In-situ simulation of one-piece metacarpophalangeal joint implants using finite element analysis

The silicone metacarpophalangeal joint arthroplasty: An in-vitro analysis

BACKGROUND

Silicone is still the gold standard implant in metacarpophalangeal arthroplasty. Whereas the clinical results are acceptable, in follow-ups with >10 years, high rates of implant fracture are common, and 5 to 7% of implants required revision. This work's purpose is to analyse the hypothesis that the joint flexion amplitude has a relevant effect on bone strain level, implant stress and bone-implant micromotion, which can reflect an increase in the risk of bone resorption/fatigue failure, implant fracture and osteolysis.

METHODS

To experimentally predict the cortical loading behaviour, composite metacarpals and proximal phalanges were used in intact and implanted states. A finite element model was developed to evaluate the structural behaviour of cancellous bone and implant. This model was validated by comparing cortical strain and load-displacement curve with experimental measurements.

FINDINGS

Bone strain changes between the intact and the implanted states showed a load transfer effect from the cortical to the cancellous bone that increases significantly with the flexion's amplitude rise. The peak implant stress occurred in the flexion amplitudes further away from the implant neutral angle. The highest implant pistoning motion and the highest phalanx cancellous-bone strain occurred simultaneously at the maximum flexion amplitude.

INTERPRETATION

Limiting joint flexion range will be helpful to reduce the strain-shielding effect on cortical bone, minimizing the overload effect on cancellous bone and decreasing the stress levels and the pistoning motion on the implant, ultimately contributing to the longevity of silicone arthroplasty.

Structural assessment of pre-flexion in silicone implants for arthroplasty of the first metatarsophalangeal joint

The development of numerical models to analyze pathologies and implants related to the first metatarsophalangeal joint of the foot remains an issue for attention. The structural effects of implants pre-flexion have been discarded in several finite elements analyses due to complexities to achieve these positions. This work aims to evaluate if the pre-flexion stress state should be included or could be discarded when only flexion is applied in two different silicone commercial implants, Swanson and Tornier, during a gait cycle. Finite element models were created for silicone implants. Both models were discretized using high-order finite elements. The hyperelasticity constitutive material model of Arruda-Boyce was used, based on experimental data; its behavior was compared with linear elastic models reported and used frequently assuming small and large deformations and applying to the Swanson and Tornier implants a flexion angle of 64°, which corresponds to in vivo measurements reported after implantation. Comparison between models, regarding hyperelastic model, showed mean variations of up to 32.5% for stresses and 14.01% for bending moment in Swanson implant, while for Tornier implant mean variations of 29.73% and 632.55% was obtained for stress and bending moment respectively. The maximum stress value obtained for the hyperelastic model in the Swanson implant reached a value of 22.82% of the tensile strength of the implant material while in the Tornier implant reached a value of 25.92%, the above values were evaluated at a flexion angle of 64°. The results suggest considering in finite element analyses not only the stress state generated to achieve critical flexion position in pleflexed implants models but also the hyperelastic material behavior of silicone for implants to avoid dismissing the non-linear structural behavior of hyperelastic materials.

Is Swanson prosthesis better than Sutter prosthesis for metacarpophalangeal joint arthroplasty? A meta-analysis

The aim of this meta-analysis is to compare the outcomes of the Swanson and Sutter prostheses (previously the Avanta prosthesis) used for metacarpophalangeal joint arthroplasty, and provide a powerful and rational conclusion regarding the use of prosthesis in MCP joint surgery. The literature search was based on PubMed, Cochrane Library, MEDLINE, EMBASE, and the Chinese National Knowledge Infrastructure. Data were evaluated using a generic evaluation tool designed by the Cochrane Bone, Joint, and Muscle Trauma Group and analysed using RevMan, version 5.0. Six randomised controlled trials were contained in this review, and five of them involving 143 patients were included in the meta-analysis. The results suggested that using the Sutter prosthesis could significantly decrease the rates of recurrence of drift when compared with the Swanson prosthesis for metacarpophalangeal joint arthroplasty (OR = 2.05, 95% Confidence interval (CI) = 1.31-3.20, p = 0.002). No significant difference in the outcomes of prosthesis fracture was found in two groups (OR = 1.07, 95% CI = 0.41-2.79, p = 0.88). Due to the limited data, the outcomes of range of motion, correction of ulnar deviation, pain, grip strength, and radiographic osteolytic changes could not be included in the meta-analyses. Theoretically, recurrence of drift was more common with Swanson prosthesis when compared with the Sutter prosthesis. No significant difference in the outcomes of prosthesis fracture was observed in two groups. More high-quality studies are required in long-term follow-up.

Influence of high-frequency cyclical stimulation on the bone fracture-healing process: mathematical and experimental models

Mechanical stimulation affects the evolution of healthy and fractured bone. However, the effect of applying cyclical mechanical stimuli on bone healing has not yet been fully clarified. The aim of the present study was to determine the influence of a high-frequency and low-magnitude cyclical displacement of the fractured fragments on the bone-healing process. This subject is studied experimentally and computationally for a sheep long bone. On the one hand, the mathematical computational study indicates that mechanical stimulation at high frequencies can stimulate and accelerate the process of chondrogenesis and endochondral ossification and consequently the bony union of the fracture. This is probably achieved by the interstitial fluid flow, which can move nutrients and waste from one place to another in the callus. This movement of fluid modifies the mechanical stimulus on the cells attached to the extracellular matrix. On the other hand, the experimental study was carried out using two sheep groups. In the first group, static fixators were implanted, while, in the second one, identical devices were used, but with an additional vibrator. This vibrator allowed a cyclic displacement with low magnitude and high frequency (LMHF) to be applied to the fractured zone every day; the frequency of stimulation was chosen from mechano-biological model predictions. Analysing the results obtained for the control and stimulated groups, we observed improvements in the bone-healing process in the stimulated group. Therefore, in this study, we show the potential of computer mechano-biological models to guide and define better mechanical conditions for experiments in order to improve bone fracture healing. In fact, both experimental and computational studies indicated improvements in the healing process in the LMHF mechanically stimulated fractures. In both studies, these improvements could be associated with the promotion of endochondral ossification and an increase in the rate of cell proliferation and tissue synthesis.

Biomechanical validation of finite element models for two silicone metacarpophalangeal joint implants

Silicone implants are used for prosthetic arthroplasty of metacarpophalangeal (MCP) joints severely damaged by rheumatoid arthritis. Different silicone elastomer MCP implant designs have been developed, including the Swanson and the NeuFlex implants. The goal of this study was to compare the in vitro mechanical behavior of Swanson and NeuFlex MCP joint implants. Three-dimensional (3D) finite element (FE) models of the silicone implants were modeled using the commercial software ANSYS and subjected to angular displacement from 0 deg to 90 deg. FE models were validated using mechanical tests of implants incrementally bent from 0 deg to 90 deg in a joint simulator. Swanson size 2 and 4 implants were compared with NeuFlex size 10 and 30 implants, respectively. Good agreement was observed throughout the range of motion for the flexion bending moment derived from 3D FE models and mechanical tests. From 30 deg to 90 deg, the Swanson 2 demonstrated a greater resistance to deformation than the NeuFlex 10 and required a greater bending moment for joint flexion. For larger implant sizes, the NeuFlex 30 had a steeper moment-displacement curve, but required a lower moment than the Swanson 4, due to implant preflexion. On average, the stress generated at the implant hinge from 30 deg to 90 deg was lower in the NeuFlex than in the Swanson. On average, starting from the neutral position of 30 deg for the preflexed NeuFlex implant, higher moments were required to extend the NeuFlex implants to 0 deg compared with the Swanson implants, which returned spontaneously to resting position. Implant toggling within the medullary canals was less in the NeuFlex than in the Swanson. The differential performance of these implants may be useful in implant selection based on the preoperative condition(s) of the joint and specific patient functional needs.

Frequency dependence of viscoelastic properties of medical grade silicones

Cylinders of medical grade silicone elastomers, (29 mm in diameter and 13 mm thick), immersed in physiological saline solution at 37 degrees C, were investigated by dynamic mechanical analysis (DMA). A sinusoidal cyclic compression of 40 +/- 5 N was applied over a frequency range, f, of 0.02-100 Hz. Values of the storage, E', and loss, E'', moduli for the cylinders were found to depend on f; the dependence of E' or E'' on the logarithm (base 10) of f was represented by a third-order polynomial. Above about 0.3 Hz, the cylindrical specimens appeared to be undergoing the onset of a transition from the rubbery to the glassy state. There was no significant difference between results obtained at 37 and 23 degrees C; pretreatment of specimens in physiological saline at 37 degrees C for 24 h and 29 days had no appreciable effect on the results.

Wear of medical grade silicone rubber against titanium and ultrahigh molecular weight polyethylene

The replacement of arthritic small joints, such as the fingers and wrist, has typically involved the use of one-piece silicone rubber implants. Newer designs have involved the silicone moving against either a titanium or ultrahigh molecular weight polyethylene (UHMWPE) component. The aim of this study was to investigate the wear of medical grade silicone rubber against titanium and UHMWPE. A pin-on-disc apparatus was used to slide a titanium and UHMWPE pin against a silicone disc, in the presence of either a Ringer's solution or bovine serum lubricant. Testing was undertaken at a sliding speed of 0.079 m/s and was continued for 10 km. Wear factors for titanium against silicone were 40.0 x 10(-6) mm(3)/N m and 66.5 x 10(-6) mm(3)/N m for bovine serum and Ringer's solution, respectively. The wear factors for UHMWPE against silicone were higher with values of 84.4 x 10(-6) mm(3)/N m and 88.3 x 10(-6) mm(3)/N m for bovine serum and Ringer's solution, respectively. The results of this study will be useful in future designs of finger and wrist implants that combine silicone rubber with either titanium or UHMWPE.

Titanium basal joint arthroplasty: a finite element analysis and clinical study

PURPOSE

To define the mechanics and determine the clinical outcome of titanium implant arthroplasty in trapeziometacarpal arthritis.

METHODS

For the finite element analysis (FEA) a 2-dimensional FEA mesh of the titanium arthroplasty was constructed with 8-node quadrilateral elements and analyzed. Flexion-extension displacement of the metacarpal was analyzed. For the clinical study, between 1996 and 2003, 47 patients (50 thumbs) with Eaton stage 3 trapeziometacarpal arthritis were treated with titanium basal joint arthroplasty. The average follow-up period was 2 years. Disabilities of the Arm, Shoulder, and Hand questionnaire answers and grip and pinch measurements were obtained before surgery and at the final follow-up evaluation. Failure was defined strictly as the point when revision to the standard soft-tissue interposition arthroplasty became inevitable.

RESULTS

In the FEA the titanium implant showed pistoning behavior with maximum stress concentration in the midmetacarpal shaft of 1.92 MPa. The convex sphere of the implant rotates and lifts out of the trapezial crater with a high stress concentration of 0.51 MPa at the radial and ulnar corner of the trapezium. In the clinical study treatment failed in 10 of the patients before 9 months. All were converted successfully to a standard ligament reconstruction tendon interposition. The remaining 80% of the patients showed significant improvement in Disabilities of the Arm, Shoulder, and Hand questionnaire scores albeit with continued weakness at the 2-year follow-up evaluations. The reconstructed thumbs never attained the strength of the contralateral thumbs; even in the success group residual swelling was not uncommon with any increase in activity level.

CONCLUSIONS

Anecdotal quotations show success rates for titanium implant arthroplasty for basal joint arthritis to be as high as 97%. Our results are quite to the contrary in that high failure rates were common early in the follow-up period. Our FEA results are confirmed by the clinical study. Titanium implant arthroplasty may have a role in low-demand patients with good bone stock; however, we have stopped offering titanium hemiarthroplasty to patients at our institution.

FEA analysis of silicone MCP implant

This paper discusses finite element study of silicone rubber prosthesis for the metacarpophalangeal joint of the hand. Based on the experimental data, a material model which incorporates test data available for different stress states was chosen and calibrated. Finite element models for three commercially available silicone joint prosthesis were developed. All models incorporated the same material model and allowed for large deformations. These models were validated against the experimental data and analyzed under demanding loading conditions. Results such as highly non-linear material behavior, dependence on the loading history and large deformations near wrinkle formation in the hinge area of the joint clearly show the necessity and importance of using multi-stress- state non-linear material models and accounting for large deformations.