Title: Clinical and Wear Analyses of 9 Large Metal-on-Metal Total Hip Prostheses

Abstract

All patients in this study had the Biomet Magnum (M2a-Magnum™) hip implant with a Recap cup and Taperloc (Taperloc1 Hip Stem) or Mallory stem.

This study attempts to figure out the underlying reasons for implant failure by looking at the wear analyses and clinical symptoms of 9 patients with revised Metal-on-Metal (MoM) total hip arthroplasty (THA) implants.

The authors conclude that is very important for doctors to monitor the indicators of implant failure very early on; these include pain, decreased range of motion, and high levels of cobalt and chromium in the blood. The long-term effects of keeping failed implants in place can seriously decrease a patient’s quality of life.

Introduction

All MoM hip implants were recalled in 2011 in the Netherlands. This decision was made after many studies showed MoM THA implants had high rates of revision and caused pseudotumors. Despite this, MoM hip implants remained quite popular in the United States and England.

MoM THA implants shed metal debris into the body, which in turn can lead to high levels of cobalt and chromium in the blood. This can also lead to the formation of pseudotumors around the implant. Pseudotumors are enlargements of tissue that resemble tumors.

The positioning of the implant can impact the amount of wear suffered by implants. For instance, cup inclination angles greater than 55 degrees are known to increase wear due to edge loading. Edge loading occurs when the cup is malpositioned on the head, causing excessive stress and friction, which leads to higher rates of wear.

The wear rates of MoM bearings in this study varied a lot. The authors of this study tried to relate each bearing’s wear rate to its patient’s symptoms in order to understand the underlying mechanisms of implant failure.

Materials and Methods

Of the 9 implants in this study, 7 were revised due to pseudotumor formation. Two of them were revised because of increasing pain. Six of the patients were female, and the average age of the patients was 57.

This study had two main parts: patient examinations and implant wear analysis.

The patient exams consisted of X-rays, MARS-MRI scans, CT scans, ultrasounds, cobalt and chromium serum measurements, and the Harris Hip Score (HHS) survey. The HHS has a series of questions that are intended to measure the performance of an implant; after answering these questions, the patient is given a score from 0-100 to describe how well an implant is working.

Implant wear analysis involved studying the implants themselves after they were removed from the patients in order to determine how much metal was lost by each component of the implant.

Results

The patients’ average HHS score was “fair” (in the 70-80 range). Patients whose implants were revised due to pain had lower HHS scores (in the 60-70 range). Seven patients experienced groin pain, and 2 patients noticed their hip joint was swollen. None of the patients had any neurological problems, deafness, or dizziness.

Four of the patients with pseudotumors experienced groin pain, while three of the patients with pseudotumors (all females) did not experience any groin pain.

According to the X-ray scans, only two patients appeared to have loose cups. Additionally, no patients suffered from bone loss, bone resorption, or fractures. The median inclination angle of the implant cup was 41.2 degrees. However, for the patients with pseudotumors, the median inclination angle of the implant cup was much higher at 47.7.

Moreover, eight patients received ultrasounds. Pseudotumors were visible in 2 ultrasounds. Liquid accumulations in the hip joint were visible in 4 ultrasounds.

In this study, the median cobalt levels were 24.8 nmol/l, and median chromium levels were 47.3 nmol/l. However, patients with pseudotumors had median cobalt levels of 53.8 nmol/l and median chromium levels of 63.0 nmol/l.

A total of 6 MARS-MRIs and CT scans were obtained. Two CT scans showed possible pseudotumors. Five MARS-MRI scans showed possible pseudotumors.

In addition to assessing the patients, the authors of this study also looked at the implants themselves in order to measure the level of wear in each implant. Wear indicates the amount of metal that was shed from a particular component of an implant. Overall, the amount of wear differed a lot from one bearing to another, ranging from 0 mm³ to 24.424 mm³ in the heads, and 0 mm³ to 20.575 mm³ in the cups, and 0 mm³ to .172 mm³ in the tapers. Severe wear was seen in only one bearing.

Discussion

Large-head MoM THAs frequently lead to early failures and pseudotumor formations. However, wear rates and pseudotumor formations vary widely. A better understanding of the relationship between wear rates and pseudotumor formations is needed in order to predict the outcome of MoM THAs in the future.

Some studies have found a link between high wear, patient’s metal sensitivity, and the appearance of pseudotumors. However, other studies have found that pseudotumors can still occur without a lot of wear. In these patients, it is possible that a hypersensitivity to metal caused the implant failure. In hypersensitive patients, reducing the amount of wear experienced by their implant could possibly prevent the implant from failing.

The most important predictor of a bearing’s wear rate is a type of wear called edge loading. Edge loading is caused by a lot of factors, the most significant of which is high cup inclination.

The effects of metal wear particulars and elevated levels of cobalt and chromium have been documented, but they are still not completely understood.

This study did not find a clear relationship between serum metal ion levels and wear rates. However, this study did show a correlation between high cup inclination and increased metal ion levels. For instance, two patients had a cup inclination angle over 55 degrees, and these patients also had the highest amount of wear in their cups.

The patients with the highest amount of head wear also had increased levels of cobalt and chromium in their serum. These findings show that implant positioning is very important to preventing high metal ion levels.

All patients had high levels of cobalt and chromium in their serum. Two patients also had very elevated levels of titanium in their serum. Elevated titanium levels indicate the presence of trunniosis, which is wear of the femoral head–neck interface. It is only seen in large-diameter head bearings. Trunniosis is typically only seen in head bearings larger than 40mm in diameter.

The exact causes of implant failure and pseudotumor formation are not yet known. They could be caused by high cup inclination, edge loading, trunniosis, patient characteristics, or a combination of all of these factors.

The authors provide some recommendations to doctors who treat people with MoM hip implants. They first recommend very close and frequent monitoring of patients with MoM implants. Doctors should not just ask the patient questions to determine if their implant is working; they should also measure patients’ cobalt, chromium, and titanium levels, take X-rays, and conduct MARS-MRI scans. More information is needed to understand the relationships between failed implants, metal ion levels, pseudotumor formations, and patient symptoms.

Reference

Koper, M.C., Mathijssen, N.M.C., Witt, F., Morlock, M.M., Vehmeijer, S.B.W. Clinical and Wear Analyses of 9 Large Metal-on-Metal Total Hip Prostheses. PLoS ONE 11(10):e0163438. doi:10.1371/journal.pone.0163438 (2016).

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