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Surgeons, scientists discuss benefits, potential faults of highly crosslinked PE
Investigator: Polyethylene offers greater wear resistance but decreased mechanical properties.
At a workshop on wear-resistant polyethylene for joint replacement, surgeons and scientists compared the predictions of early lab studies to the current clinical results of highly crosslinked polyethylene components.
They also discussed whether issues associated with the use of these implants stem from the bearing materials.
Harry A. McKellop, PhD, said that data from laboratory hip simulation studies conducted nearly 10 years ago accurately predicted the clinical results seen with highly crosslinked polyethylene today.
“The clinical wear rates to date with increased crosslinking are comparable to what was predicted by the hip simulators,” he said. “More importantly, lysis is either absent or much less extensive with increased crosslinking than with historical the controls. The concerns that smaller debris from crosslinked polys will be more reactive, offsetting the wear reduction have not been supported by the clinical data. Component fractures have been rare and those that have occurred have often been a function of implant design or positioning, rather than the type of polyethylene.”
Specifically, in a database study up to May 2008, he found that only 22 of the 141 reported failures of hips using highly crosslinked polyethylene cups were attributed to liner fractures. “Since several million have been implanted the total number of fractures to date is fortunately small,” McKellop told Orthopedics Today.
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Image: Ries MD |
Trade-offs
However, Michael D. Ries, MD, cautioned that the trade-offs for wear-resistance are decreases in the mechanical properties.
“Many of the in vivo fractures are design-and surgical-related which may be solved by modifications in design,” he said. “However, we need a better understanding of the basic science driving polyethylene fatigue and fracture mechanisms and the development of standard tests that can be used for characterization of fracture resistance and design.”
He noted that irradiation followed by annealing above 130° decreases crystallinity and neutralizes free radicals, while irradiation followed by annealing below 130° maintains a higher crystallinity but allows free radicals to remain and potentially lead to oxidation in vivo.
“Many factors affect the fatigue, crack propagation resistance and mechanical properties in vivo,” Ries said.
In his own study, Ries and his colleagues found four fracture cases that occurred in patients with highly crosslinked polyethylene liners. All had crescent-shaped fracture fragments and the breaks started on the outer surface of the liner.
“Some of the cups were malpositioned, but there were also some implant design factors such as 2 mm in thickness in the rim and 7.5 mm of overhanging polyethylene, which likely contributed to the fracture rate,” he said.
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Image: Schmalzried TP |
He also noted that surgeons are using larger heads.
“For the cup size, in order to maintain thickness at the dome in what has been considered the weight-bearing region, the head has been moved outward a little bit so there is some extra polyethylene in place of the rim,” Ries said. “That means the thickness of the locking mechanism may be reduced and the impingement occurs in result of the stress concentration at the locking mechanism.”
Material limitation?
Thomas P. Schmalzried, MD, a member of the Orthopedics Today Editorial Board, acknowledged that there are some limitations of highly crosslinked polyethylene, but he held that these shortcomings do not stem from the bearing material.
“Modularity is part of the problem,” he said. “Most surgeons look at it as a benefit or as a solution, but we need to recognize that it plays a role in the current problems we are discussing.”
Surgical technique is a problem, he told Orthopedics Today. “The diameter of the bearing is a double-edged sword. We would all like to use the largest diameter possible because of the functional benefits it gives to the patient, but it does create some issues when we start looking critically at the materials and component position.”
Schmalzried highlighted his research on the blowhole-lysis phenomenon, which illustrated that modular pistoning of the liner could create hydraulic forces. In one case, the apex hole eliminator threaded through and fell into the lytic lesion behind the cup. The pistoning forces created backside wear.
“It is important to recognize that this is an issue of modularity and not an issue related to the polyethylene,” Schmalzried said.
In a multicenter study with 1,024 hip cases performed with either highly crosslinked or conventional polyethylene components, Schmalzried and his colleagues found that the surgeon was one of the most important variables for influencing wear rates. “The surgeon or the center had the strongest effect on wear rates, more than three-times stronger than any of the other factors including the bearing materials,” he said.
Schmalzried’s work has also illustrated the interplay of vertical sockets, modular components and large heads in creating pelvic osteolysis. “Iliac osteolysis was associated with a lateral opening angle of more than 50°,” he said. “For each 5° increase in the lateral opening of more than 50° there was a four-times greater risk of osteolysis.”
More recently, Schmalzried and colleagues reported a dramatic reduction in femoral and pelvic osteolysis with crosslinked polyethylene.
For more information:
- Harry A. McKellop, PhD, is a professor at the J. Vernon Luck Orthopaedic Research Center, UCLA- Orthopaedic Medical Center, 2400 South Flower St., Los Angeles, CA 90007; 213-742-1000; e-mail: hmckellop@laoh.ucla.edu. He is a consultant for DePuy.
- Michael D. Ries, MD, can be reached at UCSF Medical Center, 500 Parnassus Ave., MU-320-WestSan Francisco, CA 94143-0728; 415-502-2235; e-mail: riesm@orthosurg.ucsf.edu. He has intellectual property with Smith & Nephew products.
- Thomas P. Schmalzried, MD, is the medical director at the Joint Replacement Institute, 2200 W. Third Street, Los Angeles, CA 90057; 213-484-7600; e-mail: schmalzried@earthlink.net. He has a consulting and research relationship with DePuy, a Johnson & Johnson company, and Stryker Corp.
References:
- McKellop H. Crosslinked polyethylenes: A convincing laboratory performance.
- Ries MD. Crosslinked polyethylenes: Where the faults may lie.
- Schmalzried TP. Clinical performance of crosslinked polyethylenes: Where the future is! All presented during Workshop 5 at the 54th Annual Meeting of the Orthopaedic Research Society. March 2-5, 2008. San Francisco.