Total shoulder arthroplasty (TSA) has become the gold standard treatment option for patients with glenohumeral osteoarthritis, an intact rotator cuff, and failed nonoperative management. In 1974, Charles Neer modified his Neer I prosthesis to add a cemented, keeled, onlay, polyethylene glenoid component, known as the Neer II prosthesis.¹ Early outcome studies demonstrated promising results but with a high level of radiographic glenoid loosening.² Since then, the use of TSA has continually increased over time, with good long-term survival outcomes. The glenoid component remains the “weak link” in the construct, with glenoid component loosening being one of the most common complications.³ In a systematic review of 27 articles and 3853 TSA, authors found asymptomatic radiolucent lines occurred at 7.3% and symptomatic loosening at 1.2% per year following TSA, indicating the high prevalence of these findings.⁴

Glenoid loosening is an even more significant issue with worsening glenoid deformity. Increased incidence of radiolucent lines is observed in cases of posterior glenoid wear,⁵ increased retroversion,⁶ rotator cuff deficiency,⁷ superior tilt, preoperative posterior subluxation, and excessive reaming⁸ (FIGURE 18.1). The preservation of subchondral bone is important for minimizing the risk of loosening and migration, thus limiting the amount of correction a surgeon can make during surgery. It is for these reasons that many surgeons choose to utilize a reverse total shoulder arthroplasty (RTSA) in cases of excessive glenoid wear.

The primary mechanism of failure of onlay glenoids in TSA is due to micromotion of the implant over time. This has been termed the “rocking-horse” phenomenon due to edge loading of the implant (FIGURE 18.2).⁷ Due to edge loading, there is microscopic compression of one end of the component, with subsequent elevation of the opposite side. Through repetitive cycles, the micromotion can lead to compromise of the polyethylene-cement-bone interface and subsequent loosening. Glenoid wear can compound this loosening through polyethylene debris, initiating a foreign body reaction with osteolysis.¹

Early attempts to address glenoid loosening included metal-backed glenoids. Early versions eventually fell out of favor as a result of a high incidence of backside wear with 83% of glenoids demonstrating radiolucencies by 2 years.⁹ More recently, newer designs of metal-backed implants are again being utilized in an effort to reduce the issues of loosening (FIGURE 18.3). Other design features have also been employed to reduce the incidence of loosening, including pegged glenoids(with varying peg configurations), different glenoid shapes, and enhanced cementation technique.

The notion of decreasing edge loading has led to the design of an “inset,” in which part of the implant is inset within the glenoid, or an “inlay” glenoid, in which all of the implant is placed within the glenoid so that it is flush with the bone, with the hope of dispersing the contact forces between the implant and the native glenoid. In a biomechanical study of eight matched pairs of glenoids, authors looked at glenohumeral contact forces and fatigue testing at 4000 cycles using a cemented onlay glenoid and a cemented inlay glenoid. They found that the edge loading of the onlay glenoid increased compared to the native glenoid, presumably from lateralization of the contact area. In contrast, the inlay glenoid had a decrease in the edge-loading contact forces, as a result of the force being distributed between the implant and the native glenoid. With fatigue testing, all onlay glenoid components demonstrated loosening at a mean 1126 cycles (range 749-1838). None of the inlay glenoids demonstrated loosening at 4000 cycles¹⁰ (FIGURE 18.4).

In another biomechanical comparison of an inset glenoid compared to both a keel and pegged onlay glenoid, authors looked at displacement and loosening at 100,000 cycles. They found that an inset glenoid showed 87% less displacement compared to the pegged and keel onlay glenoids. They did not demonstrate a significant difference in loosening at 100,000 cycles.¹¹ These two studies provide strong biomechanical evidence that an inlay or inset glenoid has less micromotion and edge loading compared to an onlay glenoid.
Of course, the unanswered question is whether the biomechanical evidence translates to improved clinical outcomes and decreased loosening, especially in more active patients.