“Floating‐socket” total shoulder replacement: Anatomical, biomechanical, and surgical rationale

Abstract

A prosthesis based on a new concept of gleno-humeral motion has been developed and clinically tested for the past 2 years. The "floating-socket" total shoulder replacement contains a nondislocatable, dual spherical bearing system consisting of a small sphere within a larger sphere with their centers offset to provide the physiologic "floating fulcrum" for mechanically advantageous motion. This geometric configuration allows a prosthetic range of motion in excess of anatomical limits to allow soft tissue structures to limit motion rather than mechanical impingement. The device is constructed from Co--Cr--Mo alloy and RCH-1000 polyethylene such that no metal--metal contact can occur. Experimental results in a 112-lb adult male chimpanzee and six clinical cases have revealed superior fixation strength of both glenoid and humeral components, as well as functional adaptation of the prosthesis without fracture of fixturing bone or prosthetic dislocation.

A History of Reverse Total Shoulder Arthroplasty

“…Proponents for this design argued this change would allow improved motion and strength without the increased risk of dislocation and loosening. A number of reverse implant systems were designed beginning in the 1970s with variable designs for scapular fixation [1,3,4,6,7,13,16,17,25,41,42] (Table 1). These prostheses created a foundation for reverse shoulder arthroplasty and made important contributions to our understanding of the reverse concept.…”

“…Liverpool/Beddow and Elloy [3] 1975 Glenoid component included a stem fixed into the scapular pillar to improve glenoid fixation Buechel et al [7] 1978 A ''floating fulcrum''; glenosphere decreased in size to increase shoulder motion Trispherical/Gristina and Webb [17] 1978 A glenosphere and a small sphere on the humerus, both articulating in a larger polyethylene cup Grammont Version 1 [16] 1985 Center of rotation medialized but remained lateral to native glenoid surface; glenoid baseplate had a press-fit central peg Delta III/Grammont 1991 Center of rotation further medialized to the native glenoid face; glenoid baseplate coated with hydroxyapatite to improve fixation theoretically allowed anatomic ROM, in vivo study did not demonstrate an improvement in ROM [6]. A series of 17 shoulders (all with rheumatoid arthritis) followed for 87 months noted active motion was unchanged and all shoulders had radiolucent lines seen within a year [6].…”

“…The design by Buechel et al [7] in 1978 was based on a floating fulcrum, with characteristics designed to allow supraphysiologic motion. This system included a small glenosphere that articulated with a larger intermediate polyethylene cup.…”

“…This system included a small glenosphere that articulated with a larger intermediate polyethylene cup. Buechel et al [7] postulated over time the muscle forces across the glenohumeral joint would lead to failure at the bone-cement or humerus-glenoid interfaces of the arthroplasty. Their early results in a small series were good, without fractures or dislocation [7].…”

“…Buechel et al [7] postulated over time the muscle forces across the glenohumeral joint would lead to failure at the bone-cement or humerus-glenoid interfaces of the arthroplasty. Their early results in a small series were good, without fractures or dislocation [7]. Similar to the Buechel prosthesis, Gristina and Webb's Trispherical system [17] from 1978 was also designed to afford a greater ROM.…”