Modeling age-related changes in the mechanical behavior of the fracture-fixated human tibia bone during healing.

The evolutionary healing phenomenon of fractured tibia bone was investigated by comparing the bio-mechanical response of the human tibia following fracture fixation for two ranges of patient ages, when a body weight pressure (BWP) is applied. Three-dimensional finite element models have been developed by adopting the biomechanical characteristics of cortical and trabecular tibia bones, and considering the time-varying callus properties during the healing process for the two patients. The stress and strain levels generated within the fractured tibia bone by the screw tight fit during the assembly process revealed its dependence on the bone stiffness that degrades with age. They have an impact on primary stability of the implants prior to the osseointegration. The gap capacity to resist and allow a gradual BWP load transfer, through the callus for the tibia bone models, was analyzed. In fact, from 10 weeks after surgery, the callus allowed the BWP transfer for young patients, which guarantees sufficient structure stabilization of the fractured tibia. However, an insufficient load was transferred to the fracture gap for the old patient, even beyond 16 weeks, which delayed the bone consolidation.

A comparative study of tapped and untapped pilot holes for bicortical orthopedic screws - 3D finite element analysis with an experimental test.

The aim of this study was to compare the screw-to-bone fixation strength of two insertion techniques: self-tapping screw (STS) and non-self-tapping screw (NSTS). Finite element analysis (FEA) was used for the comparison by featuring three tests (insertion, pull-out and shear) in a human tibia bone model. A non-linear material behavior with ductile damage properties was chosen for the modeling. To validate the numerical models, experimental insertion and pull-out tests were carried out using a synthetic bone. The experimental and numerical results of pull-out tests correlated well. Thread forming was successfully simulated during the insertion process of STS and NSTS. It is demonstrated that the STS generates higher insertion torque, induces a higher amount of stress after the insertion process and relatively more strength under the pull-out and shear tests than the NSTS. However, the NSTS induces more stiffness under the two tests (pull-out and shear) and less damage to the screw-bone interface compared to the STS. It is concluded that the use of STS ensures tighter bony contact and enables higher pull-out strength; however, the use of NSTS improves the stiffness of the fixation and induces less damage to the cortical bone-screw fixation and thus minimum risk is obtained in terms of bone necrosis.

Mechanical response of polyethylene tibial component using compression loading contact test: experimental and finite element analysis.

An experimental apparatus has been developed to study the mechanical response of Total Knee Replacement (TKR) prosthesis under compression loading test (Tibial HDPE component and femoral Vitallium component). Analysis of experimental results indicates that the load-displacement curve depends on the initial contact location. The position of this curve varies between two curves of lower and higher stiffness. Experimental compression loading curve for femoral part resting on flat HDPE is located between the extreme curves. This result allows us to model load-displacement curve of TKR prosthesis under compression loading using femoral component resting on flat polyethylene surface. For simple contact geometry of spherical cap on flat HDPE specimen, Hertz elastic theory can be used to describe the load displacement curve for loads under 635 N. This value corresponds to the load which induces plastic flow of polyethylene. In the case of femoral component on flat HDPE specimen, Hertz elastic theory can be considered for loads under 800 N. Finite element 2D is also used to model the compression load displacement curve for femoral component resting on flat HDPE specimen. The finite element results are similar to the Hertz elastic theory analysis for compression loading under 1200 N. Damage of tibial component can be related to the critical load which induces plastic flow of polyethylene. This load depends on the contact geometry and can be estimated with the Hertz elastic theory.

Visualisation of tibiofemoral contact in total knee replacement using optical device.

An in situ optical visualization technique (OVT) has been developed to identify the tibiofemoral contact (TFC) area during prosthesis indentation. An artificial total knee replacement (TKR) with borosilicate glass femoral component has been reproduced similarly to the original one. The medial and lateral contact areas have been observed, located and measured by means of in situ OVT. Therefore, it was experimentally possible to ensure a good axial alignment of the femoral component and tibial polyethylene insert. In addition, experimental measurement for load-displacement curves became reproducible. Furthermore, the evolution of the medial and lateral TFC areas as a function of the normal load was established. Finally, this study has shown that the in situ OVT is a simple in vitro method that provides comparable results with well-known methods such as Fuji film technique.