Bony Increased-offset Reversed Shoulder Arthroplasty: Minimizing Scapular Impingement While Maximizing Glenoid Fixation

Abstract

Background Scapular notching, prosthetic instability, limited shoulder rotation and loss of shoulder contour are associated with conventional medialized design reverse shoulder arthroplasty. Prosthetic (ie, metallic) lateralization increases torque at the baseplate-glenoid interface potentially leading to failure. Questions/purposes We asked whether bony lateralization of reverse shoulder arthroplasty would avoid the problems caused by humeral medialization without increasing torque or shear force applied to the glenoid component. Patients and Methods We prospectively followed 42 patients with rotator cuff deficiency treated with bony increased-offset reverse shoulder arthroplasty. A cylinder of autologous cancellous bone graft, harvested from the humeral head, was placed between the reamed glenoid surface and baseplate. Graft and baseplate fixation was achieved using a lengthened central peg (25 mm) and four screws. Patients underwent clinical, radiographic, and CT assessment at a minimum of 2 years after surgery. ResultsThe humeral graft incorporated completely in 98% of cases (41 of 42) and partially in one. At a mean of 28 months postoperatively, no graft resorption, glenoid loosening, or postoperative instability was observed. Inferior scapular notching occurred in 19% (eight of 42). The absolute Constant-Murley score improved from 31 to 67. Thirty-six patients (86%) were able to internally rotate sufficiently to reach their back over the sacrum. Conclusions Grafting of the glenoid surface during reverse shoulder arthroplasty effectively creates a long-necked scapula, providing the benefits of lateralization. Bony increased-offset reverse shoulder arthroplasty is associated with low rates of inferior scapular notching, improved shoulder rotation, no prosthetic instability and improved shoulder contour. In contrast to metallic lateralization, bony lateralization has the advantage of maintaining the prosthetic center of rotation at the prosthesis-bone interface, thus minimizing torque on the glenoid component.

Does lateralisation of the centre of rotation in reverse shoulder arthroplasty avoid scapular notching? Clinical and radiological review of one hundred and forty cases with forty five months of follow-up

“…Radiographic analysis of a third case of loosening, which occurred at two years from the index procedure, showed that primary fixation had never been achieved. Therefore, it seems that-as opposed to some literature reports [1,3,36]-glenoid loosenings in our series were not due to scapular notching and subsequent progressive glenoid destabilisation. Special attention must be paid to primary fixation of the glenoid baseplate, eventually adding a bone graft and/or using a long-peg glenoid implant.…”

“…Lateralisation of the COR leads to a theoretical risk of increasing the shear forces applied on the glenoid [3]. This, however, did not lead to an increase in glenoid loosening in our study.…”

“…Half of the scapular notches are evolutive and are responsible for decreased function and greater risk of glenoid loosening [37]. To avoid this complication, many solutions have been proposed: bony lateralisation [3]; different techniques of baseplate implantation, such as a lower implantation, flush to the inferior rim of the glenoid cavity [4,38] and/or applying an inferior tilt [22,23,26,31]; modification of the shape of the glenosphere (eccentric [49] or oversized [10,49]); modification of the humeral-neck-shaft angle [33]. Some authors chose metallic lateralisation of the COR, which keeps the humeral bearing away from the pillar of the scapula.…”

Implant Design Variations in Reverse Total Shoulder Arthroplasty Influence the Required Deltoid Force and Resultant Joint Load

“…This effect is particularly concerning because despite this testing protocol only testing basic arm elevation, glenosphere lateralization elicited a large increase in deltoid loading (ie, approximately 16 pounds [71 N] for an average North American male weighing 178 pounds [31]) which would be further exacerbated by more demanding tasks such as lifting. The effect of lateralizing the glenosphere by 10 mm also may be concerning as this level of offset has been popularized (eg, the bony increased-offset reversed shoulder arthroplasty technique of Boileau et al [6]) and is endorsed in some product technical manuals [10,29] as a means to reduce scapular notching. Conversely, humeral lateralization is the only parameter in this model that decreased deltoid force and thus should be an important consideration when designing implants to address the clinical problems of deltoid fatigue and acromial fracture, which are both associated with increased deltoid force [8,15,30].…”

“…Fig. 3 A computer rendering of the in vitro muscle loading-driven active motion simulator with a right shoulder mounted shows: a scapula and humerus implanted with the custom adjustable, instrumented RTSA prosthesis (1); rotating scapula pot (2); motor and linkage mechanism to drive scapula pot rotation (3), low-friction deltoid and rotator cuff cable guide system which routes sutures from the muscle attachment to low friction pneumatic actuators (out of frame to the right) (4), optical trackers used to provide real-time kinematic feedback to the control system (5), and weight used to replace the mass of the resected distal arm (6). Soft tissues are omitted for clarity.…”