Interview with Geoff Ball Founder of Symphonix
AO/Beck: Hi Geoff. Thanks again for spending some time with me.
Ball: Hi Doug. Happy to help. What are we talking about this time?
AO/Beck: Geoff, I'd like to discuss some more technical issues this time. In particular, Laser Doppler Vibratomy and Reverse Transfer Functions. However, before we do that, may I re-introduce you to the new readers? Let me tell them a little about you, and please feel free to correct me if I mess this up!
Ball: Sure Doug, go for it!
AO/Beck: Geoff Ball is the founder of Symphonix and he wears two Middle Ear Implants. Additionally, Mr. Ball is a vice-president and the Chief Technology Officer of Symphonix. Geoff has a master's degree in systems management from the University of Southern California and he has worked as a biomedical engineer for many years. How'd I do?
Ball: You nailed it.
AO/Beck: Thanks, I'd like to tell you that I remembered all that...but of course, I read it from my notes! OK, let's start with Reverse Transfer Functions. What are they, and how do they relate to hearing and middle ear implants?
Ball: Reverse Transfer Functions are basically the observation that any transducer in the middle ear generates vibrations. Although most of the vibrations go into the cochlea, which is generally the goal, some of the vibrational energy is transferred into the tympanic membrane and the ear canal. The tympanic membrane is not a very good speaker, but you can measure the sound that comes through it, by recording from the ear canal. This is useful for middle ear implants because it allows us to determine that the middle ear implant is functioning. Additionally, the reverse transfer function has been used as a performance measure.
AO/Beck: Geoff, before we go too much further, this is starting to sound a lot like Otoacoustic Emissions (OAEs). Can you please compare and contrast OAEs and
Reverse Transfer Functions?
Ball: I'm not an expert on OAEs at all, but from what I understand, an OAE originates with an audible tone, and the tone is carried across the ossicles and into the cochlea, and then if the outer hair cells are functioning normally, they respond, and their response is the OAE. The reverse transfer function is less dependent on the cochlea or the outer hair cells. The reverse transfer is VORP exhaust, it's what comes out of the middle ear and it's mechanically generated.
AO/Beck: And VORP refers to the Vibrating Ossicular Prosthesis?
Ball: Yes, that's correct. So in brief, the reverse transfer function has little do to with the cochlea, and the OAE is essentially dependent on normal, or nearly normal cochlear function.
AO/Beck: OK, then let's go backwards a little, you said earlier that the reverse transfer function allows us to verify that the middle ear implant is working. How does that work?
Ball: The reverse transfer function can be used intraoperatively, before closing up the patient's ear, to verify that everything is working properly.
AO/Beck: I see. So you stimulate the patient intraoperatively using the VORP to deliver an acoustic stimulus such as a click, pure tone whatever, and then if you record the appropriate response from the outer ear canal with a probe mic, you can conclude that the system is working properly?
Ball: Yes, that's the idea. Of course the resonance characteristics of the middle ear will impact the recording, as will the transducer itself, and the transducer's position too.
AO/Beck: Very good. So this is similar to the testing that occurs in the operating room to confirm intraoperative cochlear implant integrity?
Ball: Correct. The idea is that it's much better to determine that the whole system is working properly before you finish the operation, rather than finding out a few weeks later that the system isn't working for some reason, which could have been addressed, if only it was detected. Of course, there is a downside to intraoperative recoding of reverse transfer functions, which is that it takes time at the end of the procedure, and OR time is very expensive. Adding 15 minutes to the procedure can add 1500 dollars to the case. So we'd like to really make the procedure quicker and more efficient, but we're just at the beginning of these developments.
AO/Beck: I understand. Another stumbling block which is a good news/bad news issue is that because the success rate is darn near 100 percent anyway, it may be a difficult sell to get the audiologist and the otologist to do any extra testing in the OR. We used to do promontory stimulation tests as part of the pre-op evaluation for cochlear implant patients, but because the results were pretty much always excellent, few people bothered to pursue that, and these days it's pretty much not done, as far as I know. Geoff, This really is very exciting work. Has anyone written this up?
Ball: Yes, as a matter of fact, we just won the Politzer prize for this work in France a few weeks ago.
AO/Beck: Geoff, if you don't mind, at the end of this interview, I'll insert the hyperlink to that news story, from the Audiology Online News Banner archives, so interested folks can learn more about that.
Ball: That would be great.
AO/Beck: OK, if you don't mind, can we spend a few minutes on Laser Doppler Vibrometry (LDV) before you run?
Ball: Sure, what would you like to know?
AO/Beck: Let's start with some history and an overview of Laser Doppler Vibrometry.
Ball: For the last 15 years, we've used LDV as the basic tool to help describe and measure the action or motion of the tympanic membrane and the ossicles. You could say that LDV is used to characterize the vibrational patterns of the human middle ear. So practically, we can use LDV technology to analyze how a prosthesis attached to the ossicular chain changes the normal vibrational characteristics of the human ear. LDV actually provides a real-time, computer generated 3-D picture of the vibrational characteristics of the middle ear.
AO/Beck: I think many people picture the stapes within the oval window as a piston. But your work indicates something very different doesn't it?
Ball: Yes, the stapes has a piston-like relationship with the oval window, but it also moves in many other directions too, and it does so simultaneously. In other words the stapes may be going in and out of the oval window, but it may also be turning a little, and it may change the angle from which the piston motion originates, in response to the acoustic stimulus. Additionally, the tympanic membrane has multiple vibrational characteristics, and these too, are impacted by the stimulus, and other anatomic characteristics of the ear itself. For example, after you go above 4000 Hz, the tympanic membrane breaks into multiple modes, and it's not very good at transferring very high frequency sounds into motion. And so you see the ossicles moving, and you can detect slippage in the normal incudo-stapedial joint, and also at the malleo-incudial joint. As one of my medical colleagues says, some ears are simply better than others. In effect, you lose energy at these soft cartilaginous joints and we can observe what happens when you add a prosthesis to the ossicular chain.
AO/Beck: So I can only assume that adding mass to the ossicles must create a damping phenomena?
Ball: Yes. In fact we know the mass of the prosthesis has to be below 50 milligrams, or it will negatively impact the vibrational pattern of the middle ear. Additionally, we know there are some locations for the transducer that are fine, and there are others that will impede the transfer of energy. Lastly, we know that the middle ear's primary function is actually NOT to transfer sound to the inner ear, rather, the middle ear acts primarily to protect the cochlea from pressure changes!
AO/Beck: So do you think LDV will be the next clinical gotta have it test?
Ball: I can't envision that at this point. Despite the advantages of LDV, it is expensive and complicated, and has limited clinical day-to-day utility currently. It is a great tool, and it has helped us a great deal, but I don't think it's ready for day-to-day clinical decision making or diagnostic implications.
AO/Beck: Geoff, once again it's a pleasure to spend time with you. Thanks for sharing your thoughts on these topics.
Ball: Thank you too Doug. It's always fun to talk about these issues.
For more information on Reverse Transfer Functions, CLICK HERE.
FOR MORE INFORMATIONON SYMPHONIX MIDDLE EAR IMPLANTS, CLICK HERE.
