Metal Ion Testing
updated 10/8/2023 tpg
All artificial implants will wear at the moving bearing surface. A low level of any type of wear particle is usually well tolerated. Total hips also have a morse cone connector between the ball and the stem that corrodes and liberates debris. Corrosion or excessive wear of any artificial bearing can lead to failure and may require revision surgery.
Plastic bearings
Plastic bearings release plastic particles that mostly accumulate around the local implant area and can cause bone destruction called osteolysis. These particles cannot be measured in the blood. Wear is best assessed by appearance on routine follow-up x-rays.
Ceramic bearings
Ceramic bearings exhibiting abnormal wear chiefly fail by causing loud squeaking. Soft tissue or bone reaction is rare.
Metal bearings
Metal bearings release cobalt and chromium particles that are deposited in the local tissues. They are absorbed in the blood and excreted in the urine. Wear cannot be determined from x-rays. The wear rate can be gauged very reliably from blood ion measurements.
We have learned that implant design flaws (DePuy ASR recall) and certain implant positions can cause abnormally high wear rates. High wear rates result in larger particle overload of the local tissues that causes pain and swelling. We call this an Adverse Wear Related Failure (AWRF). It is diagnosed by a triad of findings: high ion level above 15ug/L, a steep cup position (acetabular inclination angle AIA), fluid collection or inflammatory mass on MRI/CT, and extensive metal debris in the tissues (metalosis) seen at the time of revision surgery. We can screen for this problem by checking ion levels in the blood.
Extensive scientific studies of this problem have allowed us to determine safe implant positions to prevent this problem. We have had only 10 AWRF in over 7000 cases and have not had a single case in surgeries performed since 2009. But we still advocate a monitoring protocol for early detection.
There is a period of run-in wear that lasts from 1-2 years, thereafter levels steadily decline in most patients, except those that have AWRF. Systemic harm such as cancer has been disproven in multiple studies. Neurologic signs such as neuropathy, tinnitus, hearing loss, or visual disturbances have rarely been seen with levels above 20ug/L. Heart damage has been rarely anecdotally reported at extreme levels well over 100ug/L. 80% of patients that I have measured show a normal level for patients without implants (<1.5 ug/L). 19% show a slightly higher level. Less than 1% have levels above 20ug/L; none since 2009.
Metal ion release is a double edged sword. If the levels are extremely high they can be damaging. But low levels have not been shown to be harmful. In fact, metal-on-metal resurfacing patients have a far lower all-cause mortality than matched THR patients at 10 years reported in 5 studies to date. There does seem to be something life-preserving about a metal on metal hip resurfacing when compared to a THR. But many "thought leaders" in THR refuse to acknowledge this scientific data and still try to create a climate of fear around metal bearings and ion release. In fact studies on Total Knees show higher mean cobalt levels than in metal-on-metal hip resurfacing, but this paradoxically is not at all concerning. But when metal ion levels are high in hip resurfacing this can alert us to excess wear so that we can take action much sooner than corrosion or wear cases in standard THR where no blood markers are reliable.