MoM Hip Implant Failure Linked to Manufacturing Variation

Diana Phillips

May 02, 2016

An "unacceptably high" risk for failure and revision surgery associated with a commonly used metal-on-metal hip implant may be partly attributable to the use of components that were outside of stated manufacturing tolerances, investigators report in a study published online April 29 in BMJ Open.

In a first-of-its kind investigation combining clinical data, blood ion concentrations, and explant analysis, David Langton, MBBS, MRCS, from University Hospital of North Tees, Stockton, United Kingdom, and colleagues sought to better understand the performance of metal-on-metal (MoM) hip prostheses by looking specifically at outcomes of patients implanted with the most common of these devices, the Pinnacle MoM 36-mm implant (DePuy).

"There is general acceptance that large diameter [MoM] total hip arthroplasty (THA) has not lived up to clinical expectations," the authors write. "Although the use of MoM hip devices has declined dramatically in the past 5 years, hundreds of thousands remain in situ, with the long-term future uncertain." Gaining insight into the mechanisms that lead to failure can inform management strategies and improve the design of future devices, they note.

For the current study, the investigators reviewed data from a cohort of patients from one hospital trust in northern England who were implanted with the Pinnacle device in conjunction with an SROM modular or Corail femoral uncemented stem between 2003 and 2009. From this subgroup, only those patients whose surgery was performed by the study's two senior authors, both of whom are specialist consultant lower-limb arthroplasty surgeons, were included in the cohort. Patients were identified through National Joint Registry reports, surgical records, and DePuy sales records. Explanted components of failed devices (femoral head, head tapers, and acetabular liners) were evaluated for wear using dimensional and volumetric analysis.

Of 434 identified patients (489 Pinnacle hips), including 243 women and 191 men, in the review, 352 attended recall clinics during which they underwent clinical examination, blood metal ion measurement, X-rays, and more targeted imaging as needed. The mean postsurgery follow-up period for the cohort was 89 months.

Across the full cohort, 71 metal hips required surgical removal and replacement, contributing to an "unacceptably high" revision rate, the authors state.

Although the observed failure rate "does not change the approach to continuing surveillance on patients with MoM total hip arthroscopy, for patients with this implant system, it does provided [an implant] survival rate that can be shared with patients," American Academy of Orthopaedic Surgeons spokesperson David Jevsevar, MD, MBA, said in an interview with Medscape Medical News. Dr Jevsevar, who is chair of the Department of Orthopaedics at the Geisel School of Medicine at Dartmouth University, was not involved in the current study.

Analyses of explanted devices showed that signs of taper-junction failure were more common than signs of bearing failure. Specifically, 50 (70%) of the 71 implant failures showed signs of taper-junction failure, and 34 (48%) of the 71 showed signs of bearing failure.

The results showed higher failure rates in female hips, with a predicted 9-year survival of 77.9% compared with 91.4% for male hips, the authors report. They note, however, that implant survival was significantly lower for patients with bilateral (73.7%) vs unilateral (86.2%) hip implants, and "there were only 31 bilateral male hips compared with 69 bilateral female hips."

Further findings included:

  • damage to the abductor musculature in 38 cases,

  • moderate to severe aseptic lymphocyte-dominated vasculitis-associated lesion observed on retrieved tissue in 36 cases (51%), and

  • histologic findings of metallosis with no lymphocytic infiltration in 13 cases (19%).

Manufacturing Variability Increased

In addition, results of explant analyses generated from the pool of Pinnacle MoM devices received at the Northern Retrieval Registry, which is directed by one of the study authors, showed that a significant number of the devices were manufactured with lower diametrical clearances than the manufacturer intended. In that analysis, the authors explain. "The rate of non-conformance appeared to increase over time, with Pinnacles manufactured from the year 2006 onwards significantly more likely to have a clearance value lower than stated to regulators," they write.

Specifically, measurement of the diametrical clearances of all of the head and cup combinations used in the 161 Pinnacle MoM implants received at the Northern Retrieval Registry up until the time of the study showed that 48 devices were below the lower tolerance band, including five that were implanted before 2006 and 43 that were implanted from 2006 and later.

With respect to all-cause revision across the full cohort, the presence of bilateral joints and the late cohort emerged as significant risk factors, and "there was a trend towards increasing risk of revision with smaller liner sizes," the authors report.

Repeating the model with liner sizes as categorical variables found that 50- and 52-mm liner sizes were significant risk factors for early revision. Multiple analyses confirmed the association between smaller liner size and revision risk.

Further, failures related to bearing failure were most common in patients with smaller liners. "Larger liners and earlier date of implantation were associated with a significant reduced risk of revision," according to the authors.

The current study revealed an association between Corail stems with greater blood Co concentrations and grater taper wear rates than SROMs. This association "did not equate to an increase in taper failure identified on explant analysis or revision rate," the authors report. "This result is at odds with DePuy’s own internal studies which found a 5-year revision rate of the Corail Pinnacle of 14.1% vs 4.78% for the SROM Pinnacle MoM systems. We have yet to identify clear evidence of mechanically assisted crevice corrosion — the classical theory of taper failure — and thus do not believe that a CoCr on CoCr head stem combination protects from this mode of failure," the authors stress.

"Other factors appear to be more important than stem type in the success or failure of the device."

One such important factor appears to be the observed manufacturing variation, the authors contend. Lower clearances such as those observed primarily in the post-2006 components "render bearings vulnerable to clamping/lubricant starvation should the cups deflect greater than expected when press fit into the acetabulum."

The investigators also observed substantial variation in the as-manufactured surface of the female taper surfaces, and the surface finish appears to be a "critical factor in material loss at this junction." However, without a large enough sample of sterile implants from pre- and post-2006 manufacture years, they write, "it is currently impossible to know whether variation in taper surface finishes in and of itself explains the difference in failure rates between the patient cohorts pre-2006 and post-2006."

Importantly, the absence of a strong relationship between acetabular cup orientation and blood ion concentrations indicates that failures in this cohort were not the result of inaccurate cup placement.

The results of the study "are not necessarily surprising, but the mechanism of failure in this study is attributed more to taper-neck junction failure [trunnionosis,] rather than bearing surface wear and debris generation," Dr Jevsevar said. "The authors also make a compelling case that failure rates are higher in implants produced after 2006 and suggest that implant production processes changed after that time, leading to the increased failure rate."

Although component positioning has been implicated previously in implant failure, "the authors found that failure was not related to component positioning, and this supports the findings of several other studies," Dr Jevsevar said. "This study demonstrates that MoM failure in this hip system was multifactorial and related to trunnion failure and bearing surface wear."

Further investigation using larger data sets are warranted to confirm the role that variations in manufacturing play in prosthetic failure, according to the authors. The high midterm failure rate of the Pinnacle in this cohort suggests that "tens of thousands of patients around the world are at risk of early revision surgery," they write. "Taper failure appears to be time dependent and a rising Co level should alert the clinician."

The relatively low wear rates of the MoM bearing surfaces in this study, together with the clinical performance of unilateral hips produced before 2006, are "encouraging" observations and suggest that "MoM technology can in certain circumstances be used successfully if taper junctions are suitably optimised," the authors conclude.

Last month, a Texas federal jury ordered Johnson & Johnson and its DePuy unit to pay a total of $500 million in compensatory and punitive damages to five plaintiffs who said they were injured by the Pinnacle metal-on-metal implants.

Dr Langton and three coauthors are retained experts for plaintiffs in ongoing MoM device litigation. Dr Langton and one coauthor have worked with the US Department of Justice in litigation involving DePuy, manufacturer of the prosthesis investigated in this study. Dr Jevsevar has disclosed no relevant financial relationships.

BMJ Open. 2016;6:e007847. Full text

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