We noticed you’re blocking ads

Thanks for visiting CRSTEurope. Our advertisers are important supporters of this site, and content cannot be accessed if ad-blocking software is activated.

In order to avoid adverse performance issues with this site, please white list https://crstodayeurope.com in your ad blocker then refresh this page.

Need help? Click here for instructions.

Today's Practice | Apr 2011

The Basics of Ultrasound

Fluidics and phaco power control have revolutionized ultrasound techniques.

Using ultrasound energy to break and remove lens material was proposed by Charles D. Kelman, MD, and Anton Banko in 1967.1 Since their pioneering attempts, cataract surgery has evolved to routinely incorporate phaco machines and techniques.2-5 However, the mechanisms underlying the emulsification of cataracts are still debated. This article provides an overview of how phacoemulsification works and discusses some recently introduced ultrasound technologies that have enhanced the efficiency of the procedure and improved postoperative results.

TIP MOVEMENT

The core of the phaco handpiece (Figure 1) is a transducer that converts electric energy into mechanical movement of the phaco tip.2,3 This is possible due to the piezoelectric effect, whereby some crystals exhibit properties that produce electricity in response to applied mechanical stress. The piezoelectric effect is a reversible process, in that materials exhibiting the direct piezoelectric effect also exhibit the reverse piezoelectric effect— the internal generation of a mechanical force (deformation) resulting from an applied electrical field. Applying a high-frequency current induces these crystals to oscillate at a particular frequency. The oscillation is transmitted to the phaco tip via an ultrasonic horn, which also acts as an amplifier, and thus longitudinal backward and forward movement of the phaco needle (stroke) is achieved. This is the traditional longitudinal form of phacoemulsification. 5-10 Some of the energy applied is dissipated into heat. The frequency at which a material naturally vibrates is called its resonant frequency, or the optimal frequency required to obtain the maximum mechanical amplitude and to reduce heat production.

The characteristics of stroke are length and frequency. Longitudinal phaco needle movement can reach a maximum of 100 to 120 μm, and the working frequency ranges from 28 to 45 kHz. The stroke frequency is defined by the manufacturer and cannot be modulated by the surgeon. Stroke length can vary linearly with application of the footpedal. Some argue that longer stroke length may generate more heat and corneal damage.11

HOW IT WORKS

The aim of phaco technology in cataract surgery is to break the lens material into pieces and then emulsify and aspirate it through the phaco tip. In traditional longitudinal phacoemulsification, the phaco needle moves back and forth, mechanically cutting the lens material during the forward movement. This is known as the jackhammer effect. Microcavitation bubbles form during the high-speed back and forth movement.9-13 The role of cavitation is debated among the ophthalmic community. Some suggest that cavitation plays an active role in lens fragmentation, but others believe it has no useful function.14-16

During forward movement of the phaco tip, the lens material is repulsed, the vacuum at the tip circumference decreases, and the bulk of the lens material may chatter. This can be compensated for by increasing the flow rate and the tip movement in the anterior chamber to regain occlusion.

The term followability is is used to describe a relatively feathery and continuous aspiration of lens fragments into the phaco tip. A high level of followability implies less movement in the anterior chamber, less stress on the internal structures of the eye, and an overall increase in phaco efficiency. Longer strokes may provide a higher degree of lens-disrupting capability due to the greater jackhammer effect on the material. On the other hand, a longer longitudinal stroke may cause a higher degree of repulsion and chattering of the lens material, which may lead to longer surgical times. Smaller but more frequent strokes may provide better followability, resulting in good lens fragmentation.13

Corneal endothelial damage and wound tunnel burns are two potential side effects of phaco energy that must be minimized.17 These events are related to the amount of energy delivered and the amount of heat produced. In recent years, phaco power modulation controls such as micropulse energy delivery systems have been developed to reduce repulsion and increase followability, limiting the total amount of the energy delivered to the eye. More recently, other removal modalities have been introduced.

TAKE-HOME MESSAGE

• The core of the phaco handpiece is a transducer that converts electric energy into mechanical movement of the phaco tip.
• The term followability describes a relatively feathery and continuous aspiration of the lens fragments into the phaco tip.
• Corneal endothelial damage and wound tunnel burns are two potential side effects of phaco energy that must be minimized.

NEW-GENERATION PHACO MACHINES

In 2005, Alcon Laboratories, Inc. (Fort Worth, Texas), introduced the Infiniti OZil torsional phaco tip. This phaco tip exhibits an oscillatory movement along the longitudinal axis. If a bent tip is mounted, the torsional oscillation produces a side-to-side movement of the end of the tip. The cataract is cut by shearing the material rather than by the jackhammer effect. The repulsion of the lens bulk is minimized, improving followability. Interestingly, the rubbing effect of the tip against the corneal tunnel is reduced because the tip rotational movement is limited to approximately 40 μm, and the length of the side-toside movement at the end of the tip is up to 100 μm.18

In 2007, Abbott Medical Optics Inc. (Santa Ana, California) released the Ellips Transversal Ultrasound handpiece for the company’s Whitestar Signature phacoemulsification system. The Ellips technology is based on an elliptical path of tip movement, combining side-toside and longitudinal strokes. The stroke of the FX version, introduced in 2010, follows an elliptical pattern in a complex 3-D fashion.19

The transversal stroke of the Ellips can be achieved with straight or bent phaco tips. The torsional stroke of Alcon’s Infiniti OZil requires a bent tip to obtain the sideto- side movement. Torsional and transversal phacoemulsification systems improve phaco efficiency by reducing the chattering effect, the total phaco energy delivered, and overall surgical time. Corneal endothelial damage and wound tunnel burns are also minimized.18-21

CONCLUSION

Phacoemulsification has significantly evolved in the more than 40 years since its first description, becoming a faster and safer procedure. Recent-generation phaco machines offer novel removal modalities with extraordinary fluidics and phaco control. Surgeons have a wide range of settings to choose from according to their preferences and to the nature of the cataract they are extracting, leading to an almost customized phaco procedure for every patient.

Paolo Cecchini, MD, practices at the Eye Clinic, University of Trieste, Ospedale Maggiore, Italy. Dr. Cecchini states that he has no financial interest in the products or companies mentioned. He may be reached at e-mail: paolo.cecchini@libero.it.

Pia Leon, MD, is a resident at the Eye Clinic, University of Trieste, Ospedale Maggiore, Italy. Dr. Leon states that she has no financial interest in the products or companies mentioned. She may be reached at e-mail: pialeon@libero.it.

Giuseppe Ravalico, MD, is Director and Chairman of the Eye Clinic, University of Trieste, Ospedale Maggiore, Italy. He states that he has no financial interest in the products or companies mentioned. Professor Ravalico may be reached at tel: +39 407 72449; e-mail: giuseppe.ravalico@virgilio.it.

Daniele Tognetto, MD, is Associate Professor of Ophthalmology at the Eye Clinic, University of Trieste, Ospedale Maggiore, Italy. He states that he has no financial interest in the products or companies mentioned. Professor Tognetto may be reached at tel: +39 407 72449; e-mail: tognetto@univ.trieste.it.

  1. Banko A,Kelman CD,inventors;Cavitron Corporation assignee.US patent 3 589 363.July 25,1967.
  2. Boukhny M.Phacoemulsification tips and sleeves.In:Buratto L,Werner L,Zanini M,and Apple D,eds. Phacoemulsification Principles and Techniques.2nd ed.Thorofare,NJ:Slack;2003:247-254.
  3. Fishkind WJ,Neuhann TF,Steinert RF.The phaco machine:the physical principles guiding its operation.In:Steinert RD, ed.Cataract Surgery:Techniques,Complications,and Management.2nd ed.Philadelphia,PA:Saunders;2004:61-77.
  4. Seibel BS.Phacodynamics:Mastering the Tools and Techniques of Phacoemulsification Surgery.3rd ed.Thorofare,NJ: Slack;1999:98.
  5. Bond LJ,Cimino WW.Physics of ultrasonic surgery using tissue fragmentation:Part II. Ultrasound Med Biol.1996;22(1):101-117.
  6. Ensminger D.Ultrasonics: Fundamentals,Technology,Applications.2nd ed.New York,NY:Marcel Dekker;1988:66.
  7. Ensminger D.Ultrasonics: Fundamentals,Technology,Applications.2nd ed.New York,NY:Marcel Dekker;1988:394.
  8. Pacifico RL.Ultrasonic energy in phacoemulsification:mechanical cutting and cavitation.J Cataract Refract Surg. 1994;20(3):338-341.
  9. Davis P.Cavitating microbubbles create shock waves that emulsify cataract.In:Mehta KR,Alpar JJ,eds.The Art of Phacoemulsification.New Delhi:Jaypee Brothers;2001:45-50.
  10. Brennen CE.Cavitation and Bubble Dynamics.New York,NY:Oxford University Press;1995.
  11. Frizzell LA.Biological effects of acoustic cavitation.In:Suslick KS,ed.Ultrasound: Its Chemical, Physical, and Biological Effects.New York,NY:VCH Publishers;1988:287-303.
  12. Fine IH,Packer M,Hoffman RS.Power modulations in new phacoemulsification technology:improved outcomes.J Cataract Refract Surg.2004;30(5):1014-1019.
  13. Packer M,Fishkind WJ,Fine IH,Seibel BS,Hoffman RS.The physics of phaco:a review.J Cataract Refract Surg. 2005;31(2):424-431.
  14. Zacharias J.Role of cavitation in the phacoemulsification process.J Cataract Refract Surg.2008;34(5):846-852.
  15. Tognetto D,Sanguinetti G,Sirotti P,Brezar E,Ravalico G.Visualization of fluid turbulence and acoustic cavitation during phacoemulsification.J Cataract Refract Surg.2005;31(2):406-411.
  16. Pacifico RL.Ultrasonic energy in phacoemulsification:mechanical cutting and cavitation.J Cataract Refract Surg. 1994;20(3):338-341.
  17. Walkow T,Anders N,Klebe S.Endothelial cell loss after phacoemulsification:relation to preoperative and intraoperative parameters.J Cataract Refract Surg.2000;26(5):727-732.
  18. Liu Y,Zeng M,Liu X,et al.Torsional mode versus conventional ultrasound mode phacoemulsification;randomized comparative clinical study.J Cataract Refract Surg.2007;33(2):287-292.
  19. Fishkind W,Bakewell B,Donnenfeld ED,Rose AD,Watkins LA,Olson RJ.Comparative clinical trial of ultrasound phacoemulsification with and without the WhiteStar system.J Cataract Refract Surg.2006;32(1):45-49.
  20. Zeng M,Liu X,Liu Y,et al.Torsional ultrasound modality for hard nucleus phacoemulsification cataract extraction. Br J Ophthalmol.2008;92(8):1092-1096.
  21. Davison JA.Cumulative tip travel and implied followability of longitudinal and torsional phacoemulsification.J Cataract Refract Surg.2008;34(6):986-990.

TAKE-HOME MESSAGE
• The core of the phaco handpiece is a transducer that converts electric energy into mechanical movement of the phaco tip.
• The term followability describes a relatively feathery and continuous aspiration of the lens fragments into the phaco tip.
• Corneal endothelial damage and wound tunnel burns are two potential side effects of phaco energy that must be minimized.

NEXT IN THIS ISSUE