Wyce Sahady, OMS 2027, MS; Benjamin Brooks, Ph.D., MBA; Jared LeCuyer, OMS 2028
Rocky Vista University College of Osteopathic Medicine

Abstract

Reverse total shoulder arthroplasty (rTSA) has expanded rapidly in indications and volume, with increasing use for cuff-tear arthropathy, fractures, instability, and revision surgery. Glenoid baseplate fixation remains the primary determinant of implant survivorship. Biomechanical strategies including inferior baseplate tilt, longer anteroinferior screws, and cortical-engaging central fixation, improve stability and reduce micromotion, but may increase proximity to critical neurovascular and cortical structures, increasing the risk of iatrogenic injury.

This focused narrative review synthesizes biomechanical, cadaveric, imaging, and clinical studies identified through a structured search of PubMed, Embase, Scopus, and Web of Science from 2005 to 2025. Studies evaluating screw trajectory, fixation mechanics, cortical breach, and neurovascular proximity were qualitatively integrated due to heterogeneity. Across studies, safe screw placement was often limited to narrow margins (<5 mm), with superior and posterior trajectories demonstrating the highest risk of cortical breach and nerve proximity, alongside substantial interpatient variability in anterior bone corridors.

Collectively, evidence suggests that strategies maximizing fixation strength may narrow an already limited margin of anatomic safety. Although patient-specific instrumentation (PSI), navigation, and 3D-planning improve accuracy and reproducibility, they do not inherently prevent iatrogenic injury without defined safety constraints.

We propose a two-dimensional, anatomy-based “Safe Triangle” for glenoid screw trajectory planning, bounded by the coracoid base anteriorly, suprascapular notch posteriorly, and inferior glenoid rim inferiorly. Within this corridor, inferior and anteroinferior screws may optimize fixation while minimizing iatrogenic injury. This framework provides a reproducible method to balance fixation and safety, with future work needed for prospective validation and integration into surgical planning platforms.

Keywords: Reverse total shoulder arthroplasty, Glenoid baseplate fixation, Screw trajectory, Neurovascular injury, Safe Triangle.

Introduction
Total shoulder arthroplasty represents one of the fastest growing segments of joint replacement surgery, with reverse total shoulder arthroplasty (rTSA) now accounting for the majority of shoulder arthroplasty procedures performed annually in the United States. Over the past decade, rTSA utilization has increased at annual rates ranging from 6% to 15%, with estimates from 2014 to 2019 suggesting that procedure volume nearly doubled (increase of 93%) to 15,398 cases annually in 2019 alone.¹ As rTSA becomes an increasingly common and costly operation, even relatively infrequent complications can translate into significant patient morbidity and healthcare expenditure, underscoring the importance of durable, complication minimizing surgical strategies.

Once reserved primarily for cuff-tear arthropathy in elderly patients, rTSA is now widely used for proximal humeral fractures, avascular necrosis, inflammatory arthropathy, failed anatomic arthroplasty, and complex revision cases.¹ Importantly, the procedure is increasingly being performed in younger and more active patients, extending implant longevity expectations well beyond those of earlier cohorts. As rTSA prevalence expands across a broader demographic spectrum, durable glenoid fixation has emerged as a central determinant of long-term success.

Despite advances in technique, achieving durable glenoid fixation remains a key challenge, particularly in revision settings and in the presence of bone loss. Although aseptic baseplate loosening is relatively uncommon in primary rTSA, its incidence increases substantially in revision settings and in the presence of glenoid bone loss.² To enhance early fixation and minimize micromotion, contemporary surgical practice favors strategies such as inferior baseplate tilt, longer anteroinferior screws, increased screw number, and bicortical purchase. While biomechanically advantageous, these techniques move screws closer to critical neurovascular and cortical structures, including the suprascapular nerve, axillary nerve, coracoid base, and anterior scapular cortex.^3,4 Cadaveric and CT-based studies increasingly demonstrate that the margin for safe screw placement is often only a few millimeters, raising concern that efforts to maximize fixation strength may inadvertently increase the risk of iatrogenic injury.^3,5,6 

At the same time, advances in preoperative 3D planning, navigation, and patient-specific instrumentation (PSI) have improved the accuracy and reproducibility of baseplate positioning and screw trajectories. However, emerging evidence suggests that these technologies frequently optimize fixation without explicitly incorporating neurovascular or cortical safety constraints, at times recommending longer or more aggressive screw paths that approach anatomic danger zones. Despite an expanding literature on rTSA complications, most large reviews continue to focus on acromial and scapular spine fractures, offering limited insight into screw-related neurovascular risk. 

Consequently, the current field lacks standardized, anatomy-based criteria to define safe screw corridors. This review synthesizes biomechanical, anatomic, cadaveric, and clinical evidence to examine the fixation-safety trade-off inherent to rTSA baseplate screw placement and proposes a two-dimensional “Safe Triangle” framework that defines a reproducible anatomic corridor for screw trajectory planning. The proposed Safe Triangle is bounded anteriorly by the coracoid base, posteriorly by the suprascapular notch, and inferiorly by the inferior glenoid rim. Within this corridor, inferior and anteroinferior screws may optimize fixation while minimizing iatrogenic injury. This framework is designed to guide screw trajectory planning, inform implant design, and improve patient safety without compromising fixation durability.

Methods

Protocol
This study was conducted as a focused narrative review to synthesize biomechanical, anatomic, imaging-based, and clinical evidence related to glenoid baseplate screw fixation and safety in reverse total shoulder arthroplasty (rTSA). A narrative methodology was selected due to the heterogeneity of study designs, outcome measures, and reporting across the available literature, which precluded formal quantitative meta-analysis.

The review was guided by structured search and screening principles informed by PRISMA recommendations to enhance transparency and reproducibility. Literature identification, screening, and study selection were performed through a staged review of titles, abstracts, and full texts. Emphasis was placed on rTSA studies evaluating screw trajectory, fixation mechanics, cortical breach, neurovascular proximity, and technology-assisted planning (e.g., navigation and patient-specific instrumentation).

Inclusion/Exclusion Criteria
Studies were included if they met the following criteria: 1) Evaluated reverse total shoulder arthroplasty 2) Examined glenoid baseplate fixation variables, including screw length, trajectory, number, cortical engagement, or baseplate positioning 3) Assessed biomechanical outcomes, anatomic relationships, imaging findings, or clinical complications related to screw placement 4) Were cadaveric, biomechanical, imaging-based (CT or 3D modeling), or clinical studies.

Exclusion criteria included: 1) Studies focused exclusively on anatomic total shoulder arthroplasty (TSA) 2) Non-English language publications 3) Case reports without relevance to screw trajectory or fixation safety 4) Studies lacking primary data on fixation mechanics or neurovascular/cortical risk. Reference lists of included articles were manually reviewed to identify additional relevant studies not captured in the initial database search.

Search Databases
A comprehensive literature search was performed across multiple databases, including PubMed, Embase, Scopus, and Web of Science, to identify relevant studies published between 2005 and 2025. The search timeframe was selected to capture modern rTSA techniques, implant designs, and advancements in imaging, navigation, and patient-specific instrumentation. Seminal studies published before 2005 were also included when necessary to provide historical context or describe foundational anatomic concepts. All identified records were screened for relevance through title and abstract review, followed by full-text evaluation when appropriate.

Search Strategy and Quality Appraisal
Search strategies were developed using combinations of controlled vocabulary terms and keywords related to reverse total shoulder arthroplasty and glenoid fixation. Core search terms included: reverse total shoulder arthroplasty, glenoid fixation, baseplate fixation, baseplate screw, screw trajectory, safe zone, cortical breach, suprascapular nerve, axillary nerve, suprascapular notch, neurovascular risk, iatrogenic risk, navigation, 3D planning, patient-specific instrumentation, and psi.

Boolean operators (AND/OR) were applied to refine search combinations and maximize sensitivity. The initial search yielded 280 studies across all databases. After removal of duplicates, 98 studies remained for title and abstract screening, with 32 studies ultimately included in the final synthesis. Search results from PubMed were supplemented and cross-referenced with additional databases to ensure comprehensive coverage. Title and abstract screening were performed to identify studies meeting inclusion criteria, followed by full-text review for final eligibility determination.

Given the narrative design and heterogeneity of included studies, formal risk-of-bias scoring tools were not systematically applied. Instead, study quality was assessed qualitatively based on study design, methodological rigor, sample size, and relevance to fixation mechanics or anatomic risk. Greater emphasis was placed on higher-level evidence, including biomechanical testing, cadaveric analyses, imaging-based modeling studies, and prospective clinical data.

Results

Biomechanical Rationale for Inferior Fixation
Biomechanical studies demonstrate that current fixation stability in rTSA is improved by inferior baseplate tilt, longer screws, increased screw number, and cortical engagement. Gutierrez et al. demonstrated that a 15° inferior baseplate tilt significantly reduces micromotion and produces the most favorable compressive forces across the bone-implant interface, supporting mild inferior tilt as a fixation-enhancing technique.⁷ Roche et al. showed that both longer screws and greater screw quantity (four-screw constructs) reduces baseplate displacement under cyclic loading and provides superior fixation compared to shorter or fewer screws, independent of baseplate size.⁸ Torkan et al. found that central screws engaging cortical bone markedly decrease micromotion compared with short central fixation or pegged screws, making cortical engagement a key determinant of primary stability.⁹ Collectively, these studies provide a strong biomechanical basis for contemporary fixation techniques, yet they also underscore the importance of understanding how such strategies intersect with the anatomic constraints that define safe screw placement. 

In pursuing maximal cortical purchase, these strategies introduce important anatomic considerations. Hart et al. reported consistent anterior cortical perforation and subscapularis violation when anterior trajectories were maximized. In many cases, the anterior screw tip sat only millimeters from the underlying anterior cortex, emphasizing how minimal error or additional length would lead to full anterior cortical breach.⁶ Complementing these findings, DiStefano et al. mapped scapular cortical thickness and identified substantial variability in optimal bone corridors, revealing that the width and orientation of safe osseous corridors differ markedly among patients.⁴ Taken together, both cadaveric and imaging studies confirm that inferior screw fixation, especially when screws are long or directed too far anteriorly, can significantly increase the risk of anterior cortex perforation near the coracoid base or glenoid notch. These findings highlight the need to balance fixation strength with anatomic safety (Table 1).

Table 1. Key Biomechanical Studies Supporting Inferior Tilt, Screw Length Optimization, and Cortical Engagement for Glenoid Baseplate Fixation in rTSA.
This table summarizes key cadaveric, biomechanical, and CT-based studies evaluating fixation variables such as glenoid baseplate tilt, screw length, screw number, and cortical engagement.

Neurovascular & Cortical Complication Evidence
Cadaveric dissections, postoperative CT analyses, and prospective clinical studies demonstrate that the margin for error in screw placement is often only a few millimeters. Vance et al. showed that 66% of superior screws and 50% of posterior screws contacted the suprascapular nerve, with minimum measured distances of only 2-4 mm.⁵ Leschinger et al. similarly reported frequent proximity of superior and posterior screws to the suprascapular and axillary nerves.¹⁰ Li et al. further demonstrated close anatomic proximity between the suprascapular nerve and the superior/posterior glenoid rim, particularly near the suprascapular notch.¹¹ In some specimens, the distance between the nerve and potential screw paths was only a few millimeters, underscoring the narrow safety margins inherent to rTSA fixation.

CT-based studies further corroborate these risks. Jang et al. reported posterior screw vault penetration rates up to 64% and identified 12% of patients with screws located <5 mm from the suprascapular nerve.¹² Yang et al. identified superior and posterior screw trajectories spanning approximately the 2:00-8:00 positions in reference to the right glenoid clockface as a primary danger zone for suprascapular nerve injury.³ 

Importantly, clinical data increasingly demonstrate that these radiographic and cadaveric risks translate into symptomatic nerve injury. Lopiz et al. prospectively linked out-of-vault screw penetration with EMG confirmed suprascapular neuropathy, and reported that screws breaching the glenoid vault were associated with EMG confirmed suprascapular neuropathy.¹³ Further clinical evidence is provided by Kahan et al., who reported two cases of persistent shoulder pain following primary rTSA in which extensive workups for infection, fracture, and instability were negative. Advanced imaging, nerve studies, and diagnostic injections ultimately localized symptoms to suprascapular nerve irritation caused by malpositioned baseplate screws contacting the nerve.¹⁴ 

Anteriorly directed screws present additional risk due to substantial variability in coracoid morphology and limited anterior bone stock. Bhatia et al. demonstrated marked variability in coracoid thickness, curvature, orientation, and anterior tilt, creating narrow and inconsistent anterior osseous corridors.¹⁵ Terra et al. similarly identified a limited “safety margin” for fixation, particularly in the mid-portion of the coracoid where bicortical fixation risks cortical violation.¹⁶ Together, these findings suggest that anterior screw placement in rTSA must be individualized and support the coracoid base as a critical anterior boundary for safe fixation.

A systematic review by North et al. reported nerve injury rates of approximately 1.3% following primary rTSA and 2.4% following revision rTSA, with the axillary nerve most commonly affected.¹⁷ Although uncommon, cumulative cadaveric, imaging, and clinical evidence suggests that screw-related nerve injury may be underrecognized and underreported. Collectively, this body of literature underscores that the margin for safe screw placement is exceedingly small and highlights the need for a clearly defined, anatomy-based safety zone when optimizing fixation in rTSA.

Role of PSI and 3D Planning
A growing body of research has evaluated how PSI and 3D planning influence baseplate accuracy, screw placement, and potential neurovascular risk. Multiple studies demonstrate that PSI and 3D planning improve baseplate positioning accuracy and reduce variability compared with conventional instrumentation. Heylen et al. reported reduced outliers in glenoid inclination with PSI when compared with conventional implantation, emphasizing the reproducibility of PSI-guided placement.¹⁸Kwak et al. demonstrated fewer cases of screw penetration into risk zones with PSI-guided screw placement.¹⁹ These findings support prior work by Iannotti et al., who showed that 3D preoperative templating improved glenoid component positioning accuracy compared with traditional 2D planning.²⁰ 

Yoon et al. further demonstrated that PSI also improved peripheral screw trajectory precision and reduced screw prominence and glenoid notch involvement compared with conventional instrumentation.²¹ Additional studies suggest that PSI and navigation may reduce neurovascular risk through more precise screw trajectory control. Lee et al. demonstrated minimal deviations in screw length and angulation with PSI and reported no suprascapular notch violation on postoperative CT, although posterior screws were abandoned in more than 90% of cases because of concern for nerve proximity.²² Holzgrefe et al. similarly observed lower rates of scapular notching and reoperation in navigation-assisted rTSA compared with conventional techniques.²³ Venne et al. additionally demonstrated improved screw endpoint accuracy with computer-assisted navigation compared with freehand placement.²⁴ 

Despite improved accuracy and reproducibility, several studies indicate that PSI and navigation systems frequently recommend longer and more aggressive fixation strategies that may approach anatomic danger zones. Lilley et al. reported that 3D planning software was commonly associated with longer screw lengths and altered fixation patterns emphasizing maximal bone purchase.²⁵ Similar findings were reported by Sprowls et al., Hones et al., and Nashikkar et al., who demonstrated increased screw length and more aggressive fixation strategies in navigated or templated rTSA cohorts.^26–28 Collectively, these findings suggest that improved accuracy alone does not eliminate the need for standardized anatomy-based safety constraints (Table 2).

Collectively, these studies demonstrate that while PSI and navigation improve fixation accuracy and reproducibility, they do not inherently prevent cortical or neurovascular injury in the absence of anatomy-based safety constraints. Taken together, the biomechanical and technology-assisted fixation studies summarized in Tables 1 and 2 highlight the need for a standardized, anatomy-based framework to guide safe screw placement in rTSA.

Table 2. Studies Evaluating Patient-Specific Instrumentation (PSI), Navigation, and Three-Dimensional (3D) Planning in rTSA.
This table summarizes clinical, cadaveric, and imaging-based studies assessing the effects of PSI, navigation, and 3D preoperative planning on baseplate positioning accuracy, screw trajectory precision, and fixation characteristics.

Discussion
To resolve the competing demands of strong glenoid baseplate fixation and neurovascular safety, this review proposes a two-dimensional “Safe Triangle” defined by consistent anatomic boundaries that constrain screw trajectory selection in rTSA. Rather than relying on generalized screw orientations or maximal bone purchase strategies, the Safe Triangle concept emphasizes patient-specific anatomy, predictable danger zones, and reproducible imaging landmarks to guide safe and effective fixation. Based on data from multiple studies, the Safe Triangle can be defined by three anatomic boundaries.

Proposed “Safe Triangle” for rTSA Glenoid Screw Placement

Anterior Boundary – Coracoid Base
The anterior boundary is defined by the coracoid base. Cadaveric studies by Bhatia et al. and Terra et al. demonstrate substantial variability in coracoid thickness, curvature, and cortical distribution, creating narrow and inconsistent anterior osseous corridors.^15,16 These findings indicate that even modest overestimation of bone stock, screw length, or trajectory may result in anterior cortical breach or neurovascular injury and support defining the coracoid base as a firm anterior boundary for screw planning.

Posterior Boundary – Suprascapular Notch
The posterior boundary is defined by the suprascapular notch, which represent one of the most clinically relevant danger zones in rTSA fixation. CT-based modeling by Yang et al. identified a posterior danger zone spanning approximately the 2:00–8:00 positions in reference to the right glenoid clockface, where superior and posterior screw trajectories carry an elevated risk of suprascapular nerve injury.³ Bigliani et al. suggests a relative safe zone of the posterior glenoid neck for procedures requiring dissection of the posterior glenoid.²⁹ Cadaveric studies similarly demonstrate that superior and posterior screws frequently contact or pass within only a few millimeters of the nerve.⁵ Mathews et al. further showed substantial variability in posterior screw corridor length, with minimum distances to the scapular notch as short as 15 mm in some specimens, particularly in smaller glenoids and female donors.³⁰ Hart et al. also reported posterior screws averaging 15mm violated the suprascapular neurovascular structures in several specimens, highlighting that a careful posterior screw trajectory and length selection remain important to minimize iatrogenic injury.⁶ Notably, glenoid size did not reliably correlate with the length of the posterior screw corridor, underscoring that larger glenoids do not guarantee safer posterior trajectories. Collectively, these findings reinforce the suprascapular notch as a firm posterior boundary and highlight the need for patient-specific imaging when planning posterior or superior screw placement. 

Inferior Boundary – Inferior Glenoid Rim
The inferior boundary is anchored by the inferior glenoid rim. Kelly et al. established the “12-mm rule,” demonstrating that placing the central guide pin approximately 11.5–12 mm superior to the inferior glenoid rim optimizes glenosphere overhang while preserving adequate bone stock for stable fixation and preventing inferior breach.³¹ Within this inferior constraint, peripheral screws, particularly inferior and anteroinferior screws, can often achieve strong fixation while remaining safely distant from major neurovascular structures. Three-dimensional vault mapping studies further support the inferior pillar and scapular neck region as a favorable fixation corridor when patient anatomy permits.³²

Angular and Depth Parameters Within the Safe Triangle
Within these anatomic boundaries, multiple studies provide quantitative guidance for safe screw angulation and depth. Three-dimensional CT-based analyses demonstrate that inferior and anteroinferior screws angled approximately 15°–30° inferiorly can achieve lengths in the 30–35 mm range while remaining intraosseous in many patients, provided anterior and posterior limits are accounted for.⁴ However, screw length and trajectory are highly sensitive to baseplate orientation. Miyatake et al. showed that baseplate rotation and inferior tilt significantly alter both achievable screw length and proximity to the suprascapular notch, with a 1–7 o’clock baseplate orientation in reference to the right shoulder and neutral tilt maximizing distance from the nerve at the expense of shorter inferior screw length.³³ These findings highlight that safe screw corridors are dynamic spaces shaped by patient anatomy and glenoid implant orientation.

Recommended CT-Based Measurement Protocol
Implementation of the Safe Triangle requires standardized preoperative imaging and measurement. Thin-slice (≤1 mm) CT scans with three-dimensional reconstruction should be obtained to evaluate glenoid vault morphology, coracoid base thickness, and suprascapular notch position. Screw trajectories should be planned virtually, with measurement of anticipated exit points relative to the coracoid base anteriorly, suprascapular notch posteriorly, and inferior glenoid rim inferiorly. Baseplate orientation (tilt and rotation) should be simulated, as changes in implant position substantially influence available safe corridors. Postoperative CT may be used to confirm adherence to planned trajectories and document vault containment or breach, facilitating standardized reporting and correlation with clinical outcomes.

Practical Application of the Safe Triangle
In clinical practice, the Safe Triangle is intended to function as a boundary-based decision framework rather than a rigid geometric template. During freehand rTSA, surgeons may use the inferior glenoid rim as a reproducible reference point, prioritize inferior or anteroinferior screw trajectories, and avoid posterior or superior trajectories unless preoperative imaging confirms an adequate corridor. When PSI or navigation is used, the Safe Triangle may be applied preoperatively by reviewing proposed screw trajectories relative to the coracoid base anteriorly, suprascapular notch posteriorly, and inferior glenoid rim inferiorly. If a planned trajectory violates or approaches these boundaries, surgeons may elect to shorten the screw, alter angulation, rotate the baseplate, or omit the screw entirely in favor of alternative fixation points. 

Contemporary fixation strategies, including inferior baseplate tilt, longer screws, and cortical engagement remain biomechanically sound strategies for enhancing early stability.^7–9 However, cumulative cadaveric, imaging, and clinical evidence demonstrates that aggressive fixation may narrow already limited anatomic safety margins, particularly near the suprascapular notch and coracoid base.^4–6,13,14 The central challenge is therefore not whether aggressive fixation should be pursued, but how it can be achieved safely within patient-specific anatomic constraints.

A recurring theme across the literature is the limited and highly variable nature of safe osseous corridors within the scapula. Cadaveric and CT-based investigations of superior and posterior screw trajectories consistently demonstrate close proximity to the suprascapular nerve, with reported nerve contact or near-contact distances often measuring only a few millimeters.^3,5,29 Importantly, this risk is not purely theoretical. Clinical studies have linked out-of-vault screw penetration to electromyography-confirmed suprascapular neuropathy, and case reports have identified malpositioned baseplate screws as a direct cause of postoperative nerve irritation and persistent pain following rTSA.^13,14 Although the overall incidence of nerve injury after rTSA remains relatively low, systematic reviews report pooled nerve injury rates of approximately 1.3% in primary cases and higher rates in revision procedures, suggesting that these complications may be under-recognized.¹⁷

Anterior screw placement presents an additional and often under-appreciated source of risk. Detailed anatomic analyses of the coracoid process reveal substantial interpatient variability in coracoid thickness, curvature, and orientation, resulting in narrow and unpredictable anterior bone corridors.^15,16 These studies demonstrate that even modest overestimation of available bone stock or screw length may result in anterior cortical breach, placing adjacent neurovascular structures at risk of injury. Together, these findings challenge generalized fixation strategies that do not incorporate individualized anatomic constraints.

The increasing adoption of PSI, navigation, and 3D preoperative planning has improved the accuracy and reproducibility of baseplate positioning and screw trajectories.^18–20 Navigation-assisted techniques have also been associated with longer screw lengths, altered fixation patterns, and improved endpoint accuracy compared with conventional freehand methods.^26–28 However, systematic and comparative studies suggest that these technologies frequently prioritize maximal bone purchase and may recommend longer or more aggressive screw trajectories that approach cortical or neurovascular danger zones unless safety constraints are explicitly incorporated.²⁵ It is worth noting that many baseplate peripheral screws are locking screws and therefore do not require bicortical purchase for adequate fixation; however, this does not completely eliminate the risk of screw proximity to adjacent neurovascular danger zones. Improved accuracy alone does not guarantee improved safety if the planned trajectory itself lies near critical anatomic boundaries.

These findings also have implications for implant manufacturers and developers of preoperative planning and navigation software. Current PSI and navigation platforms primarily optimize fixation parameters such as screw length, trajectory, and bone engagement, but generally lack embedded safeguards related to neurovascular proximity or cortical boundaries. Incorporation of anatomy-based safety constraints, such as automated alerts when planned trajectories approach the suprascapular notch or coracoid base, or color-coded risk mapping within planning software, may enhance the clinical utility of these technologies. Integration of standardized safety frameworks, like the Safe Triangle, into digital planning represents a logical next step toward aligning technological precision with patient safety following clinical validation.

In this context, the proposed Safe Triangle may provide a practical framework to reconcile fixation optimization with neurovascular protection. By defining reproducible anterior, posterior, and inferior boundaries, the Safe Triangle translates anatomic, biomechanical, and imaging data into a practical guide for screw trajectory planning.^31–33 Unlike fixed-distance “safe zone” rules or generalized clockface recommendations, this framework explicitly acknowledges patient-specific variability and the influence of baseplate orientation on available screw corridors.

Several limitations warrant consideration. The Safe Triangle is derived primarily from cadaveric, CT-based, and biomechanical studies rather than large prospective clinical trials and therefore remains a conceptual framework rather than a validated predictive tool. Much of the available anatomic data is derived from elderly, arthritic specimens, or demographically distinct populations which may not fully reflect a diverse younger population or those with preserved bone quality. Additionally, intraoperative variables including bone quality, implant-specific screw options, and surgeon technique may influence final screw placement beyond preoperative planning. While the framework emphasizes avoidance of neurovascular structures, it does not define a threshold for “clinically significant” proximity or breach, which remains poorly standardized across the literature. Application of the Safe Triangle relies on high-quality three-dimensional imaging and planning tools that may not be universally available. Finally, the proposed Safe Triangle lacks validation regarding whether the triangle improves fixation or limits it.

Prospective validation of the Safe Triangle using postoperative CT analysis correlated with clinical outcomes, including nerve function, implant survival, revision rates, screw trajectory, vault containment, and neurovascular outcomes, is necessary to refine and clinically validate this framework. Incorporation of standardized screw trajectory reporting into registries and clinical studies may further clarify the relationship between fixation strategies and iatrogenic risk. Finally, integrating explicit anatomic safety constraints into PSI and navigation software represents an important next step toward translating technological precision into meaningful improvements in patient safety.

Conclusion
Reverse total shoulder arthroplasty continues to evolve toward stronger and more aggressive glenoid fixation strategies aimed at improving implant durability. However, the collective evidence reviewed here demonstrates that maximizing fixation strength frequently shrinks an already limited margin of anatomic safety, placing critical neurovascular and cortical structures at risk. While PSI, navigation, and 3D planning improve accuracy and reproducibility, they do not inherently protect against iatrogenic injury in the absence of standardized safety constraints.

By explicitly defining anterior, posterior, and inferior anatomic boundaries, the Safe Triangle may provide a reproducible framework for safe screw trajectory planning without abandoning established fixation principles. Prospective validation, standardized reporting of screw trajectories, and integration of safety constraints into PSI and navigation platforms are necessary next steps to ensure that advances in fixation technology translate into durable and safe clinical outcomes in reverse total shoulder arthroplasty.

Ethical and Transparency Statements

Submission Declaration and Verification
The work submitted in this manuscript was previously presented as a poster at a medical school research day in May 2026. No part of the manuscript has been previously published in a journal, and the work is not currently under consideration for publication elsewhere.

IRB/Ethics approval or exemption: Not applicable

Consent to participate (or waiver): Not applicable

Consent for publication: Not applicable

Conflict of Interest statement: The authors have no conflicts to disclose.

Funding statement: No funding was received for this work.

Declaration of AI Use:
During the preparation of this work the authors used OpenAI to assist with manuscript clarity, grammar, and improvement of readability. After using this tool, the authors reviewed and edited the content and take full responsibility for the content of the publication.

References:

1. Rupani N, Combescure C, Silman A, Lübbeke A, Rees J. International trends in shoulder replacement: a meta-analysis from 11 public joint registers. ActaO. 2024;95:348-357. doi:10.2340/17453674.2024.40948 
2. Rojas J, Choi K, Joseph J, Srikumaran U, McFarland EG. Aseptic Glenoid Baseplate Loosening After Reverse Total Shoulder Arthroplasty: A Systematic Review and Meta-Analysis. JBJS Rev. 2019;7(5):e7-e7. doi:10.2106/JBJS.RVW.18.00132 
3. Yang Y, Zuo J, Liu T, et al. Glenoid morphology and the safe zone for protecting the suprascapular nerve during baseplate fixation in reverse shoulder arthroplasty. International Orthopaedics (SICOT). 2018;42(3):587-593. doi:10.1007/s00264-017-3646-4 
4. DiStefano JG, Park AY, Nguyen TQD, Diederichs G, Buckley JM, Montgomery WH. Optimal screw placement for base plate fixation in reverse total shoulder arthroplasty. Journal of Shoulder and Elbow Surgery. 2011;20(3):467-476. doi:10.1016/j.jse.2010.06.001 
5. Vance DD, O’Donnell JA, Baldwin EL, et al. Risk of suprascapular nerve injury during glenoid baseplate fixation for reverse total shoulder arthroplasty: a cadaveric study. Journal of Shoulder and Elbow Surgery. 2021;30(3):532-537. doi:10.1016/j.jse.2020.07.008 
6. Hart ND, Clark JC, Wade Krause FR, Kissenberth MJ, Bragg WE, Hawkins RJ. Glenoid screw position in the Encore Reverse Shoulder Prosthesis: an anatomic dissection study of screw relationship to surrounding structures. Journal of Shoulder and Elbow Surgery. 2013;22(6):814-820. doi:10.1016/j.jse.2012.08.013 
7. Gutiérrez S, Greiwe RM, Frankle MA, Siegal S, Lee WE. Biomechanical comparison of component position and hardware failure in the reverse shoulder prosthesis. Journal of Shoulder and Elbow Surgery. 2007;16(3):S9-S12. doi:10.1016/j.jse.2005.11.008 
8. Roche C, DiGeorgio C, Yegres J, et al. Impact of screw length and screw quantity on reverse total shoulder arthroplasty glenoid fixation for 2 different sizes of glenoid baseplates. JSES Open Access. 2019;3(4):296-303. doi:10.1016/j.jses.2019.08.006 
9. Torkan LF, Bryant JT, Bicknell RT, Ploeg HL. Central fixation element type and length affect glenoid baseplate micromotion in reverse shoulder arthroplasty. Journal of Shoulder and Elbow Surgery. 2022;31(7):1385-1392. doi:10.1016/j.jse.2022.01.120 
10. Leschinger T, Hackl M, Buess E, et al. The risk of suprascapular and axillary nerve injury in reverse total shoulder arthroplasty: An anatomic study. Injury. 2017;48(10):2042-2049. doi:10.1016/j.injury.2017.06.024 
11. Li J, Zhou J, Wang D, Li D, Zhang W. Applied anatomical study on suprascapular nerve protection in reverse total shoulder arthroplasty. J Orthop Surg. 2020;15(1):524. doi:10.1186/s13018-020-02061-2 
12. Jang YH, Oh SY, Kim SH. Three-dimensional analysis of baseplate screw penetration in reverse total shoulder arthroplasty: risk of iatrogenic suprascapular neuropathy by screw violation. Journal of Shoulder and Elbow Surgery. 2022;31(5):940-947. doi:10.1016/j.jse.2021.10.024 
13. Lopiz Y, Rodriguez-Gonzalez A, Martín-Albarrán S, et al. Suprascapular nerve injury after reverse total shoulder arthroplasty: correlation with screw out of vault penetration and functional situation: prospective study. Journal of Shoulder and Elbow Surgery. 2023;32(12):2421-2429. doi:10.1016/j.jse.2023.06.026 
14. Kahan JB, Iannotti J, Donohue K. Suprascapular Neuropathy From Malpositioned Baseplate Screws in Primary Reverse Shoulder Arthroplasty: Two Case Reports. JBJS Case Connector. 2020;10(4):e20.00096-e20.00096. doi:10.2106/JBJS.CC.20.00096 
15. Bhatia DN, De Beer JF, Du Toit DF. Coracoid process anatomy: Implications in radiographic imaging and surgery. Clinical Anatomy. 2007;20(7):774-784. doi:10.1002/ca.20525 
16. Terra BB, Ejnisman B, De Figueiredo EA, et al. Anatomic Study of the Coracoid Process: Safety Margin and Practical Implications. Arthroscopy: The Journal of Arthroscopic & Related Surgery. 2013;29(1):25-30. doi:10.1016/j.arthro.2012.06.022 
17. North D, Hones KM, Jenkins P, et al. How common is nerve injury after reverse shoulder arthroplasty? A systematic review. Journal of Shoulder and Elbow Surgery. 2023;32(4):872-884. doi:10.1016/j.jse.2022.10.022 
18. Heylen S, Van Haver A, Vuylsteke K, Declercq G, Verborgt O. Patient-specific instrument guidance of glenoid component implantation reduces inclination variability in total and reverse shoulder arthroplasty. Journal of Shoulder and Elbow Surgery. 2016;25(2):186-192. doi:10.1016/j.jse.2015.07.024 
19. Kwak JM, Jeon IH, Kim H, Choi S, Lee H, Koh KH. Patient-specific instrumentation improves the reproducibility of preoperative planning for the positioning of baseplate components with reverse total shoulder arthroplasty: a comparative clinical study in 39 patients. Journal of Shoulder and Elbow Surgery. 2022;31(7):1488-1498. doi:10.1016/j.jse.2021.12.012 
20. Iannotti JP, Weiner S, Rodriguez E, et al. Three-Dimensional Imaging and Templating Improve Glenoid Implant Positioning. The Journal of Bone and Joint Surgery. 2015;97(8):651-658. doi:10.2106/JBJS.N.00493 
21. Yoon JP, Kim MS, Kim HT, et al. Effect of the patient‐specific instrumentation guidance on the accurate positioning of the baseplate and screws in reverse total shoulder arthroplasty: Multicenter comparative study. Knee surg sports traumatol arthrosc. 2025;33(10):3715-3727. doi:10.1002/ksa.12781 
22. Lee W, Yu W, Lee H, Kim GB, Jeon I, Koh KH. Evaluation of the Baseplate Position and Screws in Reverse Total Shoulder Arthroplasty Using 3D Printed Patient‐Specific Instrumentation. Journal Orthopaedic Research. 2025;43(10):1695-1704. doi:10.1002/jor.70023 
23. Holzgrefe RE, Hao KA, Panther EJ, et al. Early clinical outcomes following navigation-assisted baseplate fixation in reverse total shoulder arthroplasty: a matched cohort study. Journal of Shoulder and Elbow Surgery. 2023;32(2):302-309. doi:10.1016/j.jse.2022.07.007 
24. Venne G, Rasquinha BJ, Pichora D, Ellis RE, Bicknell R. Comparing conventional and computer-assisted surgery baseplate and screw placement in reverse shoulder arthroplasty. Journal of Shoulder and Elbow Surgery. 2015;24(7):1112-1119. doi:10.1016/j.jse.2014.10.012 
25. Lilley BM, Lachance A, Peebles AM, et al. What is the deviation in 3D preoperative planning software? A systematic review of concordance between plan and actual implant in reverse total shoulder arthroplasty. Journal of Shoulder and Elbow Surgery. 2022;31(5):1073-1082. doi:10.1016/j.jse.2021.12.006 
26. Sprowls GR, Wilson CD, Stewart W, et al. Intraoperative navigation and preoperative templating software are associated with increased glenoid baseplate screw length and use of augmented baseplates in reverse total shoulder arthroplasty. JSES International. 2021;5(1):102-108. doi:10.1016/j.jseint.2020.09.003 
27. Hones KM, King JJ, Schoch BS, Struk AM, Farmer KW, Wright TW. The in vivo impact of computer navigation on screw number and length in reverse total shoulder arthroplasty. Journal of Shoulder and Elbow Surgery. 2021;30(10):e629-e635. doi:10.1016/j.jse.2021.01.017 
28. Nashikkar PS, Scholes CJ, Haber MD. Role of intraoperative navigation in the fixation of the glenoid component in reverse total shoulder arthroplasty: a clinical case-control study. Journal of Shoulder and Elbow Surgery. 2019;28(9):1685-1691. doi:10.1016/j.jse.2019.03.013 
29. Bigliani LU, Dalsey RM, McCann PD, April EW. An anatomical study of the suprascapular nerve. Arthroscopy. 1990;6(4):301-305. doi:10.1016/0749-8063(90)90060-q
30. Mathews S, Burkhard M, Serrano N, et al. Glenoid morphology in light of anatomical and reverse total shoulder arthroplasty: a dissection- and 3D-CT-based study in male and female body donors. BMC Musculoskelet Disord. 2017;18(1):9. doi:10.1186/s12891-016-1373-4 
31. Kelly JD, Humphrey CS, Norris TR. Optimizing glenosphere position and fixation in reverse shoulder arthroplasty, Part One: The twelve-mm rule. Journal of Shoulder and Elbow Surgery. 2008;17(4):589-594. doi:10.1016/j.jse.2007.08.013 
32. Ghobrial PG, Burra R, Evans DA, Chudik SC. Exploring Alternative Sites for Glenoid Component Fixation Through Three-Dimensional Digitization of the Glenoid Vault: An Anatomic Analysis. JAAOS Glob Res Rev. 2020;4(12):e20.00199-9. doi:10.5435/JAAOSGlobal-D-20-00199 
33. Miyatake K, Fukuta S, Tonogai I, et al. Influence of the glenoid baseplate position on the direction and length of the superior and inferior locking screws. J Med Invest. 2022;69(3.4):185-190. doi:10.2152/jmi.69.185