Medical devices such as heart pacemakers, stents, dental implants, spinal cord simulators, artificial knees, and hips are being implanted in the human body to replace or support damaged organs, tissues, or bones. However, several major news outlets have brought the issue of medical implant safety to the attention of the public recently [1][2]. Horrifying stories of implant failure in patients and lax medical regulations that evidently let insufficiently tested implants into the market are the subjects of the eye-opening Netflix documentary “The Bleeding Edge” [3]. This blog will address what could happen in the body if a total hip replacement implant fails.

Hip implants design and materials

In my earlier blog, I discussed the total hip replacement surgery recommended to end-stage arthritic patients as well as hip implant designs and materials available. Currently, there is no standard implant alloy or design in the market. Orthopedic surgeons choose a suitable hip implant device based on the age, activity level, and body measurements of each patient. The cost and availability of the implant also factor into this decision.

Metal-on-metal implants, metal-on-polymer implants, and ceramic-based implants are some of the hip implant designs currently in the market. Modern hip implant designs are “modular,” i.e., they are manufactured as separate components and “locked” together during the surgery [4]. Modularity has several benefits and is preferred by surgeons. However, modular implants have additional interfaces that are susceptible to wear and corrosion.

The biomedical alloys commonly used to create the “ball” and “socket” in artificial hips are stainless steel, cobalt-chromium alloys, and titanium alloys [5]. None of these alloys have all the properties required to be used as an implant, namely biocompatibility, strength, bone integration properties, fatigue, and corrosion resistance. Hence, modular hip implants are made up of a cobalt alloy “head” and titanium alloy “stem” in order to utilize the superior wear resistance of cobalt alloy articulating surfaces [6] as well as elastic modulus [7] and bone integration properties [8] of the titanium alloy.

Risks involved when a hip implant is in the body

Every type of hip implant carries the risk of infection, hip dislocation, and possible difference in leg lengths after the surgery. In addition, the biomedical alloys used in hip implants are under increasing scrutiny due to the metallic wear fragments and corrosion products that release into the body.

• Corrosion and wear products released from the implant are usually metal ions and metal or polymer fragments.
• Metal or polymer fragments can accumulate in the hip joint cavity, leading to Adverse Local Tissue Reactions (ALTRs), pseudo tumors [9][10][11], osteolysis (degeneration of bone), aseptic loosening (loosening of the implant from bone) or fracture of the implant [12].
• Metal ions released from implants due to corrosion can be transported into the bloodstream [13][14], leading to local and systemic effects. For example, cobalt, chromium, and nickel ions (components of cobalt-chromium alloys) have a toxic effect on human cells. Long-term exposure to cobalt has been associated with thyroid, cardiac, and neurological dysfunction [15], while chromium ions are carcinogenic [16].

Regulatory and clinical concerns about total hip replacement implants

• Medical device regulatory agencies in the United States and the United Kingdom have raised alerts on metal-on-metal hip implants due to the metallic fragments released into the body from articulating cobalt alloy bearing surfaces [17][18].
• Cobalt-titanium alloy junctions present in modular implants have been documented to release corrosion products and cause ALTRs [19][20][21].
• A recent clinical study concluded that the above-mentioned adverse reactions were significantly higher in patients carrying a modular metal-on-metal implant (46%) compared with a non-modular metal-on-metal implant (16%) [22]. However, in spite of its risks, modularity is still being favored by surgeons. In a survey of members of the American Academy of Orthopedic Surgeons, 87% of the respondents indicated they had used modular implants [23].

Recommendations for those who are undergoing hip replacement surgery

In the United States, the most commonly used hip implant was a metal-on-polymer bearing couple with a cobalt-chromium-molybdenum alloy “head,” titanium alloy “stem” and a UHMWPE (ultra-high molecular weight polyethylene) acetabulum liner. Cobalt-chromium-molybdenum implants are cheaper, which is an important factor that hospitals and patients have to consider while choosing an implant.

Corrosion problems lead to a decline in the use of cobalt-chromium-molybdenum “heads” in favor of ceramic “heads.” In 2017, the American Joint Replacement Registry reported for the first time that more ceramic heads (52.3%) were used in total hip replacement surgeries compared to cobalt-chromium-molybdenum heads (47.7%) [24]. However, it has yet to be seen if this trend will continue or whether it will eventually plateau. As of today, the ceramic-on-polyethylene (ceramic-on-polymer) implant appears to be the new gold standard in the U.S and what you should probably ask your doctor.

References

[1] https://www.bbc.com/news/health-46318445

[2] https://medicaldevices.icij.org/

[3] https://www.netflix.com/de-en/title/80170862

[4] Hernigou. P et al., International Orthopaedics, 37, 2013, 2081-2088.

[5] Eliaz. N, Corrosion of Metallic Biomaterials: A Review, 2019, 12, 407.

[6] Narushima. T et al., Advances in Metallic Biomaterials, 2015, 3, 157-178.

[7] Niinomi. M, Materials Science and Engineering. A, 1998, 243, 231-236.

[8] Acero. J et al., Journal of Cranio-Maxillofacial Surgery, 1999, 27, 117-123.

[9] Hart. A. J. et al., Acta Biomaterialia, 2010, 6, 4439 – 4446.

[10] Higgs. G.B. et al., Journal of Arthroplasty, 2013, 28, 2-6.

[11] Huber. M. et al., Acta Biomaterialia, 2009, 5, 172-180.

[12] Gilbert. J. L. et al., Journal of Bone and Joint Surgery, 1994. 76a, 110-115.

[13] Lardanchet. J. F. et al., Orthopaedics & Traumatology: Surgery & Research, 2012, 98, 265-274.

[14] Di Laura. A. et al., Scientific Reports, 2017, 7, 10952.

[15] Devlin. J. J. et al., Journal of Medical Toxicology, 2013, 9, 405-415.

[16] Cohen. M. D. et al., Critical Reviews in Toxicology, 1993, 23, 255-281.

[17] https://www.federalregister.gov/documents/2016/02/18/2016-03331/effective-date-of-requirement-for-premarket-approval-for-total-metal-on-metal-semi-constrained-hip

[18] https://www.gov.uk/drug-device-alerts/all-metal-on-metal-mom-hip-replacements-updated-advice-for-follow-up-of-patients

[19] Whitehouse. M. R. et al., Bone & Joint Journal, 2015, 97B, 1024-1030.

[20] Scully. C. W. F. et al. Orthopedics, 2013. 36, E666-E670.

[21] Cooper. H. J. et al., Journal of Bone and Joint Surgery, 2012, 94A, 1655-1661.

[22] Vendittoli. P. A. et al., Hip International, 2019, 29, 262-269.

[23] W. M. Mihalko, Survey results concerning Modular Taper Junctions among Fellows of the American Academy of Orthopaedic Surgeons, ASTM Work. Modul. Tapers Total Jt. Replace. Devices Jacksonv. FL, 2013.

[24] http://ajrr.net/images/annual_reports/AAOS-AJRR-2018-Annual-Report-final.pdf. AJRR 2018 Annual Report, 2018.