Risk Factors and Prediction of Medial Canthal Angle Regression after Modified V-Shaped Concealed Flap Canthoplasty

Summary

This protocol aims to standardize intraoperative measurement of folding parameters of the anterior limb of the medial canthal tendon during modified V-shaped concealed flap canthoplasty and to assess their association with postoperative medial canthal angle regression.

Abstract

This study aimed to define a reproducible intraoperative measurement protocol for modified V-shaped concealed flap canthoplasty and to characterize the association between quantitative folding parameters of the anterior limb of the medial canthal tendon (MCT) and postoperative medial canthal angle regression. In this prospective cohort, 331 consecutive patients undergoing the procedure were enrolled. Intraoperative folding length (L), folding angle (θ), fold-point position (P), fixation plane, and suture tension grade were recorded using a standardized protocol. Medial canthal angle regression at 6 months was assessed from standardized photographs relative to the 1-week postoperative baseline. Multivariable logistic regression was used to explore associations between intraoperative measurements and postoperative regression, and the resulting model was internally validated with bootstrap resampling. The 6-month incidence of medial canthal angle regression was 24.6%. Greater L, θ, and P were protective, whereas tendon/soft-tissue fixation, low suture tension, greater epicanthal fold severity, thick soft tissue, larger preoperative medial canthal angle, and longer operative time were associated with increased risk. The apparent area under the receiver operating characteristic (ROC) curve (AUC) of the internally validated model was 0.85 (95% confidence interval [CI], 0.81–0.89), and the optimism-corrected AUC was 0.81 (95% CI, 0.77–0.85). These findings support the feasibility of intraoperative quantitative assessment but should be interpreted as internally validated single-center evidence requiring external validation before broader clinical application.

Introduction

Epicanthal folds are common medial canthal anatomical features in East Asian populations and represent an important consideration in Asian eyelid surgery¹. Previous studies have reported a high prevalence of epicanthal folds among East Asian populations and evaluated postoperative cosmetic outcomes following epicanthoplasty². The fold can visually shorten the horizontal palpebral fissure, widen the perceived intercanthal distance, and obscure the lacrimal caruncle or medial canthal contour. Modified V-shaped concealed flap canthoplasty is performed to improve medial canthal morphology, but postoperative medial canthal angle regression may reduce the durability of the initial correction³. Because even small changes in medial canthal position can affect aesthetic satisfaction, reproducible measurement of postoperative regression and intraoperative factors associated with that regression is clinically important.

Previous studies of epicanthoplasty have mainly addressed flap design, scar concealment, and postoperative scar optimization^4,5. These refinements are important, but they do not fully explain why some patients experience delayed medial canthal angle regression during scar remodeling. Recent ophthalmic outcome studies and evidence syntheses have further emphasized the importance of standardized imaging, quantitative endpoints, and reproducible risk stratification methods for evaluating surgical stability and predicting postoperative outcomes⁶. Recent studies in ophthalmic outcome assessment have highlighted the importance of standardized imaging, quantitative measurements, and reproducible evaluation frameworks for improving the objectivity and comparability of surgical outcome assessment^7,8,9. These approaches provide a methodological basis for developing more reproducible surgical evaluation protocols. In this context, the anterior limb of the MCT may be considered a deep structural support that can be measured and modified intraoperatively, rather than an implicit technical detail left solely to surgeon's experience^10,11.

The modified V-shaped concealed flap technique combines hidden incision placement with reconstruction of medial canthal curvature, consistent with recent anatomical approaches aiming to restore medial canthal configuration¹². The principal methodological contribution of the present protocol is the intraoperative measurement sequence used to quantify deep support, including identification of the anterior MCT limb, standardized traction, measurement of L, θ, and P, and documentation of fixation plane and suture tension (Figure 1). The predictive model represents a secondary application of these reproducible measurements rather than the primary focus of the protocol. Standardizing these steps may reduce inter-surgeon variability and allow postoperative regression to be analyzed against measurable intraoperative parameters¹³.

Therefore, this study was designed to standardize the intraoperative quantification of anterior MCT limb folding during modified V-shaped concealed flap canthoplasty and to examine how these measurements relate to 6-month medial canthal angle regression. The protocol links procedural steps, measurement definitions, postoperative image-based outcome assessment, and internal model validation, providing a structured workflow that can be reproduced and further evaluated in different surgical settings.

Protocol

Perform all human-participant procedures in accordance with the Declaration of Helsinki and institutional requirements. Written informed consent was obtained from all participants before enrollment. Separate written consent for the publication of clinical photographs was obtained from participants whose images are included in the manuscript. Images were cropped or limited to the periocular region whenever possible to minimize patient identifiability. The study was approved by the Ethics Committee of Tianjin Medical University Eye Hospital (approval No. 2025KY-35). 

1. Participant selection and preoperative evaluation

1. Screen consecutive adult patients (18 years or older) who seek modified V-shaped concealed flap canthoplasty for epicanthal fold correction or medial canthal contour improvement. Eligible cases may include unilateral or bilateral procedures and mild, moderate, or severe epicanthal fold severity when standardized photography and 6-month follow-up can be completed.
2. Exclude patients presenting a history of surgery affecting medial orbit anatomy.
3. Exclude cases with severe congenital or post-traumatic deformities requiring simultaneous complex medial orbital reconstruction.
4. Exclude individuals with active infection, significant inflammatory skin conditions, or a confirmed abnormal keloid tendency.
5. Capture standardized anterior and local close-up photographs using the same digital camera system equipped with a fixed lens, a fixed patient-to-camera distance of approximately 1.0 m, diffuse frontal lighting, a neutral background, and the camera lens aligned horizontally with the interpupillary line. Record the camera model, focal length or zoom setting, and distance so that the same setup can be repeated during follow-up.
6. Maintain uniform light intensity and exposure settings. Ask the patient to sit upright with the Frankfort horizontal plane level, the head centered, both eyes naturally open, and facial expression relaxed. Use the medial canthal point, pupil center, medial orbital rim reference, and palpebral margin as repeatable landmarks for image measurement.
7. Instruct the patient to maintain a centered head position and a level gaze.
8. Ask the patient to keep their eyes naturally open and facial expressions relaxed to minimize fluctuations in soft tissue tension.

2. Preoperative marking and surgical preparation 

NOTE: Figure 2 shows the step-by-step intraoperative photographs of the procedure, and Figure 3 shows the intraoperative quantification workflow for anterior MCT limb folding parameters and medial canthal angle measurement.

1. Mark the intended new medial canthal point, the existing medial canthal point, and the planned concealed V-shaped incision limbs with the patient seated upright. 
2. Design each limb of the concealed V-shaped incision with a length of approximately 5–8 mm and an included angle of approximately 60–90°. Adjust limb length and angle according to epicanthal fold severity, tissue redundancy, and the intended medial displacement of the new medial canthal point. 
3. Record the selected incision dimensions before incision and reproduce the same parameters on the contralateral side in bilateral procedures. 
4. Confirm left-right symmetry using the facial midline, interpupillary line, palpebral fissure length, and intercanthal distance before local anesthesia or incision.
5. Position the necessary reference points to establish the baseline for subsequent symmetry assessment and medial canthal angle regression measurement (Figure 2A).
6. Verify the anatomical landmarks and confirm the designated fold axis (Figure 2B).
7. Ensure the planned folding direction and fixation points align with predefined anatomical references.

3. Surgical incision and ligament exposure

1. Create the incision strictly along the preoperative design line.
2. Dissect through the subcutaneous and orbicularis oculi layers in the premarked plane, staying superficial to the lacrimal drainage structures and avoiding unnecessary medial orbital trauma. 
3. Identify the anterior MCT limb as a firm fibrous band extending from the medial canthal soft tissue toward the medial orbital rim/periosteal region, while keeping the lacrimal canaliculus, medial canthal vessels, and caruncular tissue outside the dissection field.
   CAUTION: Carefully identify and preserve the lacrimal canaliculus, medial canthal vessels, and adjacent soft tissues throughout the dissection. Excessively deep or misdirected dissection may result in injury to the lacrimal drainage system, increased bleeding, postoperative scarring, or distortion of the medial canthal contour.
4. Identify the anterior branch of the medial canthal tendon within the medial orbital tissues (Figure 1A).
5. Expose the anterior fibrous band clearly prior to initiating the folding procedure (Figure 2D).

4. Intraoperative folding and parameter quantification

1. Apply standardized medial-to-lateral traction with fine forceps or a skin hook until the medial fold is flattened without visible blanching, tearing, or eyelid-margin distortion. Because direct force measurement is not routinely available during surgery, define acceptable traction operationally as the minimum force required to achieve complete fold flattening while maintaining stable anatomical landmarks.
2. Execute the anterior branch folding maneuver along the confirmed axis (Figure 2E).
3. Define folding length (L, mm) as the linear distance from the folding origin on the exposed anterior MCT limb to the intended fixation point along the folded vector; measure it with a digital surgical caliper to the nearest 0.1 mm. 
4. Define fold-point position (P, mm) as the distance from the medial canthal point reference to the fold apex or fixation-related turning point, measured intraoperatively using the same digital surgical caliper. 
5. Measure folding angle (θ, degrees) using calibrated image analysis based on an intraoperative photograph obtained perpendicular to the surgical field, with the angle defined between the folded MCT limb vector and the medial orbital rim/fixation reference line. 
   NOTE: The measurement geometry and anatomical reference points are shown in Figure 1C and Figure 3C,D. Repeat each measurement three times and use the mean value for analysis.

5. Fixation and wound closure

1. Fix the folded anterior MCT limb using 6-0 nylon or 6-0 polypropylene on a fine ophthalmic needle. Place one or two interrupted fixation sutures according to tissue thickness and intraoperative stability, anchoring the fold to the periosteal/deep fascial plane when safely accessible or to the tendon/soft-tissue plane when deep anchoring is not feasible. 
2. Tie knots away from the skin edge and verify that the medial canthal contour is stable without excessive blanching, eyelid-margin distortion, or canalicular traction.
3. Record the specific fixation plane utilized during the procedure.
4. Grade suture tension before closure using predefined categories: low tension indicates visible recoil or unstable fold apposition after knot tying; moderate tension indicates stable apposition with preserved capillary color and no eyelid-margin distortion; high tension indicates firm fixation accompanied by tissue blanching, fold compression, or contour distortion. 
   NOTE: For modeling, moderate and high tension were grouped as moderate/high tension and compared with low tension.
5. Redrape the flap and perform wound closure while ensuring tension equilibrium (Figure 2G).
6. Capture immediate postoperative photographs to document the initial anatomical reduction (Figure 2H).
   NOTE: At this stage, the surgical procedure has been completed, and the patient may recover in accordance with institutional postoperative care protocols. Standardized follow-up photography and outcome measurements should be performed at scheduled postoperative visits, beginning approximately 1 week after surgery.

6. Postoperative follow-up and image measurement

1. Schedule postoperative follow-up visits at approximately 1 week, 1 month, 3 months, and 6 months.
2. Obtain standardized anterior and close-up images at each follow-up appointment utilizing the standardized imaging protocol described in section 1.
3. Define the measurements obtained at the 1-week follow-up point as the baseline reference state.
4. Define the medial canthal angle (MCA) as the angle formed at the medial canthal point by the superior and inferior palpebral-margin reference lines extending along the upper and lower eyelid margins. Identify the medial canthal point and the corresponding eyelid-margin reference landmarks according to the predefined anatomical landmark system (Figure 3A)¹⁴.
5. Calibrate each photograph using the standardized imaging conditions described in section 1, including the fixed patient-to-camera distance and camera alignment. Measure MCA using ImageJ software (version 1.54) based on the calibrated image. 
6. Have two independent masked observers perform all measurements. When the difference between observers is ≤2°, use the mean value for analysis; when the difference exceeds 2°, obtain a third independent measurement and use the median of the three values.
7. Define medial canthal angle regression as a postoperative increase in MCA of at least 3° and/or posterior displacement of the medial canthal point of at least 1.0 mm at 6 months compared with the 1-week postoperative baseline. 
8. Grade severity as mild (1.0 to <2.0 mm displacement or 3° to <5° MCA increase), moderate (2.0 to <3.0 mm displacement or 5° to <8° MCA increase), or severe (at least 3.0 mm displacement or at least 8° MCA increase). When angle and displacement categories differ, assign the higher severity grade.

7. Statistical analysis and predictive modeling

1. Express continuous variables as mean and standard deviation or median and interquartile range.
2. Represent categorical variables as frequencies and percentages.
3. Compile baseline demographic, anatomic, and surgical characteristics to compare the regression and non-regression cohorts (Table 1).
4. Summarize intraoperative quantitative folding parameters and calculate measurement reliability metrics (Table 2).
5. Document the incidence and severity grading of postoperative medial canthal angle regression across all evaluation timepoints (Table 3).
6. Conduct univariable analysis to identify preliminary associations between candidate variables and the primary endpoint (Table 4).
7. Perform multivariable logistic regression to identify independent predictors and estimate adjusted effects (Table 5).
8. Assess missing data before modeling. Exclude variables with more than 20% missing data from the primary model. 
9. For variables with ≤20% missingness and considered plausibly missing at random, perform multiple imputation using chained equations with 20 imputed datasets, including the outcome variable and all candidate predictors in the imputation model. Compare the pooled estimates from the imputed datasets with results from a complete-case sensitivity analysis.
10. Select candidate predictors using clinical pre-specification and univariable screening (P < 0.10), while avoiding highly collinear variables in the same model. Evaluate collinearity with variance inflation factors and preserve an acceptable event-per-variable ratio for the 74 regression events.
11. Perform internal validation with 1,000 bootstrap resamples to estimate optimism-corrected discrimination, calibration slope and intercept, Brier score, and decision-curve net benefit. Conduct all analyses in an appropriate data analysis software application (here, SPSS version 26.0) and verify graphics and additional model diagnostics using the corresponding statistical software workflow.
12. Evaluate the discriminative ability of the model using the area under the ROC curve (AUC).
13. Summarize predictive performance, calibration parameters, and net clinical benefit through decision curve analysis (Table 6).

Results

Baseline characteristics

According to the primary outcome definition, 331 patients underwent modified V-shaped concealed flap canthoplasty and were included in the analytic cohort. At the 6-month primary endpoint, 74 patients (24.6%) met the criteria for medial canthal angle regression and 257 did not. The cohort was predominantly female (286/331, 86.4%), with a mean age of 26.8 ± 5.9 years. Patients with regression were older (28.4 ± 6.2 vs 26.3 ± 5.7 years, P = 0.012). Age s...

Discussion

This study establishes a standardized intraoperative measurement workflow for quantifying anterior MCT limb folding during modified V-shaped concealed flap canthoplasty. The protocol integrates reproducible anatomical landmark identification, controlled traction, quantitative assessment of folding geometry, and structured documentation of fixation characteristics. The predictive model represents a secondary application of these standardized measurements rather than the primary methodological contribution of this work. Th...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Digital Surgical Caliper	Mitutoyo Corporation	Series 500	Measurement of intraoperative folding length (L) and fold-point position (P)
ImageJ Software	National Institutes of Health (NIH)	Version 1.54	Measurement of medial canthal angle and landmark coordinates
SPSS Statistical Software	IBM Corporation	Version 26.0	Multivariable logistic regression and predictive model construction
Standardized Digital Camera System	Canon Inc.	EOS 90D	Acquisition of standardized perioperative photographs
Surgical Protractor	Shanghai Medical Instruments Co., Ltd.	Standard Surgical Grade	Measurement of intraoperative folding angle (θ)