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L. Pinchuk, F. Fantes, Y. Zhou, J.B. Martin, J.–M. Parel; The BPEI–InnFocus Glaucoma Drainage Implant – The MiDi: Concept, Physics and Pressure Control . Invest. Ophthalmol. Vis. Sci. 2006;47(13):36.
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Drainage tubes perform well in the long term for the chronic treatment of glaucoma; however, they are difficult to implant, often cause acute hypotony, encapsulate and hinder mobility, and are difficult to remove or add devices in the event that they fail. A novel biomaterial, poly(styrene–b–isobutylene–b–styrene) ("SIBS"), that does not provoke scar tissue or encapsulation in the eye has enabled the development of a valveless micro–tubular device that that can easily be implanted in the eye through a 27g needle.
The MiDi is made from SIBS, a biocompatible biostable material that is very soft and flexible and conforms to the curvature of the eye. It is 11mm long, 250µm in OD and nominally 70, 100 or 150µm in lumen diameter. Integral to the tubular device is a 1mm2 tab which prevents the device from migrating. The shunt is implanted through a 27g needle tract formed in a conjunctival dissection beginning 2mm below the limbus and exiting in the angle. The MiDi is passed through the tract using a special inserter and rests with its proximal tip 2mm into the AC and the distal end in the flap near the equator. The Hagen–Poiseuille equation was used to calculate lumen diameter for a given length to reduce IOP without causing hypotony. Calculations through the 70, 100 and 150µm MiDi’s, at a pressure head of 20mmHg, predict flow rates of 11, 70 and 360µL/min., respectively.
Actual flow measurements are 6.5, 60 and 347µL/min, respectively, which deviate from theoretical by 40, 14 and 3.6%, respectively. The decreasing error trend is predominantly due to 1), breakdown of the Hagen–Poiseuille equation at very small diameters where the surface tension of the fluid begins to play a predominant role and 2) the sensitivity of flow rate as a function of the fourth power of lumen diameter with the inability to measure lumen diameter accurately. Pressure decay curves through the 70, 100 and 150µm tubes from an initial 20mmHg pressure head with concomitant flow from a syringe pump of 2.5µL/min through the device; indicate asymptotes at 9.0, 1.5 and 0.3mmHg, respectively when the exit of the device is exposed to air.
The Hagen–Poiseuille equation is sufficiently accurate to show the trends of pressure drop and flow rate for the three sizes tested without the need for a pressure control valve.
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