Metal-on-metal bearings solved some but caused other major THA problems

Metal-on-metal implants addressed osteolysis, dislocation issues; concerns abound about metal ion release.

Metal-on-metal implants offer benefits such as low wear rates, rare osteolysis and decreased dislocation rates;^1,2 however, with concerns mounting about metal ion release, the popularity of this hip bearing surface is waning.

Michael A. Mont

According to a 2009 study,³ at their peak, metal-on-metal implants were used in about 35% of all total hip arthroplasties (THAs), said Michael A. Mont, MD, Rubin Institute for Advanced Orthopedics, Sinai Hospital of Baltimore, Md. However, their use is “rapidly declining … because of concerns about metal ions.”

Metal-on-metal total hip prostheses first emerged back in the 1970s, Mont said. They fell out of favor for several reasons: the technology was not up to par, the metallurgy was not satisfactory and patients experienced fixation failures.⁴ Metal-on-polyethylene implants stole the spotlight because they were doing well, Mont said. However, early metal-on-metal retrievals more than 20 years later showed minimal wear of these components, which sparked renewed interest in this technology.

Things started changing in the 1990s. Metallurgy and manufacturing techniques improved, giving way to new metal-on-metal implants, he said.

Problems surfaced with early generation polyethylenes

Major wear problems began to emerge with some of the early generation metal-on-polyethylene bearings. Surgeons were seeing wear between the metal and plastic components, leading to osteolysis. They were also seeing particle-induced wear, loosening of prostheses and bone loss. Traditional polyethylene had a considerable osteolysis rate at 5 to 10 years in various studies,⁵ Mont said.

The thin polyethylene used at the time was also a problem, according to a 1994 study by Daniel Berry, MD.⁶ His results revealed catastrophic failures of polyethylene liners because of wear-through and liner fractures. These failures were seen often in cups with a minimum polyethylene thickness of 5 mm or less.

Significantly lower wear rates

Newer metal-on-metal implants largely solved the wear problem, lowering wear rates compared to the older polyethylenes,¹ Mont said. They also carried the promise of greater longevity.

“The metal-on-metal technology allowed for larger heads and thus, larger jump distances before the device would dislocate.”

— Michael A. Mont, MD

In addition, the large head used in metal-on-metal technology addressed the catastrophic issue of dislocation² — the No. 1 reason for THA revision — causing approximately 22% of all revision cases, according to a 2009 study by Bozic.⁷ Mechanical loosening, infection and osteolysis/wear are additional causes of revision.

“The metal-on-metal technology allowed for larger heads and thus, larger jump distances before the device would dislocate,” Mont said.

Multiple studies have shown that a large femoral head (> 36 mm) with a metal-on-metal implant produces “pretty negligible” dislocation rates,² he said.

Historically, metal-on-metal implants performed well. The McKee and Watson-Farrar devices had good early results with 95% survival. The 20-year survival rates were at about 85%, according to Mont. These devices were eventually abandoned in favor of better results with the Charnley prosthesis. Modern metal-on-metal prostheses have shown 20 years of use with good to excellent clinical outcomes.¹

Important concerns

Despite the benefits of metal-metal bearings (larger heads, higher head/neck ratio, greater range of motion before impingement and a lower dislocation risk) surgeons have concerns about using them. The most prominent concern is metal ion release, which can trigger metal hypersensitivity or altered soft tissue reactions, some of which are described as pseudotumors, Mont said.

“If somebody has a known metal allergy, they should not have a metal-on-metal device interface.”

— Michael A. Mont, MD

“In my view, some of these people have an inherent, probably rare, hypersensitivity to metal,” he said. “If somebody has a known metal allergy, they should not have a metal-on-metal device interface.” Mont added that the risk for metal hypersensitivity is higher in women and the frequency remains controversial.

Another concern is that metal-on-metal implants do not tolerate malalignment. “There is a small window for putting these in correctly,” Mont said. In fact, they have a lower tolerance for prosthesis malpositioning than most other implants. Implant malpositioning can lead to impingement and accelerated wear, which can cause adverse reactions.

When implanted optimally, metal-on-metal implants probably have a very low incidence of altered soft tissue reactions, Mont said.

Fortunately, there have been purported improvements in the polyethylenes that were used in modern THA, which may help solve some of the prior problems of wear and dislocation. In his recent series, Mont has used large femoral heads against new, highly crosslinked polyethylene with extremely low wear rates and excellent stability.

References

1. Silva M, Heisel C, Schmalzried T. Metal-on-metal total hip replacement. Clin Orthop Relat Res. 2005; 430: 53-61.
2. Smith T, Berend K, Lombardi A, Emerson R, Mallory T. Metal-on-metal total hip arthroplasty with large heads may prevent early dislocation. Clin Orthop Relat Res. 2005; (441):137-142.
3. Bozic K, et al. The epidemiology of bearing surface usage in total hip arthroplasty in the United States. J Bone Joint Surg (Am). 2009; 91:1614-1620.
4. Dumbleton J, Manley M. Metal-on-metal total hip replacement. What Does the Literature Say? J Arthroplasty. 2005; 20(2):174-188.
5. Dumbleton J, Manley M, Edidin A. A literature review of the association between wear rate and osteolysis in total hip arthroplasty. J Arthroplasty. 2002; 17(5):649-661.
6. Berry DJ, et al. Catastrophic failure of the polyethylene liner of uncemented acetabular components. J Bone Joint Surg (Br). 1994; 76(4):575-578.
7. Bozic KJ, et al. The epidemiology of revision total hip arthroplasty in the United States. J Bone Joint Surg (Am). 2009; 91(1):128-33.

Michael A. Mont, MD, can be reached at the Rubin Institute for Advanced Orthopedics, 2401 Belvedere Ave., Baltimore, MD, 21215; (410) 601-8500; email: mmont@lifebridgehealth.org.