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Oxford® Partial Knee System

Medial compartment osteoarthritis of the knee is one of the most common ailments seen in orthopedic clinics. Traditionally, these patients received a partial knee prosthetic as a temporary measure before inevitable total knee replacement, but orthopedic surgeons at OrthoMemphis are hoping a newly approved mobile bearing partial knee replacement will yield more superior outcomes than the traditional fixed prosthesis.

The Oxford® Partial Knee System designed by leading Oxford, England surgeons and Biomet, Inc. engineers, has become the first FDA-approved free-floating mensical partial knee system available in the United States. It has been utilized throughout Europe for more than two decades.

Knee osteoarthritis usually occurs first in the medial compartment as this side of the knee bears most of the weight. In knees that are otherwise healthy, a partial replacement approach allows the outer compartment and all ligaments to remain intact. By maintaining the undamaged parts of the knee, the joint may bend better and function more naturally.

Traditionally, orthopedic surgeons have used fixed bearing prostheses, a single unit with metal fixed to the femur and a plastic fitting fixed to the tibia, but there was no mensical fitting between them. Because it is one singular piece, it didn’t allow for full, natural flowing movement of the knee. Typically within five to seven years, the metal would eventually wear out the plastic.

“We used the fixed bearing as a stop-gap measure for younger patients, for about five to seven years, before they would need a total knee replacement,” explained Dr. Kenneth Weiss, an orthopedic surgeon specializing in knees and shoulders at OrthoMemphis.

According to literature on the Biomet Web site, the Oxford is the first partial implant with an artificial mensical bearing designed to glide freely throughout the knee’s range of motion, more closely replicating normal movement.

In a healthy knee, the meniscus serves as a shock absorber between the ends of the bones. With the Oxford® mobile bearing prosthesis, two metal fittings are attached to the femur and tibia and a plastic meniscus sits in between to more closely replicate normal movement. The plastic actually moves back and forth and bends as the knee moves. The free floating nature of the device also improves durability of the implant.

“We as a group were doing a fair number of partial knee replacements,” said Weiss. “Some did great, but we were seeing enough failures that as a group we weren’t happy with the design of the prosthesis and decided to stop doing [fixed bearing].”

Surgeons in the group looked into the newly approved Oxford prosthetic and decided to take the four month FDA required training course.

“We see so many patients with medial compartment arthritis and we thought, there’s got to be a better way,” Weiss said.

This month, they are performing the first partial knee replacements with the Oxford free-floating mensical system.

“I think we’ll see more and more of these as the use increases,” he estimated.

But it’s not a solution for everyone. While the medial side of the knee is usually the first to go, that doesn’t mean everyone with medial damage is a candidate for partial knee replacement. Weiss said he works with patients to identify where their pain is coming from. Patients are asked to keep a pain diary over a week and x-rays are taken to determine that the narrowing is only occurring on the medial side of the knee.

“We’ll do a stress view on the outside part of the knee to see if the medial side will open,” he explained. “The patient also has to be able to fully straighten their knee or get very close to fully straightening it. If there is too much flexion, there is too much damage.”

Published long term clinical results on the Oxford knee demonstrated a 98 percent success rate at 10 years, equaling the results of the most successful total knee replacements. Studies also show most patients experience a rapid recovery and more normal joint function.

As reported in the November 2001 issue of Clinical Orthopaedics and Related Research, complications with the Oxford unicompartmental replacement are typically from bearing dislocation and an increase in loosening rate when the components were placed in knees that had anterior cruciate ligament (ACL) deficiency.

That is why surgeons won’t consider candidacy for partial knee replacement in patients who don’t have normal, functioning ligaments.

“No torn ACLs,” maintained Weiss.

If a patient has had a stable ACL replacement, they may be considered, although it’s preferred they have no damage.

Weiss said he believes the Oxford partial knee prosthesis is superior to fixed bearing replacements. Patients can expect an easier operation, less pain and shorter hospital stay and recovery time. The group’s problem with the fixed design wasn’t just that it had less mobility, but that it was a temporary fix.

“Now we can tell patients, ideally, this could be a final measure for them,” explained Weiss, who added that they have never been able to say that to patients before. “This could be their one operation.”




October 2007

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