Thermal Effects of Polyethylene on Shell Liner Congruency
Profile Series 4, Number 1 September 2003
Most materials exhibit measurable changes in dimensions due to changes in temperature. The material’s “coefficient of thermal expansion (α)” is a measure of this phenomenon.1,2 Ultra high molecular weight polyethylene (UHMWPE), a common bearing material in orthopedic implants, is known to have a much higher coefficient of thermal expansion than metallic materials.3,4 The coefficient for UHMWPE is more than 20 times the coefficient for titanium alloys.
In the case of the A-Series liner and mating acetabular shell, Exactech designers have used this difference in thermal expansion to control the fit between the liner and shell.
The AcuMatch® A-Series design capitalizes on the difference in fit between room temperature and body temperature to tighten the fit in vivo. This was confirmed by laboratory testing as detailed below.
A 50mm Multi-Hole shell was fixed on an Instron Universal Testing Machine frame and a 15° liner was inserted into the shell. A dial indicator capable of measuring motion to within .0005" was positioned at two different points along the liner/shell interface to quantify the movement of the liner within the shell as rim loads were applied (Figure 1). Increasing load was applied until deflection ceased.
The first measurements were taken at room temperature (68°F) to quantify actual behavior during the implantation of the cup and liner. Motion measurements were repeated after components were submerged in a temperature controlled water bath and heated to body temperature (98.6°F).
As seen in Table 1, when the liner was loaded perpendicular to the cup face, the movement measured at the cylindrical region decreased significantly from room temperature to body temperature. The average movement recorded at room temperature was .0060" compared to .0005" at body temperature.
Table 1. The effect of temperature on observed liner-shell micromotion
|Spherical Region Movement*||Cylindrical Region Movement|
|Room Temperature||.0035" (0.09mm)||.0060" (0.15mm)|
|Body Temperature||<.0005" (0.01mm)||.0005" (0.01mm)|
*In the spherical region indicator motion was mathematically converted to radial motion at the interface to allow comparison. This yielded approximately .0035" (0.09mm) of movement at room temperature compared with an unmeasurable amount of movement at body temperature.
Based on these observations, one can see that the predictable change in temperature has been used to significantly reduce the clearance at the polyethylene-metal interface, minimizing micromotion at body temperature while allowing initial assembly of the components.
1. Material properties provided by Westlake Plastics Company, Lenni, PA.
2. American Industrial Plastics, Daytona Beach, FL.
3. MIL-HDBK-5G, November 1, 1994.
4. Materials Properties Handbook: Titanium Alloys, ASM International