Subject UHMWPE

Although some polymers may be satisfactory when used under the stress of static loads, they may fail when subjected to impact. The impact resistance, or resistance to brittle fracture, is a function of the molecular weight of a polymer. Thus uhmwpe is much more resistant to impact failure than general purpose high-density polyethylene (hdpe). The impact resistance of brittle polymers is also increased by the addition of plasticizers. Thus polyvinyl chloride (PVC), plasticized by relatively large amounts of dioctyl phthalate, is much less brittle than unplasticized rigid PVC. 

Each test specimen was subjected to a constant load and a fixed cycle of reciprocating motion. Long term wear tests were performed with the total sliding distance in each case extending to several hundred km. The material removed by wear was monitored by periodic measurements of the weight of the UHMWPE wear pins on a Mettler microbalance with a sensitivity of 1 g Density measurements enabled volume loss (V) against sliding distance (X) relationships to be established at each load (P) and wear factors (k) were then determined from the relationship. 

Measurement of moleeular weight by solution viscosity is only accurate for virgin UHMWPE powder that has not been subjected to temperatures above its crystalline melting point (138-142 °C). UHMWPE, which has been processed into solid shapes by means of heat and pressure, is too insoluble, due to auto-crosslinking, to allow for measurement by solution techniques. The degree of autocrosslinking induced in the polymer by exposure to heat is low but sufficient to inhibit complete dissolution of the material in hot solvent. The solution technique is described by the ASTM test method D4020. 

In summary, UHMWPE was subjected to wear testing and biocompatibility testing prior to its first use in patients, during November 1962. After its clinical introduction, Chamley continued his research on the wear properties of polymers, but he never foimd a material better suited for joint replacements than UHMWPE. Although it is clear that he evaluated different polymers, including Hi-Eax UHMWPE material from the United States, his cups were always fabricated from the RCH 1000 compression molded UHMWPE material produced in Germany by Ruhrchemie (now Ticona). 

UHMWPE, as well as other thermoplastics, exhibit a complicated nonlinear response when subjected to external loads. Their behavior, as demonstrated previously, is characterized by initial linear viscoelasticity at small deformations, followed by distributed yielding, viscoplastic flow, and material stiffening at large deformations until ultimate failure occurs. The response is further complicated by a dependence on strain rate and temperature. It is clear that higher deformation rates and lower temperatures increase the stiffness of the material. 

There are a variety of polymers used in joint replacement implants that can be subject to wear. In this section UHMWPE, crosslinked polyethylene, poly(ether ether ketone) (PEEK), silicone and polyurethan are discussed. Some of the mechanical properties for these polymers are shown in Table 7.1. 

Table 3 (71) defines the threshold stress intensity and the range over which the Paris equation is valid for ultrahigh molecular weight polyethylene (UHMWPE) subjected to different processing conditions. From Table 3, it is clear that subtle processing conditions can alter the threshold level and the onset of FCP. 

Table 3. Fatigue Threshold Stress Intensity Factor and Paris Regime or UHMWPE Subjected to Different Processing Conditions ...

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