Background
Osteonecrosis of the femoral head is a pathological process that can result from internal or external destruction of the blood supply to the femoral head, accompanied by avascularity, cell death, and cartilage collapse, finally results in deformation and avascular necrosis of subchondral bone [
1]. As the disease progresses, the femoral head can develop cystic lesions and collapse of the articular surface, with further progression resulting in osteoarthritis. Conservative, non-operative treatment is only suitable in the early stages of osteonecrosis, and surgical intervention is often required as the disease progresses to prevent further damage to the femoral head and to avoid or delay the potential need for an artificial joint replacement [
2‐
4].
The classic surgical treatment of osteonecrosis of the femoral head is core decompression (8 mm/6 mm drill), which can effectively reduce the pressure on the femoral head, improve local blood circulation and relieve hip pain [
5‐
7]; however, this can increase the risk of postoperative fracture and further collapse of the articular surface due to a lack of mechanical support. Currently, core decompression combined with bone impaction grafting or implantation is the main operative approach used in Steinberg stage I-II and can increase the strength of the femoral head and reduce the risk of articular surface collapse [
8‐
11]. Implantation options include porous tantalum rods, vascularized fibular grafting, and nonvascularized fibular graft, etc. [
12,
13]. However, neither provides strong structural support, good bone ingrowth, or an uncomplicated operation. The implant design and material, surgical technique, clinical indication and application, and the clinical characteristics of candidates should all be carefully considered and monitored prior to any procedure [
13].
Silk protein has now also been widely used as a medical biomaterial in products such as silk protein fibers, silk protein films, and silk protein sponges [
14]. As a biodegradable material, silk protein differs from other more conventional materials, while still demonstrating high strength and resilience due to its unique molecular structure [
15]. The rate at which the silk protein wall degrades can be regulated and programmed into the material during its preparation process [
14], providing a great advantage for its use as a bone replacement material. Processed silk protein can also be embedded with drugs or cell morphogens, like bone morphogenetic protein-2 (BMP-2), has good biological compatibility, is minimally pro-inflammatory [
16], and is easier to design and manufacture in a range of shapes, angles, and dimensions as required.
Currently, silk protein is only used with other materials, like hydroxyapatite, as a composite bone filler. A recent study reported that silk protein could be manufactured as silk screws and inserted into rat models with femoral fractures [
17]. These screws demonstrated good efficacy with no screws failing during implantation and no postoperative adverse events or screw displacements. Histological results demonstrated osteoclast activity, suggesting early resorption of the silk screws; and osteogenesis of the surrounding tissues, indicating continuous bone ingrowth; demonstrating the potential application of silk protein as a bone substitute material [
18‐
20].
With advances in manufacturing technology, silk protein can now be produced as stronger and larger silk protein rods [
21,
22]. The present study aimed to assess the biomechanical properties of the silk protein materials to understand its mechanical properties better and establish a three-dimensional (3D) finite element model of osteonecrosis of the femoral head. To the best of our knowledge, this is the first study investigating the clinical application of the silk protein rod design in the treatment of osteonecrosis of the femoral head, with comparisons made between models of silk protein rod implantation, fibula implantation, porous tantalum rod implantation, and simple core decompression.
Discussion
Osteonecrosis of the femoral head is common in China and typically affects young to middle-aged adults, with the potential to cause significant morbidity [
1]. Controlling the progression of the disease is important to prevent progression into osteoarthritis, which if severe enough may require an artificial joint replacement. While conservative management is appropriate in the early stages of the disease, implantation following surgical core decompression is the preferred treatment option in more advanced osteonecrosis, including non-vascularised bone grafting, vascularized bone grafting, and tantalum rods, etc. [
4]. However, there has been no great success as yet in regards to the ideal implantation biomaterial to use. Silk protein is a new biomaterial that is demonstrating promising results as a potential bone substitute material [
25,
26]. The present study assessed the biomechanical properties of the silk protein materials to understand its mechanical properties better and established a 3D finite element model of osteonecrosis of the femoral head. We investigated the clinical application of the silk protein rod design in the treatment of osteonecrosis of the femoral head, with comparisons made between models of silk protein rod implantation, fibula implantation, porous tantalum rod implantation, and simple core decompression.
The first aim of this study was to determine the biomechanical properties of silk protein. Our results of the silk protein demonstrated that the average elastic modulus was 0.49 GPa, and the average shear modulus was 0.66 GPa. For the cadaver fibula samples, the average elastic modulus was 2.06 GPa, and the average shear modulus was 0.55 GPa. Macroscopically, the elastic modulus is an indicator of tensile elasticity. The greater the elastic modulus, the greater the force needed to deform it, indicating greater material stiffness. Our results demonstrated that the silk protein had a lower elastic modulus than the fibula. Despite this reduced stiffness, the silk protein material has been demonstrated to be strong, making an investigation of its application in the treatment of femoral head osteonecrosis worthwhile.
The 3D finite element models produced demonstrated that the amount of surface displacement of the weight-bearing area of the femoral head was less following silk protein rod implantation compared to simple core decompression (p < 0.05). This was the case when the osteonecrosis range was 60°, 90°, and 120°, indicating that silk protein rod implantation was better at preventing further collapse of the femoral head. The silk protein did demonstrate greater external displacement than both the fibula and tantalum rod implantation groups, suggesting that the hardness of the silk protein was insufficient to prevent surface deformation. However, there were no differences in the surface stress of the femoral head weight-bearing area, regardless of the range of osteonecrosis, when compared to the fibula or tantalum rod implantations (p < 0.05). Surface stress represents the degree of damage to the surface of the weight-bearing region of the femoral head, with our 3D finite element models demonstrating that the silk protein rod can provide good mechanical support under static load-bearing forces, such as when standing.
One limitation of this study was the small sample size. Due to their high production costs, we could only obtain and test a small number of silk protein samples. Unfortunately, one of the silk protein rods was inadvertently damaged due to incorrect use during the torsion test. The samples were also slightly asymmetrical in size and imbedding, potentially resulting in some error when under concentric compression and torsional strain. In the three-point bending test, the diameter of the silk protein samples proved too small, resulting in friction when axial force was applied, potentially affecting the results. Furthermore, the solid denture mix used to fix the silk protein samples during torsion testing had lower ultimate stress than the samples, resulting in it occasionally failing first, which is why the linear torsion results were not fully available. Finally, the irregular geometrical characteristics of the fibula, which we assumed to be approximately elliptical in cross-section, caused a minor error during data processing. Overall, the results of our biomechanical testing, in conjunction with known reference values, were adequate to conduct 3D finite element analyses.
Silk protein has been demonstrated to have good biocompatibility and clinical applications. Although silk protein rod implantation did not prove to be superior to fibula or tantalum rod implantation in preventing further collapse of the femoral head after decompression, it still has the potential for clinical application in osteonecrosis treatment as it can be strengthened, degraded and embedded with bone morphogenic proteins [
27,
28]. The silk protein samples used in our biomechanical tests were semi-finished, and not the hardest finished product. The primary objective of our study was to understand the biomechanical properties of silk protein rods and provide a reference for additional improvement of silk protein rods. Therefore, further investigations into silk protein rods as a potential treatment option for early-stage osteonecrosis of the femoral head are worthwhile.
Future research would aim to increase the hardness of the silk protein rod by decreasing the molecular weight of the silk protein constituents. We would propose adding a porous agent to the preparation to roughen the surface and produce a more porous structure, which would produce a design closer to the ideal implant material. A roughened surface increases friction and enhances the stability of the silk rod after implantation, and a more porous structure can better promote new bone ingrowth and embed bone morphogenic proteins and other cytokines.
Publisher’s Note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.