PhacoVR: A Phacoemulsification Simulator

MMVR 7, Jan 22 1999

J. Terry Ernest MD/PhD Department of Opthalmology and Visual Science, Colin Davis, Jerome Jahnke, Janna Ore Nugent, Biological Sciences Division Information Services, Center for Research Technology, University of Chicago

What Is PhacoVR

PhacoVR is an immersive surgical simulator designed to simulate a small portion of cataract replacement surgery while accurately scoring surgical performance and providing a safe practice environment.

Phacoemulsification

We wish to model an entire cataract replacement surgery, and have begun by modeling phacoemulsification, the removal of the lens through a tiny incision in the cornea.  Phacoemulsification requires the bulk of the time in the surgery, and must be done with great care as the phacoemulsifier can cause a great deal of damage to the structures within the eye.  We began with phacoemulsification because it is both the most straightforward part of cataract surgery to model, and also presents the greatest difficulty for Ophthalmology residents learning to perform the surgery. The entire replacement surgery is roughly as follows.
  1. Initial Incision
  2. Capsulorhexis
  3. Hydrodissection
  4. Phacoemulsification
  5. Removal of Cortex
  6. Insertion of New Lens
  7. Close Incisions
After modeling phacoemulsification we hope to continue with the second stage of the surgery, capsulorhexis.

The Phacoemulsifier

The Phacoemulsifier is the instrument used to remove the lens from the eye. Once the lens is removed a new one may be put into the eye. The phacoemulsifier transmits ultrasound waves from it's tip which breaks up the lens material. It also has aspiration which removes the lens fragments from the capsule and irrigation which makes sure that there is a constant volume of fluid underneath the cornea.

The lens is delicately positioned in the eye. It is held in place by a ring of ligaments, which also apply force for focusing. Pressing too hard on the lens will tear the ligaments causing it to fall back into the eye. Therefore when using the phacoemulsifier the surgeons must make sure that their movements are not faster than the ability of the ultrasound and aspiration to remove material from the tip.

In addition to the threat of pushing the lens back into the eye, the surgeon must also be quite careful with the ultrasound that eminates from the phacoemulsifier's tip. It is under the surgeons control, but if it is activated too near a structure other than the lens it can result in severe damage to the patient's eye.

The Simulator Task

User's of Phaco-VR must use the phacoemulsifier to remove the lens from the capsule of the eye. The protocol followed by Dr. Ernest's team at the University of Chicago requires first that the user sculpt four grooves into the lens.  Using a second tool, the user breaks the lens into four pieces by "cracking" the four grooves.  The user than carefully removes each of these four pieces from the capsule using ultrasound and suction from the phacoemulsifier. At the end of the exercise the capsule will be empty and ready to recieve the new lens.

What we will use the Simulator to examine

Our goal is initially to develop a tool that will help us predict how well a surgeon will perform this portion of the surgery. Currently there is no objective analysis possible on these types of surgery. Patient recovery and surgery time are two of the performance metrics used, and it is widely held that patient recovery has much more to do with the individual patient than it does the surgeon, and time is not really the best metric to gauge a resident's surgical skills. At present, resident surgical proficency is evaluated by faculty using their years of experience (both doing the surgery as well as teaching the surgery) to gauge the students, but these scores are much more subjective.

The particular input device used for this simulator is the Phantom Desktop, from Sensable Technolgies. It's data resolution is in the neighborhood of 1 kHz, the simulated portion of the surgery should not take more than 10 minutes so we are able to collect all of the positional data from the arm, giving us all the movements by the surgeons hand. At the same time, we will update the world in which the haptic arm resides on the order of 120 Hz (the speed at which the monitor moves) and we will collect all of that positional data as well.

Our first subjects using this instrument will be skilled surgeons. We hope to create a gold standard, a data set the students will be judged against. Our hope is that this gold standard will be demonstrably different than the data we collect from the students, and demonstrably "better" as well.

This huge morass of data can then be navigated and we will look for a couple of types of events.

  1. Overall control of the device. Doing surgery at this small a scale requires a steady hand with precise control. We will examine the data sets looking for tremors, and precision.
  2. Proximity to non lens elements when ultrasound is activated. When the ultrasound is activated in the phacoemulsifier the tip should not be near any non lens elements. If they are there is a potential for damage.
  3. Force exerted on lens. The haptic devices are able to derive a force value when the arm is moved. We want to make sure very little force at all is applied to the lens, as it will tear it loose and push it back into the eye.
  4. WHAT ABOUT SUCTIONING CAREFULLY, NOT GETTING LENS BITS STUCK ON THE TIP?
  5. ANGLE OF THE LENS THROUGH THE INCISION?