This study examined whether TiNbN surface characteristics can reduce corrosion and wear of Chrome Cobalt Molybdenum Metal-on Metal bearings. Two series of patients had plasma concentrations of chromium and cobalt at intervals following surgery. The First Series comprised a retrospective analysis of 52 consecutive cases (49 patients, 73–96 months following operation; age at surgery: 33–78) who had undergone an ACCIS (Implantcast, Germany) Modular Large Head hip replacement. The Second Series comprised a prospective, consecutive series of 125 cases (109 patients, 1–61 months following operation; age at surgery: 24–75) who had undergone an ACCIS Resurfacing Hip Replacement in whom pre-operative samples and periodic post-operative metal ion analysis was obtained. Cup inclination and anteversion angles, patient outcome and Harris hip scores at last follow-up were also recorded. The first series revealed medians for [Cr] of 1.2 (range <0.5–2.4) ug/l and [Co] of 3.3 (range <0.15–8.18) ug/l. Four patients were not available for measurement. The second series gave one year [Cr] of 0.8 (range <0.5–1.6) ug/l and [Co] of 0.2 (range <0.15–0.9) ug/l and at two years [Cr] of 0.2 (range <0.5–1.5) ug/l and [Co] of 0.8 (range <0.15–1.0) ug/l. There was no correlation with cup inclination (38° to 62°) or anteversion (0° to 32°) in either group. Mean Harris Hip Scores were 80.9 and 92.3 respectively. Low median levels of metal ions were found in the First Series (despite differing stem type usage). The low median ion levels were more consistent in the Resurfacing patients of the Second Series. The Titanium Niobium Nitride Ceramic Surface Engineering Metal-on-Metal bearing implants appear to protect against raised plasma [Cr] and [Co] both over time and with outlying cup positions. The Harris Hip Scores suggest a good patient outcome for the hip replacements in both series. Further study by a randomised controlled prospective analysis is suggested.
The ACCIS system comprises a bearing of a 5 micron surface ceramic upon a Chrome Cobalt Molybdenum (CrCoMo) substrate which allows for a homogeneous couple. The Titanium Niobium Nitride (TiNbN) microceramic applied by Plasma Vapour Deposition. In comparison with CrCoMo alloy, TiNbN gives a hard (2800 vs 489V), smooth (0.23 vs 0.55Rz), low friction (0.079–0.1 vs 0.11–0.56mu pin on disk test), wetable surface which when combined allows for the potential of gaining fluid film lubrication (lambda>3) on a smaller head diameter than an untreated surface. These properties are postulated to reduce wear and hence lower the release of Cr and Co ions in vivo. The surface microceramic also prevents exposure to release of Cr and Co by corrosion. The surface modified implants were first used in 2003 in large head arthroplasty and later in 2005 with resurfacing implants. Two series of patients implanted by a single surgeon were examined to elucidate the metal ion release of the ACCIS system. The first series retrospectively examined 52 consecutive Large Head Arthroplasty cases for [Cr], [Co] and [Mo] levels. A mean follow-up time of 7.5 yrs (77–101mths) with 9 patients being lost to revision and death by the time of the study. Median levels of [Cr] 1.6, [Co] 4.76 and [Mo] 2.5 µg/l were obtained. The second series prospectively examined the [Cr] and [Co] levels with the pre-operative values as controls in 125 resurfacing cases with the ACCIS microceramic. The second series gave no rising trend observed at up to 5 years ([Cr & Co] (range < 0.5–1.6 µg/l). It is postulated that the ACCIS surface microceramic reduces wear and also reduces the effective patch size for a given load thus allowing for a greater Patch to Edge Distance allowing a greater tolerance to cup positioning. A RCT is currently underway.
Surface engineering of metal articular surfaces effectively minimizes corrosion and metal ion release. The absence of increase of metal ion levels indicates that metal wear is minimal.
