AMERICAN SOCIETY OF BIOMECHANICS
Presented at the Twenty-First Annual Meeting
of the American Society of Biomechanics
Clemson University, South Carolina
September 24-27, 1997
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| AMERICAN SOCIETY OF BIOMECHANICS
Presented at the Twenty-First Annual Meeting |
The use of structural autografts for fracture stabilization, deformity reconstruction, and arthroplasty is well accepted in orthopaedic practice. However, for natural bone grafts, the associated donor site morbidity (including motor weakness, sensory defects, and pain) is often a concern [1]. As such, porous ceramics and metals provide an attractive alternative to naturally derived autografts. Typically, porous metals are a thin (about 0.5 to 1.0 mm) coating of sintered titanium or Co-Cr-Mo alloy wire or beads onto a like solid substrate, resulting in a 20-35% porous surface. A new porous tantalum material characterized by 75 to 85% porosity, pore size between 500 and 600 mm, 3-D interconnecting pores, and fabricated in bulk form (not a coating) is an attractive alternative to traditional porous metals, and in substitution for autograft and allograft bone. A primary function of any porous bone substitute material is to provide for initial stabilization of the bone-implant interface to permit bone ingrowth and long term fixation. The purpose of this study was to measure the friction of porous tantalum in contact with periosteum intact and denuded cortical bone and cancellous bone using an established model for bone-on-bone friction, and to compare these values to those for bone-on-bone, and sintered bead porous coating on bone. The primary utility of this information the defining of boundary and interface conditions for FEA computer modeling of devices and loaded fabricated from porous metals and in comparison to bone.
An inclined plane apparatus based on ASTM Specification D4518-91 was used to determine the coefficients of friction. The apparatus consisted of a horizontal stationary plate which was rigidly mounted to an MTS Bionix( machine, with a second hinged plate attached to the MTS actuator. Movement of the actuator arm permitted adjustable inclined planes of up to 90°. The substrate block (either cancellous bone or porous tantalum) was secured to the inclined plane with a smaller slider block (either porous tantalum or cortical bone) placed on top. By gradually raising the incline from 0°, the angle of slippage (ø), and thus the coefficient of friction (tan (ø), was determined.
Ten fresh bovine radii were obtained from a local abattoir. Cortical bone squares measuring 2 x 2 x 0.6 cm were machined from the anterior radial diaphysis. The overlying soft tissue was carefully dissected until only periosteum remained. Two mutually perpendicular holes, 3 mm in diameter, were drilled through each specimen parallel to the periosteal surfaces. A metal rod was placed through the hole and weights attached to each end. A normal force of 4.4 N was applied to the slider specimens, and even at steep angles near 90°, sample tilting did not occur. Cancellous bone specimens 4 x 4 x 2 cm were obtained from the proximal epiphyses, subsequently potted in a circular mold with PMMA, and milled flat.
Testing of the cancellous-porous tantalum interface was accomplished using 2 x 2 cm sliders placed on 4 x 4 cm cancellous bone substrates. The two surfaces were copiously wetted with normal saline prior to testing. The angle of inclination was increased from 0° at a rate of 1.5° s-1 until the point of slippage. The tangent of the slip angle provides for calculation of the coefficient of friction. Five trials were performed for each of the four possible "downhill" orientations of the cancellous substrate. It was not possible to obtain flat cortical blocks measuring greater than 2 x 2 cm, therefore, testing of cortical specimens was done with the porous tantalum as the substrate and the cortical bone as the slider. With this exception, the same testing protocol was used for both periosteum intact and denuded specimens.
The average friction coefficient of porous tantalum against cancellous bone was 0.88 (n=100 trials, s.d.=0.09). There was no visually perceptible bone residue on the porous tantalum surfaces after testing. The determination of friction between the periosteum-intact cortical bone and the porous metal proved complex because the periosteum was extremely adherent to the porous metal. For three of the five cortical blocks, the periosteum was more that 50% defaced from the underlying bone before testing was completed. The friction coefficient calculated from the two specimens for which periosteum-intact testing was performed was 1.75 (n=40 trials, s.d.=0.33), indicating an adhesive character between the porous tantalum and periosteum. There was visually perceptible periosteal residue on the porous tantalum after testing. For periosteum-free cortical bone against the porous metal, the average coefficient of friction was 0.74 (n=100 trials, s.d.=0.07).
The friction coefficients for this novel porous tantalum material were higher than those previously reported for cortical bone against cancellous bone (0.608) [2] and for traditional porous coated materials on cancellous bone (0.50 - 0.66) [3]. Table 1 below summarizes the coefficient of friction (COF) data from this and previous studies.
| Friction Couple | COF |
|---|---|
| Porous Ta - cortical bone+periosteum | 1.75 |
| Porous tantalum - cancellous bone | 0.88 |
| Porous tantalum - cortical bone | 0.74 |
| Cortical bone - cancellous bone | 0.61 |
| Sintered beads - cancellous bone | 0.5-0.66 |
Table 1: Summary of coefficient of friction data
The supra-unity friction coefficient for periosteum against porous tantalum was an interesting finding in that soft tissue fibers have a tendency to be “snagged” by the small spicules at the surface of the porous tantalum, resulting in what appeared to be an adhesive effect. The coefficient of friction for the porous tantalum material was higher than that exhibited by bone-on-bone and sintered beads-on-bone. The results of this study will be applied to future FEA modeling, and specifically for prediction of the implant-bone stability based on the parameters of design, loading, and interface friction.
[1] Parker and Urbaniak, JBJS, 78A:2, 1996.
[2] Ahn et al., Trans. 42nd ORS:603, 1996.
[3] Shirazi-Adl et al., J Bio Mat Res, 27, 1993.
Financial support (grant) and porous tantalum samples provided by Implex Corp.