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Patient-specific uni-knee implants and instruments may simplify procedures
Potential disadvantages of patient-specific devices include limited intraoperative variability.
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Patient-specific unicompartmental knee arthroplasty is a viable treatment for osteoarthritis that can facilitate both implantation and sizing issues.
Such was the focus of a presentation by Thomas S. Thornhill, MD, at the Current Concepts in Joint Replacement Spring 2009 meeting.
Thornhill said the platform technology behind patient-specific implants is based on the ability to use MRI or CT to measure patient-specific contours of the cartilage, bone and menisci. The advantages of patient-specific unicompartmental implants include disposable, patient-specific instruments, a customized fit and a potentially easier operative procedure. Potential disadvantages include a limited intraoperative variability due to a lack of size options and the need to take a preoperative CT to ensure alignment.
For the procedure itself, Thornhill outlined his steps, which include: fitting the femoral jig, marking it and scraping off the remaining articular cartilage. He then removes the remaining tibial cartilage and balances the knee, using four balancer/navigation chips contoured to the femur and the tibia in flexion and extension. This determines the alignment and soft-tissue tension and he then makes the axial and sagittal cuts.
The balancer chip on the tibial side can be used to give an alignment to the tibia. A reciprocating saw may be used to make the medial cut, and the blade should be left in to make the horizontal cut. Thornhill noted that the thicker the chip used, the less bone will be removed.
Linking this to the femur can be accomplished via an L-guide that will now fit to the proper rotation of the femur and the proper tension of the flexion space. The anterior femoral lug can then be pinned, followed by the removal of the tibial template and the L-guide.
“You then pin the posterior lug and make the posterior cut, which is really important for femoral component stability,” Thornhill said. “Finally, tibial preparation is rather easy – it is a matter of two lugs and a fin keel and then cementing as you would in all unicompartmental replacements.”
Easier to perform?
Following his presentation of the procedure, Thornhill raised the question of whether or not it was actually easier to perform.
“Most knee surgeons are either sports medicine surgeons who are very skilled at arthroscopy, ACLs, instability patterns etc. or arthroscopy surgeons,” he said. “Arthroplasty deals with the instruments, bone cuts and component implantation in unicompartmental and total knee replacement. Will this be easier because it is patient-specific for people who do not perform arthroplasty on a routine basis?”
A “transition operation”
“Unicompartmentals are indicated in about 10% of osteoarthritics,” Thornhill said in conclusion. “I do them bilaterally in elderly people. I think it is a great transition operation. The patient-specific unicompartmentals may facilitate both sizing issues and component implantation.”
Keith R. Berend, MD, moderator for the session, labeled the concept of a customized resurfacing partial knee replacement “very interesting.”
“Dr. Thornhill has clearly identified the key factors in the long-term outcome of any partial knee replacement, surgical technique and indications,” Berend told Orthopedics Today. “As we further address the issues of wear and loosening, and as we define the pathoanatomic condition of anteromedial osteoarthritis, this percentage is likely to approach 40% or even 45% of degenerative knees. We will all wait with great anticipation to see if this customized approach will make implantation easier and surgical results more consistent.”
For more information:
- Keith R. Berend, MD, can be reached at Joint Implant Surgeons, 7277 Smith’s Mill Road, Ste. 200, New Albany, Ohio 43054; 614-221-6331; e-mail: BerendKR@joint-surgeons.com. He has no direct or financial interest in any companies or products mentioned in this article.
- Thomas S. Thornhill, MD, is head of the Department of Orthopedic Surgery at the Brigham and Women’s Hospital. He can be reached at 75 Francis St., Boston, MA 02115; 617-732-5383; e-mail: tthornhill@partners.org. He has no direct or financial interest in any product or company mentioned in this article.
Reference:
- Thornhill, TS. Patient specific UKA resurfacing options. Paper #43, presented at the Current Concepts in Joint Replacement Spring 2009 meeting. May 18, 2009. Las Vegas.