Hierarchy of Stability Factors in Reverse Shoulder Arthroplasty

Abstract

Reverse shoulder arthroplasty is being used more frequently to treat irreparable rotator cuff tears in the presence of glenohumeral arthritis and instability. To date, however, design features and functions of reverse shoulder arthroplasty, which may be associated with subluxation and dislocation of these implants, have been poorly understood. We asked: (1) what is the hierarchy of importance of joint compressive force, prosthetic socket depth, and glenosphere size in relation to stability, and (2) is this hierarchy defined by underlying and theoretically predictable joint contact characteristics? We examined the intrinsic stability in terms of the force required to dislocate the humerosocket from the glenosphere of eight commercially available reverse shoulder arthroplasty devices. The hierarchy of factors was led by compressive force followed by socket depth; glenosphere size played a much lesser role in stability of the reverse shoulder arthroplasty device. Similar results were predicted by a mathematical model, suggesting the stability was determined primarily by compressive forces generated by muscles.

The effect of component positioning on intrinsic stability of the reverse shoulder arthroplasty

“…The glenosphere articular contact surface is spherical, so a change in version is similar to revolving a ball about its centre, which does not affect the incidence angle, defined as the angle between the glenosphere and the humerosocket edge. 2,17 However, for the resting-arm position, with the glenosphere in 20° retroversion, a statistically significant drop in stability ratio was measured for all humeral versions. In this particular configuration, the hole on the surface of the glenoid component for the central screw that fixes the glenosphere to the base plate is very close to the border of the polyethylene cup, and may affect the contacts between the two components.…”

“…7,17 Intrinsic stability of the semiconstrained reverse prosthesis plays a dominant role to maintain a stable joint because of the mostly deficient active muscular stabilizers. 4,13 The purpose of this study was to test the hypothesis that intrinsic stability of a reverse prosthesis varies with the degree of version of the humerus and glenoid components.…”

“…34 This high rate of dislocation and the current lack of knowledge on possible causes call for a better understanding of the factors involved in instability. 7,17 Distortion of the osseous and softtissue anatomy by prior trauma, poor deltoid tensioning, inappropriate ratio between the central depth and the diameter of the concave component, leverage of the humeral component against the glenoid bone and components malpositioning have been identified as possible causes of instability. 4,13,17,23,25,35,40 Revision surgery for correction of instability has been advocated in case of component malpositioning, 23 However until now, no clear and explicit recommendations for adequate component positioning are available.…”

“…7,17 Distortion of the osseous and softtissue anatomy by prior trauma, poor deltoid tensioning, inappropriate ratio between the central depth and the diameter of the concave component, leverage of the humeral component against the glenoid bone and components malpositioning have been identified as possible causes of instability. 4,13,17,23,25,35,40 Revision surgery for correction of instability has been advocated in case of component malpositioning, 23 However until now, no clear and explicit recommendations for adequate component positioning are available. 20 A primary factor in resisting dislocating forces is the geometry (conformity and constraint) of the implant itself, 2,11,32,36 especially in the semi-constrained reverse setting.…”

“…20 A primary factor in resisting dislocating forces is the geometry (conformity and constraint) of the implant itself, 2,11,32,36 especially in the semi-constrained reverse setting. 4,5,13,17 However, within a defined design, the stability then depends on relative positioning of the components, a factor that is directly controllable by the surgeon during implantation. Our hypothesis is that intrinsic stability of a reverse prosthesis varies with the degree of version of the humerus and glenoid components.…”