control rotation, especially when an additional derotational screw is not used, with varus subsidence and femoral neck shortening, which alter hip offset, have been reported in the literature [18]. This is mainly because the cephalic screw gradually slides, causing impaction of the fracture focus, which is greater in a malreduced fracture and in cases where the anti-rotation screw is not used. Thus, our hypothesis was that an implant which retained the main characteristics of existing systems (such as the cephalic screw and an intra- or extramedullary anchorage stop) but prevented the progressive collapse of the femoral neck during the healing process could minimize the rate of complications observed in young adult FNF. Indeed, some authors showed a reduced load-to-failure with the fixed-angle proximal femoral locking plate (PFLP), potentially minimizing femoral neck shortening and other complications [16,19,20]. Liporace et al. reported a nonunion rate of 8% for Pauwels type-3 FNF treated a PFLP, compared with 19%i n those treated with multiple cannulated screws [20].

The main objective of this study is to evaluate the biomechanical behavior of a locking autocompression screw, called X-PIN, and a variant (X-PIN+P) in P3 FNF using a finite element model (FEM). The secondary objective was to compare the results of this model with clinically established fixation methods for FNF.
