As the most common congenital facial malformation globally, cleft lip with or without cleft palate has a prevalence of approximately 0.3 per 1000 live births (Aycart and Caterson, 2023,Salari et al., 2022). This condition, resulting from failed facial fusion during embryogenesis (Hammond and Dixon, 2022), can lead to severe lifelong complications affecting feeding, speech, hearing, and social adaptation. Surgical repair remains the primary treatment, with the Millard rotation-advancement technique being the most prevalent for unilateral cases (Matthews et al., 2024; Roussel et al., 2015). Its design conforms to anatomy, yielding a natural lip shape and minimal scarring (Manlove and Linnebur, 2022; Xue et al., 2021). Alternative procedures include Z-plasty for tissue lengthening and the Fisher technique for superior philtral and alar reconstruction (Kang, 2022; Kwong et al., 2019, Rogers and Tse, 2025). Surgeons often combine or modify these techniques to meet individual anatomical and aesthetic needs (Ye et al., 2024).

While largely superseded by these modern methods due to limitations in vermilion and muscle repair, the straight-line closure technique remains relevant. Valued for its anatomical accuracy, predictability, and discreet scarring, it is particularly suitable for mild unilateral deformities and avoids extensive dissection (Baek et al., 2016; Jin and Li, 2010; Nakajima et al., 2008; Xue et al., 2021). Furthermore, its strategic use in a staged approach with subsequent Millard repair can significantly improve outcomes like alar width and lip symmetry (Grewal et al., 2021).

Postoperative stress and tissue deformation critically impact wound healing and aesthetic results. Excessive tension risks wound dehiscence and scar hyperplasia, while inadequate repair can lead to asymmetry and functional deficits (Hattori et al., 2023; Rautio et al., 2017). Patient-specific three-dimensional finite element (FE) models offer a powerful tool to simulate biomechanical responses, predicting tension distribution and tissue deformation preoperatively (Bola et al., 2021; Cen et al., 2023,Zhu and Forman, 2022). This simulation helps identify potential issues like poor flap alignment or excessive tension, which are difficult to assess intuitively (Taub and Collins, 2012). Consequently, preoperative FE analysis is significant for optimizing surgical design and improving repair quality.

This study employs the straight-line closure technique as an example. By constructing a patient-specific 3D FE model, we simulate mechanical changes to predict incision tension and tissue deformation, thereby assisting in optimal incision design and suture strategy (Chen et al., 2020; Phellan et al., 2021; Sarrami et al., 2021; Tsai et al., 2021).

As the leading cause of congenital facial malformations globally, cleft lip with or without cleft palate have an average prevalence of 0.3 cases per 1000 live births (Aycart and Caterson, 2023,Salari et al., 2022), mainly caused by failed facial fusion during embryogenesis (Hammond and Dixon, 2022). Due to issues such as facial appearance, feeding, speech, obstructive apnea, hearing, and social adaptation, cleft lip can lead to severe lifelong complications (Hammond and Dixon, 2022)seriously affecting the physical and mental health of affected children. Currently, the radical treatment methods for cleft lip mainly rely on several repair surgeries (Carlson and Taylor, 2022). The Millard rotation-advancement technique is considered the most commonly used and classic surgical procedure for treating unilateral cleft lip (Matthews et al., 2024; Roussel et al., 2015). This method reconstructs the philtral column by designing a rotating flap on one side of the cleft lip and simultaneously advances the flap on the opposite side to fill the defect area, achieving natural repositioning of soft tissues and restoring muscle continuity. Due to its incision design conforming to the anatomical structure, resulting in a natural lip shape and less visible scars after surgery, it is widely used globally (Manlove and Linnebur, 2022; Xue et al., 2021). In addition, the Z-plasty is also a common alternative surgical procedure, which mainly uses a Z-shaped incision to lengthen the lip tissue and correct tissue tension (Kang, 2022). In recent years, the Fisher technique has also gradually gained attention due to its aesthetic reconstruction effect on the repair of the philtrum and alar (Kwong et al., 2019, Rogers and Tse, 2025). In response to the individual anatomical differences and aesthetic requirements of different patients, surgeons often combine multiple surgical procedures or adopt modified plans to achieve the best repair effect (Ye et al., 2024). In the development history of cleft lip repair, as an age - old basic surgical procedure (Sykes and Tollefson, 2005), the straight - line closure has gradually been replaced by modern surgical procedures such as Millard, Z - plasty, and Fisher (Rogers and Tse, 2025), due to its drawbacks such as poor vermilion reconstruction and incomplete orbicularis oris muscle reposition. However, it was widely used in the early 20th century because it was simple to operate and time – saving (Shkoukani et al., 2013), through symmetric resection and direct approximation suture of the cleft margin tissue. As the original model for cleft lip repair, straight-line closure and its modified techniques based on it are still widely used in modern times due to its anatomical accuracy, predictability, and the ability to conceal scars (Baek et al., 2016; Jin and Li, 2010; Nakajima et al., 2008). The scar of straight-line closure is closer to the natural philtrum crest, making it particularly suitable for unilateral grade Ⅰ deformities and avoiding extensive dissection of the alar base in the rotation-advancement method (Xue et al., 2021). Additionally, studies have shown that applying straight-line closure in the first stage and combining it with the modified Millard repair in the second stage can significantly improve the alar width and lip symmetry (Grewal et al., 2021).

In clinical practice, the stress and tissue deformation borne by tissues during and after surgery have a significant impact on wound healing and appearance restoration (Raghavan et al., 2018). If the stress at the sutured site after surgery is too high, it may lead to wound dehiscence, scar hyperplasia, and even secondary nasal deformity; while insufficient deformation or inadequate muscle reconstruction may cause problems such as asymmetric lip shape, blurred philtrum, and upper lip collapse (Hattori et al., 2023; Rautio et al., 2017). In cleft lip surgery, a realistic three-dimensional finite element model can dynamically simulate the biomechanical responses of complex anatomical structures (such as the nasal base, columella, and lip peaks) under different surgical procedures, which has great guiding significance for reality. For example, the lead surgeon can set different parameters for three-dimensional finite element simulation based on the different skin and muscle conditions of patients before surgery to predict the biomechanical responses of different tissues (Bola et al., 2021; Cen et al., 2023,Zhu and Forman, 2022), providing a quantitative basis for surgical procedure design. Especially in simulating the behavior of complex soft tissues, the three-dimensional finite element model helps to visually display the mechanical problems that are difficult to intuitively evaluate with traditional surgical procedures, such as incomplete deformity correction due to insufficient deformation, lip peak deviation caused by poor flap alignment, and suture breakage due to excessive tension (Taub and Collins, 2012). So, the preoperative comprehensive assessment of the tissue biomechanical state through finite element analysis is of great significance for optimizing surgical design and improving repair quality. Meanwhile, the finite element model can also be used to compare the differences in mechanical distribution between different surgical methods, guiding the selection of surgical methods and the formulation of individualized surgical plans.

This article takes straight-line closure for cleft lip repair as an example, by constructing a patient-specific three-dimensional tissue model, simulating the response of different structures to mechanical changes before and after surgery, researchers can predict the tension distribution of incisions and the trend of tissue deformation before surgery, so as to assist in determining the optimal incision design and suture strategy (Chen et al., 2020; Phellan et al., 2021; Sarrami et al., 2021; Tsai et al., 2021).