Individualizing Glenoid Lateralization to Optimize Range of Motion after Reverse Shoulder Arthroplasty with the Arthrex Univers Reverse: A Virtual Assessment of the Influence of Scapular Neck Length Using a Statistical Shape Model.

BACKGROUND

Preoperative scapular neck length varies widely due to anatomical factors and pathologic glenoid wear. A short scapular neck length (SNL) can lead to early impingement, decreased range of motion (ROM) and increased rates of scapular notching after reverse shoulder arthroplasty (rTSA). Glenoid-sided implant lateralization can avoid these issues, but it is unclear how much is necessary for varying scapular neck lengths. The objective was to use a statistical shape model (SSM), varying SNL, to evaluate impingement-free rotational ROM across different glenoid positions and lateralizations.

METHODS

100 scans were randomly chosen from a clinical database of over 10,000 shoulder computed tomography (CT) scans uploaded for shoulder arthroplasty preoperative planning. These 100 scans were utilized to create and validate an SSM. Modes corresponding to scapular size and scapular neck angle (SNA) were identified as producing the greatest variance in scapular neck length (SNL), from which the mean SNL and two standard deviations (positive and negative) were evaluated for the study. For each scapula, a single glenosphere diameter (33-42mm) was selected by a consensus of three surgeons. A 135° neck shaft angle (NSA) inlay humeral prosthesis was utilized for all simulations (Arthrex Univers rTSA). Impingement-free rotational ROM was then assessed for each of the 5 scapulae for the following virtual implantation variables: lateralization (0-12mm), inferior glenosphere eccentricity (0-2.5mm) and posterior glenosphere eccentricity (0-2.5mm).

RESULTS

Both scapular neck length and glenoid lateralization had significant contributions to rotational ROM. Progressive glenoid lateralization improved external rotation at 0° of abduction (ER0), external rotation at 60° of abduction (ER60) and internal rotation at 60° (IR60) across all 5 standard deviations of SNL. IR60 was greatest for the longest SNL, and the value of progressive glenoid lateralization was the greatest for improving IR60 for the shortest SNL, where 6mm of lateralization achieved maximum IR60. ER0 was greatest for the shortest SNL, and the value of glenoid lateralization for ER0 was not seen for the longest SNL until >4mm of lateralization. ER60 was largely not influenced by lateralization or SNL, where 2-4 mm of lateralization maximized sufficiently.

CONCLUSIONS

Both scapular neck length and glenoid implant lateralization influence rotational ROM after rTSA with a 135° NSA and should be considered as covariates when determining appropriate implant selection and positioning. For smaller SNL, 6mm of implant lateralization is needed to maximize impingement-free internal rotation. For larger SNL, a minimum 6mm of implant lateralization is needed to maximize impingement-free external rotation.