Materials Strength Requirement for Artificial Finger Joint

The stress fields on phalangeal bones under various physiological loading conditions are not very well-understood since most empirical stress data on bone loading interactions are limited to large joints such as the hip and knee. Although the finite element method (FEM) can estimate the stress field in a real situation, complicated tendon kinematic chains induce uncertainties which prohibit application in small joint simulation. At present, the loading capacity of finger joints is mostly predicted from simplified 2D theoretical models [62, 63] or more recently by 3D modelling [64, 65]. In An's model [62], the joint loadings in an index finger can be estimated from the unit applied force normal to the distal phalanx. Using this mathematical model with tip pinch strength of 66N estimated from clinical data [66], the maximum internal joint forces of an index finger can be calculated as 179N, 331N and 299N for the DIP, PIP and MCP joints respectively. The joint force from a power grip can also be estimated from this model. According to empirical data, the mean contributions of individual finger from the total grip strength were found to be 30% at index finger, 30% at long finger, 22% at ring finger and 18% at small finger [63]. In particular, forces exerted normal to the distal, middle and proximal phalanx of a single digit were evaluated in the ratios of 1:0.34:0.66 from a grip function [62]. The force component on the distal phalanx of an index finger from a power grip was thereby determined as 81N based on average maximum grip strength of 55.2 kg from a clinical study [67]. Substituting this force component into An's model, the maximum internal joint forces of an index finger from a gripping function were estimated at 281N, 442N and 391N for the DIP, PIP and MCP joints respectively. The joint force at the MCP joint is slightly lower than that in the PIP joint, contradictory to common claims that MCP joints experience the highest joint load [68].

Examination of the required strength of materials for the new design was conducted based on loads and positions that produce the maximum detrimental effect. The effect is considered most critical when maximum load from the middle phalanx is applied in a direction perpendicular to the long bone axis of the proximal phalanx. This perpendicular joint load essentially produces a turning moment at the proximal component.

The maximum bending stress for the new design can be determined from a bending moment of 1.66 Nm due to a maximum joint load of 442 N. With a neutral axis distance of 0.8 mm and a moment of inertia of 1.81e 12 m4 taken from the previously determined geometry of the best-fit fixation stem, this bending stress was calculated to be 734 MPa. To prevent failure from occurring, the strength of the candidate material for the fixation stem has to be higher than this maximum bending stress.

With a bending stress of 734 MPa at the fixation stem region, polymeric biomaterials were excluded due to their insufficient mechanical strength. Three groups of metallic biomaterials, stainless steels, cobalt chromium and titanium alloys, can satisfy such a strength requirement. The biocompatibility of these three metallic materials has been proven at the acceptable level which allows them to be extensively applied in orthopeadic applications. To accommodate the additional requirement as a bearing material, the superior wear resistance of CoCr was considered to be more suitable for the new design. Apart from metals, bioceramics are also good candidate materials for wear-resistant applications.

Thus, in the new joint design, CoCr alloy and Al2O3 ceramic were employed for the following reasons:

1. Simulator studies of hip joints with MOM and COC articulations have shown good wear resistance of at least 50 to 100 times better than polyethylene [69].

2. Retrieval of hip joint components and respective periprosthetic tissues indicated that the wear and wear debris from MOM and COC articulations were substantially less than polyethylene on CoCr.

3. Accumulated debris may mediate cascade osteoclastic bone resorption at periprosthetic tissues resulting in progressive osteolysis and ultimately aseptic loosening to the implant [70-72].

Standard medical-grade CoCr rods were purchased for the fabrication of the new joint design. F90 CoCr alloy was chosen in preference to other variants due to superior mechanical properties and being more biologically compatible with less nickel than some others. Cold rolling resulted in a 99% increase in hardness from the original annealed state. The corresponding tensile strength at this hardness value was estimated to be around 1.6 GPa.

Fabrication of the new design (Fig. 9.9) was performed by electrical discharge machining (EDM) which can machine extremely hard metals when conventional tool-cutting machining becomes infeasible. The EDM process

Fig. 9.9 Prototype of the new design manufactured in CoCr

also offers good dimensional accuracy and complicated geometries can be produced by proper design of tooling.

Production of ceramic components requires geometry shaping and sintering. The properties rely on the purity of the ceramic powder, the grain size and the density free from entrapped porosity. Due to its brittleness, the physical characteristics of alumina ceramics for implant application are subject to stringent demands as outlined in ASTM F603. Additional processes such as hot isostatic pressing at high pressure (200 MPa) and high temperature (2500°C) are normally used to make alumina components with the desired density and strength. These complicated manufacturing processes and the demanding physical properties have limited the application of alumina to simple geometries for hip prosthetic components.

In this work, trial alumina articulating components of proximal and distal joints were produced as shown in Fig. 9.10 by a manufacturing process known as ceramic injection molding (CIM). CIM is a near-net-shape production process for complex geometries with high dimensional accuracies [73]. CIM is still a relatively new production process and much fine tuning of the processing parameters is required to produce high-quality products. From the several trial runs attempted, manufacturing flaws such as cracks and porosity commonly observed in CIM-produced parts [74] were also found in the components produced. Assessment of the mechanical properties of the ceramic components by micro-indentation revealed an average hardness value of 13.4 GPa which is below the minimum limit of 18GPa required in ASTM F603. The trial runs reported here show the possibility of using CIM to make ceramic parts of artificial finger joints. Further fine-tuning of the process is necessary to make parts of sufficient strength. This is left as a potential topic for future investigation.

Fig. 9.10 Prototype of the new design manufactured in Al2O3 ceramic

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