Since its first use in 1967, phacoemulsification technology has undergone significant advances, resulting in decreased intraoperative and postoperative complications.1,2 The development of phaco machines, handpieces, tips, and irrigation systems have established this approach as the standard for cataract extraction in developed countries.3 Rapid development of phacoemulsification technology warrants careful evaluation of how these changes affect clinical outcomes. For example, previous studies4–6 evaluated the impact of the phaco tip angle on nuclear emulsification. However, there is a lack of peer-reviewed literature evaluating the effect of tip diameter on phacoemulsification efficiency.
Aspiration flow rates, vacuum levels, and phaco tip movement are the 3 main factors influencing cataract extraction efficiency,6 which is defined as the inverse of the amount of time required to remove nuclear material. Efficiency is reduced by microchatter and macrochatter events during phacoemulsification. Chatter results from inadequate hold as counteracted by the tip action’s repulsive force at the phaco tip and leads to decreased efficiency and an increased risk for complications.7–10 Previous studies have shown that reduced repulsion from the phaco tip improved followability and reduced chatter.7 Similarly, torsional ultrasound (US) movement has been said to reduce lens fragment repulsions at the phaco tip compared with conventional longitudinal movement, resulting in improved followability and efficiency.11,12
In this study, we determined whether different phaco tip diameters affect efficiency and chatter. Smaller diameter tips should produce less hold at a given level of vacuum and have less material consumed per tip movement cycle, resulting in decreased efficiency. Because the parameters used have a considerable impact on phacoemulsification efficiency and chatter, we used the most efficient combinations as determined by our 2 recent studies of this subject.13,14
Materials and methods
Porcine lenses were prepared in the previously described manner.14 In short, whole pig eyes were purchased from a supplier (Visiontech, Inc.) and dissected within 48 hours of arrival at the Moran Eye Center. Lens nuclei were soaked individually in 10 mL of 10% neutral buffered formalin for 2 hours. After soaking in formalin, all nuclei were placed in 10 mL of a balanced salt solution for 24 hours to increase the uniformity of the formalin’s hardening effect. A lens-cutting apparatus13,14 was used to cut the lenses into 2.0 mm cubes. They were placed in a pool of balanced salt solution in a moisture chamber until the study procedures took place no more than 36 hours after cutting. Porcine lenses prepared in this manner are comparable to human cataractous lenses in terms of density and behavior during phacoemulsification.14 All lens cubes were placed in a single container, which was shaken before random selection of cubes for each experiment.
Phacoemulsification experiments were performed with torsional US, transversal US, and micropulsed US. All parameters were at the settings previously determined to be most efficient.13,14 Torsional was set at 50% power, 100% amplitude, and 550 mg Hg vacuum (The same vacuum level was used for all experiments.) Micropulsed was set at 50% power, with a 6 ms on-duty cycle and 12 ms off-duty cycle. Transversal was set at continuous 50% power. All systems had a 50 cm bottle height and 40 mL/min flow. All 3 US modalities used the peristaltic pump setting. This setting allows independent control of aspiration and level. Ultrasound and vacuum were always at their maximum setting (full pedal on).
All phaco tips had a 30-degree tip angle; 1.1 mm, 0.9 mm, and 0.7 mm tips were studied with the micropulse and transversal experiments. Because 1.1 mm tips could not be used for torsional experiments, only 0.9 and 0.7 mm tips were used in the studies of this US mode. Torsional and transversal experiments used a 30-degree bend, while micropulsed experiments used a straight tip.
Efficiency and Chatter Comparisons
Efficiency and chatter comparisons were consistent with previously described methods.13,14 In short, 1 randomly chosen lens cube was placed in a chamber filled with a balanced salt solution. The pedal was depressed, and once the lens fragment occluded the phaco tip, the pedal was fully depressed to initiate US. A stopwatch recorded the time from US initiation to fragment removal and was stopped if the particle bounced from the tip; each bounce was counted as a chatter event. When the particle bounced from the tip, the pedal was again depressed to vacuum until the particle was aspirated to the tip. Once the particle reoccluded the tip, the pedal was fully depressed and the timer restarted. This allowed the chatter delay time to be distinct from the total particle removal time.
Efficiency was measured in terms of the amount of time in seconds that US was used to remove the lens particle.
Efficiency times were averaged, and a standard deviation (SD) was calculated. Efficiency times that were more than 2 SDs from the mean were considered outliers and removed from the data set. Then, the new means and SDs were calculated. A paired Student t test was used to compare the efficiency arms between the different tip diameters for each US variation tested, with significance set at a P value of less than 0.05. A chi-square test was used to compare chatter events. Finally, an unpaired Student t test was used to compare chatter and phacoemulsification efficiency. All statistical analyses used Stata data analysis and statistical software (Statacorp LP).
Influence of Tip Diameter Phacoemulsification Efficiency
The lens removal time was longer using the 0.7 mm tip than using the 0.9 mm tip with all systems. These differences were statistically significant for micropulsed and transversal phacoemulsification but not for torsional phacoemulsification (Figure 1). Combining all studies, the 0.7 mm tip was highly statistically significantly less efficient than a 0.9 mm tip (P<.0001). There was no statistically significant difference in lens removal times between the 1.1 mm tip and the 0.9 or 0.7 mm tip for micropulsed phacoemulsification (P=.3003 and P=.1681, respectively) or transversal phacoemulsification (P=.1435 and P=.3153, respectively); however, the efficiency was less than for the 0.9 mm tip and was statistically less significant when micropulsed and transversal results were combined (P<.05) (Figure 1).
Influence of Phaco Tip Diameter on the Incidence of Chatter
There were no statistically significant differences in chatter with micropulsed or torsional phacoemulsification between any of the phaco tips (Figure 2). However, transversal phacoemulsification statistically significantly increased chatter events in the 0.7 mm tip group compared with the 1.1 mm tip group (P=.0268) and the 0.9 mm tip group (P=.0041).
Effect of Chatter on Phacoemulsification Efficiency
There was a statistically significant decrease in efficiency from chatter for transversal phacoemulsification (P<.05) but not for torsional or micropulsed phacoemulsification (Figure 2). The phacoemulsification efficiency P values for the 0.9 mm and 0.7 mm torsional tips were then recalculated after excluding all the trials that had at least 1 chatter event. There was still no statistically significant difference between the 2 groups.
Phaco tips have evolved to include a variety of bore sizes that influence vacuum power and flow. According to Poiseuille’s equation, flow is proportional to the radius of the tube to the fourth power. A small change in needle tip size can result in a very large change in the flow. When 2 common-sized phaco needles (0.9 mm and 1.1 mm) are compared, with all other factors equal, the flow through the larger 1.1 mm needle is more than twice that of the 0.9 mm needle.15 Therefore, peristaltic pumps have to work much harder to provide equivalent flow with smaller tips and may not be able to maintain the same flow.15 A smaller tip presents a challenge to delivering the adequate outflow necessary to clear the nucleus. Less flow would equate with less followability.
A second concern would be the total hold created by a specific level of vacuum. For any level of vacuum, this would be directly related to the square of the size of the opening if the entire tip is occluded. Vacuum levels can also vary depending on tip diameter.16 This factor should strongly favor increased tip size because vacuum alone greatly influences the rapidity with which lens particles are removed and often is all that is needed to remove many lens fragments. Furthermore, the significantly increased hold that results from increasing the tip size should powerfully counteract chatter. However, we did not see a geometric relationship between tip size and increased efficiency or decreased chatter. The results in our study suggest that there is an optimum tip size that allows maximum vacuum power and lens removal ability.
An additional dimension is the work product created at the tip. Although this can be very complex, in general, for the same amount of tip excursion, work would be directly related to the size of the tip edge. This should also favor a larger tip if we can make the reasonable assumption that the tip excursion distance is the same for any power setting regardless of tip size. However, the actual actions of the phaco tips are not known at this time. Again, we expect this should favor a larger tip when we look at efficiency; however, this would result in greater repulsive force and thus should increase chatter.
With all 3 systems tested, 0.7 mm diameter tips required the most time for lens removal, a difference that was statistically significant for micropulsed phacoemulsification and transversal phacoemulsification. This suggests that, relative to a larger bore size, the 0.7 mm tip generates less vacuum hold and maybe less fluid flow to engage and maintain a lens fragment at the tip. This is supported by the number of chatter events observed with the 0.7 mm tip compared with the 0.9 mm tip. The 0.7 mm phaco tip does not form as strong a hold to a lens fragment, resulting in higher chatter and less efficiency (Tables 1 to 3). This would be the case only if the lens fragment were fully engaged and the maximum vacuum attained in a peristaltic system. Thus, although we should expect a 1.1 mm tip to be better in terms of efficiency and chatter than a 0.9 mm or a 0.7 mm tip, we suspect that the larger tip decreases the odds of complete tip occlusion and creation of a maximum vacuum event through the time of nuclear fragment removal. If this were the case, we would see very different results with different lens fragment sizes or with a venturi vacuum system, both of which we intend to study in the future. For the lens fragment size, vacuum system, and flow parameters we studied, a 0.9 mm tip was the best choice (Figure 3).
Only transversal phacoemulsification showed a significant difference in chatter between the different tip sizes. The SD was much larger with the 1.1 mm tip and 0.7 mm tip than with the 0.9 mm tip. For 2 totally different reasons, we suspect that the increased variability seen with the 1.1 mm and 0.7 mm tips was a result of microchatter due to reduced engagement of the lens fragment at the tip edge. For the 1.1 mm tip, it was due to less occlusion, hence the decrease in total maximum vacuum during the removal event. For the 0.7 mm tip, it was due to a decrease in net vacuum hold. A weak seal at the tip–lens interface likely increases lens movement, adding variability and decreasing efficiency. This was particularly evident when we had clear outliers. The fragment would sit at and rattle around the tip until it was finally engaged and then was promptly removed, at which time there was typically no further rotational movement, representing incomplete suction hold of the lens fragment.
Microchatter is a very small vibration (approximately 5 μm) that occurs even when the tip is engaged with the lens material. Poor contact between the lens and tip creates these low-amplitude vibrations, which affect efficiency. Inadequate suction and a small surface area are thought to increase the presence of microchatter. We theorize that microchatter creates variability in the measurement of efficiency. This is supported by the higher SD values and the increased number of outliers observed with the 0.7 mm and 1.1 mm tips compared with the 0.9 mm tip.
We were unable to find a 1.1 mm diameter phaco tip to test with torsional phacoemulsification. Although no significant difference was found between the 0.9 mm tip and 0.7 mm tip in torsional phacoemulsification, our results follow the same trend observed with micropulsed and transversal phacoemulsification. The 0.9 mm tip in torsional phacoemulsification had a smaller SD and better efficiency than the 0.7 mm tip. Furthermore, in a statistical power analysis, the results would be significant with a small increase in the number of testing runs; thus, we expect that this finding simply represents a power issue. It also would be interesting to see how a 1.1 mm tip behaves with torsional phacoemulsification.
Limitations of our study include its in vitro nature as well as the single size of fragment that we used to test efficiency. It is possible that the decreased efficiency of the 1.1 mm tip was due to its diameter being too large for the size of lens fragment we tested. Nonetheless, control of variables to accurately measure the impact of US modulation alone is not possible in a clinical setting. We believe our study mirrored the clinical setting with much better control of the variables involved.
Our results suggest several potentially promising avenues for future research. Assessing efficiency and chatter outcomes with varying tip sizes used in conjunction with a venturi vacuum system might be worthwhile. The venturi system might cause a decrease in chatter and efficiency given its unique ability to maintain constant vacuum at the tip. In addition, the lens cubes we used for our study were cut to approximately the same 2.0 mm size. Varying the size of the cubes may change the efficiency and chatter results. It would also be useful to determine the impact of various phaco tip sizes on vacuum and flow measurements; there may be an optimum phacoemulsification system setting that depends on the size of the tip used. Last, tip size and mass may affect the actual US work produced for each of the machines we tested. Our experimental design used the US frequencies programmed in the 3 phaco machines. The working frequencies of the 3 machines are torsional, 32 kHz; micropulsed, 29.0 kHz; transversal, 38.0 kHz.17 Further studies to elucidate the effect of various tip diameters on the working frequency are needed.
In conclusion, effective phacoemulsification requires a tip diameter that is large enough to maintain appropriate aspiration and suction but small enough to produce a sealed grip of the lens to the phaco tip. Although we expected the largest bore to be the most efficient given advantages of vacuum hold and possibly some increased fluid flow and greater US work, we found that the 0.9 mm tip may be more efficient than the 1.1 mm tip. We found that the 0.9 mm tip diameter required the least amount of time to clear the lens with all 3 systems and was significantly more efficient than the 0.7 mm tip.
What Was Known
- The angle of the phaco tip has an impact on nuclear emulsification. However, the effect of tip diameter on phacoemulsification efficiency and chatter has not been evaluated.
What This Paper Adds
- It was anticipated that the largest bore size would be the most efficient, given the possible advantages of vacuum hold, increased fluid flow, and greater US work. However, with all 3 phacoemulsification systems, the 0.9 mm tip was more efficient than the 0.7 mm tip and the 1.1 mm tip.
1. Shah PA, Yoo S. Innovations in phacoemulsification technology. Curr Opin Ophthalmol
2. Hoffman RS, Fine IH, Packer M. New phacoemulsification technology. Curr Opin Ophthalmol
3. Riaz Y, Mehta JS, Wormald R, Evans JR, Foster A, Ravilla T, Snellingen T. Surgical interventions for age-related cataract, Cochrane Database Syst Rev. (4) (2006 Oct 18) CD001323.
4. Coelho RP, Raskin E, Paula JS, Cruz AA., 2008. Tip position during phaco [letter], Ophthalmology, 115, 2315-2315.e1.
5. Yow L, Basti S. Physical and mechanical principles of phacoemulsification and their clinical relevance. Indian J Ophthalmol. 45, 1997, p. 241-249, Available at: http://www.ijo.in/article.asp?issn=0301-4738;year=1997;volume=45;issue=4;spage=241;epage=249;aulast=Yow
. Accessed December 8, 2013.
6. Watanabe A. New phacoemulsification tip with a grooved, threaded-tip construction. J Cataract Refract Surg
7. Davison JA. Cumulative tip travel and implied followability of longitudinal and torsional phacoemulsification. J Cataract Refract Surg
8. Liu Y, Zeng M, Liu X, Luo L, Yuan Z, Xia Y, Zeng Y. Torsional mode versus conventional ultrasound mode phacoemulsification; randomized comparative clinical study. J Cataract Refract Surg
9. Miyoshi T, Yoshida H. Ultra-high-speed digital video images of vibrations of an ultrasonic tip and phacoemulsification. J Cataract Refract Surg
10. Blodi BA, Flynn HW Jr, Blodi CF, Folk JC, Daily MJ. Retained nuclei after cataract surgery. Ophthalmology
11. Miyoshi T, Yoshida H. Emulsification action of longitudinal and torsional ultrasound tips and the effect on treatment of the nucleus during phacoemulsification. J Cataract Refract Surg
12. Zeng M, Liu X, Liu Y, Xia Y, Luo L, Yuan Z, Zeng Y, Liu Y. Torsional ultrasound modality for hard nucleus phacoemulsification cataract extraction. Br J Ophthalmol. 92, 2008, p. 1092-1096, Available at: http://bjo.bmj.com/content/92/8/1092.full.pdf
. Accessed November 8, 2013.
13. DeMill DL, Zaugg BE, Pettey JH, Jensen JD, Jardine GJ, Wong G, Olson RJ. Objective comparison of 4 nonlongitudinal ultrasound modalities regarding efficiency and chatter. J Cataract Refract Surg
14. Oakey ZB, Jensen JD, Zaugg BE, Radmall BR, Pettey JH, Olson RJ. Porcine lens nuclei as a model for comparison of 3 ultrasound modalities regarding efficiency and chatter. J Cataract Refract Surg
15. Devgan U. Phaco fluidics and phaco ultrasound power modulations. Ophthalmol Clin North Am. 2006;19(4):457-468.
16. Payne M, Waite A, Olson RJ. Thermal inertia associated with ultrapulse technology in phacoemulsification. J Cataract Refract Surg
17. Köse S, Menteş J, Üretmen O, Topçuoğlu N, Köktürk U, Yılmaz H. The nature and origin of intraocular metallic foreign bodies appearing after phacoemulsification. Ophthalmologica