Prospective randomized study using EBRA-FCA to compare bone fixation between cementless SL-PLUS Zweymüller versus SL-PLUS MIA femoral implants in primary total hip arthroplasty with clinical assessment at a minimum 5years' follow-up

Introduction: Sometimes the slightest changes in implant design can lead to failure, even for a validated prosthesis. A minimally invasive cementless model, the SL-PLUS MIA™, in which the lateral shoulder is eliminated, was developed from the Zweymüller SL-PLUS™ implant. After satisfactory in-vitro tests, it required in-vivo assessment to ensure that bone fixation is good. We therefore conducted a prospective randomized study comparing the two versions of the Zweymüller femoral stem, with the aim of (1) comparing bone fixation up to 2 years' follow-up on EBRA-FCA radiography; and (2) assessing any difference in clinical or radiographic performance.

Hypothesis: Primary stability assessed on EBRA-FCA does not significantly differ between the SL-PLUS MIA™ and SL-PLUS™ implants.

Patients and method: A single-center multi-surgeon prospective randomized study included 80 patients (79 were operated on) between April 2009 and October 2012, with a mean 6 years' follow-up. Radiographic assessment used the EBRA-FCA application up to 2 years' follow-up; clinical assessment, with a minimum 5 years' follow-up, was performed by a single observer, using the Harris and Oxford-12 scores. The two groups, SL-PLUS™ (n=38) and SL-PLUS MIA™ (n=41), were comparable in gender, age, indications, body-mass index and preoperative functional status.

Results: At a minimum 2 years' follow-up, 24 SL-PLUS™ and 27 SL-PLUS MIA™ implants were analyzed on EBRA-FCA. Mean migration was respectively -0.3mm±0.8 [range, -1.6 to 1.3] and -0.5mm±0.7 [range, -2.2 to 0.5] (p=0.21). There was likewise no significant difference in varus tilt. The number of ectopic ossifications did not differ, despite the absence of shoulder: 7 with SL-PLUS™ (23%), and 10 with SL-PLUS MIA™ (32%), without clinical impact. Oxford score improved from 43±6.8 to 19±7 at 5 years' follow-up with SL-PLUS ™ and from 44±8.8 to 20±7.4 with SL-PLUS MIA™: i.e., no significant inter-group difference. Likewise, Harris score at 2 years' follow-up did not differ: 91.6±8.7 and 89.7±10.2, respectively. Implant survival did not differ: SL-PLUS MIA™, 41/41 (100%); SL-PLUS™, 36/38 (94.7%) (p=0.13).

Conclusion: There was no significant difference in fixation quality between the SL-PLUS™ and SL-PLUS MIA™ implants. Elimination of the shoulder did not jeopardize primary or secondary fixation, but neither did it reduce the rate of ossification. The modified Zweymüller implant appeared risk-free at 6 years' follow-up.

Level of evidence: II, low-power prospective randomized study.