PSI - Issue 42

N.S. Hennicke et al. / Procedia Structural Integrity 42 (2022) 404–411 Hennicke et al. / Structural Integrity Procedia 00 (2022) 000 – 000

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Keywords: Periprosthetic femoral fractures; Subject specific finite element analyses; Osteoporosis; Aging

1. Introduction Total hip arthroplasty (THA) is one of the most successful orthopedic procedures conducted in clinics today (Learmonth et al., 2007). However, the average time a patient is in need of an endoprosthesis is rising with an increasing life expectancy (Tsiridis et al., 2003). In order to reduce the stress of multiple revision and correction surgeries on patients, these need to be reduced and avoided as long as possible. But, after THA there is a risk for periprosthetic femoral fractures (PFF) (Sidler-Maier and Waddell, 2015; Spina and Scalvi, 2020). The treatment of PFF requires very complex reconstructive surgeries which are especially difficult for patients suffering from osteoporosis since the healing properties of the bone are reduced due to the decelerated bone metabolism (Augat et al., 2005). Also, the changes of the bone tissue due to the advancing age of the patient effect the stability and load bearing capacity of a femur with an endoprosthesis which increases the fracture risk (Sidler-Maier and Waddell, 2015). To prevent PFF and develop possible countermeasures for elderly and osteoporosis patients, investigating the influence of osteoporosis and aging on the development of PFF form a biomechanical view point is essential. Most previous numerical studies on osteoporotic bone fractures focus either on small tissue samples or on whole osteoporotic bones at a certain time point (Lee et al., 2019). But, none have simulated osteoporotic bone in its previously healthy stage or after further progression of osteoporosis at a later point in time. The objective of this work is to integrate statistical data on aging and osteoporosis into a previously developed and validated bone model (Hennicke et al., 2022) that is able to predict PFF on a subject specific basis. 2. Materials and methods 2.1 Original model construction In a previous study a subject specific finite element (FE) model was developed to simulate the development of PFF and validated (Hennicke et al., 2022). This modelling approach served as a basis for the presented work here and is illustrated in Fig.1. To create a model, the geometry of a human femur was generated with the approach by Kluess et al. (Kluess et al., 2009) from quantitative computed tomography (QCT) data provided by the Rostock University Medical Center, Germany. The subject specific bone geometry was imported to the FE software ABAQUS/CAE 2017 (Dassault Systèmes Simulia Corp., Johnston, RI, USA) and virtually implanted with a cementless hip stem of the Zweymueller design via Boolean operations. The model was meshed with linear, tetrahedral finite elements of the type C3D4 with an average element size of 3 mm. To generate a heterogeneous bone model, the density data from the original CT scans was mapped onto the generated mesh with the in-house algorithm AbaCTmat (Mauck et al., 2016; Vogel et al., 2020). The algorithm assigned a specific density value to each node of the mesh, which was afterwards used to calculate the corresponding density dependent material parameters (Cong et al., 2011; Keller, 1994; Keyak et al., 1994). For the material definition, a bilinear elastic-plastic constitutive law was used, where the tangent modulus was set to 5% of t he calculated Young’s modulus. A critical plastic failure criterion was applied with a ductile damage model to implement element deletion. In the numerical analyses the femur was loaded quasi-statically in the load case “ stumbling ” . For this purpose, a vertical displacement was applied incrementally at the implant head in constant steps. A penalty based contact with a friction coefficient of 0.5 was defined at the implant-bone interface. For the presented investigations, FE models generated with QCT scans from two left femurs originating from two different donors were used and labeled 1L and 3L.