Total shoulder arthroplasty: Glenoid component design

Although Charles Neer’s original glenoid component underwent several modifications, the all-polyethylene, keeled component with a radius of curvature that conformed to the humeral radius of curvature and that was implanted with cement became the glenoid implant of choice. Neer reported approximately a 30% incidence of radiolucent lines; however, only 2 of 615 glenoids were revised for loosening. Other surgeons have reported radiolucent lines in up to 90% of glenoid components and have correlated symptoms with increasing radiolucencies. This has led to the development of alternative glenoid components for unconstrained total shoulder arthroplasty. Variations in component design include component shape, articular conformity, method of fixation, and material composition. The purposes of this presentation are to review the performance of the original Neer design, as well as other more recent glenoid designs, to identify factors that may influence the performance of glenoid components, and to provide a rationale for future changes in glenoid component design.

Introduction

Charles Neer initially described shoulder arthroplasty without resurfacing the glenoid; however, he subsequently introduced resurfacing of the glenoid because of the possibility of persistent pain, weakness, and loss of motion after hemiarthroplasty in patients with diseased and flattened glenoids.¹⁷ Although Neer’s original glenoid component underwent several modifications, the cemented, all-polyethylene, keeled component with conforming humeral and glenoid radii curvature became the glenoid implant of choice.9, 17, 18, 21, 32, 39, 40, 56

Glenoid loosening is the most common prosthesis-related cause for revision surgery after total shoulder arthroplasty, with rates ranging from 0% to 12.5%.6, 43, 60 The incidence of radiolucency at the glenoid-cement interface reported in the literature is high, ranging from 30% to 90%.5, 6 This variation is attributed to nonstandardized methods of measurement, inconsistent definitions, and limitations of conventional radiography.3, 4, 9, 11, 12, 15, 21, 39, 57Although Neer³⁸ reported an approximately 30% incidence of radiolucent lines among 615 arthroplasties performed from 1973 to 1988, only 2 were revised for glenoid loosening. Neer postulated that the lines were the result of poor cement technique. Although the correlation between glenoid lucent lines and clinical loosening is uncertain, the association between loosening, declining patient satisfaction, and increasing shoulder pain with long-term follow-up is documented.3, 6, 9, 29, 31, 49, 51, 54 Reported survivorship of the Neer cemented, all-polyethylene glenoid component ranges from 83% to 93% at 10 years and 73% to 87% at 15 years after implantation.51, 54

Multiple factors contribute to implant stability, including glenoid preparation, soft-tissue balancing, wear debris, cellular mediators of osteoclastic bone resorption, and the availability of glenoid bone stock for prosthetic fixation. Prosthetic design considerations, such as the articular surface geometry, glenohumeral conformity, biomaterials, and the glenoid keel or peg morphology, are also critically important.³³ As new components are being designed and marketed, the Neer prosthesis should be the benchmark upon which comparison studies are made. This report reviews the performance of glenoid designs, identifies factors that may influence the performance of glenoid components, and aims to provide a rationale for future changes in glenoid component design.