Dual-Energy X-Ray Absorptiometry (DEXA) Evaluation of the Bone Remodeling Effects of a Low-Modulus Composite Hip Stem After 2 Decades of Follow-Up

Abstract

Background: The Epoch FullCoat Hip Stem (Zimmer) was an isoelastic composite femoral stem developed to address stem stiffness concerns. Purpose: We sought to evaluate the long-term bone mineral density (BMD) of a cohort of patients who underwent total hip arthroplasty (THA) using the Epoch isoelastic stem and having more than 2-decade follow-up. Methods: We conducted a retrospective chart review of all patients who were study subjects at our institution in a multicenter prospective trial for the Food and Drug Administration of the Epoch implant in the mid-1990s. Through this, we identified 16 patients who had dual-energy X-ray absorptiometry (DEXA) scans, with which we could determine BMD preoperatively and at 3 points postoperatively. Of these, 5 agreed to participate in the study (the others were deceased, unable or declined to participate, or were lost to follow-up) with mean follow-up of 22 years. These participants underwent clinical and radiographic evaluation consisting of a Harris hip score, anteroposterior (AP) pelvis and AP and lateral hip X-rays, and DEXA evaluation of both hips. BMD in the 7 Gruen zones at last follow-up was compared with immediate postoperative and 2-year follow-up. Results: At last follow-up, all stems were well-fixed with signs of extensive osteointegration. In proximal Gruen zones 1 and 7, patients underwent a decrease in BMD with more modest losses in Gruen zone 1. All patients demonstrated an increase in BMD in zones 2 through 6 at latest follow-up, except for 1 patient in Gruen zone 6. BMD changes were not limited to the first 2 years of follow-up. Conclusion: This small follow-up cohort study found excellent long-term clinical results, no plain radiographic signs of notable stress shielding, and general maintenance of BMD at a follow-up of over 20 years for this isoelastic stem. Long-term bone remodeling after implantation of the isoelastic stem resulted in increased BMD in Gruen zones 2 through 6, suggesting that composite implant designs may still have a role in THA.