Chow and Nystrom 12

Redesign of Otologic

Surgical Instruments

Vanderbilt University

BME 273

April 24, 2001

Biomedical Engineering Student: Brian Chow

Biomedical Engineering Student: Joy Nystrom

Mechanical Engineering Student: Ryan Josefovsky

Advisor: Russ Johnson,Ph.D. Smith and Nephew, ENT division

Abstract

Approximately 120,000 middle ear reconstructive surgeries occur in the U.S. annually. Movements in these procedures must be very precise since confined spaces are involved, with open spaces less than one centimeter across. Several fragile structures are close-by. Current instruments for this procedure require large movement compensation by the surgeon to keep the tip of the instrument steady. This goal is to design an instrument that removes the need for this compensatory motion while keeping cost similar to current instruments and keeping size (diameter of 5mm) and weight similar to existing picks. Per a surgeon's request, the movement required to activate tip motion is a fingertip operated push button mechanism. Two designs that fit these criteria were developed. One leveraged the spring properties of a curved metal band. The other, derived from a side-advancing mechanical pencil, involved a wedge that would convert a push-button motion to the desired motion at the tip. Both of these designs assumed that existing tip configurations would be kept the same. The designs were rendered in three-dimensional computer aided drawings. The metal-band design was selected for prototyping. Evaluation of the designs involved cost, simplicity, ease of manufacturing, and safety. In both cases, the main safety concern was being unfamiliar with the instrument, potentially corrected with further training. The metal-band design was deemed easier to manufacture, but the pencil-wedge design was a simpler, more versatile design. Both designs offer a significant cost reduction over the current design by eliminating the joint, which is difficult to manufacture. The cost of developing the prototype was less then $5000, with potential revenue from the sale of the product potentially reaching $1.35 million. Future work on this project will include refining the prototype, testing the materials rigorously, gaining FDA approval, and marketing the instrument set.


Introduction:

This project addresses a redesign of an existing set of otologic surgical instruments used in middle ear reconstructive surgery.

The tympanic cavity, or middle ear (figure 1), is located in the temporal bone in the head and is lined by mucosal cells. It is an air-filled cavity, with the air originating from the nasal cavity by way of the pharyngotympanic, or Eustachian, tube. This tube serves as a protective mechanism to help equalize the pressure on both sides of the tympanic membrane.

The tympanic cavity houses the three small bones that are responsible for transducing sounds waves into mechanical vibrations that can be detected by portions of the inner ear. These bones, called auditory ossicles, are the malleus, incus, and stapes. The malleus is attached to the tympanic membrane, and the stapes is attached to the fenestra vestibuli. The incus connects the other two bones. The bones are connected to the walls of the cavity by ligaments, which can carry blood vessels to the bones.

The cavity consists of two parts: the tympanic cavity proper and the epitympanic recess. The former is located opposite the tympanic membrane, and the latter is located above this level. The epitypmanic recess houses the upper part of the malleus and most of the incus.

The pharyngotympanic tube connects the anterior tympanic wall with the nasopharynx. Air can pass through this tube during swallowing, allowing pressure to equalize on both sides of the tympanic membrane. The tube passes through about 12 mm of bone at the tympanic end, and 24 mm of cartilage at the nasopharyngeal end. The tube is lined with muscosa, which runs together with the mucosa of the nasal cavity and the tympanic cavity (Williams 1995).

Middle and inner ear surgery takes place for a number of reasons. The procedures are aimed at correcting hearing loss or treating infection. Some of these procedures include stapedectomy, cochlear implants, and ossiculoplasty (implantation of prosthetic ossicles) (Chole 1999). Upwards of 320,000 procedures requiring precise movements in the middle ear take place yearly in the United States. Worldwide, this number nearly doubles to 600,000. The proposed instrument would be used in about 120,000 of these cases (Johnson 2001).

The goals for this project were to redesign the drive system of the surgical instrument (figure 2) so that movement is fingertip controlled. The tip was to remain unchanged. It was desirable that the cost be comparable to current technology, between $300 and $1500 per set of instruments. The instrument would be easier to manufacture, but the market would be more limited, resulting in a lower production. However, if a disposable version of the tool were to be made, the price per unit would drop significantly, and the volume of sales would likely rise. Since this is a new product, there is no definite timeline for development. The creation of a disposable instrument is a long-term goal, which is beyond the scope of this design project.


Methodology

Initial research began with a patent search. This search revealed that the problem being solved is a relatively new problem that has not had solutions previously. An application where maneuvering in limited space is an issue is in laparoscopic surgery. In this case, designs exist for a series of pulley mechanisms. This design led to the creations of several ideas, such as a gear design, a screw design, and a concentric wheel design. Initial motivations for these designs were driven by a desire to achieve an acceptable mechanical advantage. In the concentric wheel model (figure 3), the outer radius moves a greater distance than the inner radius so that the tip does more work than the sliding finger.

After the intellectual property rights were negotiated, contact was made with the ENT surgeon, Dr. Moíses Arriaga, who proposed this idea to Smith and Nephew. From this conversation arose a series of specifications, including the size, weight, and specific types of movement. The size was to be kept about the same diameter (5 mm) and weight as existing surgical picks. Dr. Arriaga requested an index finger push-button mechanism. No specifications were set for any mechanical properties, however the current range of movement involved a 2:1 ratio for mechanical advantage.

From the size specification, the large concentric wheel design was eliminated from consideration. A replacement design, consisting of a system of levers (figure 4) was developed as a replacement. After consultation with manufacturing, it was determined that the weakest section of the design would be the numerous pins holding the levers together. The design was simplified to a one-piece construction that eliminated these levers and pins (figure 4 and 5). The metal band that replaced the levers would have spring properties that would allow it recoil under its own power without the need for an extra spring. Further modification of the design would allow the instrument to exist in either a steady-state open (forceps) or steady-state closed (posigator) configuration.

In further research, another alternative was found in a side-advancing mechanical pencil (figure 6). This uses a design that translates a downward push-button motion to a transverse motion by way of an inclined plane, with recoil power provided by a spring. Reversing the position and orientation of the plane would allow for the same versatility with the output being reversed.

Results

In evaluating the different designs, several factors were considered, including meeting demands, ease of use, ease of manufacturability, durability, and cost. These criteria were applied to both of the designs.

The metal band design (figures 7 and 8) met the most important demand that the device could fit inside an instrument 5 mm in diameter. It also employed a pushbutton mechanism, and was the correct approximate weight. It is also the easier of the two final designs to manufacture because existing parts could be used for the entire model. However, without materials analysis and testing, the lifetime and durability of the instrument is not known. There is no easy way to determine the spring constant of the band. The size and therefore spring properties of the band will have to be determined by experimentation. These design problems make selection of materials a non-trivial task, which in turn makes cost analysis more difficult.

The wedge design (figure 9), derived from a side loading mechanical pencil, met the size constraints. However, it is the more difficult design to manufacture because the part to make the wedge does not exist in the ENT manufacturing field. The design analysis is much easier; as long as the wedge portion is made of a non-deformable material, it does not matter what material should be used in its manufacturing. The recoil properties are based completely on a spring constant of a single spring, and simply altering the slope of the wedge can easily change the necessary exerted force. Because the design analysis is more intuitive, the wedge design seems like a better concept.

Safety Issues

Design Safe was used to assess the safety issues of the two designs (Appendix A). For the most part, the safety issues are similar to the existing tools in terms of biohazards and normal surgical precautions. Notable differences include a surgeon's unfamiliarity with the tool and the unknown durability. Surgeons would need to be retrained in order to feel comfortable handling the tool. For full analysis, see appendix A.

Economic Analysis

Middle ear surgery requires much delicate and precise control over the instrument. Some Ear, Nose, and Throat (ENT) surgeons have expressed interest in developing a new set of surgical instruments that would allow for fingertip control of the tip mechanism instead of hand control. In the case of hand control, the surgeon must compensate for additional movement of the instrument caused by the movement of the hand. Switching to a fingertip controlled system would afford the surgeon greater control over the mechanism, less room for error, and easier access to the delicate inner ear. Extensive searching of medical literature, professional societies, legal literature, and consumer advocacy groups reveal that specific numbers do not exist for surgical errors, much less surgical errors that can be attributed specifically to the surgical instrument. The redesign of this tool must thus be considered more of a comfort and ergonomic improvement and a possible reduction in manufacturing cost rather than specifically addressing any particular economic issue. Smith and Nephew, ENT division is currently only one of very few surgical instrument manufacturers attempting to meet this need. Competition appears to be minimal.

This product would be targeted at mostly younger generation surgeons. A best-case scenario, where the instrument set is introduced in a major teaching center, would convert about 10% of ear surgeons in five years. The number of otolaryngologists is 9,000 in the US and 20,000 worldwide (Johnson 2001).

Current instruments cost $300 per tool and there are generally five tools in a set of otologic instruments in conjunction with many more general surgical tools. The high cost associated with these tools comes not only from the cost of the material, but also from the intricacy of manufacturing the drive system. Eliminating this intricacy would reduce the cost of manufacturing, assuming the types and amounts of material would be similar. The cost of maintaining the instrument is projected to be no different than for current instruments.

With the previously cited number of approximately 9,000 ENT surgeons in the U.S. and 20,000 ENT surgeons worldwide, a potential market penetration of 10% in the first five years would result in potential revenue upwards of $1.35 million. This projection is contingent upon the introduction of the surgical instrument set in at least one major teaching institution.

The cost of developing the prototype is about $4770, calculated as follows:

3 students x $15/hour x 75 hours per student (on average) / $3375 /
Cost of travel / $270
Cost of prototype / $125
Computer Access/Maintenance / $400
Overhead / $600
Presentation Materials
Supplies
Sample Tools / $50
$50
$500
Total / $4770

The benefits of this tool are greater control during surgery and better ergonomics, potentially leading to a reduced surgery time. Surgeons may have more down time, resulting in lowered stress levels and fewer personal leaves of absence. These savings are not quantifiable.


Conclusions

A new fingertip controlled otologic surgical instrument is feasible both in manufacturability and cost. Both the metal band design and the pencil-wedge design meet the surgeon's demands and show promise for further development. The prototype proves that the concept of using a single metal band is feasible. However the prototype exaggerates the proportions of the instrument; scaling down of the actual instrument will be needed before marketing.

The pencil-wedge design, while simpler in concept, remains a prototyping challenge as the design calls for much innovation on the part of the manufacturer. If successful, the pencil-wedge design can potentially be made of disposable materials. This would eliminate the risks associated with potentially unsterile instruments. In addition, the manufacturing cost would be greatly decreased as material cost decreases. As a result, production volume would likely increase.

The most expensive section to manufacture in the previous design, the joint between the tip and handle, is eliminated in both of the new designs. This associated cost savings would potentially offset any initial startup costs to begin production of the new instrument set.


Recommendations

Further work on these designs should begin with filing for a design patent. In addition, more refined prototypes must be developed, and these prototypes must be tested rigorously for reliability and durability. Once these prototypes have been proven safe and effective, FDA approval to market the instrument should be obtained. Since the devices are very similar to existing instruments, approval should be relatively easy to obtain.

Once the product is closer to gaining market approval, the market conditions must be reassessed in order to develop a marketing strategy. Key teaching institutions must be identified to begin market penetration.