Reducing posterior capsular rupture

Reviewed by Jan Venter

Accidental perforation of the capsular membrane during surgery may cause posterior capsular rupture, leading to vitreous loss, which, in turn, may result in severe visual disability, cystoid macular oedema and retinal detachment.¹ Posterior capsular rupture is thought to occur in approximately 2% of cataract surgeries performed by skilled surgeons.² Although phacoemulsification is the standard of care in cataract surgery, it is commonly associated with posterior capsular rupture.

Innovations in phaco tip design

A potential risk factor for posterior capsule rupture is the design of the phaco needle. Typically, phaco needles are sharp in order to cut through the cataract. However, even the briefest encounter between phaco needle and capsule may lead to capsular rupture.³ Given the sight-threatening complications associated with this phenomenon, it is not surprising that phaco-needle manufacturers are designing tips with safety firmly in mind.

Early examples of efforts to improve tip safety included a design by Peter Davis, MD, who in the early 1990s developed a needle that had a rounded outer edge but retained a sharp inner edge. Joseph F. Gravlee, MD, created a needle with a blunted bevel. Takayuki Akahoshi, MD, devised the “knuckle tip”, which utilised a bulbous design with round edges on all surfaces.⁴

A more recent development in safe phaco-tip design is the Dewey Radius Tip (MicroSurgical Technologies, Redmond, Washington, USA), which was launched in 2006. The brainchild of Steven Dewey, MD, the Dewey Radius Tip represents considerable innovation in phaco-tip design. It incorporates radius edges on the inner and outer surfaces and is available in 

thicknesses of 0.7 mm and 0.9 mm with a 30° bevel (Figures 1a and 1b). The inner and outer rims of the tip are rounded with exacting precision so there are no sharp edges to come into contact with the iris or the capsule, thus affording safer phacoemulsification. Dr Dewey notes that the Dewey Radius Tip behaves and functions exactly like a sharp-edged

phaco needle and is used in the same way; that is, no further training or modification in surgical technique is required. It can also be used on any patient and with any phaco machine, although older generations of phaco equipment might not be compatible.⁵

Figure 1a: 0.9 mm Dewey Straight Tip 30

Figure 1b: 0.7 mm Dewey Straight Tip 30

Safety

Safety of sharp versus “dull” tip design

I recently evaluated the Dewey Radius Tip in porcine eyes in order to determine the risk of capsular rupture compared with that of standard sharp-edged phaco tips. In the first part of this study, I used the Stellaris phacoemulsification platform (Bausch & Lomb, USA) with a 0.9 mm Dewey Radius Tip or a 0.9 mm sharp tip applied directly onto the capsular membrane with a vacuum pressure of 250 mmHg. I then delivered phaco power of increasing strength, from 0–100%, and noted the incidence of posterior capsule rupture.

In a group of ten eyes exposed to the 0.9 mm sharp tip, the posterior capsule ruptured in all cases – all at low phaco energy levels. Specifically, the posterior capsule ruptured at a phaco power of only 10% in five eyes and at a phaco power of 16% in the other five eyes. In contrast, with the 0.9 mm dull tip (Dewey), posterior capsule rupture occurred in only four of ten eyes, and then only when subjected to much higher phaco powers. Even at a phaco power of 75%, six capsules failed to rupture (Figure 2).

Figure 2: Posterior capsular rupture with a sharp versus a dull phaco tip (0.9 mm)

In the second part of the study, I compared the effects of the 0.7 mm Dewey tip with a Packard 0.7 mm tip (sharp) (MicroSurgical Technologies). All five eyes in the sharp-tip group had capsule rupture, again, even at low phaco powers (10–16%). In contrast, none of the eyes in the dull-tip group had capsular rupture, even at a phaco power of 75% (Figure 3).

Figure 3: Posterior capsular rupture with a sharp versus a dull phaco tip (0.7 mm)

Comparing new and old tips

Finally, I compared the effect of a used versus a new tip on posterior capsule rupture. The hypothesis was that a tip used many times becomes dull and, therefore, should cause less capsular damage than a new, sharp tip. Consequently, I exposed the capsular membrane of four eyes to a new AMO 21G Tip (Abbott Medical Optics, Inc, Abbott Park, Illinois, USA), and four eyes to an AMO 21G Tip that had been used 20 times previously. Interestingly, rupture occurred in all eyes in both groups at similar phaco powers (Figure 4).

Figure 4: Posterior capsular rupture with a new versus a used AMO 21G Tip

In another study, I compared what happened when the anterior capsule was contacted by a traditional sharp-edged tip and by the Dewey Radius Tip (0.9 mm, 30°) during torsional phacoemulsification with the Centurion system (Alcon, Fort Worth, Texas, USA).

The anterior capsule tests were conducted on six porcine eyes (per tip) at 70% and 100% torsional power. With the traditional sharp tip there was instant capsule rupture in all six eyes. In contrast, with the Dewey Radius Tip there was no anterior capsule rupture in any eye at either power setting. This suggests that the Dewey Tip also provides an increased level of safety in case of inadvertent contact with the anterior capsule.

Can tip type influence EPT?

Can tip type influence EPT?

Because reducing effective phacoemulsification time (EPT) is desirable in terms of minimising endothelial cell loss during cataract surgery, I compared EPT using the Dewey Radius Tip versus the standard sharp tip for a range of cataract grades. As expected, harder lenses required longer phacoemulsification. However, across all cataract grades, there was no difference in the EPT between tips (p=0.05). These findings suggest that the Dewey Radius Tip increases capsule safety without compromising performance.

There are few published studies evaluating the effect of tip design on capsular rupture, so it is difficult to discuss our study results within the context of others. However, our data echo early observations by Dr Dewey and colleagues, who assessed the Dewey tip in cadaver eyes and found that they could apply 50% phaco power at 400 mmHg vacuum for extended periods before the capsule ruptured.⁴ Overall, these findings suggest that using a rounded phaco tip increases the safety of phacoemulsification, thus helping to reduce the burden of complications to both cataract surgeons and – more importantly – to patients.

References

1. M. Zare et al.,J. Ophthalmic. Vis. Res. 2009; 4: 208-212.

2. D.F. Chang. Cataract & Refractive Surgery Today February 2012.

3. E. Stodola. ASCRS EyeWorld March 2014.

4. S.H. Dewey. Ocular Surgery News October 2007.

5. J. Helzner. Ophthalmology Management December 2008.

Kotaro Oki, MD, PhD

E: kotaro-o@pc4.so-net.ne.jp

Dr. Kotaro Oki, PhD, is an ophthalmologist in practice at the Oki Eye Surgery Center, Tokyo, Japan. He states that he has no financial interests in MicroSurgical Technologies.