3D printing, also known as additive manufacturing, was first invented in the 1980s. Since then, applications for 3D printing have exploded. We have 3D printed toys, jet engine components, space rocket parts – and, now 3D medical printing responsible for the reproduction of middle ear bones.
In 2017, researchers from the University of Maryland School of Medicine in Baltimore successfully created the three bones of the middle ear through 3D printing. These tiny bones, known as ossicles, are responsible for transmitting sound vibrations from the eardrum to the cochlea for the sensation of hearing.
Conductive Hearing Loss
Conductive hearing loss is a type of hearing impairment that occurs when sound cannot be transmitted through the external and/or middle ear. Damage to the ossicles is one cause of conductive hearing loss, such as through trauma, but conductive hearing loss may also be a result of infection, systemic disease, congenital birth defects, or an overproduction of earwax (cerumen impaction).
The standard hearing test is known as pure tone audiometry, a series of beeps that measure the volume threshold at which you can no longer hear the tone being played at a specific frequency. Pure tone audiometry produces a graph of results depicting the softest sounds you were able to perceive across the tested range of frequencies; this graph is known as an audiogram. While this type of hearing test is useful for identifying your level of hearing, it does not necessarily differentiate between the different categories of hearing loss – conductive, sensorineural, or mixed (a combination of conductive and sensorineural hearing loss). For the audiologist or otolaryngologist to pinpoint the cause of the hearing impairment, your hearing examination will involve other components, including a good chat about anything in your history that may relate to hearing, such as head or ear trauma, occupations or hobbies that may involve noise at damaging levels, general health, and any genetic conditions in the family. A physical examination of the visible ear structures helps too, looking for signs of accumulated earwax, foreign objects in the ear canal, the condition of the eardrum membrane, or any other physical abnormalities of the ear.
The Rinnes and Weber hearing tests are specific tests that rely on bone conduction to help differentiate between conductive and sensorineural hearing loss. During the Rinnes hearing test, an active tuning fork is placed on the skull behind the ear, known as the mastoid bone. The duration of the audible sound of the tuning fork is recorded before repeating the test, this time with the tuning fork placed next to the ear canal. Those with conductive hearing loss will find they can hear the hum of the tuning fork when it is placed against the mastoid bone for a longer period of time than when the fork is next to the ear canal as this location relies on air conduction, something that doesn’t work too well in conductive hearing loss. The Weber hearing test is similar, this time with the tuning fork positioned somewhere on the midline of the head, such as the scalp, forehead, or nose bridge. In the case of conductive hearing loss, the hum of the fork will be heard most clearly in the affected ear. Conversely, if the hearing loss is sensorineural, it is the normal ear that works better during the Weber test.
Traditionally, treatment for conductive hearing loss is through invasive surgery or medications. However, surgical reconstruction of the middle ear’s ossicles typically has a poor success rate, partly due to difficulty in creating a correctly sized prosthesis of such tiny bones.
3D Printing the Bones of the Middle Ear (Ossicles)
The 3D printed ear bones at the University of Maryland were modeled from the ossicles of three cadavers. In order to test the accuracy of the printing, four independent surgeons were then asked to match each prosthesis to the corresponding cadaver’s middle ear space, something that all surgeons were able to do successfully. According to the researchers involved in the study, the test proved the strength of 3D printing in its ability to reproduce anatomical components with high precision, to a sub-millimeter level.
Since this breakthrough, middle ear transplants using 3D printed ossicles have been successfully performed on two patients with conductive hearing loss at the University of Pretoria in South Africa, one with congenital underdevelopment of the middle ear and another with middle ear damage from trauma. The ossicle prostheses were created from titanium, a biocompatible material, and inserted using endoscopy, allowing the transplant procedure to be performed quickly with minimal scarring.
Future applications of 3D printed middle ear bones on the horizon include other ear procedures, such as ossiculoplasty (reconstruction of the middle ear bones) and stapedectomy (replacement of all or part of the stapes bone, one of the three ossicles). Doctors hope the use of 3D printed prostheses will help to reduce the risk of accidental trauma induced by the surgery itself, such as facial nerve paralysis.
3D printing is already utilized in audiology to create customized hearing aids. As medical technology advances (and researchers gain the funding they need) 3D printing may one day become the standard for creating the components needed to treat hearing loss, whether it be through a hearing aid or a middle ear bone prosthesis. Though we may not be there quite yet, the future sounds promising; best to keep an ear out.