Highly Cross-Linked Polyethylene Performs Well in Implants
In This Article
- 17-year follow-up study shows that highly cross-linked polyethylene (HXLPE) hip implants reduce wear by 90%
- Rates of periprosthetic osteolysis are commensurately low
- The next follow-up study will be completed in three years, in order to take 20 years of continuous data into account
Periprosthetic osteolysis, or bone death, was the single biggest cause of late-term failure in hip and knee replacements, which are generally made of conventional polyethylene. Wear in the polyethylene over time generates particulate debris in the joint, triggering an immune response that results in bone loss around the joint and failure of the implant (see Figure 1). The incidence of osteolysis with conventional polyethylene joint implants has been reported to be as high as 37% at 8 to 10 years after surgery. To reduce wear and osteolysis, Orhun Muratoglu, PhD, director of Massachusetts General Hospital’s Harris Orthopaedics Laboratory, and his colleagues developed a material called highly cross-linked polyethylene, or HXLPE, more than two decades ago.
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Now, Dr. Muratoglu and his lab have completed a 17-year follow-up study of patients with hip replacements made of HXLPE. Their findings, which will be published in a forthcoming article in 2018, show that use of HXLPE in prosthetic hips has reduced wear by 90% versus conventional polyethylene and has almost eliminated periprosthetic osteolysis.
Fig. 1: Lysis Histology
Histologically, the tissue within areas of bone destruction is filled with macrophages in response to the sub-micron particles of polyethylene. The macrophages engulf the polyethylene and cause a series of reactions that result in large amounts of bone resorption.
Low Wear Rate Reduces Complications
The Harris Laboratory at Mass General has been developing and clinically testing improved iterations of polyethylene since the 1990s. To prevent the material from wearing out and shedding debris, Dr. Muratoglu and his colleagues tried cross-linking the polyethylene using ionizing radiation and then melting the material to increase its oxidative stability. The new HXLPE was then tested extensively in preclinical studies for durability. It was patented and licensed to two orthopedic manufacturers, after which time clearance from the Food and Drug Administration was secured. Surgeons began placing the new HXLPE hip implants in patients starting in 1998.
The 17-year follow-up study, completed by Dr. Muratoglu and his team, looks at medical data from patients given hip replacements using HXLPE to determine its clinical efficacy, as judged by patient-reported scores and radiographic analysis of osteolysis and wear. The study looked at 768 patients from eight medical centers. Currently, it is the largest study of patients undergoing total hip replacement surgery using a single type of HXLPE.
“The clinically important finding here is that there is no periprosthetic osteolysis that we can detect in these patients. That’s good news,” says Dr. Muratoglu. “The underlying reason is that the wear rate is low.”
The study also found no significant difference in wear produced by standard- and large-size artificial femur bone heads at 13 and 10 years after hip replacement surgery. Larger femur bone heads are preferred because of their lower dislocation rate.
Data Confirm Previous Results
The 2017 findings were in line with those from a 2012 study of the same cohort of patients with similar methodology, published in Clinical Orthopaedics and Related Research. In that study, the Harris Lab found no evidence of periprosthetic osteolysis around the implants in any HXLPE patient, nor any radiographic loosening, failure or fracture in any of the HXLPE components.
In addition, there were no reports from any of the centers contributing data that any patients required corrective surgery to fix changes to their artificial hips due to polyethylene wear. They did find slightly greater wear in patients who had prosthetic hips with some larger femur bone heads, but the wear was below the threshold for producing osteolysis. By comparison, the control group, whose hips were constructed from conventional polyethylene, had an overall incidence of periprosthetic osteolysis of 14%.
New Materials Must Last Even Longer
The next follow-up study will be completed in three years in order to account for 20 years of continuous data. Dr. Muratoglu and his team are hopeful that the hips they are building today will last 40 years, but they won’t know until they have 40 years of data.
“A lot of people used to outlive their hip replacement,” explains Andy Freiberg, MD, chief of the Center for Hip & Knee Replacement at Mass General. “It would wear out, requiring revision surgery. Even someone 65 or 75 years old can expect many years of active life and function with hip replacements. The concern is treatment of young people with advanced arthritis because of risk of wear or loosening.”
Dr. Freiberg continues, “We routinely do hip and knee replacements on people who are 55 and younger, and our experience is that cross-linked polyethylenes will likely allow these replacements to last a lifetime. Long-term study of our patients at Mass General is critical to our understanding of how long these implants will ultimately last.”
A 20-year-old, for instance, would probably need a prosthetic that could last at least 70 years. The Harris Lab is taking steps to get there.
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