Bone Harvester Made With Polycarbonate Resin Helps Make Spinal Fusion Procedures Faster And Less Invasive
Material Offers Strength, Biocompatibility, and other Desired Properties
•Designing the Device
•Selecting the Right Material
Often one of the most crucial steps in designing a medical device is choosing the materials that will ultimately make the product function properly. This was true for Sulzer Spine-Tech Inc., whose bone-harvester device is designed for use in spinal fusion procedures.
In the procedure, BAK interbody devices (threaded, hollow titanium implants) are placed between the vertebrae and filled with bone material taken from the patient's hip. Over time, the vertebrae, the implants, and the transferred bone material fuse together.
The procedure is used in instances in which the discs between the patient's vertebrae have degenerated and caused chronic pain. "For certain patient indications, the BAK interbody device is an alternative to the pedicle screw procedure," says Serafin Samson, Senior Product Development Engineer for Sulzer Spine-Tech.
Samson co-developed the bone harvester along with Dr. Hansen Yuan, a surgeon at Syracuse University Hospital and the SUNY Medical Center, who is a medical adviser to Sulzer Spine-Tech. "The advantage with the BAK procedure is that it provides less pain and quicker recovery for the patient."
The bone-harvesting device allows bone material to be collected through a small incision on the patient's hip and inserted directly into the BAK implant. The harvesting devices are provided in manual and powered versions and feature a clear tube, graduated in both cubic centimeters and in implant sizes so the surgeon can harvest exactly the amount of bone material needed.
Even though the device seems like a simple instrument, it is sophisticated in design. The harvester has a removable cutting head. With the cutting head removed, the collection tube mates with the titanium implant, and the bone material can be packed into the implant by a plunger which is pushed down the inside of the harvester tube.
Samson says that a key design element was the clear tube, which provides the surgeon with the ability to measure the amount of bone being collected, taking only what is needed. "This is an efficient means for collecting and handling bone material," he explains. "If the surgeon has to harvest and transfer bone materials by hand, the procedure can be very cumbersome and more invasive.
"Efficiency and simplicity is the payoff," Samson says. "We're taking a 30- to 40-minute procedure down to about 15 minutes, which is a significant amount of saved operating room time."
Samson says the criteria for materials to be used in the bone harvester were very demanding. "We had to have strength, biocompatibility, the ability to withstand sterilization, a clear tube and overall suitability for use in a surgical environment," he notes. "We designed the bone harvester for one-time use so the surgeon would have a sharp, strong, sterile product every time."
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The bone harvester kits consist of a trocar, cutting head, harvester handle and collection tube, and a plunger. The manual harvester's handle and collection tube are made using Makrolon Rx2530 polycarbonate resin from Bayer Corp. For the powered version, the handle is replaced by a fitting that adapts the Makrolon polycarbonate tube to the power connector.
"The tube is where the technology is," says Brian McKenzie, Medical Sales and Marketing Manager for McKechnie Plastic Components, which molds the bone harvester devices for Sulzer Spine-Tech.
"Bayer's material was chosen because we needed a polycarbonate that was sterilization tolerant, and McKechnie was able to identify that certain flow characteristics would be needed to succeed. Those characteristics were high flow and uniform molding," McKenzie says. "The molding for us is a piece of technology in itself. It is a 6½-in. cylinder with absolutely zero draft through the core. That makes the material's molding characteristics critical."
According to McKenzie, cycle time was less important than the flow characteristics. Also, the material had to tolerate both gamma and EtO sterilization. The resin's set-up time was also a concern.
"We needed a polycarbonate that not only had excellent flow properties but also set up quickly," says McKenzie. "Because of the zero draft, cooling is very important, and we have developed proprietary molding technology for making these types of parts. The material itself has to set quickly so we can maintain a reasonable cycle time and have a part that maintains its geometric tolerances."
Since the bone harvester is a surgical instrument, there can be no leaching with blood and bone contact. And although the device is not implanted, the bone material it harvests is implanted, so the device comes into direct contact with bone matter that is reintroduced back into the patient.
"The polycarbonate resin used in the bone harvester had to meet the biocompatibility testing requirements of ISO-10993. The biologic testing data Bayer provided to us was an enormous aid to Sulzer Spine-Tech for market clearance," McKenzie says.
With the polycarbonate material meeting all the demands of the bone-harvesting device, the design was a success. And ultimately, that makes for a successful product for the manufacturer.
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For more information on the Sulzer Spine-Tech Bone Harvester Kit, contact Sulzer Spine-Tech Inc., 7375 Bush Lake Rd., Minneapolis, MN 55439; (612) 832-5600.
For more information about medical device molding, contact McKechnie Plastic Components, 7309 West 27th St., Minneapolis, MN 55426; (612) 929-3312.
For more information on Makrolon polycarbonate resin, contact Bayer Corp., Polymers Communications Group, 100 Bayer Rd., Pittsburgh, PA. 15205; (800) 622-6004.