Computer modeling and validation testing for glenoid component rotation and optimal glenoid screw angles for reverse shoulder arthroplasty in an Asian population

Abstract

Purpose: Good initial fixation of glenoid component for reverse total shoulder arthroplasty (RTSA) relies on component placement and screw purchase in the scapula bone. This is especially difficult in an Asian population with small glenoid geometry. Optimal glenoid component roll angle and screw angulation to achieve the longest screws for best fixation has not been defined in the current literature.

Methods: Computer 3D modelling of 133 scapulas with RTSA performed were analyzed to determine patient specific optimal glenoid roll angle (GRA) for the longest bi-cortical screws attainable. The cranial-caudal angle (CCA), anterior-posterior angle (APA) and lengths for the superior and inferior screws were measured. Validation testing using calculated average (CA) angles and rounded average (RA) angles to the nearest 5 degree were recomputed for each case to determine the bi-cortical screw lengths achievable. The CA and RA screw lengths were compared against patient specific modelling using paired-sample t-tests.

Results: Average GRA was - 1.6°, almost perpendicular to the long axis of the glenoid and achieves an average bi-cortical screw length of 51.3 mm and 45.5 mm for the superior and inferior screws respectively. The CCA and APA were 9.1° cranial and 6.5° posterior for the superior screw and screw angulation of 11.2° caudal and 0.7° anterior for the inferior screw. Validation testing shows statistically shorter screw lengths in the CA and RA models compared to patient specific modelling (p < 0.01).

Conclusion: Validation testing with average angles for GRA, CCA and APA demonstrates strong patient heterogeneity and anatomical variation. Despite this, screw lengths attainable in the RA group were > 38 mm with good safety profile. Surgeons may consider the additional use of navigation-assisted, or 3D printed patient specific instrumentation to optimize baseplate and screw configuration for RTSA.

Keywords: 3D modelling; Glenoid fixation; Patient specific instrumentation; Reverse shoulder arthroplasty; Screw length.

Fig. 1

Glenoid geometrical measurements. Orange line: Cranial-caudal diameter of glenoid fossa. Purple line: Anterior-posterior width of glenoid fossa

Fig. 2

Patient specific modelling with metaglene placed nominal to the glenoid surface. Small glenoids in Asian population have inadequate or no bony purchase for anterior and/or posterior screws (green lines)

Fig. 3

Glenoid Roll Angle (GRA) is the angle between the supraglenoid tubercle and center of metaglene line (orange) and a line intersecting the center of the superior and inferior screw holes of the metaglene representing the metaglene axis (purple)

Fig. 4

(a) Three-Dimensional view from above of the superior glenoid vault demonstrating the posterior orientation (6.5 degrees) in the optimal trajectory of the superior screw (blue), attributable to the relative posterior position of the scapular spine to the glenoid and slight glenoid retroversion relative to the scapular body. (b) AP view showing the optimal cranial angle of the superior screw (9.1 degrees)

Fig. 5

Three-Dimensional view from below of the inferior glenoid vault demonstrating the slight anterior orientation (0.7 degrees) of the optimal trajectory of the inferior screw (green) due to the retroversion of the glenoid relative to the scapular body. (b) AP view showing the optimal caudal angle of the superior screw (11.2 degrees)