Hydrodynamic analysis of intraocular pressure changes during anterior chamber procedures.

Variations in intraocular pressure (IOP), observed by us and other authors in closed-system anterior chamber procedures, are explained using a simple hydrodynamic analysis based on the Bernoulli equation. Calculations show that the sudden increases in IOP (pressure spikes) measured during anterior segment surgery are due primarily to the small size of the infusion cannulas and are exacerbated by high infusion rates. Accurate measurement of these IOP variations requires placement of the pressure sensors in situ and is not possible by remote pressure sensing at the infusion line or the console. This work indicates that closed-system procedures could benefit from accurate monitoring of intraoperative IOP and could use these measurements for automatic IOP control during surgery.

Injection of a silicone prepolymer system into the gallbladder of rabbits.

The aim of this study was to assess the feasibility of a new nonsurgical treatment for cholelithiasis. This involved injecting a liquid prepolymer into the gallbladder which would polymerize in situ, entrapping existing gallstones and permanently filling the gallbladder so as to prevent subsequent gallstone formation. Initially, several silicone prepolymer mixtures were tested in vitro for viscosity, cure time, radiopacity, and consistency when polymerized. The optimal system with regard to these parameters consisted of two room temperature vulcanizing silicones (3110 and 200), a catalyst (F), and barium sulfate. This mixture was then injected into beakers containing human gallstones alone and in combination with bile, and was periodically evaluated over six months. It was also injected into the gallbladder of six rabbits and left for 12 weeks. No clinical complications were noted in any of the animals. The cured silicone system maintained its original shape and did not fragment. At necropsy, the casts were free in the rabbits' gallbladders, with small accumulations of biliary sludge adhered to their surfaces. No mucocele of the gallbladder was found in any of the rabbits. Cholecystic histology was normal in all cases, and only one animal exhibited a mild chronic portal triaditis. These promising results indicate the need for further studies of the technique and refinement of the silicone formulation.

Evaluation of drug treatments for proliferative vitreoretinopathy using vitreous microtensiometry.

The effectiveness of antimetabolic and anticollagen agents against proliferative vitreoretinopathy (PVR) was assessed by vitreous microtensiometry, a new technique that measures in situ the tensile strength of vitreous membranes. Two PVR models were produced in rabbits by intravitreal injection of bovine retinal pigment epithelial (RPE) or fibroblast cells, and the animals subsequently were treated with 5-fluorouracil (5-FU) and beta-aminopropionitrile (BAPN), administered alone or in combination. The fibroblast PVR model produced high-strength membranes that did not respond significantly to these therapies. The RPE model gave lower-strength membranes that showed marginally significant decreases in strength with intravitreal 5-FU and systemic BAPN treatments. However, combination therapy showed a highly significant decrease in membrane strength and a clinically encouraging reduction in retinal detachment.

Constant-volume polymerization of composites by addition of ammonia-modified montmorillonite.

Polymerization shrinkage is a major limitation of dental composites. It generates internal stresses within bonded restorations and leads to marginal leakage, sensitivity and recurrent decay in the absence of adequate bonding. A method is presented for formulating composites that cure at constant volume by adding to the resin system small amounts of the hydrated mineral montmorillonite (MMT), which has been modified by replacing part of its hydration water with ammonia (NH3). The polymerization exotherm of composites containing ammonia-modified montmorillonite (NH3/MMT), cured at ambient temperatures, raises their temperature to between 60 and 80 degrees C, which causes the NH3/MMT particles to swell and counteract polymerization shrinkage. The polymerization shrinkage was measured for the BIS-GMA resin system, without filler, to which had been added NH3/MMT. The addition of 4 to 5 weight percent NH3/MMT resulted in zero polymerization shrinkage. An experimental composite was formulated using the BIS-GMA resin system, hydroxyapatite filler and 4 weight percent NH3/MMT. Cured samples developed compressive strength and hardness comparable to reported values for dental composite resins. Porosity was not detected by scanning electron microscopy. The use of NH3/MMT for formulating polymeric systems that cure at constant volume should be explored for application in direct dental composites.

Effect of incision length, location, and shape on local corneoscleral deformation during cataract surgery.

The effect of incision length, location, and shape on the structural integrity of the globe was examined using finite element analysis by comparing the radial deformation of the corneoscleral tissue adjacent to the incision. The results show that incision length had the greatest adverse effect, 12 mm incisions causing deformations an order of magnitude higher than the distension of the intact globe at 20 mm Hg. The least structural disturbance was produced by 3 mm scleral pocket incisions.

The Pressure-Controlled Infusion System: a new instrument for closed-system surgery.

We present the new Pressure-Controlled Infusion System (PCIS), designed to automatically regulate intraocular pressure (IOP) during surgery. Studies were performed with rabbits in which we compared the true IOP during closed-system surgery using gravity-fed infusion with the automatic control provided by the PCIS. The results from irrigation/aspiration of the lens cortex, wound closure, and vitrectomy show that the PCIS performs two functions during closed surgery: it automatically maintains the baseline IOP at the pre-set level, and it dramatically decreases the pressure fluctuations that occur with gravity infusion.

Variations in intraocular pressure during closed-system surgical procedures.

True intraocular pressure (IOP) during closed-chamber anterior segment surgery and vitrectomy was measured in rabbits by inserting a miniature pressure sensor directly into the anterior chamber or into the vitreous. Infusion fluid pressure was also measured simultaneously with a sensor in the infusion line. Significant and rapid changes in IOP up to 110 mm Hg were observed during routine anterior segment and vitreous procedures. None of these IOP changes affected infusion line pressure, which was governed solely by the infusion bottle height. Furthermore, raising the infusion bottle produced minimal IOP elevation whenever fluid was flowing into and out of the eye via a surgical instrument or a wound. These results show the limitations of monitoring of infusion line pressure as a means for assessing IOP and stress the need for IOP control during surgery.

Effects of topical treatment with beta-aminopropionitrile after radial keratotomy in the rabbit.

This study assessed the effect of beta-aminopropionitrile treatment on the long-term curvature and compliance of corneal tissue subjected to radial keratotomy (RK). beta-Aminopropionitrile is known to inhibit cross-linking of collagen and is expected to enhance the flexibility of scar tissue, thereby reducing wound contracture and regression after RK. Seventeen adult New Zealand rabbits, weighing about 4.5 kg each, underwent RK in both eyes (eight incisions, 90% deep, 3-mm pupillary zone). Their mean preoperative corneal curvature was 44.25 diopters (+/- 0.32D at 95% confidence level). Nine of the rabbits were treated topically with beta-aminopropionitrile ointment (33 weight % in petroleum three times daily), while the control group received the petrolatum base only. The remainder of the animals received bland petrolatum gel as a control. The animals were given periodic keratometric examinations and were killed after six to eight weeks. At that time, the beta-aminopropionitrile group showed a mean reduction of 1.85 +/- 0.13 D in corneal curvature, compared with 1.18 +/- 0.08 D in the control group. The compliance and strength of the corneas were measured in vitro immediately after death. In the pressure range from 10 to 40 mm Hg, the beta-aminopropionitrile-treated corneas changed in curvature by an average of 1.4 D as compared with 0.5-D flattening for the controls. These results indicate the effectiveness of beta-aminopropionitrile treatment in enhancing longterm compliance and reducing refractive regression after RK.

Investigation of the aggregation and activation of prothrombin using quasi-elastic light scattering.

The technique of quasi-elastic light scattering was used to measure the translational diffusion coefficient, D, of purified human prothrombin in buffered aqueous solutions and to monitor for the first time the fragmentation of this protein as it is converted to thrombin. The values of D20,w, measured at two different concentrations, are 4.72 X 10(-7) CM2/S at 2MG/CM3 and 4.51 X 10(-7) CM2/S at 5MG/CM3; the corresponding molecular weights (Mw of 92 000 and 120 000), obtained by combining sedimentation velocity measurements with the diffusion data, confirm the presence of molecular aggregates of prothrombin in these solutions. These results, as well as analysis of the intensity-intensity autocorrelation functions from two-component systems with various dimer conformations, indicated the presence of end-to-end dimers in these prothrombin solutions. The values obtained for D indicate a dimer weight fraction of 0.4 to 0.5 in the 2 mg/cm3 solution and 0.6 or greater in the 5 mg/cm3 solution. The fragmentation of prothrombin was monitored in a nonphysiologic activation system, containing taipan snake venom, dihexanoylphosphatidylcholine, and CaCl2. At a temperature of 15 degrees C, conversion to thrombin proceeded very slowly and was still incomplete after 90 h. A method for determing the percentage of converted prothrombin is an activated system containing aggregates from the average value of D and light scattering data is discussed.

Ultrastructure of the human aortic valve.

The human aortic valve is examined histologically by light, transmission electron and scanning electron microscopy. The observations show that the valve leaflets are multi-laminated structures, whose laminae do not have very distinct boundaries. Through the whole thickness of the leaflets, the load-bearing collagenous fibers are preferentially oriented in the transverse direction. Elastin networks are found only in the ventricular area. The leaflets attach themselves to the ventricular musculature through very dense collagenous meshworks known as the aortic ring. The whole structure is very efficient for its function of sustaining large pressures when closed and opening under minimal pressure gradients in systole.

Mechanical and histological study of aortic valve tissue from a patient with Marfan's disease.

A comparison between a normal and a Marfan's disease aortic valve has been attempted in terms of their tissue stress-strain characteristics and their histology. The diseased valve tissue was found to be more compliant by an order of magnitude than the normal valve, which is related to the disorientation of its collagen fibers, and the proliferation of mucopolysaccharides.

Growth of cultured calf aortic smooth muscle cells on cardiovascular prosthetic materials.

The growth of cultured calf aortic smooth muscle cells on cardiovascular biomaterials was investigated, using native and oxidized polyacrylonitrile (orlon) fabrics, dacron velour, and Parylene-C coated polypropylene microfabric as substrates. By light microscopic evaluation, surface cell coverage was most complete on microfabric, followed by native orlon, dacron velour, and oxidized orlon. Native orlon supported the greatest total cell growth, as determined by chemically extractable protein, followed by oxidized orlon, dacron velour, and the microfabric. The observed differences appear to be related to the pore size and fiber thickness of the different substrates.