Wednesday, April 13, 2011

Total shoulder: the glenoid component, Part III

We have spent a substantial amount of effort in identifying causes of failed total shoulder arthroplasty; examples are the investigations of our shoulder fellows Hasan and Franta. One of the most common causes of total shoulder failure is failure of the glenoid component. In our research on this issue we have identified a number of factors, very important among which is the technique of insertion of the glenoid component. Our shoulder fellow Lazarus conducted a multicenter study in which radiographic evaluation of glenoid components demonstrated that many components inserted around the country were not well seated and or not well fixed. This may be due in part to the observation by our shoulder fellow Hasan that many surgeons doing shoulder joint replacement have less experience than those doing hip or knee surgery.

When the glenoid bone surface is not properly contoured to fit the back of the glenoid component, bone cement can be used to fill in the gaps as shown in the left hand diagram below. However, loading of the glenoid component can lead to fracture and loosening of this thin layer of cement, leading to its displacement and loss of support for the glenoid component as shown on the right hand diagram below.

In the investigation by Lazarus, glenoid component designs with pegs outperformed glenoid component designs with keels. This article also pointed out the importance of the experience of the surgeon: 80% of the glenoid components inserted by the most experienced surgeon had better cementing in comparison to 50% for the remaining surgeons. There are some other important articles relating to the effect of surgeon experience to the outcome of shoulder arthroplasty. An article by Hasan shows that 75% of shoulder arthroplasties are done by surgeons performing only one or two per year. Articles by Hammon and Jain show that surgeons performing higher numbers of shoulder arthroplasties have a lower rate of complications.

At present, the optimal approach to glenoid fixation combines (a) reaming the glenoid bone to precisely fit the back of the glenoid component

(b) drying the bone with a carbon dioxide jet to remove blood, fluid and debris before cement is inserted (as the divers among you will recognize, we use CO2 because of its solubility in water and blood, so that nitrogen bubbles (the bends) are avoided).
(c) pressurizing the cement in to the holes
(d) removing the excess cement so that none remains on the surface of the bone

(e) inserting the component so that the precise geometry of the pegs and back surface of the component meet the precise geometry of the holes and reamed surface of the glenoid bone

and (f) vigorously impacting the component into the prepared bone

This technique minimizes the amount of cement used, so that excessive heat is not generated by the curing of the methylmethacrylate, a risk pointed out by our shoulder fellow Churchill.
A detailed view of the postoperative film shown in yesterday's post reveals excellent seating of the glenoid component on the prepared bone surface with penetration of the pressurized cement into the bone (arrow) - note the absence of radiolucent lines.


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