Abstract
Introduction: After total hip arthroplasty (THA) the periprosthetic bone is loaded in an unphysiological manner (stress shielding), a major cause for periprosthetic bone resorption and aseptic loosening. Design, material and surface properties of the implant influence the stress shielding effect. This study investigates whether the design changes from the successful ABG-I to the ABG-II stem can be verified in perioprosthetic bone remodelling using Dual-Energy X-ray Absorptiometry (DEXA).
Methods: 51 THA patients (22f, 29m, avg. age: 60.8 years) were randomised to either ABG-I or ABG-II. DEXA measurements were performed preoperatively and 10 days (baseline), 3 weeks, 3, 6, 12 and 24 months postoperatively using standard Gruen zone analysis. At the same time clinical Merle d’Aubigne (MdA) scores were measured. Changes in bone mineral density (BMD) were expressed as percentage changes from the baseline for each of the Gruen zones (R).
Results: The average MdA score (25 ABG-I, 26 ABG-II) increased from 10.3 preoperatively to 17.3 at 24 months postoperative. The improvement was higher for ABG-II (7.5) than ABG-I (6.5) but not significant (p=0.15). During the first three postoperative months the average BMD of all zones combined dropped steeply for both the ABG-I (−5.5%) and ABG-II (−4.5%, n.s.). Beyond 3 months, the overall BMD change (zones combined) continued to develop without significant difference between both implant designs (plateau and slight recovery) but the individual zones showed distinct differences. The average BMD loss in the proximal Gruen zones was much lower for ABG-II (R1: −7.9%, R7: −3.7%) than for ABG-I (R1: −9.3%, R7: −11.9%) while distally the situation was reversed with better bone preservation for the ABG-I (R3: −2.9%, R4: −1.5%, R5: −1.7%) than for the ABG-II (R3: −6.0%, R4: −2.8%, R5: −4.6%). In the mid-stem region a transitional area was identified with better bone preservation for ABG-II in Gruen zone 6 (+2.7% vs −1.4%) and for ABG-I in Gruen zone 2 (-4.9% vs 7.9%). However, the p-values (two-sided t-test) ranged from 0.05–0.35 at statistically non-significant levels.
Discussion: The steep initial bone loss for both stem designs and all Gruen zones combined indicates that during this early postoperative phase surgical trauma and reduced loading dominate the bone remodelling process and not the type of implant. The different development of proximal and distal BMD for ABG-I and II in the period thereafter demonstrates the long-term effect of implant design verifies the design improvements (less proximal stress shielding). A parallel study identified the dominant influence of preoperative BMD on BMD loss. This explains our high standard deviation and the lack of statistical significance. The study is now expanded with patients matched for preoperative BMD.
Theses abstracts were prepared by Professor Roger Lemaire. Correspondence should be addressed to EFORT Central Office, Freihofstrasse 22, CH-8700 Küsnacht, Switzerland.
