These authors evaluated x-rays of 124 reverse total shoulder arthroplasties (RTSA) performed with an uncemented curved short stem humeral component with a 145° neck shaft angle (NSA); they measured the incidence of stem malalignment and related changes in the effective neck-shaft angle. They point out that higher effective neck-shaft angles risk unwanted contact between the humeral component and the scapula when the arm is adducted along with the associated risk of scapular notching.
The humeral stem axis was measured and defined as neutral if the value fell within ± 5° of the longitudinal humeral axis. Angular values > 5° were defined as malaligned in valgus or varus.
The filling ratio of the implant within the humeral shaft was measured at the level of the metaphysis (FRmet) and diaphysis (FRdia).
The average humeral stem axis angle was 4±3° valgus, corresponding to a true mean NSA of 149±3°.
Stem axis was neutral in 73% (n = 90) of implants. Of the 34 malaligned implants, 82% (n = 28) were in valgus (NSA = 153 ± 2°) and 18% (n = 6) in varus (NSA = 139 ± 1°).
The average FRmet and FRdia were 0.68 ± 0.11 and 0.72 ± 0.11, respectively. A low positive association was found between stem diameter and filling ratios (r = 0.39; p < 0.001); indicating smaller stem sizes were more likely to be misaligned.
A third (28 out of 90) of the neutrally implanted stems radiologically demonstrated contact with the humeral endocortex. In the 34 malaligned stems, an even larger proportion demonstrated cortical contact (74%, 25 of 34 cases). Of these, all stems implanted in varus demonstrated cortical contact.
They concluded that approximately 25% of the stems were implanted with > 5° malalignment. The majority of malaligned components (86%) were implanted in valgus, corresponding to an effective NSA of > 150°.
They concluded that approximately 25% of the stems were implanted with > 5° malalignment. The majority of malaligned components (86%) were implanted in valgus, corresponding to an effective NSA of > 150°.
Comment: Some authors advocate the use of shorter humeral stems believing that these stems decrease operative time, preserve more bone stock, facilitate revision and reduce stress shielding (a finding related to high filling ratios). While smaller stem sizes may reduce the risk of stress shielding, smaller stems also increase the risk of stem malalignment. These authors also observed that the smaller malaligned stem has a higher rate of endocortical contact distally, a finding that is also associated with stress shielding and bone remodeling.
Thus, while smaller stem sizes may protect against stress shielding by virtue of having smaller filling ratios, they are also associated with a higher incidence of malpositioning with a higher chance of endocortical contact.
In our practice we find that a long stemmed impaction grafted stem offers all the bone-preserving advantages of a short stem without the risks of malposition or stress shielding.
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