Lateralized reverse shoulder arthroplasty maintains rotational function of the remaining rotator cuff

Abstract

Background: Humeral rotation often remains compromised after nonlateralized reverse shoulder arthroplasty (RSA). Reduced rotational moment arms and muscle slackening have been identified as possible reasons for this impairment. Although several clinical studies suggest lateralized RSA may increase rotation, it is unclear whether this is attributable to preservation of rotational moment arms and muscle pretension of the remaining rotator cuff.

Questions/purposes: The lateralized RSA was analyzed to determine whether (1) the rotational moment arms and (2) the origin-to-insertion distances of the teres minor and subscapularis can be preserved, and (3) their flexion and abduction moment arms are decreased.

Methods: Lateralized RSA using an 8-mm resin block under the glenosphere was performed on seven cadaveric shoulder specimens. Preimplantation and postimplantation CT scans were obtained to create three-dimensional shoulder surface models. Using these models, function-specific moment arms and origin-to-insertion distances of three segments of the subscapularis and teres minor muscles were calculated.

Results: The rotational moment arms remained unchanged for the middle and caudal subscapularis and teres minor segments in all tested positions (subscapularis, -16.1 mm versus -15.8 mm; teres minor, 15.9 mm versus 15.3 mm). The origin-to-insertion distances increased or remained unchanged in any muscle segment apart from the distal subscapularis segment at 0° abduction (139 mm versus 145 mm). The subscapularis and teres minor had increased flexion moment arms in abduction angles smaller than 60° (subscapularis, 2.7 mm versus 8.3 mm; teres minor, -6.6 mm versus 0.8 mm). Abduction moment arms decreased for all segments (subscapularis, 4 mm versus -11 mm; teres minor, -3.6 mm versus -19 mm).

Conclusions: After lateralized RSA, the subscapularis and teres minor maintained their length and rotational moment arms, their flexion forces were increased, and abduction capability decreased.

Clinical relevance: Our findings could explain clinically improved rotation in lateralized RSA in comparison to nonlateralized RSA.

Fig. 1A–B

(A) Anterolateral and (B) posterolateral views of the humerus and scapula in the 3-D model are shown. The glenosphere is lateralized by an 8-mm resin block (green). To align the humerus and scapula, two coordinate systems were defined (red lines, scapula coordinate system; white lines, humeral coordinate system). The subscapularis and teres minor are each represented by three lines (subscapularis, blue lines; teres minor, green lines).

Fig. 2

Values for rotational moment arms (mm) of the subscapularis and teres minor (middle segments) before and after lateralized RSA are shown. Positive values express external rotation capability. No difference can be seen preimplantation versus postimplantation for this segment.

Fig. 3

Values for origin-to-insertion distances (mm) of the subscapularis and teres minor (middle segments) before and after lateralized RSA are shown. Postimplantation origin-to-insertion distance remained unchanged at 0° abduction. At 30° abduction and greater both muscles show increased origin-to-insertion distance values.

Fig. 4

Values for flexion moment arms (mm) of the subscapularis and teres minor (middle segments) before and after lateralized RSA are shown. Postimplantation flexion moment arms were increased at 0° to 45° abduction.

Fig. 5

Values for abduction and adduction moment arms (mm) of the subscapularis and teres minor (middle segments) before and after lateralized RSA are shown. Postimplantation abduction moment arms decreased in all positions, except for the 60° position compared with preimplantation values.