A study on the new revision total knee prosthesis development with an assessment on the biomechanical stability and the biological safety

Abstract

The knee joint consists of the femur, tibia, patella, articular cartilage, and ligaments and is one of the largest joints in the human body. The knee joint is constantly subjected to repeated weight-bearing loads and is prone to injury or functional decline. In severe cases, such as degenerative arthritis, degenerative knee joint arthritis, rheumarthritis, and trauma where conservative therapies show little improvement, total knee arthroplasty (TKA) has proven to be an efficient treatment option with good clinical outcomes. Revision TKR is required when the life span of the device implanted during the primary TKR has expired, and this method can serve as a suitable knee joint replacement procedure when different functional requirements (mechanical, clinical, and design) are needed or harsh conditions need to be overcome. In an effort to improve the service life of knee replacements, extensive studies have been conducted and a wide range of products have been commercialized. In Korea, the several primary TKR was already developed and commercialized, but the supply of revision TKR prostheses depends on imports from global companies, and revision TKR prostheses require domestic research and development. This study was conducted to analyze the clinical and mechanical design elements of revision total knee replacement systems based on the primary artificial knee joints currently in use and to develop a new revision TKR prosthesis by performing various tests and evaluations. To implement the design of a new revision TKR prosthesis, three design elements, namely stability, modularity, and safety, were taken into account in analyzing the detailed design elements and product design. In order to increase stability, the range of motion of the prosthesis was restricted with high conformity. Furthermore, the femur stem extension, tibial baseplate, and stem extension were anatomically designed to enhance the support provided by our revision TKR system. To restore bone removal and loss, reconstruction parts were introduced in order to compensate and the tibial insert post was concavely designed, which increases the jump distance and prevents dislocation. We performed a number of tests to evaluate this new revision TKR system wherein several prosthesis design parameters were considered. Structural stability was validated using finite element analysis (ABAQUS v6.10) and performed ASTM F1223-08 and F1800-07 and compared with the primary TKR system. The stress distribution results showed that the strength of the femoral component was 30% higher with the new revision TKR system than with the primary TKR system. Eccentric test results showed that the strength of the tibial baseplate was 23% lower compared with the primary TKR system, but this factor does not significantly affect stability considering the yield stress of CoCr and high load level. We conducted a tibial cyclic fatigue test and a stem extension assembly locking test to assess the tibial components. Fatigue load was applied to the proposed TKR prosthesis in accordance with the ASTM test method to investigate the failure feature. No fracture or crack was observed, even under the application of a load higher than the ASTM guideline standard. The bond strength between the stem extension and offset adapter was tested by the assembly and disassembly torque test. The result indicated an assembly-to-disassembly torque ratio ranging between 99.7% and 101% (average: 100.5%), thus demonstrating that stability was sufficiently reflected in the proposed revision TKR prosthesis. After TKR implantation, the major parameters for stability analysis include the load transfer of the knee and biomechanical characteristics. It is well known that the stress concentration and stress shielding induced by knee replacement during insertion can cause fracture, pain, and loosening induced by bone resorption. Thus, load distribution in the tibia is an important factor from the biomechanical point of view. We used computed tomography to reconstruct surgical finite element models in order to assess the influence of loading distribution with stress/strain. First, the stress analysis results showed that the new revision TKR system had lower than yield strength at the tibia baseplate, stem extension, and cortical and cancellous bone it means that the proposed TKR prosthesis does not need any additional improvement or supplement in design elements for application. As such, structural stability is sufficiently provided under the aspect of tibial fracture risk. Second, the stress-strain distribution at the cortical bone was high at the proximal posterior and distal anterior portions, whereas that at the cancellous bone was high at the proximal posterior and distal lateral portions. The pattern of the a real-life revision TKR system was similar to that of the new revision TKR system. This implies that adequate stress/strain can be generated in the cortical and cancellous bones during the insertion of the proposed revision TKR, which is expected to mitigate the loosening-related problem due to stress-shielding effects and increase in bond resorption and remodeling, which would not cause bone loss or stability. Since an artificial knee joint is a medical device that is inserted into the human body for a long period of time, its biological safety should be ensured. For the development of a new revision TKR prosthesis, the hole plug, for which metal is usually used, was fabricated with a new material, SL7870 and its biological safety was validated in the cytotoxicity, subcutaneous (intradermal) reactivity, sensitization, acute systemic toxicity, and genotoxicity tests. The cytotoxicity test revealed that the extract of the test material was non-toxic by verifying 99% cell viability. In the subcutaneous (intradermal) reactivity test using rabbits, the extract of the test material did not trigger skin reactions such as erythema, crusting, and swelling. Nor did the sensitization test reveal any abnormal reactions on the skin of the mice exposed to stimuli. The acute systemic toxicity test did not trigger any abnormal changes in skin, hide, eyes, and body weight. Finally, in the genotoxicity test, no significant differences were observed in the results of mutagenicity of microorganisms between the test material and negative controls. The biological safety of the new material was thus proved. The clinical and anatomical requirements for revision TKR prostheses were analyzed for the purpose of developing a new revision TKR prosthesis. Detailed parameters for important factors were analyzed, and a design meeting all the requirements was implemented. A variety of validation and testing methods were established in order to evaluate whether the designed product meets the functional and biomechanical requirements. We evaluated our new revision TKR system using structural, mechanical, and biological tests in order to assess whether it meets functional and key requirements. In analysis results of various performed tests, we confirmed the feasibility of our new revision TKR system and verified its clinical applicability and marketability. Furthermore, This study is significant in that it first provided the basic data for the development of a domestically produced revision TKR prosthesis. Based on the results of this study, research will be continued with the intent to obtain approval for the proposed revision TKR prosthesis through its efficacy validation and clinical trials. The achievement of this study is expected to contribute to the research and development of domestically produced revision TKR prostheses for the domestic and global market sharing, which is currently dominated by imported products. It is hoped that our results will be used as a foundation for the local TKR market, which predominantly includes foreign products.