Friday, March 16, 2018

Hybrid glenoid - possibility of stress shielding and fatigue fracture

Five-year minimum clinical and radiographic outcomes of total shoulder arthroplasty using a hybrid glenoid component with a central porous titanium post

These authors evaluated clinical and radiographic outcomes at a minimum 5-year followup in 45 shoulders that underwent total shoulder arthroplasty (TSA) using a system with a central porous titanium post to augment the cemented peripheral pegs as shown below

The mean American Shoulder and Elbow Surgeons score improved from 40.4 to 83.7 (P < .0001). Radiographs showed glenoid component radiolucency in 29 of 45 shoulders. Radiolucencies were confined to the area under the glenoid faceplate in 6 and were around the central post in 13. Nine TSAs (20%) demonstrated 2 or more columns of involvement but were not judged to be at-risk. One implant (2.2%) had glenoid component failure and was revised to a hemiarthroplasty. 

The authors point out that previous hybrid glenoid designs have manifested problems with fatigue fracture of the titanium (Results of total shoulder arthroplasty with a monoblock porous tantalum glenoid component: a prospective minimum 2-year follow-up study).

Comment: There are two potential issues with hybrid glenoid components. One is the risk of fatigue fracture - the breakage of the metal from repeated rocking forces applied to the glenoid face while the titanium peg or pegs are securely fixed to the bone of the glenoid. The second is the risk of stress shielding that arises because the titanium is stiffer than bone and polyethylene. Just as in the case of stress shielding in humeral component fixation, when the metal is stiffer than the bone in which it is inserted, the unloaded bone tends to lose mineral. Loss of bone beneath the polyethylene face of the component can give rise to increased rocking of the component with increased risk of fatigue fracture of the metal.

The reader may wish to look at these links regarding Young's Modulus for bone, titanium, and polyethylene. Correlation between Young's Modulus and Porosity in Porous Materials  and Young's Modulus for Some Common Materials and Microstructure and Mechanical Properties of Porous Titanium Based on Controlling Young’s Modulus
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