Design modifications and evaluation of the silicone artificial finger joints

Abstract

BACKGROUND:

There are many reasons that could lead to finger joint arthroplasty, and the most familiar reason is osteoarthritis. Silicone finger joint are the most commonly used implants. However, these implants might fracture with time and cause wear which will lead to chronic inflammation and synovitis for the patient and then implant failure.

OBJECTIVE:

The aim of this study is to improve the design of the silicone finger joint and simulate the different designs using finite element analysis (FEA) simulation.

METHOD:

Three different designs were drawn and FEA has been used in this study using Solidworks software. The first design is the silicone finger joint design without any modification, the second one is modified design with added ribs to the junction of distal stem and hinge and the third design was added filler material inside the body of the artificial joint. An axial force with 625 N that was applied on the upper part of the distal stem which is nearly represents the maximum value of the grip strength for normal males.

RESULTS:

The results showed improvement on the design in which the concentrated stress at the junction of the distal stem and hinge of the design was distributed. In addition, the Von Mises stress was stable for the modified design with added ribs and the added filler material designs after 15°.

CONCLUSION:

The design modification could improve the stress distribution and stability of the artificial finger joint and increase the lifetime expectancy of these implants.

Keywords

Artificial finger joints FEA design Swanson

1. Introduction

Knuckles are joints shaped by the bones of the fingers which have primary or main knuckle which is metacarpophalangeal joint (MCP) incapacitated in closed-fist activities and knuckles in the middle of the finger is the proximal inter-phalangeal joint (PIP) [1]. Distal inter-phalangeal joint (DIP) is the furthest joint of the finger [1,2]. There are many diseases affecting finger joints such as gout, psoriatic arthritis, juvenile rheumatoid arthritis, osteoarthritis and rheumatoid arthritis [3]. One of the most common chronic diseases affecting the finger joint is osteoarthritis [4]. Around 43 million in the USA are affected by this disease and around 15% of the world population [5–7]. Osteoarthritis is a disease affecting synovial joints and leading to the softening of the cartilage which will lead to cartilage break down and failure [8]. The failure of the cartilage will lead eventually to pain in the joint and cause limitation of activity which will lead to absenteeism for working adults and major decline in function for the elderly patients [8].

The treatment of the osteoarthritis could be exercise and physiotherapy or by using medications and if the patient are not responding to these treatment then the surgical treatment will be done [9]. The arthrodesis surgical treatment of osteoarthritic finger joint is done by fusing two bones around the joint [1]. This will lead to losing the range of motion and limiting the ability of normal finger movements for patients [10]. Arthroplasty is an alternative solution for joint fusion procedures where an artificial joint is used to replace the natural one. The main function of the artificial finger joint is to repair motion of the finger joint; limited motion of the finger joint can cause severe pain to the patient. There are two main categories of the finger joint prosthesis: the first one is an integral hinge prosthesis or space-filler (single-piece design) such as Swanson and NeuFlex artificial finger joint [11]. The second category is the mechanical ball and socket prosthesis (two-piece design) such as pyrocarbon finger joint prosthesis [12]. There are many challenges facing the ball and socket design such as wear, dislocation of the implant and implant loosening [13–15]. The single-piece silicone implant such as Neuflex and Swanson are the most commonly used implants [1,16–18]. These type of implants are better in ease of implantation, improving cosmetic appearance of the finger and providing wider motion than the sliding implant [16]. However, there are many challenges facing the single-piece silicone implants such as Swanson and Neuflex such as fracture which will lead to wear debris causing chronic inflammation and synovitis for the patient and implant failure [16,19,20].

Designing the silicone artificial finger joint with different specifications should be done to improve the current design; in addition, a specific criterion is needed to decide which design is the best to work with. Since artificial finger joints have many concerns, a comparison between different designs will be done. Artificial finger joints are used to restore the movement of the fingers without any disability and maintain a physiological range of motion [1]. It is very important to choose the right size of the artificial finger joint that fits perfectly inside the finger, in addition, the material used is also very important. The device must be able to sustain proper spacing, to permit the full range of motion and afford stability. It is essential to come in a variety of sizes to accommodate patients with different finger lengths and spacing needs. Like a natural finger joint, the artificial finger joint must act as proper finger with the full abilities especially if it is going to be used in several levels of load at one time. Many factors can help either causing failure by fracture or a true success for the finger joint and lengthening its life time as much as possible. The MCP (Metacarpophalangeal) joint has a passive range of motion in the sagittal plane is 90° of flexion and 20°–30° of extension (hyperextension), 40° arc in the coronal plane for abduction and adduction movement. The MCP joint ranges from 33° to 73° of flexion, with an average of 61°. For tasks such as pinching, grasping, and gripping, the flexion of the MCP joint is 58°, 33°, and 72°, respectively. For ordinary activities it can perform between 0° and 68° or between 5° and 62° [21]. The grip force for normal males has a wide range from 81 N to 672 N, according to age, hand dominance and the instrument used. The grip force for normal women ranges from 21 N to 425 N. The grip strength of women is on average 56% that of men [21]. Total active extension improved from 32 degrees prior to surgery to 18 degrees following surgery [21], which makes the silicone material a perfect candidate. The MCP joint of the middle finger has resulted to have a range of motion (ROM) of 15° flexion to 65° flexion during activities of daily living (ADL) considering activities like using the computer, using a cup, and using scissors. According to our knowledge there is no previous research studied the effect of modifying the design of the silicone finger joint and simulate it.

The aim of this paper is to improve the design of the artificial finger joint and simulate the different shapes of the silicone artificial finger joints and to compare between them to find the optimal design and to suggest future modifications on the current designs. According to our knowledge, there are no research study in the literature has done improvements and FEA simulation on the design of the artificial finger joint.

2. Methodology and materials

2.1. Methodology

In this study, a similar shaped designs of a Swanson artificial finger joints were designed using Solidworks 2020 software (SolidWorks Corp., USA), as shown in Fig. 1. A finite element analysis (FEA) simulation was done to the artificial finger joint to check the weakness points of the similar designs to try to improve it. Two modified designs were drawn using Solidworks. The first modified design was done by adding a rib to the junction of distal stem and hinge of the artificial joint, as shown in Fig. 2. The second modified design was done by adding a filler material inside the implant, as shown in Fig. 3. Convergence was done for the design with different number of meshing elements to check the stability of the model and results.

Fig. 1.

Artificial finger joint without any modifications designed using Solidworks.

Fig. 2.

Artificial finger joint with a rib to the junction of distal stem and hinge designed using Solidworks.

Fig. 3.

Artificial finger joint with a filler material inside the implant designed using Solidworks.

A tetrahedral element meshing was used in all models due to the ability of modelling irregular shapes under large deformations such as hyperelastic rubbers, as shown in Fig. 4. The axial force that was applied on the artificial finger 625 N on the upper part of the distal stem which is nearly represents the maximum value of the grip strength for normal males [21]. Proximal stem was fixed with zero displacement and the distal stem was able to move freely in all directions.

Fig. 4.

The geometric model for a similar shaped Swanson was meshed using Solidworks.

2.2. Materials

For over 60 years, silicone materials have been used in medical applications. It can withstand extreme cold and heat, is chemically resistant, and can be tailored to include a variety of special features. Silicones are simple to work with and come in a variety of haptic qualities, giving designers a wide range of possibilities. Silicon’s properties include thermal stability (temperature range of 100 to 250 °C), low heat conductivity, low chemical reactivity, low toxicity (does not allow microbiological growth), biocompatibility and bio durability. They offer different chemical and physical features that result in high biocompatibility and bio durability in a variety of applications. Silicone elastomers have an extremely low coefficient of friction and sustain their glass-transition temperatures allows flexibility across a wide temperature range. They must be able to tolerate circumstances ranging from cold storage to extreme heat such as autoclaving using steam. They have a low surface tension and are easy to work with exceptional chemical stability allows for biocompatibility and bio durability in a variety of long-term implant applications [22]. Silicone materials have a backbone structure made up of repeated units of inorganic –SiO2–, with methyl or other functional groups acting as –Si– side groups. Biocompatible, extremely stable, nontoxic, and insoluble in human fluids, these materials are excellent [22]. The SolidWorks was used to design the different artificial finger joint, this software enables a designer to design each part of the artificial finger joint. After the design is completed, simulation was applied on the artificial finger joint using silicone material. Flexspan silicone was used in this study in the design. The elastic moduli of the Flexspan is 2.3 MPa and the Poisson’s ratio is 0.45. The uniaxial tensile test results were conducted from a previous research [23]. A natural rubber material was used as a filler material in the third design with tensile strength of 20 MPa, Poisson’s ratio is 0.5 and elastic moduli of 0.01 MPa. Hyperelastic material model Mooney–Rivlin was used in the simulation of this model previously [23].

2.3. Mooney–Rivlin hyperelastic material model

Mooney–Rivlin is a hyperelastic model used in elastomers especially with the hyperelastic materials. The Mooney–Rivlin strain energy function is expressed by Eq. (1): $\begin{eqnarray}{\rho}_{0}{\psi}=C_{1}(\bar{I}_{1}-3)+C_{1}(\bar{I}_{2}-3)+\frac{1}{2}K(\ln J)^{2}\end{eqnarray}$ (1) Where, $\begin{eqnarray}\displaystyle \text{} & \text{} & \displaystyle \bar{I}_{1}=J^{-2/3}I_{1}\end{eqnarray}$ (2) $\begin{eqnarray}\displaystyle \text{} & \text{} & \displaystyle \bar{I}_{2}=J^{-4/3}I_{2}\end{eqnarray}$ (3) $\begin{eqnarray}\displaystyle \text{} & \text{} & \displaystyle J=\det F\end{eqnarray}$ (4) where C1 and C2 are material constants, K is the bulk modulus, where $\bar{I}_{1}$ and $\bar{I}_{2}$ in Eqs (2) and (3) are the first and the second principal invariants of the right Cauchy–Green deformation tensor C, J in Eq. (4) is the Jacobian (determinant of the deformation gradient, corresponding to the ratio of the current and initial volume). The Mooney–Rivlin constants for the materials were C ₁ =1.73 MPa and C ₂ = −1.39 MPa.

3. Results and discussion

Figure 5 shows the convergence results of the artificial finger joint in which the stress started to be stable after 20000 elements of meshing for the artificial finger joint. When increasing mesh density the results of the maximum Von Mises stress showed less than 1% variance for all the stresses. Also, showed less than 0.1% for the last four results of the maximum Von Mises stress for where a finer mesh were used after number of elements 14922 elements.

Fig. 5.

Number of elements and Von Mises stress for the similar shaped Swanson finger joint simulation using Solidworks.

A less than 1% variance occurred during simulation as a convergence for all the maximum Von Mises stress result, as shown in Fig. 5. This result is acceptable and comparable with previous simulation for the same joint [23]. The simulation results showed that the junction of distal stem and hinge of the design must be improved as it showed stress concentration in that region which might be the main failure cause of the real prosthetic finger joint. Moreover, many researchers [15,19] have mentioned that the failure occurred at the junction of distal stem and hinge of the design. Thus, improving the design by adding ribs to the junction could improve the stress distribution as shown in Fig. 6.

Figure 6 shows the similar shaped Swanson design after simulation. The results showed that the design has weakness points at junction of distal stem and hinge of the design which will lead to fracture. The results showed that the maximum Von Mises stress facing the junction of the stem and hinge is 0.325 MPa when the artificial finger joint reaches 23°.

Fig. 6.

FEA results of the artificial finger joint without any modifications.

Figure 7 shows the ribs modified artificial finger joint simulation. The modified design showed better stress distribution in comparison with the original design (Fig. 6), as the original design had stress concentrated at the junction of distal stem and hinge. The results showed that the maximum Von Mises stress facing the junction of the stem and hinge is 0.484 MPa when the artificial finger joint reaches 23°. The ribs plotted at the junction of distal stem and hinge improved the design by distributing the stress concentrated at the junction which might lead to fracture of the implant.

Fig. 7.

Ribs modified design of artificial finger joint simulation results.

Using filler as mentioned in the methodology section inside the finger joint will not have an improvement on the maximum Von Mises stress value or on the stress distribution at junction of distal stem and hinge of the design, as shown in Fig. 8.

Fig. 8.

Modified design with filler of the artificial finger joint simulation results.

Figure 9 shows a comparison between the three different designs for original design without any modification, one with filler and another design with ribs added to the original design. Before 6 degrees of flexion movement the results showing near results for the three designs. However, the variation of the results between the three designs started to appear clearly after 10 degrees of flexion movement. The original design without any modifications showed increasing values of during the incremental degrees of movements during flexion movement. The design with filler material showed stable behavior after 15 degrees of flexion movement and this could be as a results of the mixed mechanical behaviors of the two materials led to more stable behaviors of the Von Mises by distributing the stresses on the two materials. The third modified design with ribs showed nearly stable Von Mises stress with Flexion movements due to the stress distribution appeared clearly in Fig. 7. The results of the modified design with ribs showed higher Von Mises stress because of adding ribs at the junction of distal stem and hinge of the design but the stresses were distributed and not concentrated at that critical region were most fractures occur, as shown in Fig. 7. Therefore, the filler and the modified design with ribs showed a better mechanical behavior and stress distribution than the original design of the finger joint.

Fig. 9.

Modified design of different artificial finger joints simulation results.

The results showed that the Von Mises stress was at the highest value at the junction of distal stem and hinge of the design, as shown in Fig. 6. This is normal to happen as the moving part is the distal stem and it was approved previously [23]. Also, these results have been approved and observed clinically in which fractures at the stem-hinge region of the implant have been reported [19,24]. Adding the rib for the design improved the Von Mises stress and distributed the stress at the hinge, as shown in Fig. 7. This stress distribution could relief the stress concentration at the hinge which will reduce the possibility of the fracture. Another solution is using a filler inside the implant to improve the mechanical properties of the joint. The results showed that the Von Mises stress were increasing with the flexion angle as shown in Fig. 8. The results for the similar shaped Swanson design of the simulation in this paper are comparable with the results from the previous research study [23]. According to our knowledge, there is only one study has been done on the FEA simulation of the Swanson artificial finger joint.

4. Conclusion

Silicone finger joint implants has shown great success; however, there are still some failure cases related to the fracture that leads to wear and then inflammations in the surrounding tissues. It has been shown from the results that the weakest point of the design was at the junction of the junction of distal stem and hinge which will lead eventually to fracture. One of the suggested solutions was to apply ribs at this junction which will help in distributing the stress at that region. The results showed that the ribs have been useful in distributing the concentrated stress at that region. Both modified design with added ribs and inside filler design showed stability in the Von Misses stress after 15 degrees of flexion.

Footnotes

Conflict of interest

None to report.

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