Scapular neck length and implications for reverse total shoulder arthroplasty: An anatomic study of 442 cadaveric specimens

[Cuff Arthropathy - Inverse Total Shoulder Arthroplasty (iSTEP) - Biomechanical Principles, Complications and Outcomes]

The inverse total shoulder prosthesis is a standard treatment for cuff arthropathy. The number of implanted inverse total shoulder prostheses in Germany is significantly higher than that of the anatomical total shoulder prosthesis. Kaplan-Meier analyses show a 10-year result of inverse total shoulder prostheses after cuff arthropathy of 90%. The extent of the scapular notching correlates with the functional clinical outcome. The humeral anchoring principles of inverse total shoulder prosthesis differ considerably (from purely epiphyseal to diaphyseal). Inverse total shoulder prostheses show significant differences in humeral and glenoid offset as well as in shaft-neck angles. The large number of systems available differ considerably concerning anatomical and biomechanical parameters - the surgeons must be trained accordingly. Still, most shoulder replacements are implanted by surgeons who rarely perform this procedure.

Influence of scapular neck length on the extent of impingement-free adduction after reverse total shoulder arthroplasty

BACKGROUND

Following reverse total shoulder arthroplasty, a short scapular neck length (SNL) decreases postoperative impingement-free adduction, and impingement between the neck of the scapula and the humeral polyethylene cup may cause scapular notching. However, no reports have evaluated the influence of SNL on impingement-free adduction. The purposes of this study were to evaluate the influence of SNL on impingement-free adduction and to examine the effect of glenoid component lateralization and inferiorization on impingement-free adduction.

METHODS

By use of 3-dimensional templating software, a virtual reverse total shoulder arthroplasty model was created in 15 patients who had no osteoarthritic change or any other bony deformity. We measured SNLs separately before implant placement (preoperative SNL) and after implant placement (postoperative SNL). The implant used was the Comprehensive Reverse Shoulder System (Zimmer Biomet, Warsaw, IN, USA), and baseplate bony lateralization of 0, 5, and 10 mm, with inferior eccentricity of 0.5 or 4.5 mm, was tested for impingement-free adduction. Correlations between the preoperative and postoperative SNLs and impingement-free adduction were analyzed.

RESULTS

The mean preoperative SNL was 8.2 ± 1.9 mm (range, 5.0-11.7 mm), and the mean postoperative SNL was 6.0 ± 2.0 mm (range, 2.1-9.8 mm). There was a moderate correlation between the preoperative SNL and impingement-free adduction (r = 0.628, P = .12) and a strong correlation between the postoperative SNL and impingement-free adduction (r = 0.771, P = .001). Use of the model with 10 mm of bony lateralization and 4.5 mm of inferior eccentricity provided the best results in terms of impingement-free adduction.

CONCLUSION

There were correlations between both the preoperative and postoperative SNLs and impingement-free adduction. Although the lateralized and inferiorized center of rotation may increase the risk of loosening of the glenoid component, this offset significantly increased impingement-free adduction.

Computed tomographic evaluation of glenoid joint line restoration with glenoid bone grafting and reverse shoulder arthroplasty in patients with significant glenoid bone loss

BACKGROUND

Restoration of native glenohumeral joint line is important for a successful outcome after reverse shoulder arthroplasty (RSA). The aims of this study were to quantify the restoration of glenoid joint line after structural bone grafting and RSA, and to evaluate graft incorporation, correction of glenoid version, and rate of notching.

METHODS

This is a retrospective review of 21 patients who underwent RSA (20 primary, 1 revision) with glenoid bone grafting (15 autografts, 6 allografts). Grammont design implants and baseplate with long peg were used in all patients. Preoperative and postoperative 3D models were created using MIMICS 21.0. Preoperative defects were classified, and postoperative joint line restoration was assessed based on the lateral aspect of the base of the coracoid. Postoperative computed tomographic (CT) scans were evaluated for graft incorporation, version correction, and presence of notching.

RESULTS

Preoperative glenoid defects were classified as massive (5%), large (29%), moderate (52%), and small (14%). The average preoperative version was 8° of retroversion. The average postoperative version was 5° of retroversion. The average preoperative medialization was noted to be 8.4 mm medial to native joint line or 0.6 mm (range -16.8 to 13.2) lateral to the coracoid base. The postoperative CT scans demonstrated a mean joint line at 12.1 mm (range 1.3-22.4) lateral to the coracoid base. At the 3-month follow-up, all patients demonstrated graft incorporation on CT scans. Graft osteolysis was observed on CT scan in 4.8% of patients at a mean follow-up of 19.5 months.

DISCUSSION

Structural bone grafting of glenoid defect effectively re-creates the glenoid anatomy, restores glenoid bone stock, re-creates the true glenohumeral joint line, and corrects glenoid deformity. The use of bone grafting also allows lateralization of the baseplate and glenosphere, reducing the risk of severe scapular notching.

CONCLUSION

Restoration of the glenoid joint line was achieved in all patients. Glenoid bone grafting is a viable option for restoring glenoid joint line in cases of significant glenoid defects encountered during RSA.

Scapular Notching in Reverse Total Shoulder Arthroplasty

Scapular notching is a common radiographic finding occurring after reverse total shoulder arthroplasty, and it refers to an erosive lesion of the inferior scapular neck because of the impingement of the humeral implant during adduction. The clinical importance of notching is unclear, and the optimal treatment of severe notching is unknown. The incidence and severity of scapular notching is related to prosthetic design and surgical technique. Implant design factors include size, shape, and position of the glenosphere, inclination of the humeral neck-shaft angle, implant offset, and native scapular anatomy. Scapular notching may lead to deterioration of functional outcomes and glenoid implant loosening and failure. Lateral offset, inferior glenosphere overhang, and careful consideration of the presurgical glenoid morphology may help prevent scapular notching. Currently, there is limited evidence to direct the management of scapular notching, and further research is needed to elucidate optimal prevention and treatment strategies.

3D image-based morphometric analysis of the scapular neck length in subjects undergoing reverse shoulder arthroplasty

Scapular notching after RSA may, in part, be related to a patient's scapular morphology. The purpose of this study was to develop a novel 3 D measurement technique to describe infraglenoid tubercle morphometry. We hypothesize that the parameters describing infraglenoid tubercle would be normally distributed and would correlate with individual's demographics and glenoid morphometry. A group of 110 subjects treated with RSA were evaluated. Scapular geometry was obtained from pre-operative CT scans. The scapular neck length was defined in anterior and posterior view as the orthogonal distance between the glenoid surface and (a) the point of most significant change of curvature on lateral pillar; (b) the most lateral portion of the infraglenoid tubercle. Scapular neck angle, maximum anatomical adduction angle, glenoid width, height and version were also measured. Scapular neck length measured in the anterior and posterior view were normally distributed with mean values of 17.6 ± 2.8 mm and 7.9 ± 2.5 mm, respectively. Scapular neck angle was on average 112.4 ± 10.6° and maximal adduction angle was 17.6 ± 2.8°. No significant associations between infraglenoid tubercle morphometric parameters, demographic, glenoid size or version were identified. Improved understanding of the patient-specific risk factors for scapular notching will help surgeons with pre-surgical planning and implant selection. Parametrization of the infraglenoid tubercle presented in this study showed normal distribution in the population unrelated to gender, side or demographics. The size of the infraglenoid tubercle is a unique trait, pre-operative evaluation of the scapular neck should be always warranted to decrease the incidence of post-operative notching. Clin. Anat. 31:43-55, 2018. © 2017 Wiley Periodicals, Inc.

Medialized Versus Lateralized Center of Rotation in Reverse Shoulder Arthroplasty

Reverse shoulder arthroplasty may be performed using components that medialize or lateralize the center of rotation. The purpose of this prospective study was to directly compare 2 reverse shoulder arthroplasty designs. Two treatment groups and 1 control group were identified. Group I comprised 9 patients using a medialized Grammont-style (GRM) prosthesis with a neck-shaft angle of 155°. Group II comprised 9 patients using a lateralized (LAT) prosthesis with a neck-shaft angle of 135°. Pre- and postoperative assessment of range of motion, American Shoulder and Elbow Surgeons score, and visual analog scale pain score were performed. Radiographic measurements of lateral humeral offset and acromiohumeral distance were compared. The GRM prosthesis achieved greater forward flexion (143.9° vs 115.6°; P=.05), whereas the LAT achieved greater external rotation (35.0° vs 28.3°; P=.07). The lateral humeral offset was greater for the LAT prosthesis compared with the GRM prosthesis, but this distance was not significantly different from that found in the control group. The acromiohumeral distance was significantly greater in the GRM prosthesis group compared with both the LAT and the control groups. The results of this study confirm that different reverse shoulder arthroplasty designs produce radiographically different anatomy. Whereas the GRM prosthesis significantly alters the anatomy of the shoulder, the LAT design can preserve some anatomic relationships found in the normal shoulder. The clinical outcomes indicate that this may have an effect on range of motion, with traditional designs achieving greater forward flexion and lateralized designs achieving greater external rotation.