Under correction or over-correction is a considerable point in ptosis surgery. A large amount of levator resection may result in a shortening of the distance between the tarsus and the Whitnall’s ligament as the check ligament of the levator palpebral superioris muscle leading to eyelid lag and lagophthalmos [12].

The amount of levator resection depends on various factors such as pre-operative levator function and ptosis severity. However, accurate prediction of outcome is still difficult [13, 14], and Although a variety of formulas have been introduced to determine the amount of levator resection, there is no consensus on these formulas due to unpredictability [8, 15].

During surgery, Whitnall’s Ligament is characterized as a landmark at the most superior extent of the aponeurosis. It is located in a region almost between the muscular part of the levator muscle and its aponeurotic part. This transverse fibrous white band lied from the medial to the lateral border of the levator aponeurosis immediately behind the superior orbital rim. In some cases of ptosis correction, this structure may be utilized to improve levator resection surgery efficiency and even used as a sling to support the eyelid at a higher level. Anatomic variability in this structure may contribute to ptosis [16]. Also, these variabilities of levator aponeurosis and Whitnall’s ligament can influence surgical outcomes. The distance between the insertion of the levator muscle and Whitnall’s ligament has been reported to be 14–20 mm in previous studies [17]. However, standard values for Asians are not available.

According to the findings of our study, Preop-LF had a significant negative correlation with the amount of levator muscle resection. It was consistent with the results of Berke 1961 [9], Beard 1966 [8], and Also, the study of Cates and Tyers, which was conducted on congenital ptosis in children and adults and showed that the best predictor of the results of levator resection surgery is the pre-operative levator function [13]. The study by Nguyen and colleagues in 2017 on patients with congenital ptosis showed ptotic eyes with lower Preop-LF needed a higher amount of levator muscle resection [10]. They found there was a correlation between pre-operative levator function and the amount of levator resection; however, this correlation was not strong. Based on this, they stated that there are probably factors other than levator function that influence the amount of levator resection required to achieve a successful outcome in levator resection surgery [10]. According to the results obtained from our study, one of these factors can be Whitnall’s distance. In fact, in our study, we noticed a positive correlation between Whitnall’s distance and the amount of levator resection, so by increasing the distance between Whitnall’s ligament and the upper edge of the tarsus, the amount of levator resection required during surgery for successful outcome, increases. Perhaps this finding can be explained considering the constant length of the levator muscle by increasing the distance between Whitnall’s ligament and the tarsus, the percentage of the levator aponeurosis increases and the percentage of its muscular part decreases, so the overall strength of the muscle decreases, and therefore, a greater amount of resection is needed to correct ptosis.

Another hypothesis proposed by the authors is that this positive correlation can be explained using the lever rule. The comparison of body muscle movements to levers in humans and animals has been investigated in previous studies [18, 19]. However, no studies have discussed it in ophthalmology. In fact, the posterior part of the levator is muscular and extends horizontally from its origin to Whitnall’s ligament. The anterior part of the levator, which consists of the aponeurosis, extends vertically from Whitnall’s ligament to the insertion site. In addition, Whitnall’s ligament, which has connections to the roof and sides of the orbit, acts as a support that converts the horizontal force of the levator muscle into a vertical force [11]. The posterior (muscular) part of the levator can be compared to the effort arm of the lever, and the anterior (aponeurotic) part of the levator can be compared to the resistant arm of the lever. Accordingly, at a fixed length of the lever, by increasing the resistance arm (the distance of Whitnall’s ligament from the upper edge of the tarsus) and decreasing the effort arm, it is necessary to resect a greater amount of the levator so that the ratio of the effort arm length to the resistance arm length increases further, leading to being more effective in correcting ptosis.

Based on our results and clinical experience, W-distance can be considered an influencing factor in the surgical outcome during levator resection surgery. Patients with lower values of Preop-LF and higher values of W-distance may benefit from higher amounts of levator muscle resection (p < 0.05). Based on Figs. 2 and 3, as well as the values obtained for correlation analysis, it seems that Whitnall's distance has an almost equal and opposite effect to the effect of levator function on the amount of resection required to achieve success in levator resection surgery. To the best of our knowledge, there are few studies on the evaluation of the effect of W-distance on the amount of levator muscle resection. One may hypothesize that patients with higher values of W-distance may have a poorer LF. However, our study did not show a correlation between these two parameters, and W-distance may affect the resection amount in ways other than LF. In our study, Preop-MRD1 and T-length were not effective on the amount of levator resection.

Overall, according to Beard, success in levator resection surgery depends on several factors [8, 20]. The most important factor in this success is the perfect decision of the surgeon during surgery. In fact, the surgeon determines the amount of levator muscle resection based on the type of ptosis, the severity of ptosis, and the amount of levator muscle function. Primarily, the experience of the surgeon plays a very important role, and the number of sutures and the placement of the sutures are very important in creating an acceptable final contour for the upper eyelid. So far, no study has mentioned the decisive role of Whitnall’s ligament in the success of levator resection surgery. Therefore the anatomical position of Whitnall’s ligament, which was mentioned in our study, should not replace the role of proven variables, Like levator muscle function or surgeon experience. In fact, our study is just the beginning of a way and points to the possible but not definitive role of Whitnall’s ligament in levator resection surgery. There is definitely a need for further and more detailed studies with a larger number of cases to check the existence or absence of this relationship.

Remarkable aspects of our study were the novel idea and prospective design. The limitations of this study were the relatively small sample size. Future studies with larger sample sizes and randomization are required to show the different aspects of this topic better. Nowadays, novel automatic image processing techniques in eyelid and face parameter measurements, for example, 3D models, algorithmic innovation, multi-classification models, and deep learning methods, have made great progress and can contribute to the level of diagnosis and decision-making and help us a lot in the field of diagnosis and treatment of eyelid and face diseases [21,22,23,24].

The results of this and future studies can be considered by oculoplastic surgeons to estimate better and faster the needed amount of levator muscle resection to achieve more successful outcomes in levator resection surgery.

In conclusion, this study showed the W-distance as a significant new parameter influencing the amount of levator resection needed to achieve success in levator resection surgery. Patients with higher W-distance or weaker Preop-LF may benefit from higher amounts of levator resection.