RETINAL TECTONICS AFTER MACULAR PUCKER SURGERY: Thickness Changes and En Face Displacement Recovery.

PURPOSE: To study visual function, retinal layer thickness changes, and tangential displacement after pars plana vitrectomy for epiretinal membrane. METHODS: Retrospective series of patients undergoing pars plana vitrectomy for epiretinal membrane, with 6-month follow-up including best-corrected visual acuity, optical coherence tomography, M-charts, epiretinal membrane grading, and infrared fundus photograph at time 0 (T0, preop) at months 1 (T1), 3 (T3), and 6 (T6) postop (±1 week). Retinal layer thickness and tangential ( en face ) retinal displacement between successive times for the entire retinal surface and the central horizontal and vertical meridian were also measured. En face displacement was calculated as optical flow of consecutive images. RESULTS: Average best-corrected visual acuity improved from 0.28 ± 0.08 logarithm of Minimum Angle of Resolution at T0 to 0.16 ± 0.25 at T6 ( P = 0.05), best-corrected visual acuity improvement correlated with best corrected visual acuity (BCVA) at T0 ( P < 0.001). Vertical metamorphopsia decreased from 1.33° ± 0.70° at T0 to 0.82° ± 0.69° at T6 ( P < 0.05). Foveal thickness reduced from 453 ± 53 µ m at T0 to 359 ± 31 µ m at T6 ( P < 0.05) and reduction correlated with best-corrected visual acuity improvement ( P < 0.05). Foveal layers decreased ( P < 0.05) in all cases. The mean en face deformation was 155.82 ± 50.17 µ m and mostly occurred in the first month: T0-T1 displacement was 83.59 ± 30.28 µ m, T1-T3 was 36.28 ± 14.45 µ m, while T3-T6 was 39.11 ± 22.79 µ m ( P < 0.001) on average. Perifoveal and parafoveal deformation correlated with optical coherence tomography foveal thickness reduction at all time intervals (1, 3, and 6 months: P < 0.01). CONCLUSION: Epiretinal membrane peeling affects all retinal layer thickness and results in new force balance across the entire retina and tangential displacement. Both en face and in-depth changes correlate with visual function.

TANGENTIAL RETINAL DISPLACEMENT INCREASES AFTER MACULAR PUCKER SURGERY: An Apparent Nonsense.

PURPOSE: To measure the tangential retinal displacement and vision before and after macular pucker surgery and study if pars plana vitrectomy with epiretinal membrane peeling allows the reconstitution of previous anatomy or else it results in a different configuration. METHODS: Retrospective series of patients undergoing pars plana vitrectomy for epiretinal membrane, with >6-month follow-up before and after surgery, complete with best-corrected visual acuity, optical coherence tomography, M-Charts, and infrared retinography. Tangential retinal displacement between earliest visit (T E ), time of surgery (T 0 ), and latest available visit (T L ) of the examined retina, concentric circles at 0.5, 1.5, and 4.5 mm radii, and the central horizontal and vertical meridians were measured. Tangential displacement was calculated as the optical flow of consecutive infrared photographs. RESULTS: The study comprised 32 patients: 15 men and 17 women. Average preoperative and postoperative follow-up were 23.4 ± 27.9 months and 19.2 ± 11.8 months, respectively. Best-corrected visual acuity reduced before surgery (0.69 ± 0.16 Snellen to 0.46 ± 0.17; P < 0.001) and increased after (0.866 ± 0.16 Snellen; P < 0.001). Horizontal and vertical metamorphopsia increased between before surgery but only horizontal metamorphopsia significantly reduced after. Average tangential displacement before surgery was 35.6 ± 29.9 µ m versus 56.6 ± 41.3 µ m after ( P = 0.023). Preoperative and postoperative displacement within the fovea was less than over the entire area ( P < 0.001). CONCLUSION: Retinal tangential displacement between diagnosis and surgery (T E - T 0 ) is less than the displacement occurring after surgery (T 0 - T L ). Postoperative displacement does not represent the restoration of the anatomy existing before the disease ensued but rather the resulting equilibrium of newly deployed forces.

Phacoemulsification Tip Elongation Under Standardized Resistance: An Objective Measure of Human Crystalline Lens Hardness.

Purpose: To establish a correlation between phacoemulsification tip normalized driving voltage (NDV) and crystalline lens hardness and use it as an objective measure of lens hardness. The study used a phaco tip equipped with previously validated elongation control adjusting the driving voltage (DV) to produce invariant elongation regardless of resistance. Methods: The laboratory study measured the mean and maximum DV of the phaco tip immersed in glycerol-balanced salt solution and correlated the DV with the kinematic viscosity at 25, 50, and 75 µm tip elongation. The NDV were obtained by dividing the DV in glycerol by the DV in the balanced salt solution. The clinical arm of the study recorded DV of 20 consecutive cataract surgeries. The correlation of mean and maximum NDV to Lens Opacities Classification System (LOCS) III classification, patient's age and effective phaco time were evaluated. Results: The mean and maximum NDV correlated with the kinematic viscosity of the glycerol solution (P < 0.001 in all cases). Mean and maximum NDV during cataract surgery correlated with patients' age, effective phaco time, LOCS III nuclear color, and nuclear opalescence (P < 0.001 in all cases). Conclusions: When a feedback algorithm is running, DV variation strictly correlates with encountered resistance in glycerol solutions and real-life surgery. NDV significantly correlates with the LOCS classification. Future developments might include sensing tips that react to lens hardness in real time. Translational Relevance: The study correlates for the first time phaco tip DV and crystalline lens mechanical properties, establishing an objective and reliable measure of lens hardness. This may lead to smart phaco tips reacting to cataract hardness change in real time and sparing ultrasound dispersion.

Silicone Oil Tamponade Removal: Which Technique Is More Effective? An X-Ray Photoemission Spectroscopy Study.

Purpose: To compare the efficacy of two surgical techniques used to remove silicone oil (SiO) emulsion tamponade after pars plana vitrectomy: triple air-fluid exchange (AFX) and balanced salt solution lavage (BSSL). Methods: X-ray photoemission spectroscopy measured silicon content of the dry residue of fluid samples taken during AFX and BSSL. Ten patients underwent AFX and five BSSL. Three fluid samples were taken per patient, and the dry residue of 10 drops per sample were analyzed. A fluid sample from a patient who never received SiO tamponade was also analyzed to set a "blank" reference sample. Results: Patients' demographics showed no significant difference. Sample 1 of the two groups contained comparable silicon content while samples 2 and 3 of the AFX group contained significantly more silicon than that of the BSSL group (15.0 ± 0.1 and 12.0 ± 0.9 for the AFX group vs. 10.7 ± 1.4 and 5.2 ± 0.6 for the BSSL group, respectively; P < 0.05). The cumulative amount of silicon in the three successive samples was also significantly higher for the AFX group (42.3 ± 1.6 vs. 32 ± 2; P < 0.0001). The average silicon content ratio of consecutive samples was significantly higher for the AFX group compared to the BSSL group (0.90 ± 0.01 vs. 0.58 ± 0.06; P = 0.006). Conclusions: Triple AFX removed more silicon than triple lavage. The eye wall actively interacts with silicon emulsion retaining silicon content rather than behaving as a neutral container. Translational Relevance: Triple air-fluid exchange removed more silicon than BSS lavage. Neither technique behaved as a well-mixed box dilution, suggesting the eye walls actively retain emulsion and a dynamic equilibrium is established between silicon dispersion and the eye wall surface.

DO DAILY ACTIVITIES AFFECT GAS TAMPONADE‒RETINA CONTACT AFTER PARS PLANA VITRECTOMY?: A Computational Fluid Dynamics Study.

PURPOSE: To calculate the retinal surface alternatively in contact with gas and aqueous because of fluid sloshing during daily activities such as ocular saccade, turning the head, standing up, and being a passenger of a braking car. METHODS: Fluid dynamics of aqueous and gas tamponade was reproduced using computational methods using the OpenFOAM open-source library. The double-fluid dynamics was simulated by the volume of fluid method and setting the contact angle at the aqueous-gas-retina interface. RESULTS: Sloshing increased the retinal surface in contact with aqueous by 13% to 16% regardless of fill rate and standing up determined the largest area of wet retina, followed by car braking, head rotation, and ocular saccade ( P < 0.001). All activities except the ocular saccade determined a significant increase in the surface of retina in contact with the aqueous ( P < 0.005). Car braking induced the highest shear stress (6.06 Pa); standing up determined the highest specific impulse and saccade the lowest. CONCLUSION: Daily activities instantaneously reduce the amount of retina consistently in contact with gas tamponade and increase shear stress giving aqueous a potential access to the subretinal space regardless of patients' compliance.

Silicone Oil Tamponade-Retina Contact in Highly Myopic Eyes With and Without Encircling Bands: A Computational Fluid Dynamics Study.

Purpose: To investigate the behavior of silicone oil (SiO) at a steady equilibrium and during saccades in pseudophakic highly myopic eyes with posterior staphyloma with and without an encircling band and compare it to behavior in emmetropic eyes. The SiO-retina contact area and shear stress were calculated by computational fluid dynamics. Methods: A numerical model of an emmetropic eye and a myopic eye with and without scleral band underwent a saccade of 50°/0.137 s. The vitreous chamber surface was divided into superior and inferior 180° sectors: lens, pre-equator, post-equator, and macula. SiO-retina contact was evaluated as a function of fill percentages between 80% and 90% for standing, 45° upward tilt, and supine patients. Maximum and average shear stress were calculated. Results: Overall, SiO-retina contact ranged between 40% and 83%; fill percentage varied between 80% and 95%. Neither the encircling scleral band nor the staphyloma significantly affected the SiO-retina contact area, although the presence of a scleral band proved disadvantageous when gazing 45° upward. The inferior retina-SiO contact remained below 40% despite 95% SiO fill. The SS significantly increased at the scleral band indentation and decreased elsewhere. The staphyloma greatly reduced shear stress at the macula. Conclusions: The presence of a myopic staphyloma reduces shear stress at the macula but does not alter SiO-retina contact significantly. The apposition of a 360° scleral band may reduce SiO-retina contact at least in some postures and increases the SS at the indentation. Translational Relevance: Assessing SiO-retina contact when vitreous chamber geometry changes according to pathologic or iatrogenic modifications allows accurate prediction of real-life tamponade behavior and helps explain surgical outcomes.

Human Vitreous Collagen Fragments Dimension As a Function of Vitrectomy Cut Rate.

Purpose: To study the dimensions and distribution of human vitreous collagen type II fragments collected after vitrectomy performed at varying cut rates and to evaluate if increasing the cut rate produces smaller collagen fragments, thus reducing retinal traction and/or viscosity. Methods: Fluid was collected during core vitrectomies performed for macular surgery at cut rates from 1000 to 16,000 cuts per minute (CPM) and immediately refrigerated. Protein fractions were separated by molecular weight (MW; >100 kDa, 50-100 kDa, 50-30 kDa, 30-10 kDa, and <10 kDa) through centrifugal filters. The Human Collagen II ELISA Kit colorimetric assay was then used to measure the COL2A1 in unfiltered and filtered samples. Results: Vitreous samples collected after vitrectomy performed at 16,000 CPM contained a higher concentration of protein with MW over 100 kDa than at any other cutting frequency (P < 0.01). No significant differences were found in fractions collected with a MW between 50 and 100 kDa. Collagen type II fragments over 100 kDa were significantly more represented than smaller fragments at each cut rate. The proportion of smaller (50-100 kDa) collagen fragments compared with those over 100 kDa was higher at 2000 CPM than at higher cut rates. Conclusions: Vitreous samples collected at different cut rates do not contain a significantly different proportion of collagen type II fragments of the tested MW. The extreme variability of vitreous flow through the cutter port may explain the uncertain predictability of collagen fragment MWs. Translational Relevance: Increasing the cut rate does not produce vitreous fragments of proportionally smaller dimension. It is necessary to achieve an invariant instantaneous flow through the cutter port in order to decrease retinal traction during vitrectomy.

Computational Fluid Dynamics of Intraocular Silicone Oil Tamponade.

Purpose: To investigate the behavior of silicone oil (SiO) at the steady equilibrium and during saccades and calculate SiO-retina contact, shear stress (SS), and shear rate (SR). Methods: A 24 mm phakic eye mesh model underwent 50°/0.137s saccade. The vitreous chamber compartment was divided into superior and inferior 180° sectors: lens, pre-equator, postequator, and macula. SiO-retina contact was evaluated as a function of fill percentages between 80% and 90% for a standing patient, 45° upward gaze, and supine. SS and SR for 1000 mPa-s (SiO1000) and 5000 mPa-s (SiO5000) silicon oil were calculated. Results: SiO fill between 80% to 90% allowed 55% to 78% retinal contact. The superior retina always kept better contact with SiO, regardless of the fill percentage (P < 0.01). SiO interface thoroughly contacted the macula only in standing position. SS followed a bimodal behavior and was always significantly higher for SiO5000 compared to SiO1000 (P < 0.01) throughout the saccade. The macula suffered the highest mean SS in standing position, while throughout the saccade the average SS was maximum at the SiO-aqueous interface. SR was significantly higher for SiO1000 compared to SiO5000 (P < 0.001). Conclusions: SS on the retinal surface may instantaneously exceed reported retinal adhesiveness values especially at the SiO-aqueous interface and possibly favor redetachment. Despite 90% SiO fill the inferior retina remains extremely difficult to tamponade. Translational Relevance: Accurate assessment of retina-tamponade interaction may explain recurrent inferior retinal redetachment, silicone oil emulsification, and help to develop better vitreous substitutes.

Testing a Novel Device for Accurate Ultrasound Delivery During Crystalline Lens Phacoemulsification Surgery.

Purpose: To assess whether the use of a patented, novel feedback device intended to accurately control phacoemulsification tip elongation is effective under varying machine settings and material resistance. Methods: Sculpt mode phaco (550-mm Hg Venturi pump; elongations, 35 and 70 µm) and quadrant settings (550-mm Hg Venturi pump; elongations, 15, 30, and 60 µm) were used in agar gel of incremental density (1%, 2%, 3%, and 6% in demineralized water). Dispersed lens fragments were also simulated with 6% agar gel spherules (2-5 mm in diameter; 550-mm Hg vacuum, and 60-µm elongation). Actual phaco tip elongation was measured on voltage readings from the piezoelectric crystals and compared to nominal elongation with feedback control off and on. Results: Mismatch between nominal and actual elongation when feedback control was off in sculpt mode varied between -13.51 µm and -23.07 µm of nominal elongation; in quadrant mode, mismatch varied between -2.79 µm and -20.41 µm. When the feedback control system was switched on, mismatch varied between -0.02 µm and +0.43 µm (P < 0.001 for all matchings). When the feedback system was off, the elongation mismatch among the 1%, 3%, and 6% agar was also statistically significant (P < 0.001). Elongation was 44.72 ± 4.16 µm with feedback control off and 60.02 ± 1.63 µm with it on (nominal elongation 60 µm; P < 0.001) when emulsifying agar 6% gel fragments. Dispersion of elongation data was also significantly wider when feedback control was turned off. Conclusions: A novel feedback control system effectively controls elongation accuracy regardless of the resistance offered by incremental agar gel concentrations. Translational Relevance: Implementing feedback control in phaco handpieces dramatically improves surgical accuracy. The translational value of this research relies on its immediate applicability to routine cataract surgery, resulting in a more appropriate use of ultrasound energy.

Left Ventricular Hemodynamics with an Implanted Assist Device: An In Vitro Fluid Dynamics Study.

Left ventricle assist devices (VADs) aid the heart pumping blood into the systemic circulation and grant the required cardiac output (CO) when the heart itself cannot provide it. However, it is unclear how effective these devices are at restoring not only physiological CO values but also normal intraventricular hemodynamics. In this work, the modified hemodynamics due to a VAD implantation is studied in vitro using an elastic ventricle made of silicone, which is incorporated into a pulse-duplicator setup prescribing a realistic pulsatile flow. Thereafter, a continuous axial pump is connected at the ventricle apex to mimic a VAD and its effect on the ventricular hemodynamics is investigated as a function of the pump flow suction. Using particle image velocimetry (PIV), we observe that the continuous pump flow effectively provides unloading on the ventricle and yields an increased CO. Conversely, the continuous blood suction from the ventricle apex deeply alters the hemodynamics and, in addition, the VAD obstruction in the ventricle behaves as a bluff body that affects the vorticity distribution in the LV thus creating a stagnant region at the ventricle apex. This phenomenon is rationalized by measuring in a modified set-up the benefits on the hemodynamics of a flush-mounted device. Additionally, the suction operated by the VAD reduces the ventricular pressure and yields an increase in the swirling motion around the ventricle axis, in a similar fashion as the bath-tub vortex effect, thus further modifying the intraventricular hemodynamics with respect to healthy conditions.

Does the Bursa Pre-Macularis protect the fovea from shear stress? A possible mechanical role.

Purpose of present study is to evaluate whether the Pre-Macular Bursa (PMB) modifies Wall Shear Stress (WSS) at the retinal surface during saccadic movements. We created a mathematical model consisting of 25,000 grid cells and simulated a horizontal saccade spanning 50° in 0.17s, both in absence and in presence of the PMB. Wall Shear Stress SS was computed throughout the retinal surface and the posterior pole was divided into 3 Zones comprising 400 nodes each: Zone 1 (radius 3.5 mm; 0°-17°) corresponding to the PMB area; Zone 2 (concentric annular area 5 mm in radius; 22°) and Zone 3 (concentric annular area 5.5 mm; 28°). The PMB reduced WSS significantly at the macula and increased it in the immediate surroundings. Average WSS in Zone 1 was 1.53 ±â€¯1.01 (max 4.23 Pa) with PMB Vs 6.94 ±â€¯9.23 (max 35.83 Pa) without. Zone 2 WSS was 9.39 ±â€¯10.33 (max 48.36 Pa) with PMB Vs 6.95 ±â€¯9.40 (max 38.60 Pa) without Zone 3 WSS was 8.41 ±â€¯10.03 (max 43.16 Pa) with PMB Vs 6.88 ±â€¯9.42 (max 39.43 Pa) without (p < 0.001 in all cases). The PMB significantly reduces WSS over the retinal surface underlying the bursa region; conversely, WSS slightly increases it in the immediate neighboring areas.

Ocular perfusion pressure control during pars plana vitrectomy: testing a novel device.

PURPOSE: To study the efficacy of a novel device intended to control infusion pressure based on mean ocular perfusion pressure (MOPP) during pars plana vitrectomy (PPV). METHODS: An arm blood pressure cuff connected to a vitrectomy machine calculated mean arterial pressure (MAP), while a pressure sensor close to the infusion trocar measured intraocular pressure (IOP). MOPP was calculated in real time in 36 consecutive patients undergoing PPV, who were divided into two groups. The device lowered IOP every time that calculated MOPP fell below 30 mmHg in the Control ON group (18 patients), while no action was taken in the Control OFF group (18 patients). RESULTS: Baseline IOP and blood pressure were similar between groups. The Control ON group had significantly lower average intraoperative IOP (30.5 ± 2.1 vs. 35.9 ± 6.9 mmHg; p = 0.002) and higher MOPP (56.4 ± 5.9 vs. 49.7 ± 6.1 mmHg) than the Control OFF group. The Control ON group also spent less time at MOPP < 10 mmHg and < 30 mmHg: 0 vs. 3.40 ± 2.38 min (p < 0.001) and 9.91 ± 7.15 vs. 16.13 ± 8.12 min (p = 0.02), respectively. CONCLUSIONS: The MOPP control device effectively maintained lower IOP and higher MOPP throughout surgery. It also helped avoid dangerous IOP peaks and MOPP dips, allowing patients to spend less time at MOPP of < 10 and < 30 mmHg.

Integrated strategy for in vitro characterization of a bileaflet mechanical aortic valve.

BACKGROUND: Haemodynamic performance of heart valve prosthesis can be defined as its ability to fully open and completely close during the cardiac cycle, neither overloading heart work nor damaging blood particles when passing through the valve. In this perspective, global and local flow parameters, valve dynamics and blood damage safety of the prosthesis, as well as their mutual interactions, have all to be accounted for when assessing the device functionality. Even though all these issues have been and continue to be widely investigated, they are not usually studied through an integrated approach yet, i.e. by analyzing them simultaneously and highlighting their connections. RESULTS: An in vitro test campaign of flow through a bileaflet mechanical heart valve (Sorin Slimline 25 mm) was performed in a suitably arranged pulsatile mock loop able to reproduce human systemic pressure and flow curves. The valve was placed in an elastic, transparent, and anatomically accurate model of healthy aorta, and tested under several pulsatile flow conditions. Global and local hydrodynamics measurements and leaflet dynamics were analysed focusing on correlations between flow characteristics and valve motion. The haemolysis index due to the valve was estimated according to a literature power law model and related to hydrodynamic conditions, and a correlation between the spatial distribution of experimental shear stress and pannus/thrombotic deposits on mechanical valves was suggested. As main and general result, this study validates the potential of the integrated strategy for performance assessment of any prosthetic valve thanks to its capability of highlighting the complex interaction between the different physical mechanisms that govern transvalvular haemodynamics. CONCLUSIONS: We have defined an in vitro procedure for a comprehensive analysis of aortic valve prosthesis performance; the rationale for this study was the belief that a proper and overall characterization of the device should be based on the simultaneous measurement of all different quantities of interest for haemodynamic performance and the analysis of their mutual interactions.

Hydraulic Resistance of Vitreous Cutters: The Impact of Blade Design and Cut Rate.

PURPOSE: To measure the hydraulic resistance (HR) of vitreous cutters equipped with a Regular guillotine Blade (RB) or double edge blade (DEB) at cut rates comprised between 0 and 12,000 cuts per minute (CPM) and compare it with vitreous fragment size. This was an in vitro experimental study; in vivo HR measure and vitreous sampling. METHODS: HR, defined as aspiration pressure/flow rate, was measured in balanced salt solution (BSS; Alcon, Fort Worth, TX) (in vitro) and during pars plana vitrectomy of 20 consecutive patients aged 18 to 65, undergoing macular surgery. HR was recorded at increasing cut rates (500-6000 CPM for the RB and 500-12,000 CPM for the DEB; 5 mL/min flow). Vitreous samples were withdrawn and analyzed with Western and collagen type II and IX immunostaining to evaluate protein size. The main outcome measures were hydraulic resistance (mm Hg/ml/min [±SD]) and optic density for Western blot and immunostaining. RESULTS: RB and DEB showed identical HR in BSS between 0 and 3000 CPM. Above 3000 CPM, RB HR steadily increased, and was significantly higher than DEB HR. Vitreous HR was also similar for the two blades between 0 and 1500 CPM. Above 1500 CPM, RB offered a significantly higher resistance. Western blot and immunostaining of vitreous samples did not yield a significant difference in size, regardless of blade type and cut rate. CONCLUSIONS: DEB is more efficient, offering a lower HR than RB over 1500 CPM in human vitreous. There is no viscosity reduction as a function of cut-rate between 1500 and 12,000 CPM, as HR does not vary. TRANSLATIONAL RELEVANCE: Future vitreous cutters will benefit of a DEB; optimal cut rate needs to be defined, and the simple increase of cut rate does not provide benefits after a certain limit to be assessed.

AXIAL ROTATION VITRECTOMY: Back to the Future? the Fluidics of a Prototype Vitreous Cutter Probe.

PURPOSE: To characterize the fluidics of axial rotating vitreous cutter probe (RT) compared with the standard guillotine (regular blade), when tested in Balanced Salt Solution (Alcon Laboratories, Forth Worth, TX). METHODS: RT and regular blade (RB) cutter probes connected to the same vitrectomy console used a peristaltic pump. The authors measured instantaneous flow through aspiration tubing proximal to the handpiece, fluid velocity, and acceleration at the port by means of particle image velocimetry. RESULTS: Average flow at aspiration tubing of RT and RB did not vary significantly. Regular blade probes produced higher instantaneous flow fluctuation than RT at any considered cut rate (RB 1,600 6.4 ± 5.3 mL/minute; RB 3,000 11.8 ± 6.3 mL/minute; RT 1,600 0.9 ± 0.7 mL/minute, and RT 3,000 1.8 ± 0.8 mL/minute, respectively. P < 0.001 in all cases). Regular blade also yield significantly higher fluid velocity at cutter port compared with RT (RB 1,600 85.8 ± 70.1 mm/second; RB 3,000 81.6 ± 66.4 mm/second; RT 1,600 71.9 ± 40.3 mm/second; and RT 3,000 32.9 ± 20.8 mm/second. P < 0.001 in all cases). Fluid acceleration at the cutter port was higher when the RB was used (RB 1,600 26.85 ± 30.18 mm/second; RB 3,000 33.76 ± 34.09 mm/second; RT 1,600 24.01 ± 21.94 mm/second; and RT 3,000 16.62 ± 17.87 mm/second. P < 0.001 in all cases). CONCLUSION: RT blade design causes less instantaneous flow fluctuation within the aspiration tubing, and also lower fluid velocity and lower acceleration at the cutter port. Fluidics suggests a safer cutting action and a reduced risk of retinal incarceration.

Ocular perfusion pressure during pars plana vitrectomy: a pilot study.

PURPOSE: Unexplained visual field loss after pars plana vitrectomy (PPV) has been reported in up to 14% of all uncomplicated cases with signs varying from visual field defect and disc pallor, to optic atrophy, loss of vision, and phthisis bulbi. Among the postulated pathogenic mechanism is ocular hypoperfusion due to insufficient blood pressure (NBP) and/or elevated IOP, or to their mismatch. The purpose of this study is to assess if, to what extent, and for how long the intraoperative simultaneous variation of IOP and NBP causes mean ocular perfusion pressure (MOPP) to drop below values considered safe, during PPV. METHODS: An IOP sensor placed in the infusion cannula recorded 6 readings per second, while arm systolic and diastolic NBP were taken every 5 minutes throughout surgery and deemed stable in between readings. Supine MOPP was calculated as (115/130) mean arterial pressure--IOP. Surgical monitor video overlay displayed all data in real time and saved them for analysis. RESULTS: Average IOP significantly increased during surgery, while NBP decreased, compared to baseline. As a result, intraoperative MOPP decreased an average 37.1% compared to baseline (range, 13.8%-58.6%; P < 0.05). Of 18 patients, 16 (88.8%) had a significant intraoperative MOPP decrease; 15/18 (83.3%) spent more than 20%, and 5/18 (27.7%) more than 50% of the entire surgery below 30 mm Hg MOPP. Surgical maneuvers, such as phacoemulsification, silicone oil removal, and fluid injection, were associated with significant MOPP decrease, while peeling and vitrectomy were not. CONCLUSIONS: The MOPP may decrease significantly in course of PPV, acutely and for longer time. Surgical maneuvers, including silicone oil removal and combined phacoemulsification, pose a higher risk for MOPP reduction. Discretion should be exercised while administering deep sedation, since it may further lower MOPP through undue blood pressure reduction.

Instantaneous flow rate of vitreous cutter probes.

PURPOSE: We report on instantaneous volumetric flow rate of vitreous cutters measured by means of particle image velocimetry (PIV). METHODS: In an in vitro experimental study, vitreous cutters mounting a regular blade (RB) or modified Twedge blade (TB) engineered for higher flow were connected to a console machine equipped with a double peristaltic and Venturi pump, and immersed in balanced salt solution (BSS). Instantaneous flow was measured on aspiration tubing sections proximal to the cutter hand piece. Measures settings were as follows: (1) regular functioning at 3000 and 6000 cuts per minute (cpm) with 300 mm Hg aspiration with both pumps, (2) aspiration tubing clamped proximal to pump cassette, and (3) aspiration tubing clamped proximal to hand piece, and (4) flow fluctuation as a function of cut rate also was calculated. For main outcome measures, instantaneous volumetric flow rate in mL/min and flow fluctuation measured as the standard deviation of flow rate were measured. RESULTS: Regular functioning shows sinusoidal flow oscillating at cut rate frequency, with amplitude between ±50 mL/min at 3000 cpm and ±35 mL/min at 6000 cpm. The TB always determined a bimodal wave and neither blade nor pump type influenced the sinusoidal pattern of flow. Clamping aspiration tubing zeroes flow, but does not influence fluctuation frequency or amplitude. Clamping at the hand piece determined a significantly higher oscillation. Oscillation amplitude retain a typical resonance pattern with significant changes in function of cut rate and resonance occurs at approximately 4000 cpm. CONCLUSIONS: Cutter blade action determines instantaneous flow rate fluctuation that interferes significantly with cutter suction and hampers a steady suction through cutter port. In a surgical scenario, this translates into a higher risk of inadvertent retinal entrapment and lower predictability of cutter behavior, especially at frequency approaching resonance.

Fluid dynamics of aortic root dilation in Marfan syndrome.

Aortic root dilation and propensity to dissection are typical manifestations of the Marfan Syndrome (MS), a genetic defect leading to the degeneration of the elastic fibres. Dilation affects the structure of the flow and, in turn, altered flow may play a role in vessel dilation, generation of aneurysms, and dissection. The aim of the present work is the investigation in-vitro of the fluid dynamic modifications occurring as a consequence of the morphological changes typically induced in the aortic root by MS. A mock-loop reproducing the left ventricle outflow tract and the aortic root was used to measure time resolved velocity maps on a longitudinal symmetry plane of the aortic root. Two dilated model aortas, designed to resemble morphological characteristics typically observed in MS patients, have been compared to a reference, healthy geometry. The aortic model was designed to quantitatively reproduce the change of aortic distensibility caused by MS. Results demonstrate that vorticity released from the valve leaflets, and possibly accumulating in the root, plays a fundamental role in redirecting the systolic jet issued from the aortic valve. The altered systolic flow also determines a different residual flow during the diastole.

Introducing new vitreous cutter blade shapes: a fluid dynamics study.

PURPOSE: To assess the efficacy of novel vitreous cutter blades compared with the regular guillotine by means of particle image velocimetry. Tested blades included a regular blade (RB) and newer designs where a circular (hole blade [HB]) or a slit aperture (slit blade [SB]) had been opened proximal to the cutting edge. METHODS: Twenty-three-gauge probes were immersed in BSS or egg albumen, and high-speed video (1,000 frames per second) was recorded. Duty cycle, flow rate, and acceleration generated by Venturi and peristaltic pumps were measured under cutter settings simulating "low-speed" vitrectomy (1600 cuts per minute, 200 mmHg vacuum) and "high speed" vitrectomy (3000 cuts per minute, 300 mmHg vacuum). RESULTS: The SB and HB had a significantly more favorable duty cycle than that of the RB (P < 0.01) and higher BSS flow rate regardless of the aspiration. The SB flow rate in albumen was significantly higher than that of the HB and RB only over 1,000 cuts per minute using a peristaltic pump and at any cut rate with Venturi pump (P < 0.001). The SB also yielded the lowest fluid acceleration than both the HB and SB (P < 0.001 in all cases). CONCLUSION: The HB and SB proved to be significantly more efficient than the RB, with better duty cycle and higher flow rate. The SB generated significantly less fluid acceleration than any other tested blade, regardless of the fluid viscosity and pump technology. The SB design is significantly more efficient and possibly safer than both HB and RB.