Abstract
Post operative stability is of paramount importance to obtain bone in growth and a tight interface in uncemented implants. Although hemispherical press fit cups are widely used different opinions exists according optimal fixation and a variety of principles are preferred. Lab studies show better stability if a cup is augmented by screws or pegs. However, cups with screws and holes increases penetration of joint fluid, pressure and particles to the interface with a risk for osteolyses. HA coating is in many studies favourable to obtain a quick in growth but is by many regarded unnecessary or even a risk for increased wear. This RSA studie was done to investigate stability and wear in cups with different fixation.
Material: 80 hips in 75 patients with a mean age of 58 years (36–70) were operated with a cemented Spectron stem and a porous coated Reflection cup of titanium (Smith a Nephew) All cups were oversized 1–2 mm and fixed with press fit by experienced surgeons. Great care was taken to achieve a good rim fit. They hips were by randomisation allocated to one of four groups with different fixation methods. One group was done with only press fit technique, one with additional screws, one additional pegs and another with HA coating on the porous surface. Migration and wear was investigated with RSA at 2, 12 and 24 months and standard radiography was done post op and at 2 years.
Mann-Whitneys U-test was used on signed values for evaluation of group differencies.
Results: At 2 years the mean cup migration for the whole group was 0.2 mm longitudinally, 0.3 mm horizontally and 0.3 mm anterior-posteriorly. (SEM 0.03–0.05) The inclination changed 0.4 dgr and anteversion 0.4 dgr as well. Most cups moved laterally, proximally and anteriorly with increased anteversion and decreased inclination. No major differences in mean values were found between the groups according translations or rotations and no significant differences. Nor did we find differences between the press fit only and the augmented cups.
Wear was 0.45 mm proximally and in total 0. 6 mm without any sign of differences between the HA and porous coated groups. HA coated cups had less radiolucent lines after 2 years. (p=0.01)
Discussion: The Reflection cups were stable fixed ad we found no indication of inferior stability for cups without augmentation as might have been anticipated. The reason is probably the firm press fit fixation obtained with a stable cup, good bone and forceful impaction. Many use screws for safety but such cups are not sealed for leakage of joint fluid and pressure gradients and the risk for backside osteolyses is higher… Pegs are easier and safer to insert and tighten the holes better. A tight, no hole cup with HA is preferred by us in normal younger patients since HA gave a better interface. Such a solution should minimise the risk for penetration of particles and pressure gradients with less risk for lyses.
The abstracts were prepared by Professor Jegan Krishnan. Correspondence should be addressed to him at the Flinders Medical Centre, Bedford Park 5047, Australia.
