Background Reverse total shoulder arthroplasty (RTSA)
is widely used; however, the effects of RTSA geometric
parameters on joint and muscle loading, which strongly
influence implant survivorship and long-term function, are
not well understood. By investigating these parameters, it
should be possible to objectively optimize RTSA design
and implantation technique.
Questions/purposes The purposes of this study were to
evaluate the effect of RTSA implant design parameters on
(1) the deltoid muscle forces required to produce abduction,
and (2) the magnitude of joint load and (3) the loading
angle throughout this motion. We also sought to determine
how these parameters interacted.
Methods Seven cadaveric shoulders were tested using a
muscle load-driven in vitro simulator to achieve repeatable
motions. The effects of three implant parameters—humeral
lateralization (0, 5, 10 mm), polyethylene thickness (3, 6, 9
mm), and glenosphere lateralization (0, 5, 10 mm)—were
assessed for the three outcomes: deltoid muscle force
required to produce abduction, magnitude of joint load, and
joint loading angle throughout abduction.
Results Increasing humeral lateralization decreased deltoid
forces required for active abduction (0 mm: 68% ± 8%
[95% CI, 60%–76% body weight (BW)]; 10 mm: 65% ±
8% [95% CI, 58%–72 % BW]; p = 0.022). Increasing
glenosphere lateralization increased deltoid force (0 mm:
61% ± 8% [95% CI, 55%–68% BW]; 10 mm: 70% ± 11%
[95% CI, 60%–81% BW]; p = 0.007) and joint loads (0
mm: 53% ± 8% [95% CI, 46%–61% BW]; 10 mm: 70% ±
10% [95% CI, 61%–79% BW]; p \ 0.001). Increasing
polyethylene cup thickness increased deltoid force (3 mm:
65% ± 8% [95% CI, 56%–73% BW]; 9 mm: 68% ± 8%
[95% CI, 61%–75% BW]; p = 0.03) and joint load (3 mm:
60% ± 8% [95% CI, 53%–67% BW]; 9 mm: 64% ± 10%
[95% CI, 56%–72% BW]; p = 0.034).
Conclusions Humeral lateralization was the only parameter
that improved joint and muscle loading, whereas
glenosphere lateralization resulted in increased loads.
Humeral lateralization may be a useful implant parameter
in countering some of the negative effects of glenosphere lateralization, but this should not be considered the sole
solution for the negative effects of glenosphere lateralization.
Overstuffing the articulation with progressively
thicker humeral polyethylene inserts produced some
adverse effects on deltoid muscle and joint loading.
Clinical Relevance This systematic evaluation has
determined that glenosphere lateralization produces
marked negative effects on loading outcomes; however, the
importance of avoiding scapular notching may outweigh
these effects. Humeral lateralization’s ability to decrease
the effects of glenosphere lateralization was promising but
further investigations are required to determine the effects
of combined lateralization on functional outcomes including
range of motion.