Quantitative Measurements of Backside Wear in Acetabular Hip Joint Replacement: Conventional Polyethylene Versus Cross-Linked Polyethylene

As shown in previous studies, the modification of conventional polyethylene (CPE) to cross-linked polyethylene (XLPE) and the contribution of antioxidants result in a reduction in total wear. The aim of this study was to evaluate XLPE inserts with vitamin E and CPE regarding their resistance to the backside wear mechanism. A cementless hip cup system (Plasmafit® Plus 7, Aesculap) was dynamically loaded using CPE and XLPE inserts. The backside wear was isolated, generated and collected using the two-chamber principle. The chambers were filled with ultrapure water. After 2 × 106 cycles, the fluids were examined for wear particles according to a particle analysis. Using XLPE inserts, the backside wear was significantly reduced by 35%. While XLPE backside wear particles are significantly larger than CPE particles, they do not differ in their morphology. This study confirms the greater resistance to backside wear of XLPE compared to CPE. It can be assumed that the improved fatigue resistance of the vitamin E-stabilized XLPE inserts demonstrates XLPE’s effectiveness against micro-motion and the resulting changing tensions in interface areas like surface breakdown, pitting and the release of very small particles.

All-Polyethylene versus Metal-Backed Tibial Components in Total Knee Arthroplasty

All-polyethylene tibial (APT) implants were incorporated into the initial design of the first total knee arthroplasty (TKA) systems. Since then, a dynamic shift has taken place and metal-backed tibial (MBT) implants have become the gold standard in TKA. This has mostly been due to the theoretical advantages of intraoperative flexibility and improved biomechanics in addition to the heavy influence of device manufacturers. MBT implant comes not only with a higher cost but also with potential for complications such as osteolysis, backside wear, and thinning of the polyethylene insert, which were not previously seen with APT implant. The majority of studies comparing APT and MBT implants have shown no difference in clinical outcomes and survivorship. Newer studies from the past decade have begun highlighting the economic advantages of APT implant, especially in patients undergoing primary, uncomplicated TKA. Use of APT implants in younger patients and those with a body mass index > 35 has not been extensively studied, but the existing literature suggests the use of APT implant in these cohorts to be equally as acceptable. With modern implant design and instrumentation, rising utilization of TKA along with current and future economic strain on health care, the increased use of APT implant could result in massive savings without sacrificing positive patient outcomes.

ARTICULATION AND BACKSIDE WEAR ANALYSIS AFTER LONG-TERM IN VITRO WEAR SIMULATION OF VITAMIN E STABILIZED POLYETHYLENE ACETABULAR LINERS WITH A PRESS-FIT LOCKING MECHANISM

A previous retrieval study analyzed the backside wear of short-term implanted liners against in vitro tested liners of similar life in service and showed comparable results among both groups, with no significant backside wear due to micro-motion.The purpose — to obtain a picture of the overall wear (articulation and backside surfaces) of 0.1% vitamin e blended polyethylene liners, with a locking mechanism based on a press-fit cone in combination with a rough titanium conical inner surface in the fixation area, under a 20 million cycles hip wear simulation.Materials and Methods. A semi-quantitative method was used in order to assess the damage on the backside of the liners and a 3d measuring machine to assess the creep and wear at the articulation surface.Results. The total average backside wear score was 22.00±2.59 from a maximum total score of 147 after 5 million cycles (mc), increased to 31.92±5.57 after 10 mc, but showed no further increment after 15 and 20 mc. The reference liners (subjected only to axial load) showed similar wear scores and modes as the liners under wear simulation (axial load and movement). Small scratches produced during insertion and removal were clearly seen at the rim (fixation) area and no considerable abrasion was observed. The machining marks on the convex surface were always visible. Regarding the articulation surface, a steady state wear rate of 7 µm/year was measured.Conlusion. These results determined that most of the backside wear produced on the liners occurred during their insertion and removal rather than during their life in service. Moreover, the wear at the articulation surface was similar to that seen in vivo at short- and mid-term on highly cross-linked polyethylene liners with and without vitamin e content.

Successes and Failures of a Freedom™ Constrained Cup Used in a Major Salvage Procedure

Background: This is a case report of a 36mm constrained cup (Freedom™, Biomet IN) that performed successfully for 7-years in a salvage case involving a total-femur implanted in a leg already short by 3-4 inches. The goal was to enhance hip motion and stability using a 36mm head instead of the usual 32mm size. Templating indications were for a 50mm cup (Freedom™; Arcom™ liner). The proximal femur inserted in 2008 incorporated the 36mm constrained THA and was anchored distally to bone using the Compress™ fixator. By 2012 the fixator loosened and was replaced by hinged total-knee arthroplasty (TKA). The THA was retained at revision and patient’s clinical follow-up was satisfactory for 4 years. As indicated by Martel radiographic method, the Arcom™ liner showed minimal wear over this period. Radiographs in Feb-2016 showed the cup’s constraint ring had rotated slightly but the patient had no symptoms. By Dec-2016, the patient had experienced three falls and also had heard a popping sound in her hip. At Dec-2016 office visit, radiographs indicated additional rotation of the constraint ring and CT scans showed an eccentric head position contacting the metal shell. At revision, 50% of the Arcom rim was ablated and the remainder present as a loose fragment. Following insertion of new Freedom liner and 36mm head, her follow-up appears satisfactory 10-months later. Her leg shortening remains but she walks to office visits using a cane and doesn’t need the cane at home.Methods: Retrieved Arcom liner and detached rim fragment were reconstructed, photographed, and then bi-valved for comparison to similarly prepared exemplar liners, one identical to our revision and one with a thicker wall. Details of liner sections were taken from photographs and reconstructed by computer graphics (Canvas Draw-3™). Wear performance over the first 7 years was assessed using the Martel x-ray method.Results: Inspection of retrieved liner showed a large oval depression in the ablated rim. The contra-rim featured the large Arcom fragment and the underlying liner wall was less than 1mm thick. Comparison to exemplar liners showed that the large fragment had separated along the lower edge of the constraint groove. Exemplars demonstrated a substantial rim buttress spanning 13mm, which had been ablated in our retrieval.Discussion and Conclusion: Although this was not a high-demand patient, the considerable hip-impingement forces in a flail limb likely levered the head repeatedly against the liner’s constrained rim. Neck impingement was clearly evident in the damaged liner. A subluxing femoral head would also thin the contra-wall, as would backside wear. We do not know if the eccentric ring image in Feb-2016 radiographs depicted failure. The liner may have escaped from the shell’s locking-ring and with activity, ablated the Arcom contours and led to rim fracture. It is also possible that the liner constraint was damaged when the patient fell, thereby allowing the liner to mobilize.