[Changes in biomechanical properties of the cornea after keratorefractive surgery]

[Direct assessment of changes in biomechanical properties of the cornea after ReLEx SMILE and FemtoLASIK surgeries]

The aim of this study was to evaluate corneal biomechanical properties in myopic patients after ReLEx SMILE and FemtoLASIK surgeries using the Corvis ST analyzer.

MATERIAL AND METHODS

The SMILE group comprised 23 patients (46 eyes) with spherical refraction -3.8±1.8 diopters (D), the FemtoLASIK group included 18 patients (36 eyes) with spherical refraction -3.5±1.3 D. Analysis of the biomechanical properties of the cornea was carried out using the CORVIS ST device (Oculus, Germany) before and seven days after corneal refractive surgery.

RESULTS

In the SMILE group, with intraoperative decrease in corneal thickness of 91.43±19.43 µm, a significant increase in the following parameters was observed: deformation coefficient (DA ratio; p=0.0001), peak distance (PD; p=0.02) and inverse concave radius (ICR; p=0.003); a decrease in the stiffness parameter at first applanation (SP-A1; p=0.0001), Corvis biomechanical index (CBI; p=0.0001), intraocular pressure (IOP; p=0.001). In the FemtoLASIK group, with intraoperative decrease in corneal thickness of 75.33±32.3 µm, we observed a significant increase in DA ratio (p=0.0002), PD (p=0.04), ICR (p=0.0002), a decrease in SP-A1 (p<0.0001), IOP values (p=0.0003). In comparison with the FemtoLASIK group, the deformation amplitude DA changed significantly less in the SMILE group (p=0.04). In the FemtoLASIK group, in comparison with the SMILE group, the DA ratio (p=0.0009) and SP-A1 (p=0.0003) significantly increased. Intraoperative corneal thickness change correlates with ICR both in SMILE (R=0.52) and in FemtoLASIK (R=0.65).

CONCLUSION

Corneal biomechanical properties determined with CORVIS ST in eyes with mild to moderate myopia change to a lesser extent after ReLEx SMILE compared to FemtoLASIK.

[State of corneal nerve fibers in systemic amyloidosis]

The term systemic amyloidosis unites a group of diseases with a single pathogenetic mechanism involving diffuse deposition of a pathological fibrillar protein (amyloid) in the intercellular space of various organs. Among the systemic forms of amyloidosis, light chain amyloidosis (AL-amyloidosis) occurs most often in clinical practice, while transthyretin amyloidosis (TTR-amyloidosis) is its most common hereditary form. Laser corneal confocal microscopy (CCM) allows for in vivo and non-invasive assessment of the state of corneal nerve fibers (CNF).

PURPOSE

To assess the state of CNF in systemic amyloidosis by confocal microscopy data obtained in vivo.

MATERIAL AND METHODS

The main study group included 16 patients (6 men and 10 women, mean age 60.5±11.6 years) with morphologically confirmed primary AL-amyloidosis, and 14 patients (5 men and 9 women, mean age 59.4±11.3 years) with genetically and morphologically confirmed hereditary TTR-amyloidosis. The control group included 23 healthy volunteers of the same age range without any neurological pathologies. The state of CNF was assessed by in vivo CCM data recorded on the HRT III system and its consequently processing using authors' self-developed program Liner 1.2. The criteria for neuropathy intensity was the degree of CNF tortuosity characterized by coefficients of anisotropy (K_ΔL) and symmetry (K_sym) of CNF orientation.

RESULTS

According to the NIS scale, the manifestations of neuropathy in the subgroup of patients with TTR-amyloidosis were significantly more pronounced compared to AL-amyloidosis patients. The severity of clinical manifestations of neuropathy did not depend on the duration of TTR-amyloidosis and AL-amyloidosis (Spearman R r_s=0.21, p=0.58 and r_s= -0.49, p=0.055, respectively). Changes in the quantitative indicators (a decrease in the anisotropy coefficient and an increase in the symmetry coefficient of the fibers orientation) confirm increased tortuosity of CNF in systemic amyloidosis.

CONCLUSION

The clinical picture of systemic amyloidosis is characterized by polymorphism of neurological manifestations that include various symptoms of damage to the peripheral somatic and autonomic nervous system. In vivo CCM can be used to reveal qualitative and quantitative changes in CNF in patients with systemic amyloidosis. However, statistical unreliability of the identified quantitative changes allows considering the state of CNF in amyloidosis only as a component of the disease monitoring algorithm, but not as a biomarker of the disease.

Wave front aberrations induced from biomechanical effects after customized myopic laser refractive surgery in finite element model

PURPOSE

A customized myopic refractive surgery was simulated by establishing a finite element model of the human eye, after which we studied the wave front aberrations induced by biomechanical effects and ablation profile after wave front-guided LASIK surgery.

METHODS

Thirty myopia patients (i.e., 60 eyes) without other eye diseases were selected. Their ages, preoperative spherical equivalent, astigmatism, and wave front aberration were then obtained, in addition to the mean spherical equivalent error range - 4 to - 8D. Afterward, wave front-guided customized LASIK surgery was simulated by establishing a finite element eye model, followed by the analysis of the wave front aberrations induced by the surface displacement from corneal biomechanical effects, as well as customized ablation profile. Finally, the preoperative and induced aberrations were statistically analyzed.

RESULTS

Comatic aberrations were the main wave front abnormality induced by biomechanical effects, and the wave front aberrations induced by the ablation profile mainly included coma and secondary coma, as well as sphere and secondary-sphere aberrations. Overall, the total high-order aberrations (tHOAs), total coma (C₃₁), and sphere ([Formula: see text]) increased after wave front-guided customized LASIK surgery. According to our correlation analyses, coma, sphere, and tHOAs were significantly correlated with decentration. Additionally, the material parameters of ocular tissue were found to affect the postoperative wave front aberrations. When the material parameters of the sclera remained constant but those of cornea increased, the induced wave front aberrations were reduced.

CONCLUSION

All biomechanical effects of cornea and ablation profile had significant effects on postoperative wave front aberrations after customized LASIK refractive surgery; however, the effects of the ablation profile were more notorious. Additionally, the characteristics of biomechanical materials have influence on the clinical correction effect.

Simulated biomechanical effect of aspheric transition zone ablation profiles after conventional hyperopia refractive surgery

We studied the effects of the aspheric transition zone on the optical wavefront aberrations, corneal surface displacement, and stress induced by the biomechanical properties of the cornea after conventional laser in situ keratomileusis (LASIK) refractive surgery. The findings in this study can help improve visual quality after refractive surgery. Hyperopia correction in 1-5D was simulated using five types of aspheric transition zones with finite element modeling. The algorithm for the simulations was designed according to the optical path difference. Wavefront aberrations were calculated from the displacements on the anterior and posterior corneal surfaces. The vertex displacements and stress on the corneal surface were also evaluated. The results showed that the aspheric transition zone has an effect on the postoperative visual quality. The main wavefront aberrations on the anterior corneal surface are defocus, y-primary astigmatism, x-coma, and spherical aberrations. The wavefront aberrations on the corneal posterior surface were relatively small and vertex displacements on the posterior corneal surface were not significantly affected by the aspheric transition zone. Stress analysis revealed that the stress on the cutting edge of the anterior corneal surface decreased with the number of aspheric transition zone increased, and profile #1 resulted in the maximum stress. The stress on the posterior surface of the cornea was more concentrated in the central region and was less than that on the anterior corneal surface overall. The results showed that the aspheric transition zone has an effect on postoperative aberrations, but wavefront aberrations cannot be eliminated. In addition, the aspheric transition zone influences the postoperative biomechanical properties of the cornea, which significantly affect the postoperative visual quality.

[On the question of progressive hyperopia correction after radial keratotomy]

Progressive hyperopia development in the long-term period after radial keratotomy (RC) is associated with not only surgical effects, but also individual biomechanical features of the fibrous tunic of the eye, its age-related changes, and hydrodynamic disturbances. When choosing a method of correction, one should take into account a number of features characteristic of this induced refractive disorder: instability and irregularity of refraction, coexisting presbyopia, and, in some cases, anisometropia. Correction of progressive hyperopia after RC can be achieved with eyeglasses, or contacts, or intraocular lenses depending on the subjective factor - the so-called patient adherence. The most effective in terms of functionality is contact correction with scleral rigid gas-permeable lenses, and in the presence of pronounced lens opacities - intraocular correction.