The Impact of 'A Miss Is as Good as a Mile': Is Compensation Necessary for Minimal Ocular Deviation in Myopic Laser Surgery?

Angle kappa refers to the offset between the visual axis and the pupil center and may influence the centration of the treatment zone during corneal refractive surgery. Inaccurate centration can lead to decentered ablation and may affect postoperative visual quality, especially higher-order aberrations. With the development of femtosecond laser technology, more precise intraoperative alignment has become possible.

Small incision lenticule extraction (SMILE) is a commonly used corneal refractive procedure with good safety and predictability. The VisuMax 800 system incorporates machine vision-assisted centration guidance, which allows intraoperative adjustment of the treatment center according to the relative position of the corneal vertex and the pupil center. This provides the possibility of performing intraoperative compensation based on the kappa-angle offset.

Previous studies have mainly evaluated centration adjustment in eyes with relatively large kappa-angle offsets and suggested that compensation may improve postoperative optical quality. However, for patients with small offsets, the necessity of intraoperative compensation remains unclear, and prospective randomized clinical trials are limited.

The purpose of this study is to compare the visual outcomes and optical quality after SMILE with or without intraoperative kappa-angle compensation in patients with small kappa-angle offsets.

This study is a single-center, randomized, parallel-group clinical trial. Participants who are suitable for SMILE surgery and have a kappa-angle offset less than 0.2 mm will be enrolled and randomly assigned to receive SMILE with compensation or SMILE without compensation using the VisuMax 800 platform.

All surgeries will be performed by experienced surgeons using standard SMILE procedures. Participants will undergo routine ophthalmic examinations before surgery and will be followed after surgery at scheduled visits.

The main evaluation will assess postoperative visual quality and refractive outcomes. Additional assessments will include visual acuity, refractive accuracy, higher-order aberrations, contrast sensitivity, and safety outcomes.

Inclusion criteria

• Patients aged 18 to 40 years with myopia or myopic astigmatism and stable refraction for >=1 year, whose refractive errors fall within the treatable range of and meet the indications for SMILE surgery.
• Normal corneal morphology and corneal thickness meeting surgical requirements.
• Preoperative corrected distance visual acuity (CDVA) >= 1.0.
• Preoperative synthetic Kappa angle offset measured >= 0.10 mm and < 0.20 mm.
• Voluntarily agree to undergo SMILE surgery on the VISUMAX 800 platform, understand the randomization scheme, and provide signed informed consent.

Exclusion criteria

• History of prior ocular surgery, significant ocular trauma, or presence of corneal or ocular diseases that may affect the safety of refractive surgery.
• Severe systemic diseases that may affect ocular healing or surgical safety.
• Females who are pregnant or lactating.
• Inability to cooperate as required to complete the procedure or follow-up visits.