Abstract
Introduction
Metal ion and particle release, particularly cobalt, has become an important subject in total hip arthroplasty, as it has shown to induce metal hypersensitivity, adverse local tissue reactions and systemic ion related diseases. The purpose of the following study was compare the ion release barrier function of a zirconium nitride (ZrN) multilayer coated hip stem for cemented use, designed for patients with metal ion hypersensitivity, against its uncoated version in a test configuration simulating the worst case scenario of a severely debonded hip stem. The ZrN multilayer coating is applied on a CoCrMo hip stem and consists of a thin adhesive chromium layer, five alternating intermediate layers out of chromium nitride (CrN) and chromium carbonitride (CrCN) and a final zirconium nitride (ZrN) shielding layer [1].
Methods
Hip stems with a ZrN multilayer coating (CoreHip AS, Aesculap AG, Germany) were tested in comparison with a cobalt-chrome uncoated version (CoreHip, Aesculap AG, Germany). In order to create a worst case scenario, the smallest stem size with the biggest offset in combination with an XL ceramic head (offset +7 mm) was used. The stems were embedded according to the ISO 7206-6 test in a bone cement sheet. Once the bone cement was bonded, the stem was pulled out and a PMMA grain was placed inside the femoral cavity in order to uprise the hip stem above its embedding line and simulate a debonded cemented hip stem with a severe toggling condition. The dynamic test was performed under bovine serum environment with an axial force of 3.875 kN [2] at 11.6 Hz for 15 million cycles. The test was interrupted after 1, 3, 5, 10 and 15 million cycles and the surfaces of the stems were analyzed through scanning electron microscopy (SEM) with energy dispersive X-Ray (EDX). Moreover, the test medium was analyzed for metal ion concentration (cobalt, chromium and molybdenum) using ICP-MS.
Results
The SEM/EDX analysis demonstrated that the ZrN multilayer coating kept its integrity, as no trace of the substrate material (CoCrMo) could be detected. Furthermore, the taper of the ZrN group showed less fretting and corrosion than the taper of the CoCrMo stem (Fig.1). Moreover, the ion concentration analysis showed a reduction of up to two orders of magnitude in the release of cobalt, chromium and molybdenum in the ZrN coated stems in comparison with the uncoated version.
Discussion
The results showed that, even in a worst case scenario of high micro-motion due to a severe stem debonding within the cement mantle, the hip stems with a ZrN multilayer coating substantially reduce the release of ions from the substrate material.
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