Aware of the possibility of creep of the UHMWPE cup, when under load in the human body, Charnley set up an experiment. Cemented cup was loaded through 22.225 mm diameter stainless steel head. The load was approximately one body mass; in the early 1960s this was taken to be approximately 76 kg. Displacement of the head into the cup – over many years, was micrometers only. It occurred early and did not progress with time (Fig. 33.1).

If creep did occur to any extent then “displaced” plastic would either heap up at the margin of the loaded area or be compressed into the subsurface of the contact area. No such changes were found. Creep, as a contribution to volumetric change, was considered not to be important. Dimensional changes were considered to be primarily due to wear.

Rose and colleagues 1980 [1] studied total prostheses of the Charnley – Mueller design by total joint simulation for the equivalent of 1 year of use using the “recovery of debris” method. They concluded that “… wear accounted for only small fractions (between 1 % and 30 %) of the dimensional changes”, and that “… most of the changes previously ascribed to wear, were in fact due to creep or plastic flow.” They did not consider that “… dimensional changes due to wear will be large enough to impair mechanical function.” “Thus, the chief clinical question is the biological effect of the debris and not the mechanical problems due to dimensional changes.” [1].