Design and strength analysis of a novel spherical hinge bio-Implant for the Human Metacarpophalangeal Finger Joint 

Technical Summary: Design and Strength Analysis of a Novel Spherical Hinge Bio-Implant for the Human Metacarpophalangeal Finger Joint

Problem Statement
The metacarpophalangeal (MCP) joint is critical for hand function, enabling grasping, pinching, and writing. However, conditions like rheumatoid arthritis and osteoarthritis severely compromise joint mobility and strength. While arthroplasty using one-piece flexible implants (such as the Swanson, Avanta, and Neuflex) is a standard treatment, existing designs face limitations. Clinical data indicates high fracture rates (up to 67% for Swanson implants) and stress concentrations at the hinge-stem junction or the central hinge. Furthermore, the functional range of motion (ROM) achieved by current implants often falls short of the natural joint's capacity (approx. 90° flexion), and there is a lack of comprehensive studies predicting the fatigue life and strength of these flexible bio-implants.

Methodology
This research focuses on the design and finite element analysis (FEA) of a novel one-piece flexible implant featuring a spherical hinge, intended to replace the rectangular box-type hinge found in the Neuflex implant.

• Design Development: A 3D model of the novel implant was developed in SolidWorks. Key design features include:

  • Spherical Hinge: Replaces the box-type hinge to better mimic the natural spherical shape of the metacarpal head and potentially enhance the tendon moment arm.
  • Cutouts: An elliptical cutout at the top of the hinge facilitates hyperextension, while an inverted 'V' cutout at the bottom facilitates flexion.
  • Pre-flexion: The stem is pre-flexed by 30° to replicate the natural resting position of the finger, reducing strain during movement.
  • Dimensions: The model utilizes dimensions based on the Swanson implant (Size 7) and anatomical data for metacarpals and proximal phalanges to ensure comparability with existing literature.

• Material and Simulation:

  • Material: Medical-grade Anasil silicone rubber was modeled using the two-parameter Arruda-Boyce hyperelastic constitutive model to account for incompressible, large-deformation behavior.
  • Meshing: A 10-node quadratic tetrahedral element (C3D10H) with a hybrid formulation was used, with a fine mesh applied to the critical hinge region.
  • Boundary Conditions: The metacarpal stem was fully fixed. The proximal stem was coupled to a reference point at the hinge center, constrained to rotate about the z-axis.
  • Loading: Displacement loading was applied to simulate realistic joint motion: 30° hyperextension followed by flexion from the neutral position (30°) to 90°.
  • Fatigue Analysis: Fatigue life was predicted using the FE-SAFE/Rubber plug-in (Endurica solver) within ABAQUS, focusing on flexion cycles.

Key Contributions

1. Novel Design: Introduction of a spherical hinge geometry with specific cutouts to improve the range of motion and anatomical fidelity compared to the standard box-type hinge.
2. Comparative Stress Analysis: A direct finite element comparison between the novel spherical hinge and the established Neuflex implant under identical loading conditions.
3. Fatigue Life Prediction: Utilization of specialized rubber fatigue solvers to estimate the cycle life of the new design versus the Neuflex implant, a metric often lacking in previous flexible implant studies.

Results
The finite element analysis yielded the following quantitative comparisons between the novel spherical hinge implant and the Neuflex implant:

• Stress Reduction:

  • During hyperextension (30°), the novel implant exhibited a maximum von Mises stress of 0.705 MPa, compared to 0.922 MPa for the Neuflex implant, representing a 23.54% reduction.
  • During flexion (90°), the novel implant showed a maximum von Mises stress of 2.106 MPa, compared to 3.121 MPa for the Neuflex implant, representing a 32.52% reduction.
  • In both cases, the highest stress remained concentrated in the central hinge region, but the magnitude was significantly lower in the new design.

• Reaction Moments:

  • The reaction moment during hyperextension was 43.49% lower in the spherical implant (4.17 N-mm) compared to the Neuflex (7.38 N-mm).
  • The reaction moment during flexion was 36.49% lower in the spherical implant (7.69 N-mm) compared to the Neuflex (12.11 N-mm).

• Fatigue Life:

  • The fatigue analysis indicated a Log-Life value of 9.225 for the spherical implant versus 5.627 for the Neuflex implant.
  • This translates to a fatigue life of approximately $10^{9.225}$ cycles for the new design, significantly exceeding the $10^{5.627}$ cycles of the Neuflex implant.

Significance and Claims
The paper concludes that the novel spherical hinge design offers superior mechanical performance compared to the Neuflex implant. Specifically, the design demonstrates:

• Enhanced strength due to significantly lower von Mises stresses during both extension and flexion.
• Reduced reaction moments, suggesting lower resistance to motion and potentially less stress on surrounding bone and tissue.
• A substantially longer predicted fatigue life, indicating greater durability under repetitive loading.

The authors state that these findings suggest the spherical hinge implant is a viable candidate for finger arthroplasty with improved strength and longevity. However, the paper modestly notes that these results are based on simulation. It explicitly states that experimental research is required to validate the design for practical clinical application. Furthermore, the authors identify the search for alternative biocompatible elastomers with superior mechanical properties as a necessary future scope to further enhance implant life.