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Pre-cooling acetabular liners helps make snapping into shells easier
Cooling liner temperature helps lock them into their shells without intraoperative fracture risk.
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Pre-cooling an acetabular polyethylene liner, which causes temporary shrinkage, makes it easier for surgeons to snap the liner into its metal shell.
In study recently published in the Journal of Arthroplasty, Richard F. Kyle, MD, and his colleagues compared the benefits of heating and cooling liners in an effort to find a more effective way of fitting the liners into their shells.
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In total hip arthroplasty (THA), modular acetabular components require implantation. Surgeons implant metal shell into the acetabulum, and then insert a polyethylene liner into the metal shell. The liner locks into the shell with one of several locking mechanism designs, such as an interference fit between a slot in the shell and a circumferential polyethylene flange or lip on the liner. To get the two to fit together, a surgeon needs to use a mallet through an insertion tool to impact enough force on the liner to make it lock into place.
However, if the patient undergoing THA has osteoporotic bone or any type of bony defect, the force of impact can either disrupt the fixation of the acetabular component or can cause intraopertive fracture, said Kyle, of the Orthopaedic Bimechanics Laboratory, Midwest Orthopaedic Research Foundation and the department of orthopedic surgery at Hennepin County Medical Center, Minneapolis.
Kyle, a former Orthopedics Today editorial board member, and his colleagues realized that if a liner with this type of locking mechanism was pre-cooled, the dimensions of the liner would temporarily shrink. “That would allow surgeons to use less force to snap it into the shell, which would also decrease the risk of damage to the acetabular component or even the patient,” wrote Kyle, who is the current president of the American Academy of Orthopaedic Surgeons.
The researchers studied the rates of heating and cooling a range of different-sized polyethylene liners to gauge how long it would take to shrink the liners in an operating room environment.
Kyle and his colleagues also measured the amount of force required to lock the liners into their respective metal shells to see if surgeons could put them together using the force of their thumb alone.
They drilled a 1.6-mm diameter hole with a depth of 1 cm into the flat edge of two ultra-high-molecular-weight polyethylene liners, one being 58 mm in diameter and the other, 70 mm diameter. A 1.6-mm thermocouple was inserted into the hole and connected to a computer data system.
“We then left the liners to reach room temperature (22°C), and then we immersed them into an ice water bath (0°C). It took up to eight minutes to cool the liners to near 0°C,” he said.
The liners were then given “push-in” and “push-out” tests at three different temperatures — 0°C, 22°C and 37°C — to see if they would stay in the shell after insertion at each temperature.
Researchers used four different sized acetabular liners: 58 mm, 62 mm, 66 mm and 70 mm. The two liners used in the previous cooling/warming test were not included.
All of the liners had 28-mm diameter shells to fit the femoral head component.
60-mm, 70-mm liners fit best
At 0°C, the 60-mm and 70-mm liners fit into their shells without force, but the 58-mm liner needed a thumb force of 290 N and the 62-mm liner required a force of 50 N. When at 22°C, stronger forcers were needed to make the liners fit into their shells, ranging from 514 N to 686 N. The liners needed even stronger forces at 37°C, ranging from 595 N to 850 N.
Pushing out a liner took less force than it did to push in. “We found that this indicated that some of the liners would need to be held in the shell to allow adjustment for warming and expansion, especially in larger components,” he said. None of the liners cracked as a result of the warming or cooling.
From these results, the researchers concluded surgeons could manually insert the liner merely by using the force of their thumbs if the liners are cooled to 0°C.
Although this study was based on an interference fit between a circumferential lip and a corresponding retaining groove in the shell, it is possible that other locking mechanisms may require more force to push in or out, the researchers said.
For more information:
- Kyle R, Bourgeault C, Lew W, Bechtold J. Precooling an acetabular liner makes its interstion into a metal shell easier. J Arthroplasty. 2006;21:249-254.