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STEFANO ZANASIVILLA ERBOSA HOSPITAL

GRUPPO SAN DONATOORTHOPAEDICS DEPARTMENT

IIIRD DIVISION – JOINT ARTHROPLASTY OPERATIVE CENTERCHIEF: STEFANO ZANASI M.D.

MEDITERRANEAN KNEE MEETING 2011

ISTANBUL SEPTEMBER 15-17 2011

DO MOBILE-BEARING DESIGNS OFFER A CLINICAL ADVANTAGE OVER FIXED BEARING

DESIGNS FOR UNICOMPARTMENTAL KNEE ARTHROPLASTY (UKA)?

In expert hands, “both of these techniques work very well”

WITH A MOBILE BEARING:

Although wear is admittedly low, the surgical technique is more demanding,

the risk of bearing dislocation is real, and all the other modes of failure are still possible.

WITH A FIXED-BEARING DESIGN, particularly one that has a metal-backed tibial component,

loosening is rare,

reoperations for wear are present but infrequent, and survivorship is high.

 

No clinical advantage found

“Although the idea of a mobile-bearing design is very appealing, I do not believe that it offers a clinical

advantage over a fixed-bearing design,” Craig J. Della Valle, MD, and Richard D. Scott, MD, 

during the COMBINED KNEE SOCIETY/AMERICAN ASSOCIATION OF HIP AND KNEE

SURGEONS 2011 SPECIALTY DAY PROGRAM.

The mobile-bearing design has two important potential benefits

a decreased rate of wear as well as

the promise of decreased stresses at the interfaces between the cemented implant

and bone that could translate into a lower rate of

prosthetic loosening.

Similar to a mobile-bearing total knee replacement (TKR), the mobile-bearing uni has increased conformity between the

femoral component and the polyethylene.According to Berend

the mobile-bearing uni knees may also improve knee kinematics.

However, the replacements do not change

how surgeons select patients, the anatomy of the knee or the importance of surgeon performance:Challenges to performing a mobile-bearing uni knee include

determining the amount of acceptable arthrosis in the patellofemoral joint, potentially jeopardizing the high

success rate of TKRand operating on obese patients

“But the real question is whether there is something magical about a mobile-bearing UKA that makes the outcomes better or in some way different

or makes survivorship longer? And is there a clinical benefit that leads to better patient outcomes?

t

Although mobile-bearing UKAs have been shown to have very low polyethylene wear rates,

the promise of decreased rates of prosthetic loosening has not been realized

Almost every series in the literature includes tibial component loosening as a failure mode as well as problems with

femoral component loosening

There are obviously risks to any moving parts and that namely is

dislocation of the bearing through impingement, either through retained bone

or retained cement. Long-term studies show less than a 1% rate of this

occurrence.

“The jump distance of the bearing is 3 mm in the back and 5 mm in the front, so it snaps in nicely and it is preserved as long as the bearing is maintained against the

lateral rail.” Berend said and this allows for anterior and posterior movement; however, if the bearing spins the jump distance may decrease to 1 mm.

Therefore, eliminating bearing spin is important.

However

In an analysis of his fixed-bearing UKAs, Berend (2010) found that

half had anterior tibial collapse in less than 1 year: these cases were associated with all-polyethylene components

and excessive or under-tibial sloping

“The conformity afforded by a mobile bearing, much like in total knees, will result in less loading and less wear compared to fixed-bearing designs,”.

“And I would submit a hypothesis that the wear pattern in arthrosis, the area of the tibial collapse in and the predicted loading pattern are all in the same location, and this may be underlying some of the failure mechanisms that we have observed and

are due to publish in the Journal of Arthroplasty

The literature supports the concept that mobile-bearing UKAs offer excellent longevity,

even in the middle-aged patient.

According to Scott our personal experience confirms that the early results of both mobile- and fixed-bearing UKAs

are similar for both range of motion and pain.

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B.A, F 67yrs old CONCURRENT MEDIAL UNI - 2/06/10

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video

B.A, F 67yrs old CONCURRENT MEDIAL UNI - 2/06/103 MS. F. UP.

We identified poor patient selection,

prosthetic design, and surgical technique

as reasons for UKA failures, With wear, loosening, and degeneration of the opposite

compartment as the modes of failure.

Wear is often design-relatedThe wear pattern of the prosthesis reproduces the

preoperative wear pattern of the arthritic knee. Prosthetic designs, therefore, must accommodate this

wear pattern.

CONCLUSIONSFIXED- VS MOBILE UNI BEARING

-survival rates between 90 and 98% at 10 to 15 yrs f.up-overall comparative data remain mixed

-larger, long term f-up studies may be needed to determine any true difference

PATIENT SELECTIONSURGICAL TECHNIQUE

SURGICAL EXPERIENCE do the differences

FROM 1/2009 NO COMPLICATIONS/REVISION OCCURRED IN 196 FIXED BEARING UNI WITH

EXCELLENT SHORT-MEDIUM TERM F.UPTHREE REVISIONS (2 FOR EXTENSION OA AND 1 FOR PAIN AND INSERT

IMPINGEMENT) OCCURRED IN 41 MOBILE BEARING UNI