These authors sought to investigate the relationship between glenosphere size and revision rates among 28,817 primary reverse total shoulder arthroplasty (RTSA).
They concluded that glenosphere sizes <38mm had a higher revision rate compared to 38-40mm glenospheres (HR =1.28)) and >40mm sizes (HR=1.35).
Males with <38mm and 38-40mm glenospheres had significantly higher revision rates compared to >40mm glenospheres (HR = 1.49 and HR = 1.28).
Females with <38mm and >40mm glenospheres had higher revision rates compared to females with 38-40mm glenospheres (HR 1.38 and HR 1.41).
For patients aged 65-74 years, glenospheres >40mm had a significantly lower revision rate than both the <38mm glenospheres and 38-40mm glenospheres.
The Delta Xtend with 38-40mm glenospheres had higher revision rates compared to >40mm glenospheres (HR=1.49).
The SMR L1, 38-40mm glenospheres had a lower rate of revision compared to <38mm (HR= 0.50) and >40mm glenospheres (HR=0.60).
They concluded that revision rates were lower for females with 38-40mm glenospheres and lower for males with >40mm glenospheres.
Comment: The Australian Orthopaedic Joint Replacement Registry is an invaluable resource in that it is centrally administered and that systematically captures data from essentially all of the joint replacements in the country, avoiding the problem of transfer bias and selection bias that confounds many case series. The AOAJRR provides important information on rates of revision and factors associated with revision arthroplasty. It does not, however, enable analysis of the relationship between factors - such as glenosphere size - and patient-reported measures of comfort and function.
In this study the authors recorded over twenty different reasons for revision of RTSA ("revision diagnoses") ranging from dislocation to heterotopic bone formation. The strategies for preventing glenohumeral dislocation are surely different than those for preventing infection, implant loosening, dissociation of the glenosphere from the baseplate, glenoid implant breakage, or heterotopic bone formation. Is glenosphere size really the primary determinant of each of these reasons for revisions?
The registry contains a number of important variables: patient age, sex, diagnosis, and prosthesis manufacturer in addition to glenosphere size. The large numbers of patients in the registry provides a unique opportunity to perform a multivariate analysis (MVA) for each of the major RTSA revision diagnoses. In contrast to previously published case series where the number of revisions is low, they have the numbers to do this: 339 cases of instability, 231 cases of infection, 179 cases of loosening, and 123 cases of fracture.
The question to be asked with a MVA is, "what factors (including but not confined to glenosphere size) are independently associated with each of the major revision diagnoses?"
The answers will be be interesting if not surprising.
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