1 Introduction

Acromioclavicular joint dislocation accounts for 9% to 12% of shoulder traumas ^1–3, which often leads to compromised shoulder function that impedes daily activities and work performance. Examples of modern surgical techniques include clavicle hook plates, suspensory plates, and suture anchors ^4–6. Because of its tissue-friendly design, superior biomechanical compatibility, minimally invasive nature, and preservation of physiological joint micromotion, the single-strap cortical button method has garnered interest in clinical practice.⁷ This approach relieves the patient of the burden of later hardware removal procedures and permits early recovery and shorter hospital stays. Osteolysis, pain, stiffness, reduction loss, and post-traumatic arthritis are still worrisome side effects, with reduction loss being particularly common.^8,9 According to research, the exact location of the plate has a major impact on decrease loss,^10,11 and there is evidence that centering the plate at the coracoid base may reduce this risk.^12,13

Although the coracoid fixation site has been optimized, the best clavicular fixation location for single-strap plates is still up for debate, which has a substantial impact on postoperative outcomes. In order to systematically evaluate the biomechanical features and related risks of each clavicular fixation site, this study uses finite element analysis to replicate the biomechanical responses at different sites in conjunction with clinical follow-up data (such as reduction stability, complication rates, and functional scores). The goal is to identify the best clavicular fixation site while providing evidence-based suggestions to improve surgical readiness, reduce complications, and advance the accuracy of acromioclavicular joint dislocation treatment. Finite element analysis has been used in orthopedic biomechanics for more than a decade. Its conclusion shows that, while being primarily exploratory research in the beginning, this approach has developed into a crucial tool for assessing bone and implant stress and improving design.¹⁴

2 Methods

2.1

2.1 Finite element analysis

2.1.1

2.1.1 Model construction

A single healthy 25-year-old East Asian male volunteer (height 180 cm, weight 75 kg, BMI 23.15) with no scapular or clavicular abnormalities was used to create a pilot finite-element model. Future research must establish a larger-scale morphological database due to the anatomical variability in clavicular length, curvature, and bone mineral density across populations. Thin-slice (1 mm) CT images of the right shoulder were performed following ethical approval and informed permission, as illustrated in Fig. 1. A 3D model of the acromioclavicular joint was then created by importing the DICOM data into Mimics software, as depicted in Fig. 2. To create a thorough skeletal model, the model underwent solid modeling in SolidWorks 2018 and mesh optimization using Geomagic Wrap 2017. Next, a single-strap plate fixation model was created, as shown in Fig. 3, using a 3 mm titanium alloy strap and a 3.75 mm bone tunnel in the distal clavicle.^15,16 To simulate physiological conditions, a 90 N pretension was used.¹⁷

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Fig. 1 Reconstructing the 3D model of the acromioclavicular joint involves: (a) reconstructing the humeral head from CT images, (b) generating the scapular model using segmentation algorithms, (c) deriving the clavicular model from high-resolution CT scans, and (d) creating an integrated osseous model of the entire glenohumeral joint for finite-element analysis.

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Fig. 2 Three-dimensional view of the acromioclavicular joint model: (a) forward view; (b) rear view; (c) side view.

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Fig. 3 Single-strap cortical button fixation models of acromioclavicular joint dislocation at positions (a) 5 mm, (b) 10 mm, (c) 15 mm, (d) 20 mm, (e) 25 mm, (f) 30 mm, (g) 35 mm, (h) 40 mm, (i) 45 mm, (j) 50 mm.

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2.1.2

2.1.2 Material properties

After importing the 3D shoulder joint model into Ansys Workbench 16.0 for finite element analysis, the material characteristics of each component—including cortical bone, cancellous bone, plate, and sling—were ascertained. The plate was represented by a titanium alloy, and the bones were thought of as homogenous, strong materials. Conversely, the polyethylene strap was represented as a uniform elastic material to simplify the investigation and boost computing efficiency. Material properties such as Young's modulus and Poisson's ratio were determined by prior research ^18,19 and are summarized in Table 1. To replicate a Rockwood Type III acromioclavicular joint dislocation treated with a single-strap cortical button plate, the acromioclavicular and coracoclavicular ligaments were eliminated from the model.

Table 1 Material properties of the finite element model.

Material	Young's modulus (MPa)	Poisson's ratio
Cortical bone	13400	0.30
Cancellous bone	1370	0.31
Loop plate (titanium alloy)	118000	0.30
Loop tape (polyethylene)	900	0.38

2.1.3

2.1.3 Loading and boundary conditions

A 0.5 kg cup was held with forces of 1.5 N in the X direction, 14.2 N in the Y direction, and 4.2 N in the Z direction to simulate loading ^20–23. Boundary conditions included completely restricting the acromion and anchoring the proximal clavicle (rotation free, displacement confined). A coefficient of 0.1 was used to describe the bone-plate contact as frictional.²⁴

2.1.4

2.1.4 Analysis sites

In relation to the conoid ligament's insertion point (42.68 ± 6.34 mm), ten equally spaced locations, 5 mm apart, were marked along the clavicle, ranging from 0 to 50 mm from the acromioclavicular (AC) joint surface.²⁵ By assessing the AC-CC joint distances and examining stress distribution patterns, the study sought to determine the ideal point.

3 Results

3.2

3.2 Clinical efficacy evaluation

3.2.1

3.2.1 Surgical overview

All patients underwent surgery without complications. The surgical parameters comprised operation duration (mean ± SD, 68.5 ± 12.3 min), intraoperative blood loss (mean ± SD, 85.6 ± 20.4 mL), incision length (mean ± SD, 4.2 ± 0.5 cm), and hospitalization duration (mean ± SD, 5.3 ± 1.2 days).

3.2.2

3.2.2 Imaging assessment

Both the coracoclavicular (CC) and acromioclavicular (AC) spaces showed dynamic shifts, as shown in Fig. 4. Both gaps showed a notable decrease at two days after surgery when compared to preoperative measures. Both spaces showed a small increase from the 2-day postoperative levels at 6 months postoperatively, with the AC space showing a more noticeable increase than the CC space. Nevertheless, both spaces were still below preoperative levels. Both spaces grew once more after 12 months after surgery, but they were still much smaller than they had been before. At 12 months after surgery, the inter-rater reliability was as follows: AC distance ICC: 0.88 (Rater 1: 10.12 ± 0.70 mm, Rater 2: 10.18 ± 0.72 mm) and CC distance ICC: 0.82 (Measurer 1: 11.50 ± 0.95 mm, Measurer 2: 11.62 ± 0.98 mm). At 12 months after surgery, the AC distance in Rockwood Type III (n = 10) patients (10.0 ± 0.7 mm) was considerably larger than that in Type II (n = 7) patients (10.2 ± 0.7 mm, P > 0.05), according to subgroup analysis based on Rockwood classification. Comparably, Type III's CC distance (11.4 ± 0.9 mm) was higher than Type II's (11.6 ± 1.0 mm, P > 0.05). According to these results, the 40 mm fixing site is suitable for a range of dislocations. Serial radiographs of a representative case illustrating the postoperative reduction and maintenance at different time points are shown in Fig. 11.

3.2.3

3.2.3 Functional assessment and VAS scoring

In comparison to preoperative evaluations, all postoperative tests showed a considerable improvement in Constant-Murley ratings, as illustrated in Fig. 6, which measure pain, daily activities, muscular strength, and joint range of motion. Pain levels significantly decreased two days after surgery, significantly decreased at one month, nearly returned to baseline at six months, and entirely recovered to baseline by twelve months, according to Visual Analog Scale (VAS) scores, as depicted in Fig. 5. Functional recovery of a representative patient at 12 months postoperatively, including abduction, forward elevation, forward flexion, and internal rotation, is demonstrated in Fig. 12.

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Fig. 4 Radiographic AC and CC distances measured at baseline and postoperative follow-up time points (mm). Values are presented as mean ± SD. AC, acromioclavicular; CC, coracoclavicular. ∗p < 0.001 vs. preoperative measurements (paired t-test). ICC >0.80 indicates excellent inter-observer reliability.

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Fig. 5 Temporal changes in visual analogue scale (VAS) pain scores (mean ± SD, n = 17). Values are presented as mean ± SD (n = 17). ∗p < 0.001 vs. preoperative baseline (paired t-test).

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Fig. 6 Temporal changes in Constant-Murley functional scores(A) Pain, (B) Activities of daily living, (C) Strength, (D) Range of motion, and (E) Total score. Values are presented as mean ± SD. ∗p < 0.001 vs. preoperative baseline (paired t-test).

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Fig. 7 Analysis of Von Mises stress distribution in the cortical bone of the clavicle under physiological axial loading was conducted utilizing a CT-based finite-element (FE) model with material properties (E = 17 GPa, ν = 0.3). The peak cortical stresses observed at consecutive fixation sites along the distal clavicle were as follows: (a) 5 mm – 114.0 MPa; (b) 10 mm – 55.26 MPa; (c) 15 mm – 30.35 MPa; (d) 20 mm – 210 MPa; (e) 25 mm – 179.4 MPa; (f) 30 mm – 82.42MPa; (g) 35 mm – 65.68MPa; (h) 40 mm – 48.76 MPa; (i) 45 mm – 74.81MPa; (j) 50 mm – 82.62 MPa.

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Fig. 8 Acromioclavicular distances at positions (a) 5 mm: 6.25 mm, (b) 10 mm: 7.40 mm, (c) 15 mm: 7.04 mm, (d) 20 mm: 21.09 mm, (e) 25 mm: 6.69 mm, (f) 30 mm: 7.75 mm, (g) 35 mm: 7.95 mm, (h) 40 mm: 8.39 mm, (i) 45 mm: 9.05 mm, (j) 50 mm: 8.03 mm.

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Fig. 9 Coracoclavicular distances at positions (a) 5 mm: 4.33 mm, (b) 10 mm: 2.83 mm, (c) 15 mm: 3.05 mm, (d) 20 mm: 0 mm, (e) 25 mm: 3.03 mm, (f) 30 mm: 3.52 mm, (g) 35 mm: 4.35 mm, (h) 40 mm: 3.89 mm, (i) 45 mm: 4.15 mm, (j) 50 mm: 3.50 mm.

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Fig. 10 Von Mises stress distribution in the trabecular compartment of the human clavicle under physiological axial loading. Peak stresses at 5 mm intervals from the lateral end: (a) 30.89 MPa, (b) 56.99 MPa, (c) 27.01 MPa, (d) 49.40 MPa, (e) 119.46 MPa, (f) 39.76 MPa, (g) 54.29 MPa, (h) 35.78 MPa, (i) 78.99 MPa, (j) 33.77 MPa. CT-based FE model, E = 0.1 GPa, ν = 0.3.

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Fig. 11 (a) Pre-operative anteroposterior radiograph: acromioclavicular distance increased; lateral clavicle elevated above acromion.(b) One-month post-operative radiograph: clavicle anatomically reduced; cortical button fixation positioned.(c) Six-month post-operative radiograph: joint alignment maintained; implant stable; no osteoarthritic changes.(d) Twelve-month post-operative radiograph: joint space preserved; implant intact; no subluxation or degenerative signs.

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Fig. 12 At 12 months postoperatively, shoulder motion recovered to near-normal levels: (a) abduction, (b) forward elevation, (c) forward flexion, (d) internal rotation.

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3.2.4

3.2.4 Postoperative complications

No loss of reduction, infection, re-dislocation or implant failure was observed within 12 months, giving a complication rate of 0/17 (Clopper-Pearson 95% CI 0–19.5%).

4 Discussion

Achieving and preserving anatomical reduction is the major goal of treating acromioclavicular joint dislocation.²⁶ Despite the widespread usage of the Single-Strap Cortical Button Fixation procedure, the best fixation site at the clavicle end is still up for debate, which impacts the predictability of the surgical outcome. The ideal fixation location for the clavicular end of the Single-Strap Cortical Button Fixation is 40 mm from the acromioclavicular joint surface. This study provided adequate biomechanical and clinical evidence, confirming this finding for the first time using finite element analysis and clinical research.

According to finite element study, the 40 mm insertion site ensures stable reduction with anterior displacement AC = 8.39 mm and lateral displacement CC = 3.89 mm while reducing cortical bone stress to 48.76 MPa and maintaining safe cancellous bone stress at 35.78 MPa under physiological loads. The chances of postoperative fractures and tunnel enlargement are reduced in this ideal site, indicating substantial clinical significance. This biomechanically superior location corresponds with the mechanical center of the clavicle, in contrast to the conventional 50 mm conoid ligament insertion position. The 15 mm location, on the other hand, exhibits lower cortical and cancellous bone stresses at 30.35 MPa and 27.01 MPa, respectively; however, shear resistance is compromised by its anterior position, short lever arm, and thin cortical bone.^27–30 Due to very high cortical bone stress (179.4 MPa) and severe cancellous bone stress (119.46 MPa), the 25 mm location is obviously not recommended. The 30 mm site shows noticeably higher cortical bone stress (82.42 MPa) and cancellous bone stress (39.76 MPa) than the 40 mm site, despite its frequent clinical use. Furthermore, its low lateral displacement (CC = 3.52 mm) does not provide soft tissues with enough defense. An excessive lever arm length is the cause of the increased displacement (AC = 9.05 mm) at the 45 mm site, which raises the cancellous bone stress to 78.99 MPa and presents a long-term instability risk. The 5 mm and 20 mm positions were eliminated because of abrupt changes in load or model failure. To sum up, the 40 mm site is in the "mechanical equilibrium zone." It strikes a balance between mechanical functionality and anatomical reconstruction, offering strong fixation support with a cortical bone thickness of 4.1 mm.

This study established basic benefits and clinical efficacy validation. The Constant-Murley score increased from 30.82 to 93.48 and the Visual Analog Scale (VAS) score decreased from 5.94 to 0.18 at 12 months after surgery, indicating a significant functional recovery for patients repaired at the optimal 40 mm location. These favorable results applied to all Rockwood II–III fracture subtypes. Crucially, there were no serious problems like infection, internal fixation failure, or re-dislocation, and the study's loss rate of reduction was far lower than what has been documented in the literature.³¹ The extraordinary initial stability offered by the improved attachment site is responsible for the superior results seen in this investigation. In addition to preventing fixation failure from micro-movements and promoting soft tissue healing in a low-tension setting, its stability successfully withstands early rehabilitation loads. Imaging also indicated that the decline was maintained throughout time. The stability evolution mechanism is further clarified by dynamic variations in the acromioclavicular (AC) and coracoclavicular (CC) distance. Mechanical stabilization is indicated in the early postoperative phase by a notable reduction in distance. Physiological relaxation during the scar tissue rebuilding phase is then represented by a rebound in distance at 6 months, with a more noticeable increase in AC than CC. Lastly, a three-stage pattern of "mechanical fixation-transitional adjustment-biological stability" is delineated by keeping a distance below preoperative values at 12 months, offering new insights into the biomechanical adaptation of acromioclavicular joint restoration.

There are various limitations to this study: First off, only 17 people made up the limited sample size in this single-center study. The small sample size may limit the generalizability of results and the assessment of consequences, even if considerable efficacy was consistently achieved across all injury categories (II-III). Second, in order to increase computational efficiency, the finite element models used several simplifications (such as bone homogeneity and an insufficient depiction of dynamic soft tissue constraints around the shoulder). Even though these simplifications might differ from actual biomechanical settings, they successfully accomplish the study's goal of contrasting site benefits. The effects of postoperative micromotion are, nevertheless, underestimated in the model. Long-term studies have demonstrated that micro-motion between the Strap and bone tunnel plays a significant role in the tunnel's enlargement and subsequent reduction in loss.³² This wear process was not taken into consideration by the current model. Research on cadaver biomechanics is likewise limited. As evidenced by Klabklay et al.'s ³³ finding that the absence of deltoid-trapezius fascia repair led to mechanical strength that was weaker than what is observed clinically, no model can accurately replicate the actual postoperative biomechanical stability mechanism. The 40 mm optimal site was established from a single healthy volunteer and does not adequately account for individual variability in clavicle length, shape, and bone density. Without taking into account individual variations like clavicle length, form, and bone density, the 40 mm optimum locus is a universal parameter obtained from the data of a single healthy volunteer. While this study supported the absolute distance's short-term effectiveness, Senel et al.³⁴ The reduction loss rate can be further decreased by adopting the relative placement approach, which uses the ratio of the clavicle tunnel distance to the clavicle length (ID/CL) to more individually conform to the anatomical insertion point variation of the coracoclavicular ligament. Additionally, this relative ratio verifies that the study's ideal fixation position falls within its safe ratio range. This proportion-based customized placement approach could be a crucial path for more accurate therapy in the future. This study did not establish any other stable locations or traditional treatment control groups, therefore it was unable to directly compare the advantages and disadvantages of alternative ways. In the lack of a control group comparison, it was difficult to prove the absolute benefits of optimizing the fixed locations, notwithstanding the positive outcomes. Holzer et al. ³⁵ demonstrated in the full-shoulder model that the order of surgical steps significantly affects the quality of the final reduction, suggesting that the interaction of the intraoperative technical sequence—a variable not included in this study—may have an effect on the optimization of the fixed position. For evaluating long-term efficacy and potential issues, the 12-month surgical follow-up time might not be sufficient.

Furthermore, the current debate over whether surgery is required for acute Rockwood type II–III acromioclavicular dislocations was not addressed in this study. Conflicting mid-term results have been reported by recent randomized controlled trials, with some demonstrating that surgery is not clearly superior to conservative care in terms of pain, function, or quality of life. The indications for surgery itself were not assessed because our analysis was limited to optimizing tunnel location after the decision for operational stabilization had been taken. In order to determine which patients actually benefit from any surgical method, including the 40 mm proximal fixation site suggested here, future research should take these developing data into account and ideally include a control arm of non-operative treatment.

To guarantee the validity and generalizability of results, future research should perform thorough validation across several centers. To improve our comprehension of the intricate relationships behind clavicle fractures, biomechanical models must be further developed. To further customize treatment plans, researchers should also consider a variety of anatomical and physiological parameters, such as variations in clavicle length, morphology, and bone density. Investigating cutting-edge technology, like as 3D-printed guides, can transform customized fixation techniques, resulting in more accurate and customized treatments while lowering the risk of problems. In order to provide more thorough and reliable clinical recommendations, it is essential to extend the follow-up evaluation period in order to examine the long-term results and detect any potential issues that may develop over time.

5 Conclusion

This study found that the ideal tunnel position for single-strap cortical-button fixation in acromioclavicular dislocation is 40 mm proximal to the joint surface, producing satisfactory reduction, functional recovery, and no observed complications. It did this by combining finite-element analysis with short-term (12-month) clinical validation. Confirming long-term stability and complication rates currently requires larger, multicenter cohorts with longer follow-up.

Author contributions

SH: Data curation, Investigation, Writing – original draft. YP: Formal Analysis, Writing – original draft. ZJ: Resources, Writing – original draft. YL: Resources, Writing – original draft. WD: Data curation, Writing – original draft. GX: Data curation, Writing – original draft. FR: Data curation, Writing – original draft. QW: Funding acquisition, Project administration, Resources, Supervision, Writing – review and editing.

Ethics approval and informed consent

This study was conducted in accordance with the Declaration of Helsinki and approved by the Institutional Review Board (IRB) of General Hospital of Ningxia Medical University (Approval No. KYLL-2025-2656). Written informed consent was obtained from all participants (or their guardians for participants under the age of 18 or those lacking capacity to consent) prior to enrollment. All participants were informed of the study purpose, procedures, potential risks, and benefits, as well as their right to withdraw at any time without affecting their medical care.

Data availability statement

The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding authors.

Funding

The authors declare that financial support was received for the research and/or publication of this article. Natural Science Foundation of Ningxia Province, 2023AAC02064.