Risk factors for taper corrosion in total hip arthroplasty
Research continues towards a better understanding of total hip arthroplasty taper corrosion in modular connections between metal device components. Corrosion has mainly been documented in metal-on-metal joint articulations but a growing number of cases involving metal-on-polyethylene have arisen. Metal debris due to corrosion and wear, loosened by mechanical stress inherent in the design of modular tapers, appears to be the source of clinical symptoms, such as elevated serum metal ions, local tissue reaction, and implant failure. We look at a number of suspected factors that, when combined in vivo, have contributed to the problem.
Hip joint replacement is the most common orthopedic joint surgery . In the EU it is the second most common surgical procedure after Caesarean sections . Rates for total hip arthroplasty (THA) are growing around the world and younger patients are undergoing the procedure; the American Academy of Orthopedic Surgeons (AAOS) reported a 123 percent increase between 2000 and 2009 in the 45 to 64 age group. While women are more likely to be recipients of an artificial hip, it has been estimated that more than 2.5 million Americans are living with this implant , with a further 300,000 Americans and more than 80,000 people in the UK receiving new hips each year .
Considering the frequency with which THA is performed, any tool that can assist surgeons with their tasks in the operating room, whether it be for primary or revision surgery, are usually welcome additions. Introduced in the 1980s and 1990s, modular implants were thought to help surgeons customize biomechanics. Modular implant pieces facilitated an easier achievement of offset, limb length, and anteversion, which is a complex endeavor .
The introduction of modular metal-on-metal total hip joint articulations with large heads (MoM THA) was seen as an opportunity and promoted by most device companies, embraced by many surgeons and as a result, a significant number of patients received this combination. Modular MoM THA appeared to be a great solution to stability issues, and an alternative to resurfacing, particularly for revisions due to failed femoral neck fracture .
Read an interview with Michael Morlock PhD, Director of the Biomechanics Institute at the Hamburg University of Technology in Germany. He outlines a timeline of the taper junction corrosion issue and suggests a course of action to address the issue.
It’s been asserted that surgeons only contemplate the strength of a material when choosing a modular junction. But this single consideration neglects a “totality of factors”, such as corrosion resistance, cost, and manufacturing constraints . When it comes to selecting a femoral head, cobalt/chromium (Co/Cr) alloys are low cost and also strong, which are desirable features . Coupled with the advantages of modular design (intraoperative flexibility, simplified procedures, decreased inventory) it is easy to see why modular metal implants are handy in the toolbox.
However, corrosion at the taper junction in modular hip implants is increasingly suspected to contribute to mortality, adverse tissue reaction, such as pseudotumors, and device failure [7,8,9,10], regardless of the type of articulating bearing surface used . One recalled modular head-neck stem implant has had reported failure rates of 86 percent within three to five years .
Indications of trouble
Friction and wear were the primary focus of THA implant failure in the early part of the 21st century; mortality tracking did not generally differentiate between resurfacing and large-head MoM patients. The National Joint Registry of England, Wales and Northern Ireland (NJR) had teased apart numbers for these types of implants by their 2010 7th Annual Report, which showed 5-year implant survival the lowest in patients receiving MoM THA, particularly after three years, when compared to cemented, cementless, hybrid, and resurfacing [See Fig. 2] . The previous year's NJR report did not differentiate.
A 2010 study published in Clinical Orthopedics and Related Research, authored by Garbuz et al, found serum cobalt levels to be 46 times higher from baseline at one-year postoperative in patients that received large-head metal-on-metal THA; those that had received resurfacing saw a 10-fold increase in levels. The authors “recommended against this particular large head total hip arthroplasty” .
Professor Michael Morlock PhD from the Biomechanics Institute at the Technical University Hamburg-Harburg in Germany points out that until the early 2000s only minor issues were reported with taper junctions because smaller diameter heads (up to 36 mm) were used . Smaller heads generate lower loads at the taper junction and therefore were less likely to jeopardize this area. However, he also highlights recent findings that corrosion is being seen in “nearly every joint articulation material and head size, down to metal and ceramic heads as small as 28 mm against polyethylene”.
A slow call to action
Surgeons were not acutely aware there was a potential problem. The European Union‘s Scientific Committee on Emerging and Newly Identified Health Risks (SCENIHR), in its 2011 Statement of Opinion, acknowledged the evidence against modular taper junctions was originally “anecdotal” before gaining traction “as the origin of problems” .
It is interesting to note that while explant analyses of modular metal implants showed corrosion, and one study by Goldberg in 2002 suggested using larger taper diameters to increase stiffness and decrease fretting and corrosion, there were not many recommendations that directly connected metal tapers to negative outcomes via research . Many study conclusions leaned toward design improvements over “biological consequences” .
A 2003 dissertation in German by Windler touched on the disconnect between the expectations of device performance, as established in laboratory experiments, and what was being seen in explant analyses. The author noted that 50 percent of the examined Co/Cr ball heads showed corrosion after four years . Indeed, as Michael Morlock points out, “we are still not able to reproduce the corrosion phenomenon we see in the patient in a laboratory setting, which makes systematic investigation of the influencing factors very difficult.” (See also interview)
Corrosion through a combination of risk factors:
Corrosion in the human body is an electrochemical process that can be exacerbated by friction or fretting under load. The body is an inhospitable environment that, as Hansen points out, is considered “one of the most aggressive and corrosive to metals” . However, in his paper for the Electrochemical Society’s publication Interface, he identifies variability in the fabrication methodology of alloy implants as the most likely factor to influence differences in “mechanical, physical, or electrochemical properties”.
Cartner, in a 2014 presentation to the European Hip Society Annual Congress, shared research that found the metal alloy composition, not the head size, determined corrosion and material loss in vivo. The study of 166 cobalt-chromium-molybdenum (CoCrMo) heads and 37 oxidized zirconium (OxZr) femoral heads “showed no correlation between head size or time in vivo and severity of corrosion” . This observation was also reported in 2002 by Goldberg et al .
Reduced taper diameter (from 14/16 to 12/14), decreased taper length (from 2 cm to less than 1 cm) and increased head diameter (more than 36 mm), while delivering increased flexibility and stability, have significantly increased the loads on the taper junction . As mentioned earlier, corrosion is not restricted to MoM THA. Metal-on-polyethylene implants are also displaying similar problems, which have been attributed to an increase in mechanical loading and a decrease in loading capacity. Loading amplifies material loss through fretting and corrosion by allowing fluid to enter the interface between the ball and taper during micromotions of the device components , this has been referred to as mechanically assisted crevice corrosion (MACC) .
The surface morphology of implants also plays a role. The roughness and machining mark patterns on tapers vary between manufacturers; there are no standard taper surface specifications used in the industry . It’s been shown that a single unit of roughness in a metal hip taper increases the wear rate by 73 percent . However, this depends on the metal alloys used; wear and damage rates differ between matched cobalt/chromium (Co/Cr on Co/Cr) and mismatched Co/Cr on titanium (Ti) (Co/Cr on Ti) . But device design and manufacturing are not the only contributors to corrosion.
Patient factors such as body mass index (BMI), age, and activity levels are also contributing factors. The World Health Organization has reported more than a doubling of global obesity rates between 2008 and 2016 . Heavier people inflict more stress and wear on their joints; this is influencing the growing demand for joint replacements within this group and will result in higher wear rates, revisions, loosening and dislocations .
Age and activity level influences wear and corrosion rates. Younger active patients are increasingly seeking joint replacement, which presents the need for an implant to perform for someone with an active lifestyle over a longer period.
Faced with the need for implants to function well for longer, surgeons are tasked with some important choices in the operating room. When selecting components for modular THA it has been noted that it is critical to ensure all parts are supplied from a single manufacturer . Even if they have the same name, components are not standardized between companies, nor are they interchangeable. Using dissimilar metal surfaces has been shown to increase corrosion . Failing to limit angular mismatch and center offset have also been suggested as contributors to corrosion .
The move towards smaller incisions during total hip replacements has been suggested as a corrosion factor as well; less space to work may make assembly force application more difficult . And assembly practices that are consistent (especially forces), follow manufacturers specifications, and reduce or eliminate contamination from “blood, bone, water, and fat”  have been proposed as steps that should be reinforced.
Potential co-mingling risk factors for taper corrosion
- Taper design: reduced taper diameter and length
- Increased head diameter
- Lateral offset
- Length of time in the body
- Patient body mass index (BMI)
- Patient activity level and age
- Interface contamination
- Inconsistency of assembly
- Dissimilar metal alloys at head-neck junction
- Smaller operative incisions
- Fretting and micro fractures
- Taper surface morphology
- Taper angular mismatch and center offset
- Device fabrication
A complex problem
What has emerged in recent research is the realization that it’s a co-mingling of factors, not a single feature, that is influencing corrosion in the taper junction. It is positive to note that a hefty decrease in the use of MoM modular junctions has been reported. The UK NJR 2016 Annual Report states that, “the use of metal-on-metal stemmed implants has virtually ceased…[and] account[s] for only 0.9% of implants in 2015.” Compare this to the NJR assertion that the peak use of MoM in THA occurred in 2006 in 10.8 percent of reported THA procedures .
Research will eventually look at each of the factors in turn. It takes time for a body of clinical and scientific evidence to build and offer support or recommendations for any particular change.
In Part II of this series we look at when taper corrosion should be clinically considered and how to diagnose it.
Where do we go from here? In conversation with Michael Morlock
AORecon spoke with Michael Morlock, Professor and Director of the Institute of Biomechanics at the Hamburg University of Technology (TUHH), about factors that contribute to the corrosion problem and what is needed in the field to address this challenge. Researchers haven't been able yet to reproduce similar corrosion phenomena in the laboratory, which adds complexity to the identification of definitive contributing factors.
When did the problem first come to the attention of the orthopedic community?
Michael Morlock: Taper corrosion was observed shortly after the start of the use of modular taper connections in hip arthroplasty. The first report of an adverse reaction was probably made by Svensson  in 1988 when a pseudotumor was reported in connection to a modular Lord stem with a metal-on-plastic bearing articulation. Interestingly, at that time, corrosion was viewed more as a prosthesis failure problem with less focus on the biological consequences.
Up until around 2010, there were several anecdotal reports about serious problems but none went so far as to warn the orthopedic community about the biological consequences of the corrosion products from modular taper connections. The advantages of modular head tapers were greatly appreciated by surgeons since it allowed them to readjust the hip reconstruction after placement of the stem by using heads of different lengths.
In 2010, Donald Garbuz from the Vancouver group  published a prospective study comparing resurfacing of the hip to metal-on-metal (MoM) bearing articulations on conventional stems. They found the MoM group’s blood metal ion concentrations were several times higher than the resurfacing group. The two study groups differed only by the presence of a taper, so it was clear that metal had to come from the taper connection. Suddenly everyone looked for signs of corrosion during revision surgery and these were observed in a significant number of cases, even so the revisions in most of them were not related to corrosion. Since then, corrosion has been reported for nearly all kinds of bearing articulations and prostheses.
What are the predominant risk factors, especially considering implant design and mechanical forces?
Risk factors for taper corrosion comprise everything that can induce micromotion at the taper interface. The amount of micromotion is influenced by three different factor groups: implant design; assembly by the surgeon; and mechanical loading in the patient. If micromotion can be prevented, the risk for taper corrosion disappears.
From a design point of view, large heads with possibly high friction in unfavorable situations, together with a high offset do cause high mechanical bending loads on the head-stem taper interface and results in a higher risk of micromotion. One problematic design feature that was identified early by the Goldberg group , was neck stem stiffness. More flexible necks cause higher deformation during loading, which greatly increases the risk of micromotion.
Most of the time, design is cited as a major reason for the problem. But it was eventually realized that other factors, like proper cleaning of taper interfaces and firmly affixing the head on the taper, are equally important for the long-term survival of the taper connection.
Since the original findings of the Goldberg group, smaller and shorter tapers were introduced to allow larger ranges of movement in combination with small prosthesis heads. But now these smaller and shorter tapers are also used with large heads. The correlation between a reduction in taper size and the corrosion problem has never been made, probably because we are still not able to reproduce the corrosion phenomenon we see in the patient in a laboratory setting. This makes the systematic investigation of the influencing factors very difficult.
Are some patients or products more vulnerable to these risks?
Some people seem to react to metallic debris much stronger that others and it is still not clearly understood if there are fixed thresholds for serum metal ion levels in the patient. Also, taper corrosion produces cobalt and chromium ion concentrations in a different ratio as wear at the bearing articulation of an MoM prosthesis. This might be the reason why clinical problems with metallic debris from taper articulations are reported for cases in which the metal ion levels are below the thresholds established for MoM prosthesis wear.
Does the corrosion issue also present itself in other joint replacements?
Taper corrosion can occur at any taper articulation being put into the human body. But the extent of the problem seems to be the biggest in the hip. This might be due to the unique forces on the hip taper. It is highly loaded in bending, which loads the taper in a direction it is not made for. If tapers would be loaded dominantly in axial compression, the corrosion problem would probably be much smaller.
However, everybody is looking for corrosion right now and you can be sure it will be observed in every single joint replacement in the body. This is partly because metals in a biological environment will always show some form of corrosion. Corrosion may not have any biological consequences if it stays below a certain, but yet unknown, level.
In your opinion, what changes to surgical practice or implant design are needed?
If all the mentioned factors — design, surgeon, loading — are addressed at the same time, the occurrence of taper corrosion would probably go back to the level of anecdotal reports. This would require a reduction in the mechanical loading of the taper interface by using low friction bearings with decent head diameters, and simultaneously improving the cleaning and assembly of the taper connections. 10 years ago Carsten Perka and myself founded the “The 36 and under club” (referring to the diameter of the prosthesis head) when the problems started to become visible. The invitation to join this club still stands!
Tapers were originally introduced to allow the use of ceramic heads on metal stems. The use of ceramic heads would greatly reduce the corrosion problem, at least that is what some studies analysing the amount of corrosion of ceramic heads have shown.
Maybe we should go back to the roots and use tapers for what they were made for— the use with ceramic heads. People might not remember, but originally two European ceramic companies introduced tapers in hip arthoplasty so ceramic heads could be employed. This was in the mid 1970s and only afterwards tapers were used together with metal heads.
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Michael M Morlock PhD
Michael M Morlock holds degrees in mathematics, sport science, and medical science. He is currently a full Professor and Director of the Biomechanics Institute at Hamburg University of Technology (TUHH) in Germany.
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