Mechanical models of the dynamics of vitreous substitutes

The applications of viscoelastic agents in vitreoretinal surgery

The introduction of viscoelastics in human ocular surgery dates back to 1972.Subsequntly, several substances were proposed as adjuvants, particularly for cataract surgery; however, the rheologic properties of ophthalmic viscosurgical devices (OVDs), such as viscosity, pseudoplasticity, and coatability led to their employment also in vitreoretinal surgery. OVDs can be used to separate tissues and create space, to break adhesions, to allow space for surgical manipulation, and to contain hemorrhage by dampening capillary oozing. We focus on the principles and clinical results of several viscosurgery techniques, such as viscodissection and viscodelamination for the management of proliferative vitreoretinopathy and diabetic retinopathy and suprachoroidal buckling for primary retinal detachment. Furthermore, we analyze the techniques in which OVDs served as adjuvants in macular hole surgery, particularly in the handling of the internal limiting membrane, and as volumetric substances against hypotony. Finally, we analyze recent perspectives on vitreous humor biochemical features. Preclinical research led to the belief that hyaluronic acid-based hydrogels could become ideal vitreous substitutes, thanks to their viscoelasticity, porosity, optical properties, and biocompatibility; however, although promising, long-term toxicity issues have limited studies on human subjects.

Development and preliminary evaluation of a novel preoperative index for quantitative analysis of photoreceptor loss in full-thickness macular holes

PURPOSE

To identify novel quantitative parameters for evaluating photoreceptor loss in full-thickness macular holes (FTMH), exploring their potential clinical impact on postoperative functional and anatomical recovery.

METHODS

This pilot study enrolled 38 eyes from 38 patients diagnosed with FTMH. Preoperatively, eyes underwent analysis and were subsequently followed for six months post-surgery. Best-corrected visual acuity (BCVA) was recorded, and cross-sectional images of FTMH were obtained using B-scan optical coherence tomography (OCT) and en-face OCT. Quantitative assessment of ellipsoid zone (EZ) and external limiting membrane (ELM) integrity changes was conducted and correlated with postoperative anatomical and functional recovery. The photoreceptor Integrity Index (PIIN), calculated as the ratio of photoreceptor area to lumen hole area measured at customized segmentation, was correlated with the minimum and base diameters of the hole, positive change in BCVA, preoperative EZ defect (EZd), preoperative ELM defect (ELMd), and changes in EZ and ELM over the six-month follow-up period (∆-EZ and ∆-ELM). The main outcome measures focused on evaluating the effectiveness of PIIN in predicting postoperative anatomical and functional changes.

RESULTS

A higher PIIN correlated with a greater BCVA change over six months (p < 0.001). Univariate regression analysis using the PIIN as a predictor for positive change in BCVA (|∆-BCVA| [logMAR]) over time yielded significant results (p < 0.001). Additionally, the PIIN significantly correlated with EZd at baseline, ELM at baseline, and ELMd change over the six-month follow-up period.

CONCLUSION

The PIIN shows promise as a tool for evaluating photoreceptor loss in macular holes and estimating postoperative functional and anatomical recovery.

KEY MESSAGES

What is known Previous studies have extensively used optical coherence tomography (OCT) to investigate various biomarkers for assessing patients with full-thickness macular hole (FTMH), without considering detailed MH ultrastructural features Existing indexes used to predict surgical outcomes for FTMH primarily depend on geometrical parameters and do not integrate detailed ultrastructural characteristics, such as cellular components.  What is new A novel concept introduces the quantitative measurement of residual photoreceptors located at the edge of FTMH. The Photoreceptor Integrity Index (PIIN) integrates different ultrastructural components of macular holes, aiming to become a valuable clinical tool to predict both anatomical and functional recovery outcomes following surgical intervention for FTMH.

Full-Thickness Macular Hole: Are Supra-RPE Granular Deposits Remnants of Photoreceptors Outer Segments? Clinical Implications

PURPOSE

To describe the presence of specific morphological characteristics of idiopathic, full-thickness macular hole (MH) potentially influencing postoperative best corrected visual acuity (BCVA) and surgical outcomes.

DESIGN

Retrospective, multicenter and interventional case series.

METHODS

Clinical charts and multimodal imaging pictures of 149 eyes of 143 consecutive patients diagnosed with MH, treated surgically and with a minimum follow-up of 12 months, were reviewed.

RESULTS

Supra-retinal pigment epithelium (RPE) granular deposits were diagnosed in 121 of 149 eyes (81.2%). A smooth morphology was identified in 58 of 149 eyes (38.9%), whereas a bumpy border was present 91 of 149 eyes (61.1%). Photoreceptor disruption was mainly located close to the MH aperture. In 8% of the included cases, preoperative anatomical progression from smooth to bumpy morphology was noted. The presence of supra-RPE granular deposits was a significant predictor of lower postoperative BCVA only in univariate analysis (P < .001). The presence of a bumpy border was significantly correlated with lower postoperative BCVA in both univariate and multivariate analysis (P < .001). BCVA gain was significantly lower in MH with bumpy borders (P < .001). A bumpy border was also significantly associated with poor postoperative anatomical restoration (P < .001).

CONCLUSIONS

Supra RPE-granular deposits and a bumpy morphology may be indicators of photoreceptor disruption in MH. A bumpy morphology may suggest deeper and potentially irreversible photoreceptor damage, and may negatively influence both functional and anatomical recovery.

Flow dynamics of vitreous humour during saccadic eye movements

In this work, we reveal the flow dynamics of Vitreous Humour (VH) gel and liquid phases during saccadic movements of the eye, considering the biofluids viscoelastic character as well as realistic eye chamber geometry and taking into account the saccade profile. We quantify the differences in the flow dynamics of VH gel and liquid phases using viscoelastic rheological models that are able to model the VH shear rheology, considering different amplitudes of saccadic movements (10^∘, 20^∘, 30^∘ and 40^∘). For this purpose, the computational fluid dynamics (CFD) open source software OpenFOAM® was used. The results portray a distinct flow behaviour for the VH gel and liquid phases, with inertial effects being more significant for the VH liquid phase. Moreover, the Wall Shear Stress (WSS) values produced by the VH gel phase are more than twice of those generated by the VH liquid phase. Results also show that for different amplitudes of eye movement both the velocity magnitude in the vitreous cavity and the shear stresses on the cavity walls rise with increasing saccadic movement displacement.

Study of wetting of the animal retinas by Water and organic liquids and its Implications for ophthalmology

Interfacial properties of the animal retinas are reported. Wetting of the retina-retinal pigment epithelium-choroid-sclera tissues of cow, sheep, and pig eyes by water, silicone and castor oil was explored experimentally. Both water and silicone oils demonstrated complete wetting of the retina, regardless of the viscosity of the silicone oil, whereas the castor oil demonstrated a partial wetting regime. Similar wetting regimes were observed for sheep, cow and pig retinas. The intact surface of animal retina was found to be both hydrophilic and oleophilic. Wetting experiments with double sandwich oil/water layers were performed. Water demonstrated stronger affinity to the retina than silicone and castor oils, and eventually replaced the oils at the liquid/retina interface. We conclude that aqueous solutions continuously secreted in the living eye may displace silicone oil from the retinal surface and contribute to retinal re-detachment. Study of dynamics of wetting of the animal retina by water and organic oils is reported. The exponent describing the dynamics of spreading of the castor oil is lower than that predicted by the Tanner law. Castor oil may provide more effective tamponade than silicone oil.

Vitreous substitutes: challenges and directions

The natural vitreous body has a fine structure and complex functions. The imitation of the natural vitreous body by vitreous substitutes is a challenging work for both researchers and ophthalmologists. Gases, silicone oil, heavy silicone oil and hydrogels, particularly the former two vitreous substitutes are clinically widely used with certain complications. Those, however, are not real artificial vitreous due to lack of structure and function like the natural vitreous body. This article reviews the situations, challenges, and future directions in the development of vitreous substitutes, particularly the experimental and clinical use of a new artificial foldable capsular vitreous body.