Discussion
inflammation, fracture, Fracture, THA
INFLAMMATION
This was the first study determining the severity of SS by comparing TNS and similarly designed Ti6Al4V stems. The TNS stem prevented SS in the short term. The clinical results of the TNS stem were acceptable compared to the metaphyseal-engaging cementless Ti6Al4V stems currently in use.SS is a mechanical phenomenon that occurs after stem implantation into the femur and is caused by multiple factors, including stem stiffness, area of surface treatment, and large stem size compared to the femur size [Femoral components of THA comprise articulation, structure, and fixation, with different mechanical properties. The requisite of the structural component, which couples the articulation and fixation components, is sufficient strength to overcome peak and dynamic stresses. When a stem is manufactured from a single alloy, such as the Ti6Al4V alloy, the alloy is selected to produce the stem according to the stiffness and Young’s modulus of the part that requires the most strength. The Ti6Al4V alloy is preferable as a biomedical material for stems because of its excellent biocompatibility and corrosion resistance. In contrast, its Young’s modulus is significantly higher than that of human cortical bone, leading to an imbalance in loading between the bone and the stem. The grade of SS caused by fit-and-fill stems implanted in the control group were reported previously using Engh’s classification. Kato reported that Synergy stems were shown grade 2 of SS appeared 66% and grade 3 and 4 of SS appeared 22% at 5-year follow-up [The low Young's modulus of the TNS alloy makes it useful as orthopaedic implants. Animal models have shown it to be a promising material for fracture treatment devices. Fracture healing is multifaceted, and besides material advantage, stem cell differentiation and proper inflammation are required for bone repair [This study has several limitations. First, the control group did not undergo THA during the same period as the TNS group, and two types of stems were implanted in the control group. The methods or sites of stem surface treatments did not match completely with the TNS stem. Young's modulus of stems was not measured in the control group. It was ideal that Ti6Al4V stem having the completely same shape and surface treatment of the TNS stem was implanted as control. However, this was the first trial of TNS stem in clinical use. It was an ethical problem that the Ti6Al4V stems completely matched in shape and surface treatment were implanted in the control group. We aimed to compare the stems in current use having the concept of metaphyseal-filling stems focused on stem shape. The shape of the TNS stem was categorized as metaphyseal-filling or fit-and-fill stem. The number of control group did not completely match because the control group was selected retrospectively. Second, the SS was not evaluated quantitatively, such as evaluating bone mineral density of femurs. We did not evaluate bone mineral density of femurs. SS was evaluated using Engh’s classification and Gruen zone for comparison with previous studies using radiographic analyses of control stems implanted in this study [