Background and Objectives: This study’s purpose was to examine the benefit of using a tibial extension in the primary operation of total knee arthroplasty (TKA). This is important because it is not a common practice to use the extension in a primary TKA, a standard configuration offering sufficient stability and good long-term survivorship. The following question arises: which situation requires the use of a standard configuration implant (without a stem) and which situation requires using the extension? Materials and Methods: The opportunity to use the tibial extension in the primary TKA was analyzed in correlation to the tibial bone structural properties. Using finite elements (FEs), the virtual model of the tibial bone was connected to that of the prosthetic implant, with and without a stem, and its behavior was analyzed during static and dynamic stresses, both in the situation in which the bone had normal physical properties, as well as in the case in which the bone had diminished physical properties. Results: The maximum stress and displacement values in the static compression regime show that adding a stem is only beneficial to structurally altered bone. Compression fatigue was reduced to almost half in the case of altered bone when adding a stem. Dynamic compression showed slightly better results with the tibial stem in both healthy and degraded bone. Conclusions: It was concluded that, if the bone is healthy and has good structural properties, it is not necessary to use the tibial extension in the primary operation; and if the bone has diminished physical properties, it is necessary to use the tibial extension at the primary operation, enhancing the stability, fixation, and implant lifespan.

Designing orthopedic implants with a long lifespan is essential for improving patients’ quality of life. It is necessary to develop new products with a high degree of personalization for the human body. Physicians and engineers analyzed the geometry and behavior of healthy joints’ motion under specific load conditions as well as the behavior over time and lifetime of orthopedic implants fitted to patients to improve their quality. The paper presents the way in which three-dimensional modeling techniques using specialized software (Catia, SolidWorks) can be combined with reverse engineering techniques (3D scanning) to optimize the design of orthopedic implants. The design of an implant consists of its three-dimensional modeling, as well as simulation of its integration into the human body, in order to analyze its behavior during motion. Therefore, it is necessary not only to 3D model the parts that make up the implant itself, but also to 3D model the bone to which the implant will be fitted. The paper highlights the complementarity of the classic modeling techniques with the reverse engineering techniques, which is necessary because the design of the parts that make up the implant itself can be achieved by specialized software modeling techniques, while the bones, having complex geometries, are better suited to 3D Modeling by scanning.

The hip (coxofemoral) joint is built so as to provide, at the same time, maximum stability and mobility, and is characterized by a very low wear rate and very low frictional forces thanks to the very good natural lubrication. In time, due to various reasons such as aging, joint illnesses, bone tumors, arthritis, injuries, coxofemoral joints may lose their self-lubrication ability, causing pains that make movement almost impossible. The therapeutic solution for coxofemoral joint illnesses is hip implant arthroplasty. Medical implants replacing the bone segments of coxofemoral joints are subjected to mechanical stress during walking and daily activities, which leads to wear in the long run. In the (most frequent) case of a bipolar hip implant, consisting of a femoral stem and femoral head, the area most affected by wear is the hip implant coupling, which represents the connection between the sphere-shaped proximal end of the femoral stem and the femoral head’s ball groove. Typically, the spherical proximal end of the femoral stem is made of metallic biomaterials (titanium and titanium alloys, cobalt-based alloys, stainless steels) or ceramic biomaterials (alumina, zirconia), while the femoral head’s ball groove is made of polymeric biomaterials (high-density polyethylene), metallic biomaterials or ceramic biomaterials. In the hip implant’s coupling there takes place a sliding friction whose coefficient depends on the nature of the touching surfaces (metal on metal, metal on polyethylene, ceramic on polyethylene, ceramic on ceramic). The wear that occurs in the coupling is due especially to the particles of material detached from the surfaces coming into contact. A worn coupling should be replaced by a revision surgery that the patient must go through, since loose particles generate severe joint inflammation. The objective of this paper is: -to point out the wear degree of the components forming the friction coupling of a hip implant, by presenting images, as seen on a video microscope, of two implants retrieved from patients after revision surgery -to present a new structural option for the construction of a hip implant coupling, by introducing two ball cages that replace the femoral head’s ball groove, which reduces the contact surface and replaces sliding friction with rolling friction (with a lower friction coefficient), thus diminishing the wear on the implant coupling.

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