The individualized evaluation of the ratio benefit-risk is defined by the maximal gel amount that can be effectively aspirated in the minimal amount of time (the “benefit”), without iatrogenic browsing of the retina or the ciliary epithelium (the “risk”).

For a given vitrectomy machine, the aspiration flow of the vitreous (a viscoplastic medium not susceptible to distortion) varies inversely with its viscosity (Poiseuille law of Physics).

Gel’s viscosity in vivo is variable and defines the individual “vitreal risk factor”. Individual variations of viscosity depend on the patient’s age and refraction, on the degree of gel contraction, itself dependent on the pathology and its duration (presence of blood, pus in the vitreous, diabetes, etc…).

Intraoperative variations depend on the probe’s tip location inside the vitreous cavity. In the lap of the AVB, the gel is mostly compact without liquefaction, the viscosity is the highest in the whole vitreous cavity, and significant change (increase) occur solely inside localized pockets of embedded pathology (blood clots, fibrin, pus…). At the vitreo-retinal junction, when the vitreous is detached, the probe’s port aspirates a mixture of gel of the posterior part of the AVB and of BSS, the viscosity of the material aspirated in the tubings is decreased and varies constantly.

In addition, the effective/safe aspiration flow value depends on the condition of the retina itself: it must be decreased in case of retinal fragility (ischemia, myopia, etc…), even if the retina is not detached. It defines the “retinal risk factor”.

To attract the gel into the probe’s port, it is necessary to bring an aspirating centripetal strain able to exceed the drawback strengthes due to the rigidity of the underlying tissues, and to the viscosity of the sub retinal fluid in case of RD.

It is best effected by the action of “trenching” of the vitreous gel -successive aspiration then cutting- using a flow-control vitrectomy machine.

The maneuver of “trenching” requires to reduce the cutting speed down to around 250-100 cpm in order to ensure effective significant aspiration inside the instrument’s port (video clips); in case of high gel viscosity, dissection must even be sometimes processed “cut-by-cut”.

Flow-control vitrectomy machines allow to work under a constant aspiration flow: flow-control pumps compensate for the intra-operative variations in viscosity along the procedure by self adjustments in the depression (“vacuum”) generated by the pump: the vacuum spontaneously increases when viscosity increases and conversely (video clips with overlay monitoring the intra-operative adjustements of the vacum variations in the central vitreous cavity, in the lap of the vitreous base, at the vitreo-retinal junction in case of posterior vitreous detachment).

For a given eye and its given gel viscosities in vivo, effective aspiration flow values depend in addition on the infusion gradient, the cutting mode, the vitrectomy probe’s design and the compliance forces inside the tubings.

On a case-by-case basis, a surgeon can evaluate indirectly the gel’s viscosity at the AVB at the beginning of the procedure, based on the amount of vacuum necessary to generate the maximum safe-flow he/she has determined according to the performances of the pump he/she uses, and to the individual condition.

With the previous machine we used the effective-safe flow ranged between 1 and 2–3 cc/mn inside a normal AVB, and the associated vacuum necessary ranged between 20 and 50 mmHg (video clip). In case of extreme fibrosis of the AVB the effective flow values had to be actively raised up to around 7–15 cc/mn; the vacuum would spontaneously increase, up to the maximum capacity of depression of the pump, around 500 mmHg, with all the intermediary values generated according to the gel’s viscosity variations (video clip).

With EVA, two major improvements increase the ratio benefit-risk (video clips) :

  • incrementation of the flow can be reduced as down as to 0.1 cc/mn (versus 1cc/mn only with the previous machine), which ensures an even narrower range between risk and benefit
  • the maximum flow necessary in case of extreme fibrosis of the anterior gel is reduced down to around 7cc/mn (versus 15cc:mn with previous machine)

Contact Details:

Cell Phone: +33680637752

Claude Boscher