A common location for radius fracture is the proximal radial head. With the arm in neutral position, the fracture usually happens in the anterolateral quadrant (Lacheta et al., 2019). If traditional surgeries are not enough to induce bone stabilization and vascularization, or the fracture can be defined grade III or grade IV (Mason classification), a radial head prosthesis can be the optimal compromise between bone saving and recovering the “terrible triad”. A commercially available design of radial head prosthesis such as Antea (Adler Ortho, Milan, Italy) is characterized by flexibility in selecting the best matching size for patients and induced osteointegration thanks to the Ti-Por® radial stem realized by 3D printing with laser technique (Figure 1). As demonstrated, Ti-Por® push-out resistance increased 45% between 8 −12 weeks after implantation, hence confirming the ideal bone-osteointegration. Additional features of Antea are: bipolarity, modularity, TiN coating, radiolucency, hypoallergenic, 10° self-aligning. The osteointegration is of paramount importance for radius, in fact the literature is unfortunately reporting several clinical cases for which the fracture of the prosthesis happened after bone-resorption. Even if related to an uncommon activity, the combination of mechanical resistance provided by the prosthesis and the stabilization due to the osteointegration should cover also accidental movements. Based upon Lacheta et al. (2019), after axial compression-load until radii failure, all native specimens survived a compression-load of 500N, while the failure happened for a mean compression force of 2560N. The aim of this research study was to test the mechanical resistance of a radial head prosthesis obtained by 3D printing. In detail, a finite element analysis (FEA) was used to understand the mechanical resistance of the core of the prosthesis and the potential bone fracture induced in the radius with simulated bone- resorption (Figure 2a). The critical level was estimated at the height for which the thickness of the core is the minimum (Figure 2b). Considered boundary conditions:
- Full-length prosthesis plus radius out of the cement block equal to 60mm (Figure 2a); - Bone inside the cement equal to 60mm (Figure 2b); - Load inclined 10° epiphysiary component (Figure 2c); - Radius with physiological or osteoporotic bone conditions; - Load (concentrated in the sphere simulating full transmission from the articulation) of 500N or 1300N or 2560N. Figure 3 shows the results in terms of maximum stress on the core of the prosthesis and the risk of fracture (Schileo et al., 2008). According to the obtained results, the radial head prosthesis shows promising mechanical resistance despite of the simulated bone-resorption for all applied loads except for 2560N. The estimated mechanical limit for the material in use is 200MPa. The risk of fracture is in agreement with the experimental findings (Lacheta et al. (2019)), in fact bone starts to fail for the minimum reported failure load, but only for osteoporotic conditions. The presented FEA aimed at investigating the behavior of a femoral head prostheses made by 3D printing with simulated bone-resorption. The prosthesis shows to be a skilled solution even during accidental loads. For any figures or tables, please contact the authors directly.
In revision TKA, the management of bone loss depends on location, type, and extent of bony deficiency. Treatment strategies involve cement filling, bone grafting and augments. On the market several solutions are currently available, differing for their shape, thickness and material. While the choice of the shape and the thickness is mainly dictated by the bone defect, no explicit guideline is currently available to describe the best choice of material to be selected for a specific clinical situation. However, the use of different materials could induce different response in term of bone stress and thus changes in implant stability that could worsen long-term implant performance. For these reasons, an investigation about the changes in bone stress in the femur and in the tibia when augments, with different materials and thicknesses was performed. Different configurations have been separately considered including proximal tibial, distal or/and posterior femoral augments with a thickness of 5, 10 and 15 mm. Apart the control, in which no augments were used, but only the TKA is considered, the augment in all the other configurations were considered made by three different materials: bone cement, to simulate cement filling, tantalum trabecular metal and conventional metal (titanium for the tibia and CoCr for the femoral augments). Each configuration was inserted on a lower leg model including a cruciate-retaining total knee arthroplasty and analyzed by means of finite element analysis applying the max force achieved during walking. The bone stress was investigated in the medial and lateral region of interest close to the augment (with a bone thickness of 10 mm) and in an additional bone region of interest of 50 mm thickness. The bone stress have been compared among the different models and also with respect to the control model.Introduction
Methods
We introduce the concept of total knee arthroplasty (TKA) fingerprinting as a tool to characterize and graphically convey the sensitivity of a TKA design to surgical variability in implant component position and patient-related anatomic factors. Identifying sensitive directions preoperatively which would cause undesirable effects may decrease revision surgery by informing surgical decisions and planning. To provide several examples of TKA fingerprinting, we estimated and compared the contact forces in a single TKA type for several configurations, simulating surgical variability and patient-related anatomical factors during a loaded deep squat. The purpose of this study is not to analyze the behavior of this specific TKA design but rather to illustrate a tool that could be used to show, in general, how surgical errors or anatomical factors can alter patello-femoral (PF) and tibio-femoral (TF) contact forces compared to its own reference configuration. Computed tomography images of one full cadaveric leg were used to generate 3D models of the bones and to obtain a physiological knee model assuming standard positions of the main soft tissue insertions. A fixed bearing posterior stabilized knee TKA design was considered in this study. The prosthesis was a medium size, replaced both cruciate ligaments and resurfaced the patella. Following standard surgical procedure, the TKA was virtually implanted, thus defining its reference configuration. Each derivative replaced knee model was then obtained by changing the values of one parameter, or a combination of two, in a range based on literature and surgical experience (Table 1). A 10 s loaded squat to 120° was performed for each configuration, with a constant vertical hip load of 200 N. These settings match the experimental tests performed in a previous in-vitro analysis on cadaver legs. Each replaced model was developed and analyzed using a validated musculoskeletal modeling software. The model of the knee included TF contacts and PF contacts of the TKA components, passive soft tissues and active muscle elements. The external forces (ground reaction and weights), the muscle forces (quadriceps and hamstrings) and the frictional forces are applied to the knee joint through the machine. The mechanical properties of the tissues were obtained from literature. With these settings, for each model, both the maximum PF and TF contact forces have been evaluated.Introduction
Materials and methods
