A quantitative analysis of the effect of baseplate and glenosphere position on deltoid lengthening in reverse total shoulder arthroplasty.
BACKGROUND
Optimizing deltoid tension is important to achieve maximal function after reverse total shoulder arthroplasty (RTSA), but the effects of baseplate and glenosphere positions on deltoid tension have not been quantified.


OBJECTIVE
To quantify deltoid elongation and elongation to failure under physiologic loads with three baseplate-glenosphere configurations with increasing inferior offset.


METHODS
Cadaver biomechanical study.


METHODS
Twenty-four cadaver shoulders were divided into three groups. The starting point for baseplate insertion in Group 1 was the center of the glenoid, with glenospheres placed in minimal inferior offset (0.5 mm). Groups 2 and 3 baseplates were placed 2 mm inferior to the center point and glenospheres in minimal (2.5 mm) offset (Group 2) or maximal (4.5 mm) offset (Group 3). Tensile testing was done to quantify deltoid elongation and evaluate failure.


METHODS
A one-way analysis of variance was performed to detect statistically significant differences among treatment groups. A post-hoc Neuman-Keul's comparison was conducted to perform discrete comparisons among treatment groups.


RESULTS
Deltoid elongation after loading decreased with increasing inferior offset of >2.5 mm. No significant difference in deltoid yield load was found among groups. The percent of elongation was decreased significantly between groups 2 and 3. Deltoid displacement at failure decreased from 33.3 mm for Group 2-17.3 mm for Group 3. 16 of the 24 specimens (67%) failed by anterior deltoid detachment from the acromion.


CONCLUSIONS
Increasing inferior offset in RTSA constructs appears to increase stretch forces on the deltoid, resulting in a diminished ability of the deltoid to further elongate under physiologic loads, (most pronounced when the inferior offset exceeds 2.5 mm) and significantly decreasing the yield displacement of the construct.
Department of Orthopaedic Surgery and Biomedical Engineering, University of Tennessee, Campbell Clinic, Memphis, TN 38104, USA.
Jonathan
J
Wright
Department of Orthopaedic Surgery and Biomedical Engineering, University of Tennessee, Campbell Clinic, Memphis, TN 38104, USA.
Christopher
C
Potts
Department of Orthopaedic Surgery and Biomedical Engineering, University of Tennessee, Campbell Clinic, Memphis, TN 38104, USA.
Mark P
MP
Smyth
Director of Ortho Kinetic Technologies LLC, Shallotte, NC 28470, USA.
Lisa
L
Ferrara
Department of Orthopaedic Surgery, Mayo Clinic, Rochester, MN 55905, USA.
John W
JW
Sperling
Department of Orthopaedic Surgery and Biomedical Engineering, University of Tennessee, Campbell Clinic, Memphis, TN 38104, USA.
Thomas W
TW
Throckmorton
Baseplate and glenosphere position; cadaver study; deltoid tension; failure; reverse total shoulder arthroplasty