METHODS: A three-dimensional finite element non-linear contact analysis was performed. CAD (Pro E) models were imported into PATRAN (version 8.5), enabling the FE meshes to be generated of each component; Figure 1. Each metallic component was modeled as a rigid body. The carpal and radial components are represented by 4-noded quadrilateral elements (6,710) and 3-noded triangular elements (1,366), respectively. The polymeric component was modeled via 20,130 8-noded hexagonal elements (E=634.92 MPa, v=0.45). In addition to the aforementioned user defined elements, 14,370 elements were defined internally by ABAQUS 5.8 for contact purposes.

Initiating in the ‘neutral’ position (Figure 1a), the radial component was free to translate in the radial-ulnar and volar-dorsal directions, while the carpal-poly complex was unconstrained along the vertical axis. A compressive load was maintained while the prescribed rotation (in increments of 0.1 degrees) was applied about a central axis parallel to the long stem of the carpal component. The compressive load was varied between models from 10N to 110N, in 20N increments, to simulate various degrees of soft tissue tension. The poly-radial interface was assumed frictionless for modeling purposes.

Experimental validation was performed on a BIONIX 858 Test System (MTS). Initiating in the neutral position, the carpal component was rotated +45 degrees with respect to the radial component, after which it was swept back through the neutral position to an angle of –45 degrees, at a rate of 1 deg/sec. A x-y stage was used to provide the translational freedom to the radial component. Six replicate trials were performed for each of the six compressive loads (10N – 110N; 20 N increments).

a b

RESULTS: The computational and experimental resisting moments were in good agreement. The results show an extreme sensitivity to rotational misalignment, even sub-degree rotations. Once neutral alignment was lost, there was a shift in the contact area, in terms of both position (Figure 2) and extent of engagement. This was further accompanied by an abrupt increase in peak contact stress.

DISCUSSION: Within the reasonably physiologic loading range explored, neither an identifiable efficacy threshold nor a point of diminishing return was observed in taughtness versus stability. Rather, a monotonic, almost linear relationship between axial load and resistance to rotational dislocation presented itself. The sensitivity associated with the rotational contact may be attributed to the very high conformity of curvature between the convex UHMWPE surface and the concave radial surface. Enroute to achieving a convergent FE mesh, considerable effort went into obtaining an appropriate local zoning density. The shallow concavity of the radial component, coupled with its abrupt lip dictated the mesh of the UHMWPE component. Establishing a mesh, based on differing predicted regions of contact for differing geometries, can be awkward and computationally expensive for parametric design studies. Consequently, adaptive meshing procedures are very attractive for this class of problems.

* Universal Total Wrist Implant, Kinetikos Medical, Inc., Size: Medium, +2 poly component

REFERENCES: 1. Menon, J., J. Arthroplasty, 13 (5), 1998.

ACKNOWLEDGEMENTS: Financial support was provided by a NIH Grant AR-07075 and by Kinetikos Medical, Inc. The authors would like to thank Dr. Anneliese Heiner for her technical assistance.