Improving LASIK outcomes with biomechanical analysis

Structural considerations can lead to improved surgical outcomes for patients 

This article was reviewed by William J. Dupps Jr., MD, PhD

Most technologies naturally evolve over time. However, in the case of LASIK, does the procedure need to? William J. Dupps Jr., MD, PhD, made his case for the impact of biomechanical information and how structural considerations may actually improve outcomes.

“Isn’t LASIK good enough?” asked Dr. Dupps, professor of ophthalmology, Cole Eye Institute, Cleveland Clinic. “Why do we need to worry about biomechanical or structural assessment? We are doing just fine.”

This is a common sentiment, noted Dr. Dupps, adding that he believes that most surgeons still are looking for ways to deliver even safer, more precise outcomes.  

Dr. Dupps describes what he calls a “precision gap” in refractive surgery planning. 

“There is a remarkable degree of precision available in our preoperative assessment tools on one hand and in the treatment delivery systems on the other,” he said. “But there is a striking gap in how we leverage those capabilities to develop and customize treatment plans.”

Moreover, Dr. Dupps noted that surgeons leave a lot of information on the table when they take a plan to the operating room.

“If we are seeking ultimately to reduce the number of refractive outliers, improve the ability to dial in very specific optical outcomes in individual cases, and minimize the risk of structural weakening that can lead to refractive outliers or progressive corneal instability, then we will need to develop treatment planning paradigms that incorporate preoperative corneal biomechanical information and use all pertinent outcome-driving data in our predictive models,” he said.

Current status of planning 
Dr. Dupps pointed out that most refractive surgery treatment planning-whether for LASIK, PRK, SMILE, intracorneal rings, or incisional refractive surgery-is currently retrospective, i.e., based on historical outcomes; probabilistic and not deterministic; and minimally personalized in that treatment plans are often driven by a very limited subset of the patient’s data. 

For example, most laser refractive surgery is performed using only the refractive error as an input, which some surgeons modify using nomogram software packages that generate empirical treatment adjustments based on previous outcomes for that treatment system.

A smaller number of procedures are performed using customized topography or wavefront-guided treatment patterns, but even in these treatments, large amounts of potential outcome-driving features of the patient are not used in treatment planning.  

“All corneal refractive procedures are either directly mediated by biomechanical effects (incisions, corneal ring segments and crosslinking) or affected by them (LASIK, PRK and SMILE), yet we lack a unifying clinical decision tool that uses this knowledge in a predictive way,” Dr. Dupps said.

Seeing below the surface
Specifically, Dr. Dupps said he believes that a comprehensive biomechanical assessment is crucial for better detecting the risk of ectasia and for advancing efforts to personalize refractive treatments. He points to two key components of structural assessment: biomechanical measurement and structural modeling. 

The software application, called SpecifEye, imports three-dimensional anatomy from a standard tomographer and other clinical data. A finite element model mesh of the whole eye is then built from that anatomy. Standard ocular material properties are incorporated but could be customized as measurement technologies become available for this. 

The next step is verification. The eye model is loaded with the patient’s IOP and confirms that the resulting topography of the eye is similar to that of the patient. After this, the surgeon enters details of a proposed procedure, which the finite element model then simulates in the cloud on the virtual eye. 

“This results in a predicted postoperative model complete with regional stresses and strains,” Dr. Dupps explained. “These data and ray-traced estimates of refractive change from the model surfaces are then translated into treatment guidance information to the clinician.”
 

Individual differences
To determine if individual variations in corneal stiffness affect LASIK outcomes, Dr. Dupps and Abhijit Sinha Roy, PhD, performed a study using the same corneal geometry from an eye and ran multiple myopic LASIK simulations with a range of corneal stiffness values from the experimental. 

In eye models with a stronger cornea, the limbus shifted outwardly while the central cornea displaced posteriorly, all as a result of simulated flap creation and ablation. In contrast, in a weaker cornea with low corneal stiffness, the procedure resulted in forward displacement of the corneal apex. 

“There is a warpage phenomenon that occurs as the stresses and strains redistribute after flap creation and ablation, and these can produce clinically significant differences in refractive outcome,” Dr. Dupps said. 

These results were reported in the Journal of Refractive Surgery (2009;25:875-887). 

The structural shifts observed in a stiff cornea contribute to further flattening of the refractive outcome, possibly resulting in overcorrection. In the cornea with low stiffness, the forward protrusion decreases the myopic correction, resulting in relative undercorrection. 

In the presence of an immediate undercorrection, a weaker cornea should be suspected and the patient followed before performing an enhancement. 

The results achieved with the eye model were validated in 19 actual LASIK cases and compared with simulated results. Dr. Dupps reported “a very high correlation between the actual and predicted outcomes (Invest Ophthalmol Vis Sci. 2017;33:444-453).” 

The study also showed that including an adjustment for the preoperative corneal hysteresis from the Ocular Response Analyzer further improved the prediction accuracy in LASIK.

Building on this evidence, Dr. Dupps said he believes that 3D biomechanical measurements from Brillouin imaging or optical coherence elastography (OCE) could bring even more predictive power to simulations by better charactering individual properties and the effects of different procedures.

His lab has used OCE to demonstrate “an abrupt decrease in the corneal stiffness in the region of the flap (the first 100 μm)” when comparing pre- and post-LASIK measurements in a clinical study. They also observed a slight increase in stiffness in the residual stromal bed of the post-LASIK cornea, which necessarily bears a higher tensile load after a flap is created. 

According to Dr. Dupps, these findings are consistent with a model-based comparison of the stress distributions between LASIK and SMILE (J Cataract Refract Surg. 2014;40:971-980), which showed that SMILE preserves the anterior corneal stress and thus protects the posterior stroma, whereas in LASIK, the postoperative stress is redistributed to the posterior bed, which may be less well adapted to bear stress over time. 

Dr. Dupps also showed strain results when the two procedures were compared using SpecifEye in the same eye. A 5-D LASIK procedure was associated with a 15.43% increase in central corneal strain compared with an 11.55% increase with a 5-D SMILE procedure. 

“SMILE decreases the chances of corneal destabilization,” Dr. Dupps said, but offered the following caveat. 

The presumption behind SMILE’s structural advantage is that every cornea has a normal corneal property distribution (stronger in the anterior stroma than the posterior stroma, as demonstrated by Dr. Randleman and others). 
 

Conclusion 
Dr. Dupps pointed out that the usual bridging collagen fibrils in the anterior stroma that contribute to this strength appear to be reduced or absent in keratoconus. OCE measurements performed by his research group in keratoconus patients show significantly lower anterior-to-posterior stromal strength profiles than in normal eyes. If similar anterior weakness is an early manifestation of keratoconus, the relative advantage of SMILE may be reduced in such eyes.

“Weaker corneas tend toward undercorrection of myopia and should be monitored for stability before an enhancement procedure is performed,” he said. “SMILE has a structural advantage over LASIK in normal corneas, but PRK may be superior in the presence of occult anterior weakness.”

Dr. Dupps also noted that cloud-based, 3D patient-specific structural simulations are becoming available for highly individualized clinical-decision support. 

“Emerging methods for measuring spatially resolved properties may help further individuali ze risk assessment and treatment optimization,” he concluded. 

William Dupps, MD, PhD
E: bjdupps@outlook.com
Dr. Dupps reported intellectual property interests in model and measurement applications mentioned.