Computer‐assisted analysis of functional internal rotation after reverse total shoulder arthroplasty: implications for component choice and orientation

Abstract

Purpose Functional internal rotation (IR) is a combination of extension and IR. It is clinically often limited after reverse total shoulder arthroplasty (RTSA) either due to loss of extension or IR in extension. It was the purpose of this study to determine the ideal in-vitro combination of glenoid and humeral components to achieve impingement-free functional IR. Methods RTSA components were virtually implanted into a normal scapula (previously established with a statistical shape model) and into a corresponding humerus using a computer planning program (CASPA). Baseline glenoid configuration consisted of a 28 mm baseplate placed flush with the posteroinferior glenoid rim, a baseplate inclination angle of 96° (relative to the supraspinatus fossa) and a 36 mm standard glenosphere. Baseline humeral configuration consisted of a 12 mm humeral stem, a metaphysis with a neck shaft angle (NSA) of 155° (+ 6 mm medial offset), anatomic torsion of -20° and a symmetric PE inlay (36mmx0mm). Additional configurations with different humeral torsion (-20°, + 10°), NSA (135°, 145°, 155°), baseplate position, diameter, lateralization and inclination were tested. Glenohumeral extension of 5, 10, 20, and 40° was performed first, followed by IR of 20, 40, and 60° with the arm in extension of 40°—the value previously identified as necessary for satisfactory clinical functional IR. The different component combinations were taken through simulated ROM and the impingement volume (mm3) was recorded. Furthermore, the occurrence of impingement was read out in 5° motion increments. Results In all cases where impingement occurred, it occurred between the PE inlay and the posterior glenoid rim. Only in 11 of 36 combinations full functional IR was possible without impingement. Anterosuperior baseplate positioning showed the highest impingement volume with every combination of NSA and torsion. A posteroinferiorly positioned 26 mm baseplate resulting in an additional 2 mm of inferior overhang as well as 6 mm baseplate lateralization offered the best impingement-free functional IR (5/6 combinations without impingement). Low impingement potential resulted from a combination of NSA 135° and + 10° torsion (4/6 combinations without impingement), followed by NSA 135° and -20° torsion (3/6 combinations without impingement) regardless of glenoid setup. Conclusion The largest impingement-free functional IRs resulted from combining a posteroinferior baseplate position, a greater inferior glenosphere overhang, 90° of baseplate inclination angle, 6 mm glenosphere lateralization with respect to baseline setup, a lower NSA and antetorsion of the humeral component. Surgeons can employ and combine these implant configurations to achieve and improve functional IR when planning and performing RTSA. Level of evidence Basic Science Study, Biomechanics.

Reverse shoulder arthroplasty with a 155° neck-shaft angle inlay implant design without reattachment of the subscapularis tendon results in satisfactory functional internal rotation and no instability: a cohort study

“…Our results show that the internal rotation is satisfactory when the humeral component is placed in 30° of retroversion in most cases. A recent computer-assisted study by Hochreiter et al reported the ideal component placements in order to obtain the best range of motion in internal rotation; the largest impingement-free functional internal rotation was achieved when combining a posteroinferior baseplate position, a greater inferior glenosphere overhang, a baseplate inclination angle of 90°, 6 mm of glenosphere lateralisation with respect to the baseline setup, a lower NSA and anteversion of the humeral component [ 23 ].…”

Correction of Static Posterior Shoulder Subluxation by Restoring Normal Scapular Anatomy Using Acromion and Glenoid Osteotomies

“…Experimentally, these features affect the acromion's efficacy as a posterior restraint of the humerus during glenohumeral elevation. Correction of the combined anomalies experimentally restores normal posterior translation of the humerus on elevation of the arm 23 . Our case is in agreement with the experimental data and documents that correction of scapular anatomy can restore normal glenohumeral congruity in SPSL in vivo.…”

“…In the absence of promising alternatives and with the consistent and specific observation of abnormal scapular anatomy 10,21,22 as well as the biomechanical documentation that posterior escape can experimentally be prevented by scapular correction 23 , we performed a "SCOPE" (scapular (acromion and glenoid) corrective osteotomy for posterior escape) procedure in a patient with symptomatic SPSL with mild degenerative changes and report the outcome at 2 years.…”

“…Rationally, this could potentially decrease resistance to translation in response to a posterior load on the humerus. This hypothesis was tested in a biomechanical pilot study, which showed that the documented scapular abnormalities allow for excessive posterior excursion of the humeral head on elevation correctable by restoring normal glenoid and acromial anthropometrics 23 .…”

Dome versus single-cut osteotomies for correction of long bone deformities—technical considerations

“…First, 3D bone models were extracted from the CT data (Siemens SOMATOM Definition AS, Siemens Healthcare, Erlangen, Germany) using a commercial segmentation software (Mimics 19.0; Materialise NV, Leuven, Belgium) 25 – 27 . The 3D models were then imported into a planning software (CASPA; Balgrist CARD, Zurich, Switzerland) including the reconstruction template.…”