In-vitro biomechanical evaluation of stress shielding and initial stability of a low-modulus hip stem made of β type Ti-33.6Nb-4Sn alloy
Graphical abstract
Introduction
Total hip arthroplasty (THA) is one of the most beneficial procedures in orthopedic surgery [1]. However, subsequent femoral bone remodeling affects long-term clinical performance. Progressive proximal bone loss can decrease implant stability, causing subsidence and aseptic loosening, resulting in periprosthetic fracture and hindrance to revision surgery [2], [3], [4], [5]. The etiology of proximal bone resorption is multifactorial [6], but it is mainly caused by a reduction in load transmitted from implant to bone, i.e., so-called “stress shielding” [7], [8]. Consequently, the femur remodels to accommodate the new mechanical situation, obeying Wolff's law [9], [10], [11]. To date, various types of femoral stems, including anatomic, straight, wedge-taper, customized, and short-type stems, have been introduced; nevertheless, stress shielding continues to occur in most cases [12], [13], [14].
The main reason for stress shielding is the difference in stiffness between metallic implants and host bone [15], [16]. Stress is predominantly transferred through the implant, because the Young's modulus of metallic implants is generally much higher than that of host bone. Therefore, to decrease stress shielding, low-modulus femoral stems, i.e., isoelastic stems, were first introduced 40 years ago [17], [18], [19], [20]. However, inadequate bone fixation and fatigue failures of the plastic materials used led to high clinical failure rates [21], [22], [23], [24], [25], [26]. Fixation failure associated with aseptic loosening is caused by the inferior surface properties or incompatibility with bone tissue of their constituent polymers [27]. The Epoch® stem (Zimmer, Inc., Warsaw, IN, USA) is the only successful low-stiffness stem for which long-term clinical performance has been reported [28], [29]: proximal bone loss was less than that associated with a more rigid stem design [30], [31]. However, the bone-sparing effect remains controversial. Dual X-ray absorptiometry demonstrated that the greatest decrease in mean bone density (27.5%) occurs in the calcar region at 2 years [32]. Development of femoral stems from alternative materials, more closely matched in stiffness to host bone, is desirable to prevent stress shielding.
Ti-6Al-4 V alloy (α-type and β-type) is widely used in femoral stems and other orthopedic implants because of its properties, which include excellent biocompatibility, good corrosion resistance, and high strength. However, its Young's modulus (110 GPa) is 3–4 times higher than that of human cortical bone (approximately 20–30 GPa). Therefore, the development of low-modulus Ti alloys for biomedical applications is ongoing [33], [34], [35], [36], [37], [38]. Because Young's modulus and strength are inversely proportional, the results of these studies have not met target values required for the femoral stem. The femoral neck region must be of high strength to support the patient's body weight and enable reduction of the neck radius, ensuring a wide range of stem movement around the cup.
Recently, Hanada et al. [39], [40] and Jung et al. [41] developed a new, low-modulus stem to overcome these issues using β-type Ti-33.6Nb-4Sn alloy (TNS). This stem demonstrates high tensile and fatigue strength in the femoral neck region because of local heat treatment, and a low modulus in the portion inserted into the femur. This design is based on the clinical observations that most stem fractures occur in the neck, and that bone loss due to stress shielding mainly occurs in the proximal femur. The Young's modulus of the distal region of this stem is <55 GPa, approximately half that of Ti-6Al-4V alloy, and much closer to that of bone. In the femoral neck, the tensile and fatigue strengths are >1200 MPa and >800 MPa, respectively. Using optimized local heat treatment, they decreased the Young's modulus of the TNS to 30 GPa, which is the lowest Young's modulus of all β-type Ti alloys developed for biomedical applications [38]. Moreover, the Ti–Nb–Sn alloy, which contains minimal cytotoxic elements, has a high degree of bone tissue compatibility, similar to that of Ti-6Al-4V alloy [42]. This novel, low-modulus Ti alloy could supersede the conventional Ti-6Al-4V alloy as a biomaterial for femoral cementless stems; however, the biomechanical performance of this stem has never been investigated.
In this study, we evaluated the stiffness, stress shielding, and initial stability of this low-modulus β-type TNS hip stem compared with a similar Ti-6Al-4V alloy stem. We hypothesized that the TNS stem would decrease stress shielding and negatively influence initial stability compared with the Ti-6Al-4V stem.
Section snippets
The low-modulus β-type Ti-33.6Nb-4Sn stem
The TNS stem has a fatigue strength of 850 MPa and a tensile strength of 1270 MPa in the femoral neck portion, and a Young's modulus of 55 GPa in the distal region (Fig. 1). To increase its tensile and fatigue strengths, the femoral neck was inserted into a hole in a copper rod and locally heated to 798 K for 5 h, causing fine α precipitation in the fiber structure. In the heat-treated neck region, the tensile strength of the TNS stem was higher than that of Ti-6Al-4V and other β-type Ti alloys
Axial and bending stiffness
The linearity of the axial force–displacement data (TNS stem, R2 = 0.99; Ti-6Al-4V stem, R2 = 0.99) showed that the specimens remained within the linear elastic region, incurring no permanent deformation during the mechanical tests (Fig. 5a). The axial stiffness of the TNS stem (754.8 ± 32.8 N/mm) was 55.9% that of the Ti-6Al-4V stem (1349.4 ± 158.4 N/mm; Fig. 5b), and the difference was significant (P < 0.001).
In the cantilever-bending test, the bending stiffness (Nm/mm) of both stems gradually decreased
Discussion
Stress-shielding-mediated femoral bone loss is inevitable following THA with intramedullary cementless femoral stems [12], [14]. In particular, proximal bone loss increases distal loading, and is considered to influence implant fixation. Thigh pain may also occur after THA, and its origin may be related to the relatively high bending stiffness of the implant within the femur [29], [51]. The TNS stem was designed to decrease proximal bone loss and thigh pain through decreased bending stiffness.
Funding
This work was financially supported by the Program to Disseminate Tenure Tracking System from the Ministry of Education, Culture, Sports, Science and Technology. This study was funded in part by Mizuho Corporation, Tokyo, Japan.
Ethical approval
Ethical approval not required.
Conflict of interest statement
The authors declare that they have no financial or personal relationships with any other people or organizations that could have inappropriately influenced or biased this work.
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