Technological advances in the dynamics of Phacoemulsification

Sushmita Kaushik & Jagat Ram
(Department of Ophthalmology, PGIMER, Chandigarh-160 012 (India)

Phacoemulsification represents the undoubted standard of care in the treatment of cataract. Better technology has contributed immensely to the development of this procedure. A major refinement in technology was the development of the piezoelectric crystal with a titanium tip.

Present phaco technology incorporates the use of a phaco handpiece with a piezoelectric crystal, which vibrates at a specific frequency (47 kHz) in an electric field.1 The mechanical energy is translated into axial oscillation of the titanium tip. There are 3 postulated mechanisms of action:-

Acoustic breakdown by ultrasonic waves, Micrscopic cavitation bubbles which implode with heat and shock waves and Purely mechanical effect like a sledgehammer.

Recent developments in phacoemulsification technology involve innovations in phaco tips, aspirating tips, phaco handpieces and the phaco machine itself.

The thermal effect on the cornea during the phacoemulsification procedure was previously a cause for concern only when very severe, resulting in a phaco burn. However, in the present era of refractive cataract surgery, it is fast assuming a cause for concern since even moderate levels of thermal transfer to the clear cornea may lead to undesirable astigmatism and possible wound retraction. Phaco tip sleeve constriction by a tight incision is now considered a major in its pathogenesis. Few of the newer tips1 devised to minimize this thermal effect are:-

Microseal tip (Mackool)-central teflon jacket which is rigid and does not allow compromise of fluid flow around the titanium needle, combined with an outer silicon sleeve. Microflow tip-spiral groove on the outer surface of the needle so that the sleeve is not compressed against the tip.

V-shaped inferior surface to facilitate grooving, Masket ergo tip-bent near hub-more comfortable, Mostel square chisel tip (zero degree)- facilitates chopping, Mastel power chisel-sharp edges to facilitate grooving, Kelman tip-30o downward bend results in non-axial vibration. It thus enables grooving nucleii in front of the tip-uses no forward force during sculpting and is therefore safer as regards zonular stress, Cobra tip has a distal wide shaft, narrow proximal shaft and a funnel shaped orifice. This ensures a focussed delivery of U/S energy and less power is required.

Removal of sub incisional cortex is always problematic. Allergan® has introduced the AMO Silicone tip2 which is an interesting variation of the standard tip design. It is made of silicon and is transparent, which permits actual visualization of the cortex being aspirated. It can transform to any configuration by a sleeve controller, which provides easier access to difficult areas.

A new feature introduced by the OcusystemTM (Surgical Design) machine is a small lightweight handpiece measuring just 5 inches and weighing 1.6oz. The heat generated at the incision site is markedly less due to the irrigation line which runs through the center of the handpiece in the middle of the stack of magnets. The 4-crystal transducer with increased linear resolution of ultrasonic power is a major advance over the earlier 2-crystal transducer. This is found in most modern machines like the Legacy® (Alcon), Sovereign® (Allergan), and the Millenium® (Bausch & Laumb).

In the last few years, there have been many new machines that have been introduced, each with new features designed to make phaco surgery more efficient for the surgeon and safer for the patient. However one of the foremost developments in machine technology is the ability to control surge3.

The Legacy® machine has a low compliance fluid venting system to minimize effects of surge. The vacuum created by occlusion of the tip is offset by irrigation fluid, which is microprocessor controlled. This translates into greater stability of the anterior chamber. The PrestigeTM (Allergan) machine also has an advanced microprocessor that monitors all fluid pressure changes. The pump adjusts its speed according to the feedback received from the microprocessor. For example, any increase in vacuum results in a computer generated decrease in pump speed, thus decreasing the aspiration flow rate. This guards against sudden collapse of the anterior chamber.

The MilleniumTM (Bausch & Laumb) machine incorporates a superior fluidics system called the "Concentrix" system which is again microprocessor controlled, based on active feedback of anterior chamber fluid dynamics. The DiplomaxTM (Allergan) machine has an air-vented peristaltic pump with an in-line vacuum sensor. This sense an occlusion break at the tip, and automatically vents the vacuum in the line to the atmosphere. This feature is very helpful in preventing a surge rather than trying to minimize it.

The SovereignTM (Allergan) machine has an IntellessisTM sensor which senses the phaco tip conditions 150 times/minute. The vertically oriented pump purges air bubbles from the line, and eliminates surge. The Prosync ResponseTM incorporates preprogrammed phaco power, vacuum and aspiration changes automatically, according to the state of anterior chamber dynamics and tip occlusion.

Phacoemulsification using laser energy has been investigated for nearly two decades, and is clearly the most exciting development on the horizon of new technology. Laser phaco was developed to overcome the drawbacks of ultrasonic power, chiefly heat generation. In an experimental model, Berger et al4 investigated the thermal injury caused by Er: YAG laser versus Ultrasonic power, the temperature rise was 10-15 times by Ultrasound as compared to the laser.

The Er: YAG laser has also been found to be safer with regard to the risk of a posterior capsular (PC) rupture. Snyder et al5 compared PC rents in vitro, and found that capsular tears with the laser (Er: YAG) probe were localised, with lesser chances of vitreous loss as compared to those with ultrasonic probes.

The ideal laser would be one which is capable of ablating the lens as well as be compatible with fibreoptic delivery within the eye. Infrared lasers like the Nd: YAG and Er: YAG lasers have been found to be suitable and are both marketed commercially.6

In Direct acting : The photon phaco laser,TM (Paradigm) system has a 1.5-nm diameter tip, which works by fragmentation and aspiration of nuclear material. The laser energy delivered is lower than that required for photovapourisation, and thus results in deeper penetration.

In Indirect acting : The DodickTM laser lens ablater system incorporates a Nd: Yag laser which strikes a titanium target, resulting in generation of shock waves that disrupt the lens matter, which is subsequently aspirated.

The CentauriTM (Premiere laser systems) system has been used in the first published clinical study by Stevens et al.7 He found good results, though there was a prolonged surgical time compared to the routine ultrasonic phacoemulsification technique. Other Er: Yag laser systems under investigation are:

The PhacolaseTM, (Aesculap Meditech, Germany) machine has a Zircon Fluoride Quartz fibre, delivering a pulse energy of 5-50mJ at the rate of 10-50Hz.

Technology has advanced to the point of making the phacoemulsification procedure easier to perform and safer for the patient. However, new gadgetry needs to be thoroughly studied and understood to make these goals come true.

1. Fine IH, Hoffman RS. Recent advances in phacoemulsification systems. In Gills JP, Fenzl R, Martin RG (eds.) Cataract Surgery The State of the Art. Thorofare, NJ, Slack, 1998;92-99.

2. Seibel BS. Phacodynamics-mastering the tools and techniques of phacoemulsification surgery. Thorofare, NJ, Slack, 1995;4-5.

3. Fine IH. Phacoemulsification: New Technology and Clinical Application. Thorofare, NJ, Slack, 1996.

4. Berger JW, Talamo JH, La Marche KJ, et al. Temperature measurements during phacoemulsification and erbium: YAG laser phacoablation in model systems. J Cataract Refract Surg 1996;22:372-78.

5. Snyder RW, Noecker RJ, Jones H. In vitro comparison of phacoemulsification and the erbium: YAG laser in lens capsular rupture. Invest Ophthalmol Vis Sci 1994;35:4.

6. Aasuri MK, Basti S. Laser Cataract Surgery. Curr Opin Ophthalmol 1999;10:53-8.

7. Stevens G, Long B, Hamann AM, Allen RC. Erbium: YAG laser assisted cataract surgery. Ophthalmic Surg Lasers 1998;29:185-9.

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