Optimizing Vacuum Settings in Torsional Phacoemulsification
Torsional movement is a new concept in the evolution of phacoemulsification. It is therefore of interest to evaluate its optimal settings, through subjective analysis of surgeons' impressions or systematic comparison of metrics from different settings, to reach an objective assessment.
We are performing an ongoing objective analysis to determine the precise metrics for the nucleus removal phase of cataract surgery. The removal phase is the most representative phase of cataract surgery for the overall efficiency of emulsification.
In these studies, using frame-by-frame analysis (25 frames/second) of digital video recordings, we extracted cumulated dissipated energy (CDE; Figure 1) and removal time (Figure 2). Consumption of balanced salt solution was measured by a weight scale.
One problem when comparing metrics from different cases is heterogeneity. To overcome this obstacle, we developed a model in which we compared two different settings in the same nucleus. After initial sculpting, the nucleus is divided into two halves (ie, heminuclei), each of which is emulsified with one of two methods in a randomized fashion (Figure 3). Metrics for the removal phase are determined as previously mentioned, and nucleus density is estimated by grading the nucleus (grade 1–4) and measuring the CDE for sculpting.
This study design yields reliable statistics for paired measurements in several series of 20 to 40 surgeries. We have now completed several series with this study design, comparing different settings for torsional phacoemulsification, such as pulse and burst settings versus continuous action.
Because consumption of balanced salt solution was directly proportional to removal time in our initial series, we have abandoned the measurement of balanced salt solution, which is measured with less accuracy than removal time.
REDUCING VACUUM LEVELS
When evaluating the relationship between torsional phacoemulsification and vacuum, one must remember the experience we have had with conventional phacoemulsification. In conventional phaco, higher vacuum levels enhance efficiency but also increase the amount of surge in the event of an occlusion break.
Initial experience with torsional phaco suggested that it is less vacuum-dependent, with no benefit in raising vacuum above our standard setting (500–600 mm Hg). We decided to examine the consequences of lowering vacuum and compared torsional phacoemulsification using a vacuum of either 500 or 350 mm Hg (Figure 4). In all cases, torsional was performed in continuous mode with a mini-flared phaco tip and Microsleeve (Alcon Laboratories, Inc., Fort Worth, Texas). We found no difference in CDE or removal time between the two settings. This suggests that 350 mm Hg vacuum is superior to 500 mm Hg with torsional phaco, as there will be less surge with the lower vacuum setting but still the same level of efficiency.
To verify that longitudinal phaco would behave differently, we performed a series of cases using conventional longitudinal phaco with the same handpiece, sleeve, and phaco tip as well as the same vacuum parameters (Figure 4). As expected, a prominent and significant difference favored the 500 mm Hg vacuum both in CDE and removal time, confirming our impression from daily practice with conventional phaco. Higher vacuum means less time for consuming nuclear material and less energy exposure in the eye. Both factors are beneficial. The downside is the increased degree of surge on occlusion break and the potential for capsule-related complications.
This series of studies indicated that torsional is less vacuum-dependent than longitudinal phaco, but the question remained: To what extent? To assess an even lower vacuum setting, we performed a series comparing 350 mm Hg vacuum to 200 mm Hg with torsional phaco. The result was a significant difference in CDE and removal time in favor of 350 mm Hg. Our interpretation of the result is that the optimal vacuum level, in regard to both efficiency and safety, is lower with torsional than with longitudinal phaco. In both series, we used the mini-flared tip, Microsleeve, and 110-cm bottle height. The optimal vacuum level was between 200 and 500 mm Hg, probably around 350 mm Hg.
This clinical series describes the difference in vacuum dependence between longitudinal and torsional phaco; however, it does not address the reason for this difference. We must consider the different mechanical actions of the two modalities. It has been demonstrated that torsional phaco causes less repulsion of nuclear material than conventional phaco.1 During torsional phaco, the lens material stays at the tip most of the time. As a consequence, less vacuum is needed to counteract repulsion. There is also reduced turbulence with torsional versus longitudinal,2 which along with other mechanical properties results in less removal time and energy exposure at the incision (CDE) compared with longitudinal at equal vacuum levels.3 This leads to a more advantageous relation between efficiency and vacuum with torsional.
CLINICAL BENEFITS
Less vacuum dependence with torsional phacoemulsification provides several clinical benefits. When vacuum is decreased from 500 to 350 mm Hg, there is a marked reduction in surge—the magnitude being dependent on factors including the type of cassette, sleeve, and phaco tip used. The outcome is safer surgery with the same level of efficiency.
There are, however, other ways to exploit the optimized fluidics with torsional. When vacuum is decreased from 500 to 350 mm Hg, the irrigation fluid bottle can be lowered (eg, from 110 cm to 85–90 cm) if an unchanged level of surge safety is desired. Lowering the bottle means less potential for high pressure spikes and, with an adequate pump speed, a potential for less turbulence in the eye.
The most pronounced clinical benefits result from the combination of torsional phaco with microincision cataract surgery (MICS). Because MICS incorporates a smaller incision, there is concern regarding friction and exposure of the incision to thermal damage. With its lower CDE count, torsional phaco is preferable in this regard.
Fluidics is also a consideration during MICS. A smaller incision permits less irrigation volume; less irrigation provides less stability, both under steady-state conditions with high pump speeds and when there is an occlusion break. New technology for microcoaxial surgery, including sleeves and tubing, together with high resistance phaco tips, provides enhanced fluidics for microcoaxial surgery. Still, at a vacuum level of 500 mm Hg or above, it is easier to supply sufficient irrigation through a larger incision. With optimized vacuum settings for torsional phaco, this scenario is changed. As a consequence of the lower optimal vacuum level with torsional, efficient phaco can be performed at lower vacuum levels, reducing the quantity of irrigation needed, therefore making torsional and microcoaxial surgery a competitive combination.
Christer Johansson, MD, practices in the Department of Ophthalmology at the Lanssjukhuset, Kalmar, Sweden. Dr. Johansson states that he has received travel grants from Alcon Laboratories, Inc. He may be reached at +46 48081477; christer.ptab@telia.com.
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