1 Introduction

Anatomic total shoulder arthroplasty (aTSA) is effective for pain relief and improves function in patients with glenohumeral arthritis, with a reported 10-year implant survival rate of 96 %.¹ The procedure has become increasingly common; however, revision to reverse shoulder arthroplasty (RSA) has emerged as a clinical concern. Loosening of the glenoid components is a major complication and one of the most frequent causes of implant failure.²

Glenoid radiolucencies have been reported to develop, with notable changes occurring five or more years after surgery.³ Computed tomography (CT) has been employed to assess radiolucency in greater detail,⁴ and superior glenoid inclination has been identified as a contributing factor in long-term follow-up studies of multi-peg components.⁵ In contrast, radiolucency in cemented single-peg designs has been rarely documented.

The cemented single-peg glenoid component may provide advantages for bone preservation during revision surgery owing to their simpler structure. Nevertheless, most prior studies have focused on multi-peg designs, while data specifically addressing the clinical outcomes of the cemented single-peg glenoid component remain limited. Furthermore, CT-based evaluation of radiolucency in this design has seldom been reported.

The primary aim of this study was to evaluate the postoperative clinical outcomes of aTSA using the cemented single-peg glenoid component. The secondary aim was to examine the relationship between CT-assessed radiolucency and postoperative outcomes.

2 Material and methods

2.1

2.1 Study design and patients

In this retrospective cohort study, 36 shoulders (31 patients) treated with the cemented single-peg polyethylene glenoid component between January 2006 and December 2015 were screened. Patients were excluded if they had less than 5 years of follow-up, a history of septic arthritis of the shoulder, or lacked postoperative CT. In total, 26 shoulders (23 patients) were analyzed (Fig. 1).

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Fig. 1 Flowchart of patient selection. ^a Six shoulders were excluded due to follow-up less than 5 years, one of which also had a history of septic arthritis of the shoulder. Abbreviations: aTSA, anatomic total shoulder arthroplasty; CT, computed tomography.

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All procedures were performed by a single fellowship-trained shoulder surgeon. The study protocol was approved by the Ethics Committee of Toho University Ohashi Medical Center (No. H23113_H21098_H20096).

The primary outcomes were postoperative range of motion (ROM), clinical scores, complications, revision rate, and radiolucency around the peg as assessed by CT. Secondary outcomes included comparison of clinical results according to the presence or absence of radiolucency on CT, as well as analysis of associated factors such as the critical shoulder angle (CSA), proximal humeral superior subluxation, glenoid inclination, glenoid retroversion.

2.3

2.3 Clinical and radiographic evaluation

At the final postoperative follow-up, range of motion (forward elevation, abduction, external rotation, and internal rotation), clinical scores including the American Shoulder and Elbow Surgeons (ASES)⁷ score, Oxford Shoulder Score (OSS),⁸ and University of California, Los Angeles (UCLA)⁹ score, complications, and revision surgery were recorded. Internal rotation was graded using Flurin's 7-point scale.¹⁰ Standard anteroposterior radiographs obtained at the final follow-up were assessed according to the Lazarus classification,¹¹ CSA,¹² and proximal humeral superior subluxation. Subluxation was evaluated using the Torchia classification¹³; cases classified as moderate (25–50 %) or severe (>50 %) displacement were considered to have superior subluxation. Immediate postoperative radiographs were reviewed to confirm that displacement had developed postoperatively.

Postoperative CT scans (3-mm contiguous slices) were routinely obtained at 5 and 10 years after surgery. Five zones around the peg were defined in each plane, modified from the method of Yian et al.,⁴ and each radiolucent line (RL) in these zones was scored on a 0–3 scale (0 mm = 0, 0–1 mm = 1, 1–2 mm = 2, >2 mm = 3). CT images and schematic diagrams illustrating the scoring method are shown in Fig. 2. The coronal and axial scores (range, 0–15 each) were summed to obtain an RL total score (0–30). Based on a receiver operating characteristic (ROC) analysis anchored to a Lazarus grade ≥3, an RL total score ≥9 was defined as radiolucency-positive, and individual plane scores were dichotomized at 4.5 points.

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Fig. 2 Radiolucent line scoring zones around the single-peg glenoid component. (A, B) Schematic coronal and axial views illustrating five scoring zones used for RL evaluation. (C, D) Corresponding postoperative CT images with overlaid zone numbers. Abbreviations: RL, radiolucent line; CT, computed tomography; Ant, anterior; Post, posterior; Sup, superior; Inf, inferior.

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Glenoid inclination and retroversion were measured on postoperative CT according to the methods of Maurer¹⁴ and Friedman,¹⁵ respectively. All radiographic parameters were independently measured by two board-certified orthopedic surgeons, and interobserver reliability was assessed using intraclass correlation coefficients (ICC 2,1). Because the Lazarus classification is ordinal, its mean score was used after confirming sufficient interobserver agreement (ICC = 0.930).

3 Results

4 Discussion

This study demonstrated that aTSA using the cemented single-peg glenoid component achieved favorable long-term outcomes, and radiolucency, when present, did not consistently correspond with inferior clinical performance. To our knowledge, this is the first report to focus on postoperative clinical outcomes and CT-assessed radiolucency in this design.

Raiss et al. reported no revisions with keel-type components at 11 years,¹⁶ McLendon et al. reported an 83 % survival rate at 10 years with three-peg components,¹⁷ and Matsui et al. documented two revisions in 18 shoulders with multi-peg components.⁵ Although direct comparisons are limited by design differences, the present results suggest that the single-peg glenoid component provides long-term outcomes comparable to these alternatives. Despite concerns that fewer pegs may compromise fixation strength and long-term outcomes, our findings provide little evidence to support such assumptions. Rather, the bone-preserving nature of the single-peg design may offer advantages in revision settings. Further long-term comparative studies are warranted to determine whether these structural features translate into meaningful clinical benefits.

No significant differences in clinical scores or ROM were observed between radiolucency-positive and radiolucency-negative groups. Karelse et al. described loosening as a progressive process from radiolucent lines to revision,¹⁸ supporting the view that radiolucency may represent an early finding rather than immediate clinical failure.

With respect to CSA, Tabeayo et al. reported that values > 35° were associated with glenoid loosening after aTSA,¹⁹ and Watling et al. confirmed CSA as a risk factor on plain radiographs.²⁰ Wolf et al. further showed that larger postoperative CSA predicted progressive cranialization and loosening at long-term follow-up.²¹ Consistent with these findings, our study demonstrated significantly greater CSA values in radiolucency-positive shoulders (mean 36.9°), suggesting that CSA may serve as a radiographic marker for secondary cuff dysfunction after aTSA and contribute to the progression of radiolucency.

Similarly, proximal humeral superior subluxation was significantly associated with radiolucency. Young et al. reported that secondary rotator cuff dysfunction after TSA can impose eccentric loading on the glenoid component, described as “superior migration.“²² Gonzalez et al. noted that cuff tears may lead to superior loading and the “rocking-horse effect.“²³ While some studies have suggested that superior migration is part of the natural history of aTSA and not always associated with poor outcomes,²⁴ Lafosse et al. further indicated that its predictive value for rotator cuff failure or glenoid loosening is very limited, as it has not been consistently correlated with inferior clinical outcomes or higher revision rates.²⁵ Our findings showed higher rates of superior subluxation in radiolucency-positive shoulders. This suggests that such subluxation after aTSA cannot be explained solely by natural history and warrants careful interpretation of long-term radiographic changes. Importantly, our results suggest that when superior subluxation is observed on plain radiographs, it may provide a rationale for further evaluation of radiolucency, and CT assessment for this purpose could contribute to follow-up strategies after aTSA.

Radiographic evaluation is commonly based on plain radiographs, with anteroposterior views assumed to correspond to the coronal plane on CT. Although CT provides greater detail, prior studies have primarily focused on the axial plane.^4,5 To our knowledge, no previous study has systematically compared radiolucency on both planes. In the present study, discordance between coronal and axial CT assessments occurred in one-third of cases, and notably, the majority of discordant shoulders (6 of 8, 75 %) were radiolucency-positive. This finding suggests that evaluation limited to a single imaging plane may underestimate radiolucency, and combined coronal and axial CT assessment may provide a more comprehensive evaluation.

This study has several limitations. First, it was a retrospective study with a small sample size, limiting generalizability. Second, CT scans were performed at two postoperative time points (5 and 10 years), which may have introduced variability. Third, the number of discordant cases was small, precluding detailed subgroup analysis. Additionally, because this was a single time-point analysis, causality could not be established, and further longitudinal studies are required. No revision cases occurred; therefore, loosening could not be directly confirmed, and comparisons relied on the presence or absence of radiolucency. Moreover, there is no consensus on standardized thresholds for loosening, and although our definition was derived from ROC analysis, this remains a limitation. Nevertheless, cemented single-peg glenoid components have been rarely reported, and long-term data on this design remain scarce. Therefore, even this small cohort can provide valuable insights by offering long-term clinical and radiographic data on single-peg glenoid components.

Future studies with additional cases and extended follow-up may help clarify the relationship between clinical outcomes and radiolucency.

5 Conclusion

The cemented single-peg glenoid component for aTSA demonstrated favorable outcomes, with 100 % implant survival over a mean follow-up of 132 months. Radiolucency assessed on CT did not necessarily correspond with worse clinical outcomes. Larger CSA and proximal humeral superior subluxation were associated with radiolucency.

CT assessment using both axial and coronal planes was a useful method for providing a more accurate evaluation of radiolucency.

Patient consent

Information about the study was disclosed on the institutional website, and potential participants were given the opportunity to opt-out.

Ethical approval

The study protocol was approved by the Ethics Committee of Toho University Ohashi Medical Center (Approval No. H23113_H21098_H20096).

CRediT authorship contribution statement

Misato Sakamoto: Data curation, Formal analysis, Interpretation, Writing – original draft. Hideaki Ishii: Formal analysis, Interpretation. Takanori Shintaku: Data curation. Shu Yoshizawa: Data curation. Takahiro Maeda: Data curation. Hiroyasu Ikegami: Conceptualization, Writing – original draft, Supervision.

All authors reviewed the manuscript, approved the final version, and agreed to its submission.

Funding

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.