Monday, July 7, 2014

Clinical perfomance of metal backed glenoids in comparison to all polyethylene components

Metal-Backed Glenoid Components Have a Higher Rate of Failure and Fail by Different Modes in Comparison with All-Polyethylene Components A Systematic Review

We conducted a comprehensive systematic review of twenty-one studies on radiolucency, radiographic failure, and revision after arthroplasty with metal-backed glenoid components and twenty-three studies with all-polyethylene components. Our analysis included data on 1571 metal-backed and 3035 all-polyethylene components. The mean duration of follow-up was 5.8 years in the studies with metal-backed components and 7.3 years with all-polyethylene components.

All-polyethylene components had a 42.5% rate of radiolucency compared with 34.9% for metal-backed components (p = 0.0026) and a 21.1% rate of radiographic loosening or failure compared with 16.8% for metal-backed components (p = 0.0005).
However, the rate of revision was more than three times higher with metal-backed components (14.0%) than with all-polyethylene components (3.8%, p < 0.0001). 
Although 77% of the revisions of all-polyethylene components were for loosening, 62% of the revisions of metal-backed components were for other reasons, such as component fracture, screw breakage, component dissociation, polyethylene wear, metal wear, and rotator cuff tear (p < 0.0001).

Comment: The published evidence indicates that metal-backed glenoid components require revision at a significantly higher rate and for different reasons in comparison with all-polyethylene components. It is important that the majority of cases of lucency, loosening, and revision became evident 7 years after the index surgery. 

One of the intrinsic difficulties with metal backed relate to the difference in elastic modulus for the different materials that are relevant to the use of these glenoids.
The key values for Young's modulus are
Cancellous bone     0.4  GPa
Cortical bone          8.0 GPa
PMMA cement       2.0 GPa
Titanium             112.0 GPa
Cobalt chrome    200.0 GPa
Polyethylene          0.5 GPa
The wide differences in these values indicate that the different materials will deform differently under load leading to shear at the interfaces.  These challenges cannot be avoided when glenoids are made of different materials. Note also that the closest match is between polyethylene and bone.

Some examples of metal backed glenoids are shown below. These are not necessarily the types of components represented in the study.

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