Posteroinferior relevant scapular neck offset in reverse shoulder arthroplasty: key player for motion and friction-type impingement in a computer model

BACKGROUND: Range of Motion (ROM) and prevention of notching remain a challenge for Reverse Shoulder Arthroplasty (RSA). Both may be affected by the morphology of the scapula. The purpose of this study was to define anteroinferior (a) and posteroinferior (p) Relevant Scapular Neck Offset (RSNO) and to examine the hypothesis that pRSNO is significantly smaller than aRSNO, and influences Rigid Body Motion (RBM). Adapting glenosphere implantation strategies may therefore be of value.

MATERIAL AND METHODS: In this computer model study (Blueprint software, Imascap) we used deidentified CT scans of 22 patients (11 male/female; mean age: 72.9 years) with massive cuff tears without joint space narrowing. Eight RSA glenoid configurations were tested with a constant neck-shaft angle (145°). Two base plate (BP) types (25mm; 25mm+3mm lateralized) and four glenospheres (GS) (36 mm; 36mm+2mm eccentricity; 39mm; 39mm+3mm) were used. RSNO was defined as the standardized measurement of the horizontal distance from the inferior extent of the GS to the bony margin of the scapula after BP positioning (flush to inferior glenoid extent; neutral position: 0° inclination; 0° version; both software computed).

RESULTS: There was a highly significant difference between pRSNO and aRSNO for both genders (p<0.00001). pRSNO was always smaller than aRSNO. pRSNO was strongly correlated with external rotation (ERO: 0.84) and extension (EXT: 0.74) and moderately correlated with global ROM (GROM: 0.68). There was a moderately strong correlation between aRSNO and internal rotation (IRO: 0.69). pRSNO was strongly correlated with aRSNO, EXT, ERO, IRO, adduction (ADD) and GROM (0.82; 0.72; 0,8; 0.71; 0.82; 0.76) in females and with EXT and ERO (0.82; 0.89) in males. The median pRSNO allowing for at least 45° ERO/40° EXT was 14.2mm for males and 13.8mm for females. For all patients and models, pRSNO≥14mm increased EXT, ERO, and GROM significantly compared to pRSNO<14mm (p<0.00001). The combination of lateralization and inferior overhang (eccentricity) lead to the most significant increase of pRSNO for each GS size (p<0.00001).

CONCLUSION: This is one of the first RSA modeling studies evaluating non-arthritic glenoids of both genders. The lateral scapular extent to glenoid relationship is asymmetric. pRSNO is always smaller than aRSNO for both genders, and was a critical variable for EXT and ERO demonstrating additional strong correlation with aRSNO, IRO, ADD and GROM in females. pRSNO≥14mm was a safe value to prevent friction-type impingement. Combining increased glenosphere size, lateralization and inferior overhang gives the best results in this computer simulated setting.