Human Factors Engineering



Human Factors Engineering - Corneal Membrane Transplant Injector

Purpose: The purpose of this document is to identify the possible sources of error that may occur with the usage of the corneal membrane implant injector. As these sources of error are recognized, means of minimizing or eliminating them will also be identified.

A. Device Overall

The corneal transplant surgery injector will deliver a donor cornea into a recipient’s eye without mechanical damage to the tissue or eye. It must also orient the tissue in the recipient’s eye and allow it to attach appropriately. It will simultaneously maintain the anterior chamber environment of the eye and provide suction to hold the corneal tissue in a particular shape within the injector. This injector must deliver the corneal tissue of an 8 mm diameter through a 4 mm incision, thereby eliminating the need for sutures. In general, this device has potential to make corneal transplant surgery more efficient and decrease complications during and after the surgical procedure. The device is to be used on those who require a corneal transplant. This population includes patients who have a diseased Descemet’s membrane, the innermost endothelium layer of the cornea. This can occur in such conditions as Fuch’s endothelium dystrophy. The device will be used in a treatment for this ailment known as Descemet’s stripping endothelium keratoplasty (DSEK).

The length of the device should be about 15 cm and is approximated to have a circular cross-section. It should have a diameter of about 2 cm where it is held by the hand. The tip making contact with recipient’s eye must be no more than 4 mm wide and 3 mm tall. The device should weigh about 280 grams. The device has two main components. The first is the cartridge and is made from a transparent polymer. The second is the injector, which slides back and forth within the cartridge and is made from surgical-grade stainless steel. The injector is connected to a phacoemulsfication machine, which is powered by an electrical outlet.

Current cornea transplant procedures involve folding the donor tissue (a very thin piece of tissue about the size of a dime) and pushing the folded tissue through an incision about four millimeters long using tweezers. This method of implantation often damages the cells and adversely affects the patient’s vision after the surgery. It also requires at least one suture. In contrast, the corneal membrane transplant injector has the potential to deliver the donor tissue with minimal damage while eliminating the need for sutures.

The injector must deliver the corneal tissue with minimal damage to the tissue while maintaining the environment of the anterior chamber. The suction pressure inside the injector should be minimized to avoid damage to the endothelial cells. The reusable portion of the device must be able to function successfully every time a corneal transplant is needed; about 15 times a week per hospital for a lifetime of 5 years. The parts of the injector that make direct contact with eye tissues should be sterilized or disposable.

B. Device User Interface

The device should be easy to handle by hand and in an efficient manner by a clinician. The device must be of a sufficient size to allow for comfortable use yet not so big that ease of manipulation is compromised. Since the device has two integrated moving parts, the surgeon will be required to use both hands, one for each part. Raised ridges at the contact points will be provided to increase grip.

In addition to our device, the surgeon will be required to control the phacoemulsfication machine. This will involve presetting the pressure and vacuum limits. During the procedure, the surgeon will control a foot pedal that adjusts the rate of irrigation and aspiration.

This device will be incorporated into a highly precise surgical procedure. Therefore, its use will require detailed training. Thus, the anticipated labels will warn that use is restricted only for trained individuals. Instructions will be included with the corneal membrane transplant injector to explain how to connect it to the phacoemulsification machine and how to operate the device in conjunction with the machine. Another label may be necessary to describe what proper use the device is restricted to.

C. Device Use

The user is expected to use the corneal membrane transplant injector exclusively for corneal membrane transplant surgery. The device is set up by first attaching a fresh cartridge over the injector component. Next, the irrigation and aspiration tubes from the phacoemulsifier are attached to the injector. The phacoemulsfication machine will allow the irrigation and suction to be monitored by the user while the device is manually manipulated. The plastic components of the device are disposable and the metal parts will undergo sterilization following every transplant. The phacoemulsfication machine must be primed with the saline solution used to maintain the anterior chamber in the eye.

D. Device User Population

The intended population for device users of the corneal membrane transplant injector are surgeons familiar and skilled with the new keratoplasty procedure for which it is designed. This population desires a procedure that involves a device with easy and efficient manipulation during surgery. The user population must follow all operating instructions and safety warnings contained in the packaging. The user must have significant background in ophthalmology and surgical procedures for diseased or damaged eyes. This device is not intended for those who are not certified eye surgeons or for procedures other than corneal membrane transplants.

E. Device Use Environments

This device is intended for use in an optical operating room where resources such as stereoscopic magnifiers and phacoemulsification machines are available. It should still work effectively when exposed to fluid such as BSS and after being exposed to high autoclaving temperatures (around 200 oC). The device must withstand the different pressures resulting from suction, aspiration, and irrigation. The device will be exposed to negligible humidity, dirt and dust. The device should not be used outside of the operating room or in a room that is not of room temperature as this may affect multiple aspects of the procedure.

F. Use-Related Hazards

Plausible use-related hazards include mechanical damage to the recipient’s eye or the endothelial side of the corneal tissue due to friction, the inability to maintain the proper anterior chamber environment, inadequate suction within the injector and contamination. These potential use-related hazards were heavily considered during the design stage of this device.

The most serious possible hazard related to human error is damage to the patient’s eye. The tip of the injector will extend from the cartridge only 9 mm in order to release the cornea tissue inside the anterior chamber. However, if the user is not careful he or she may damage the eye tissue by exerting too much force on the device, which could result in scratching or puncturing the eye tissue. Mechanical damage of the corneal tissue occurs when the membrane is exposed to the walls of the injector or cartridge. However, because the corneal tissue will be placed on a track that will inhibit it from touching the walls of any part of the device, this risk should be negligible.

An improper anterior chamber environment results when the amount of irrigation does not balance with the amount of aspiration. This can either cause a damaging amount of high pressure or not provide enough pressure within the chamber; both of which are detrimental to the eye. There will be tubes for aspiration and irrigation connecting to the phacoemulsfication machine, which measures the pressure. Nevertheless, the holes leading to these tubes may become blocked. If the user closely monitors the pressure displayed on the screen and pays close attention to the anterior chamber under a microscope, the probability of this hazard should be negligible. Inadequate suction occurs when there is not enough suction to keep the tissue in place on the track within the I/A tip. Tests will be performed to find a minimum vacuum pressure needed to suction the tissue, thereby holding it in place. Finding a minimum vacuum that will not be too low for some corneal tissues should assure the proper vacuum needed to hold tissues.

Several cases of contamination have been reported in similar lens implantation devices. The cases of interest involved crystallization of deposits and contamination with bacteria on lens implanted with devices that were not properly sterilized or thoroughly rinsed before use[1]. These cases demonstrate the importance of proper cleaning and autoclaving techniques.

Use-related hazards are prioritized according to the severity of harm they could potentially cause. These hazards need to be addressed to minimize the time needed for the design and revision stages of development.

G. Verification and Validation

When the first prototype is ready for testing, Dr. Al-Ghoul will perform trials on animal eye models in order to physically test the device’s potential. The trials will replicate the procedure to be used for corneal membrane transplant surgery. Dr. Al-Ghoul, along with his colleagues will provide feedback on the device’s ease of use, efficacy in the procedure compared to past techniques, and any damage caused to the eye at a tissue and cellular level. Tissue and cellular analyses of the eye post-operation will determine how well the device handles and injects the corneal membrane. Recording and evaluating the pressures experienced by the anterior chamber of the eye will provide insight on hazards associated with the irrigation and aspiration functions of the device. With the feedback from the surgeons, design improvements will be made, new prototypes will be constructed, and another round of testing will begin.

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[1] Hickman, Werner, et al. Intraoperative explanation of two single-piece hydrophobic

acrylic intraocular lenses due to surface deposits. Eye. 20:9 pp1054-1060.

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