Coronal view through the central region the glenoid demonstrating
good osteointegration. There are some regions of artefact, most
clearly seen adjacent to the humeral head.
The SMR metal back glenoid is a popular implant in total shoulder arthroplasty in New Zealand. Some metal back glenoid designs have been reported to have early problems.
How was the SMR metal back glenoid performing in New Zealand?
Specifically, I wanted the answer to these three questions:
Is there a problem with liner dissociation?
Is there a problem with early loosening or failure of the metal back glenoid?
Is osteointegration of the metal base plate reliable in the medium term?
To answer the first two questions, I reviewed data from the New Zealand Joint Register.
To answer the last question, I reviewed a series of patients at minimum three years post surgery with CT scan analysis.
Is there a problem with liner dissociation?
No, we have not identified a problem. Over a 5 year period, 191 metal back glenoids were implanted in New Zealand foranatomical
total shoulder replacement by 29 surgeons. Some were general orthopaedic surgeons with limited patient volumes in shoulder arthroplasty. There were no cases of liner dissociation.
Is there a problem with early loosening or failure of the metal back glenoid?
No, we have not indentified a problem. The 5 year cohort of 191 patients referred to above, have 2 to 7 years follow up on our National Joint Register. Nine cases have had a revision procedure, with indications for revision including rotator cuff tears and instability. In 4 of these the revision has been to a Reverse Shoulder Arthroplasty with retention of the metal base plate. There have been no cases of revision for glenoid loosening and there have been no cases of revision involving removal of the metal back glenoid base plate. The survivorship of the metal back glenoid in this group at this time is 100%.
Does reliable osteointegration occur?
Yes, we found reliable osteointegration on CT analysis. CT has been found to be a more sensitive and reliable tool for assessingloosening than conventional imaging in cemented polyethylene glenoids1. We examined 20 consecutive patients with 64-slice CT scan at a mean of 3 years 9 months from surgery (range 3-5 years). For analysis, the glenoid was divided into 8 zones, with 4 zones around the central peg and 4 zones on the surface of the metal base plate. The central peg accounts for approximately 50% of the surface area of the implant. Images were analysed with a musculoskeletal radiologist.
None of the components were loose. In 85% of the zones, there was no gap at the bone to component interface. In 11% of the zones, there was a lucency of up to 2 mm. This was sometimes likely due to artefact in some cases, especially artefact related to the humeral head superiorly. In 4% of the zones there was osteolysis, all in the one patient. This patient had a revision procedure for superior instability with polyethylene and metal wear debris 4 years 3 months after his initial procedure. The glenoid was not loose at revision and the metal back base plate was retained.
Artefact was an issue in assessing the interface, especially superiorly. Cost and additional radiation exposure would limit the clinical utility of this form of radiological assessment in routine practice. However, it was clear from our study that reliable osteointegration occurs in the SMR metal back glenoid. The central peg showed reliable osteointegration and appears to be an important part of the metal back design.
KD Mohammed, MBChB, FRACS
Consultant Orthopaedic Surgeon,
Christchurch Hospital, Senior Lecturer
in Orthopaedic Surgery
University of Otago (New Zealand)
References
“Radiographic and CT analysis of pegged polyethylene glenoid components in total shoulder replacement” Yian,
Gerber et al, JBJS(A), 2005.
