Background
Total hip arthroplasty (THA) is a reliable surgical treatment for hip deformity and pain that improves hip pain and quality of life. In recent years, the number of patients undergoing THA has increased worldwide, with the number of patients under 65 years of age predicted to increase in the future because young and active patients who had hip degeneration too extensive for osteotomy demanded to regain full activities [
1,
2]. Although THA has shown favourable clinical results, it can cause complications, such as aseptic loosening. Aseptic loosening of the femoral stem is the most common cause of THA revision [
3,
4]. Young age at surgery is associated with a high revision risk. The lifetime revision risk is significantly higher in men aged 50–54 years than in those aged 70–74 years (29.6% vs. 7.7%) [
5]. Preventing aseptic loosening is critical to improving the long-term outcome of THA.
Stress shielding (SS) affected aseptic loosening when combined with other risk factors such as polyethylene wear and low-precision installation of the prosthesis [
3,
6]. Highly cross-linked polyethylene was developed to reduce polyethylene wear [
7]. While improvements have been obtained for polyethylene wear, stress shielding of cementless hip prostheses is still an issue that needs to be improved.
The change of local strain caused by the difference in Young's moduli of bone and implant materials leads to SS [
8]. Abnormal load transmission from the proximal portion of the stem to the area in contact with the stem tip causes cortical hypertrophy (CH) [
9]. Excessive stiffness of the stem was one of the causes of SS, so an improved femoral stem material that combined low elastic modulus with strength was needed. The Young’s modulus of bone is 10–30 GPa, and the ideal Young's modulus of an implant material is similar to that of bone while maintaining sufficient strength [
10]. Femoral stems made of materials with lower Young's moduli were previously developed to approximate Young’s modulus of bone [
11]; however, their results were unacceptable because of poor strength. The femoral stem material should have biocompatibility and strength to fix the femur. Titanium alloys, particularly Ti6Al4V alloys, are most commonly used in the medical field and have adequate biocompatibility and corrosion resistance for use as orthopaedic implants [
12]. In contrast, Young’s modulus of the Ti6Al4V alloy is approximately 110 GPa, while the stiffnesses of bone and the Ti6Al4V alloy differ [
13,
14].
Hanada et al. developed the novel Ti-33.6Nb-4Sn (TNS) alloy, which has a low Young's modulus, the same tensile strength as Ti6Al4V, and functionally graded characteristics of an adjustable Young's modulus with heat treatment [
15,
16]. A novel cementless femoral stem of the TNS alloy with Young's modulus gradient properties with heat treatment has been developed [
16]. Furthermore, short-term clinical outcomes of TNS stems include a reduced incidence of SS [
17].
However, the reduction of SS by TNS stems compared to similarly designed Ti6Al4V stems has not been investigated. The aim of this study was to evaluate the reduced incidence of SS and the postoperative clinical score of TNS stems compared to similarly designed Ti6Al4V stems.
Discussion
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 [
27‐
29], with the key factor being the difference in elastic moduli of the stem and bone [
30]. SS was not necessary associated with complications and clinical results [
23,
31]. However, potential risks of SS were related with stem loosening, deficient bone stock when a revision surgery was required [
32,
33]. The reduction of SS is critical to obtain the better clinical results of THA. Several animal models have suggested that a low elastic modulus of the stem leads to lesser resorption of the cortical area than a high elastic modulus of the stem [
34,
35]. To resolve the mismatch in elastic moduli of the stem and bone, Robert Mathys developed an isoelastic stem as a low-elastic stem to avoid SS [
36,
37]. However, the results with this stem were unacceptable [
38], probably because of inadequate primary fixation. A femoral stem made of the TNS alloy, which has both low Young's modulus and comparable strength to the Ti6Al4V alloy, has been developed, and good short-term results have been reported [
17]. In the present study, we compared TNS and conventional Ti6Al4V stems, and the radiological assessment indicated that TNS stems with a low Young's modulus had a superior effect on the suppression of SS compared to Ti6Al4V stems. The frequency of CH was higher in the TNS group at 3-year follow-up comparing at 1-year follow-up, however the actual number of cases of CH did not increase. This was because one patient in the TNS group dropped out of the study during the follow-up.
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 [
39]. Nishino also reported that Synergy stems had grade 2 of SS appeared 38% and grade 3 and 4 of SS appeared 46% at 10- to 12-year follow-up [
40]. VerSys HA/TCP Fiber Metal Taper had 42% of them were shown grade 2 of SS, 7% were shown grade 3 of SS and degree 4 of SS was not appeared at 3-year follow-up [
41]. Synergy stem and VerSys HA/TCP Fiber Metal Taper caused grade 2 and 3 of SS 3 to 10 years after THA. These results previously reported were consistent with our results of control stems. In contrast, the TNS stem exhibited novel properties and a gradient change in stiffness in the same alloy with heat treatment. Stiffness could be altered in the appropriate portion of the stem to utilize this property. The results of this study suggest that the TNS stem adequately transmitted the load to the proximal femur.
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 [
42]. Mouse and rabbit tibial models have shown that TNS intramedullary nails are more effective than the Ti6Al4V alloy or stainless-steel intramedullary nails in promoting bone healing [
43‐
46]. The low Young's modulus of the TNS alloy promotes the expression of runt-related transcription factor 2, which is the key signal for osteoblast differentiation at the fracture callus site and promotes bone healing [
43]. Furthermore, the effect of the anodic oxidation method on the biocompatibility of the TNS alloy has been studied. The TNS alloy treated by anodic oxidation with sulfuric or acetic acid generates hydroxyapatite on its surface in vivo, which enhances bone conduction ability [
47‐
52]. Osteoblast progenitor cells are expressed in the endosteum of the bone marrow [
42]. The TNS alloy after anodic oxidation treatment promotes bone formation by proliferating and activating osteoblasts in the endosteum of the bone and exerts an antibacterial effect through photocatalytic performance owing to sodium tartrate and sulfuric acid [
53‐
55]. The TNS alloy is considered to be a promising orthopaedic implant material owing to the low Young’s modulus and functional improvement by surface modification.
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 [
39‐
41]. Third, different surgeons performed the surgeries in the TNS and control groups at different timing. In both groups, surgeons with over 10 years of experience performed the surgery with the same approach; however, the surgical techniques were not completely same. In this report, TNS stems and control stems were not implanted by same surgeons and not in the same period. Finally, the sample size was small. THA with the TNS stem was performed as a clinical trial; therefore, only 40 patients could be followed up. Future studies with a large sample size are required. The follow-up period was short. Although SS was prevented in the short term, whether or not it could be prevented in the long term requires further investigation. We believe that safety and durability should be examined in the future.
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