The purposes of this paper was to determine the wear rates and volumetric wears of ceramic (Zirconia) and metal (Cobalt-chrome) femoral heads, and to differentiate creep from true wear. Another purpose was to determine the extent of wear and volumetric wear per annum.

We examined 86 patients (111 hips) who were continuously followed up from 180 patients (225 hips) who received THRA with AVN of stage III or above at our hospital. Patients were followed until December 1999 for more than a minimum of 3 years from June for 1996. The hip joint AP radiographs that had been taken immediately after the operations, six weeks postoperatively and annually thereafter were compared. We used the computer assisted vector wear analysis program of Martell to measure the wear rate, volumetric wear and vector wear.

The wear rate and the volumetric wear in the zirconia ceramic head group (1.443 mm/year, 665.433 mm3/year) and metal head group (2.161 mm/year, 1271.831 mm3/year) were high immediately after operation and six weeks postoperatively, and this was significantly more than during all for other periods, giving a wear rate and volumetric wear in the ceramic head group of 0.180 mm/year and 81.101 mm3/year, and in the metal head group of 0.219 mm/year and 124.149 mm3/year. Moreover, the average wear rates and volumetric wear rates of these two groups were significantly different, and annual wear and volumetric wears showed a trend to decrease with time.

Wear rate and volumetric wear six weeks after operation were remarkably high compared with the other periods. This is interpreted as the result of a creep phenomenon, which occurred mainly during the six weeks following the operation. The Zirconia ceramic head group showed little wear, which confirms the theoretical advantage of the zirconia ceramic head in vivo.