Sunday, April 10, 2016

Glenoid components - do polyethylene and metal mix? - Young's modulus - of princesses and peas

We know that in hip surgery, the combination of a polyethylene articulating surface within a metal cup has proven long-term clinical results, in spite of the dissimilarity in the mechanical properties of the two materials. We suggest that this is because the increased deformability of the poly is contained within the less compliant metal shell and because the loading is generally concentric.

A similar situation exists for the contained poly of a reverse total shoulder

However in anatomic total shoulders were the poly is not contained and the loading is characteristically eccentric, the results are different,  as shown in Metal-Backed Glenoid Components Have a Higher Rate of Failure and Fail by Different Modes in Comparison with All-Polyethylene Components: A Systematic Review, and in Metal-backed glenoid implant with polyethylene insert is not a viable long-term therapeutic option and in this figure below from the Australian Orthopaedic Association annual report metal+poly glenoid components appear to have a higher revision rate than all poly components.

One of the most important figures in the Metal-backed glenoid implant with polyethylene insert is not a viable long-term therapeutic option paper is shown below. It indicates that the often applied standard of a minimum two year followup for arthroplasties may not come close to indicating the failure rate that occurs later on.
In looking at some of these implants we can identify areas where loading of the component may produce different amounts of deformation in the two materials with a possible increase in risk of failure.

The high rate of failure of metal-backed glenoid or hybrid components - especially with eccentric loading - may be explained in large part by examining the Young's modulus of the materials involved. This quantity reflects the relationship between stress (force per unit area) and strain (proportional deformation) in a material - in other words, how much the material changes shape when a load is applied to it.

Here are some representative values of the materials of interest in total shoulder arthroplasty. Young's modulus is expressed in Giga Pascals (GPa). It can be seen that in spite of claims to the contrary, the three metals do not have " biomechanical properties similar to native bone"
Cancellous Bone 0.4
Ultra high molecular weight polyethylene 0.5
PMMA bone cement 2
Cortical Bone 8
Titanium 112
Tantalum 186
Cobalt chrome 200

Thus with a metal backed or hybrid glenoid there are two critical mismatches of Young's modulus: (1) that of metal (100-200) to bone (0.4-8) and (2) that of metal (100-200) to polyethylene (0.5). The first may account for stress shielding of the glenoid bone and the second may account for the increased rate of polyethylene wear and polyethylene-metal dissociation seen with metal backed components. By contrast, all polyethylene components have better modulus matches with bone.

What this means is that when a component is loaded, the materials of similar Young's modulus deform similarly, whereas those with mismatched Young's modulus do not, resulting in relative sheer at the interface.

See also the post "The challenges of metal backed glenoids - why do they fail more often?" as well as the information posted here and here and here.

In summary, metal-backed glenoid components do not seem to solve -but rather exacerbate -  the problem of glenoid failure in total shoulder arthroplasty. Our preference remains the all polyethylene component shown here inserted with no cement between the back of the component and the well prepared face of the glenoid bone. The Young's modulus of poly matches that of bone, without having to use materials that are over 100 times stiffer.

Fixation is achieved with bone ingrowth between the flanges of the central peg as nicely shown in this two year x-ray.

Readers with children can explain Young's modulus using the story of the Princess and the Pea, by Hans Christian Andersen.


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