Collarless, polished and tapered cemented stems are nowadays commonly used in hip surgery. Normally, a hollow centralizer is applied to the stem tip to allow the prosthesis to sink in the cement mantel in the event of creep and loosening between stem and cement. It is believed that in this way the stem will stabilize and regain its tight bond with the cement. The prosthesis MS-30 (Zimmer) is collarless, polished and triple tapered and has a hollow centralizer, but was previously used with a solid centralizer. We hypothesised that these types of stems, exemplified by the MS-30, used with a hollow centralizer would sink more but stabilize better, become more stable in the important rotational migration and retrovert less than with a solid centralizer. In a prospective, controlled clinical study we randomised 60 patients with primary coxarthrosis into either hollow or solid centralizer used with the MS-30 stem. The effect was evaluated for a 2-year follow up period by repeated RSA examinations, conventional radiographs and clinical follow-ups with the questionnaires WOMAC, SF-12 and Harris Hip Score.

The RSA results showed small early migration in both groups and almost all of it occurred within the cement mantle, i.e. between stem and cement. The group with hollow centralizers migrated distally significantly more than the group with solid centralizers (p< 0.0001) (1.40 mm vs 0.28 mm). In rotation, however, there was no difference (retroversion 0.99° and 0.94°). Neither was there any difference regarding clinical outcome and questionnaires.

As expected the group with hollow centralizers migrated more distally, in the same magnitude as reported in earlier RSA studies for the conceptually similar prostheses Exeter and C-stem. Interestingly, there was no difference regarding the rotational behaviour, and both groups showed less retroversion than reported in the earlier reports. MS-30 seems to have a design that regardless of centralizer type well withstands rotational motion within the cement mantle. This study cannot fortify the need for a hollow centralizer for this collarless, polished and triple tapered prosthesis.

Introduction: Progressive retroversion of the stem within the femur has been suggested to be an important initial mode of hip prosthesis failure. We have assessed the relationship between postoperative stem anteversion angle, measured with CT, and the rotational stability as measured with repeated radiostereometric analysis (RSA) with 5 years follow up.

Patients and methods: 57 patients were operated on with THA using a cemented, matt and collared stem. The achieved stem anteversion angles were measured postoperatively with 3-D CT-examinations. The patients were divided into three groups depending on their ante-version angle: ≤10°, 11°–25° and ≥25°. They were followed with repeated RSA examinations for 5 years to determine the stem migration pattern.

Results: The mean postoperative anteversion angle was 20.5° (range 1°–43°). At 5 years, all except one stem had rotated into retroversion. There was a strong correlation between the postoperative anteversion angle and later rotation into retroversion (p=0.007). The group with ≤10° of stem anteversion rotated significantly more into retroversion, seen as early as 3 months (p=0.02), but more obvious at 5 years (p=0.002) with a mean of 9.9° of retroversion compared to 3.8° in the 11°–25° group and 2.4 ° in the ≥25° group. The distal stem migration results were accordant with more migration at 5 years (p=0.008) for the ≤10° anteversion group (1.6 mm subsidence compared to 0.5 and 0.3 mm respectively). Two stems have been revised because of aseptic loosening, both with a low initial anteversion angle (7° and 1°) and large retroversion at 5 years (7 ° and 31° respectively).

Conclusion: Measured by RSA, rotation into retroversion of the femoral stem was a regular finding in this study using a conventional prosthesis design. We propose that such rotation is a common finding in hip arthroplasty, but the degree may be design sensitive. Our results strongly suggest that the initial rotational position of the femoral component during surgery is decisive for the degree of later retroversion and probably prosthetic longevity; the less anteverted position the more the stem will migrate into a more retroverted position after the operation. This rotational migratory pattern is correlated to subsidence and eventual loosening. Hence, meticulous attention should be paid to the rotational position of the femoral stem during surgery, with less than 10° of anteversion appearing deleterious. However, too much anteversion will risk impingement and possibly other unwanted biomechanical effects, and an upper limit still remains to be established.

In a prospective, controlled clinical study we randomised 50 patients with primary coxarthrosis into either removal or retention of the subchondral bone plate during ace-tabular preparation in cemented total hip arthroplasty. The effect was evaluated for a 2-year follow up period by repeated RSA examinations, analyses of radiolucent lines on conventional radiographs and clinical follow-ups with WOMAC, SF-12 and Harris Hip Score. Removal of the subchondral bone plate resulted in an improvement in radiological appearance of the bone-cement interface. For the retention group the extent of radiolucent lines as measured on pelvic and AP-view, had increased from a direct postoperative average level of 3.4% to a 2-year level of 28.8%. For the group with removal of the subchondral bone plate, the direct postoperative radiographs revealed no radiolucency, and at 2 years it only occupied a mean of 4.1 % of the interface. With the classification according to Hodgkinson the retention group had 10 out of 25 patients remaining in grade 0 (no demarcation) at 2years, whereas the removal group had 23 out of 25 patients in grade 0 at 2 years. The RSA results showed small early migration in both groups, but a tendency towards better stability and less scatter of the results in the removal group. The retention group tilted from 6 months onwards slightly but continuously towards a more horizontal position, whereas the removal group stabilized in a slightly vertical position after 1 year. The mean proximal migrations for all cups taken together were 0.09 mm at 2 years with no significant difference between groups. No differences were found in clinical outcome neither pre- nor postoperatively. To optimize the bone-cement interface and thereby increase the long time cup survival, removal of the subchondral bone plate where possible appears to be advantageous, but it is a more demanding surgical technique.