Effects of intracorneal ring segments implementation technique and design on corneal biomechanics and keratometry in a personalized computational analysis

Abstract

The implementation of intracorneal ring segments (ICRS) is one of the successfully applied refractive operations for the treatment of keratoconus (kc) progression. The different selection of ICRS types along with the surgical implementation techniques can significantly affect surgical outcomes. Thus, this study aimed to investigate the influence of ICRS implementation techniques and design on the postoperative biomechanical state and keratometry results. The clinical data of three patients with different stages and patterns of keratoconus were assessed to develop a three-dimensional (3D) patient-specific finite-element model (FEM) of the keratoconic cornea. For each patient, the exact surgery procedure definitions were interpreted in the step-by-step FEM. Then, seven surgical scenarios, including different ICRS designs (complete and incomplete segment), with two surgical implementation methods (tunnel incision and lamellar pocket cut), were simulated. The pre- and postoperative predicted results of FEM were validated with the corresponding clinical data. For the pre- and postoperative results, the average error of 0.4% and 3.7% for the mean keratometry value ($$\text {K}_{\text{mean}}$$ K mean ) were predicted. Furthermore, the difference in induced flattening effects was negligible for three ICRS types (KeraRing segment with arc-length of 355, 320, and two separate 160) of equal thickness. In contrast, the single and double progressive thickness of KeraRing 160 caused a significantly lower flattening effect compared to the same type with constant thickness. The observations indicated that the greater the segment thickness and arc-length, the lower the induced mean keratometry values. While the application of the tunnel incision method resulted in a lower $$\text {K}_{\text{mean}}$$ K mean value for moderate and advanced KC, the induced maximum Von Mises stress on the postoperative cornea exceeded the induced maximum stress on the cornea more than two to five times compared to the pocket incision and the preoperative state of the cornea. In particular, an asymmetric regional Von Mises stress on the corneal surface was generated with a progressive ICRS thickness. These findings could be an early biomechanical sign for a later corneal instability and ICRS migration. The developed methodology provided a platform to personalize ICRS refractive surgery with regard to the patient’s keratoconus stage in order to facilitate the efficiency and biomechanical stability of the surgery.

Ex vivo, in vivo and in silico studies of corneal biomechanics: a systematic review

“…At the state of art, FE models of cornea and whole eye have been developed for different purposes: to prevent complications after modern laser refractive surgical procedures and iatrogenic ectasia [ 74 ], to study the mechanical response of the cornea subjected to a non-contact air-jet tonometry differentiating the contributions related to geometry, the corneal material behaviour and the loading [ 75 ], to analyse the blunt impact of foreign bodies [ 3 ], to study the effect of surgical factors on the cornea, to improve keratoconus treatment [ 41 , 76 , 77 ], to analyse the wave propagation for a better understating the differences between healthy subjects and glaucoma patients [ 78 ], to measure the deformation after airbag impact [ 79 ] and to quantify the biomechanical change caused by the LASIK flap [ 80 ]. The most ambitious goal stays on the development of a daily clinical computational tool for the planning and optimization of corrective procedures and in preclinical optimization of diagnostic procedures [ 40 ], permitting the personalization of the surgical procedure [ 81 ] by using a multi-physics model that permits not only to analyse biomechanical aspects, but also treatments, as CXL, by adding the migration of the riboflavin (i.e., the photo-initializer), UV light absorption, the photochemical reaction that forms the cross-links, and biomechanical changes caused by changes to the microstructure [ 82 ].…”

“…Numerous studies simplified the cornea as a linear, time-independent and isotropic material [ 76 , 78 , 79 ], omitting the “true” mechanical aspects of corneal tissues. More sophisticated papers described the corneas as a material presenting all or part of the following features: anisotropy, visco-hyperelasticity and inhomogeneity [ 3 , 40 , 74 , 75 , 81 ], respecting more the experimental evidences and permitting more solid simulations-derived considerations.…”

“…Constitutive model : Material defined by using experimental nonlinear stress–strain data ANSYS The sclera was imposed with displacement zero. The intraocular pressure inside the eye is fixed normal to the sclera and cornea surface from 10 to 20 mmHg [ 81 ] Development of a methodology to personalize ICRS refractive surgery regarding the patient’s keratoconus stage in order to facilitate the efficiency and biomechanical stability of the surgery 3D patient-specific model of the keratoconic cornea Linear hexahedral elements of the C3D8R. more than 25,000 elements Hyperelasticity Anisotropy Constitutive model : Modified Gasser-Holzapfel-Ogden Abaqus The contact behavior between the intracorneal layer was defined as “frictionless” and “hard” contact.…”

“…Curvature variations eventually led to changes in light refraction, which greatly affected the recovery of the patient’s visual acuity. Bagheri et al [ 81 ] and De Oteyza et al [ 41 ] studied the intracorneal ring segment as a keratoconus treatment finding that increasing ring thickness and base width along the arc of the asymmetric ring segment produced a more pronounced flattening in this part of the ring. The asymmetric ring design did find a good balance between maximizing corneal flattening at one end and minimizing it at the other end, compared to the isolated effect of ring thickness and width.…”

Ocular condition prognosis in Keratoconus patients after corneal ring implantation using artificial neural networks

“…Wide fluctuations were reported in the visual improvement following the ring implantation in KC patients. The ring characteristics can also influence surgical outcomes [ 27 , 28 ]. Therefore, one of the genuine concerns of ophthalmologists for the KC treatment is forecasting the outcome of implanting a specific type of ring segment with no need for intervention.…”