Researchers in New Zealand found that artificial hip liners made of highly-cross-linked polyethylene showed less wear 10 years after implantation than liners made of conventional ultra-high-molecular-weight polyethylene.
Patients whose hip liners were made of highly cross-linked polyethylene also had a significantly lower prevalence of osteolysis and revision surgery, according to a report in Orthopedics Today.
That’s because cross-linking ties molecules together in a lattice-like matrix, Larry Wallace, senior staff scientist at Spectrum Plastics Group, told Medical Design & Outsourcing. The structure of regular high-density polyethylene is crystalline, Wallace told us.
“It’s really good material but it isn’t chemically bonded. It’s more of a physical alignment of molecules,” Wallace said of the regular polyethylene. “But on the cross-linked polyethylene, you have chemical bonding, which holds all the molecules together. Probably the reason they see the improved performance is more of a dimensional stability.”
Cross-linked polyethylene has been around since the 1960s and is better known as PEX, a material used in domestic hot water pipes and as insulation for high-voltage electrical cables. Its greater dimensional stability makes it a good candidate for the wear and tear of an artificial hip, Wallace said.
Wellington Hospital orthopedic surgeon Dr. Peter Devane and colleagues randomly assigned 122 patients undergoing total hip arthroplasty for degenerative hip disease to receive either an Enduron conventional ultra-high molecular weight polyethylene liner (UHMWPE) or a Marathon highly cross-linked polyethylene liner (XLPE). Both hip liners are made by DePuy. The researchers collected Oxford Hip Scores and general health survey scores from the 91 patients who were available for follow-up 10 years after surgery and used validated software to analyze radiographs for osteolysis and for 2-dimensional, 3-D and volumetric wear. The Oxford Hip Score is a patient-reported survey developed to assess function and pain in hip surgery patients.
The results showed a mean wear rate of 0.03mm per year for the XLPE liner group and 0.27mm per year for the UHMWPE group. The XLPE liner group also showed a significantly lower prevalence of osteolysis, a pathological destruction or disappearance of bone tissue. Devane was surprised that any osteolysis had occurred with the XLPE.
“We believe the osteolysis seen with the highly [cross-linked polyethylene liner] XLPE in our study came from premature loosening of a less than optimal cemented femoral stem,” Devane told Orthopedics Today.
Only two of the 12 revisions performed among the 91 reporting patients occurred in the XLPE group. The Oxford Hip scores and general health scores did not differ significantly among patients, regardless of the type of liner they received.
The study was limited by size, taking place at a single location and using liners made by one manufacturer. It also had the potential for underreporting of osteolysis because researchers used anteroposterior and lateral radiographs instead of CT or MRI, but Devane told Orthopedics Today that surgeons should be confident in using highly cross-linked polyethylene in total hip revision surgery.
“Surgeons are able to offer total hip replacement with this bearing surface to patients with confidence that their prosthesis will last substantially longer than previous conventional polyethylene, which has been quoted at 15 years,” Devane said. “This allows total hip to be performed in much younger patients than previously thought, with the knowledge it may be their only operation.”
DePuy’s Marathon highly cross-linked hip liner is produced from GUR 1050 polyethylene ram extruded bar that is moderately crosslinked at 5 Mrad of gamma irradiation, then stabilized using a patented remelting process to eliminate free radicals, balancing the need for wear resistance against the damaging effects of irradiation, according to the company website.
While XLPE has outperformed metal-on-metal or ceramic-on-ceramic artificial hips, the presence of osteolysis in Devane’s patients 10 years out warrants continued monitoring of hips made with XLPE liners, added Dr. Craig Della Valle, an editorial board member of Orthopedics Today. Della Valle is a consultant for DePuy Synthes, Smith & Nephew and Zimmer-Biomet; receives research support from Smith & Nephew and Stryker; and receives royalties from Zimmer-Biomet, according to the magazine. Devane has received grants from DePuy.