A Bone to Pick with Bone Conduction

Just over 70 years ago, Georg von Békésy’s experiments laid the foundation for bone conduction (BC) testing. Over 50 years ago, Raymond Carhart researched and wrote about the clinical applications of BC assessment. Ever since the earliest testing using tuning forks, the assessment of bone-conducted hearing sensitivity has been an essential part of audiological assessments and remains so today.

Despite the historical significance, there are some scholars who have cast doubt on the validity of BC, due to variations in results introduced by different skull structures and placement of the bone vibrator. The reliability of the BC results have been doubted too, which is the discrepancy between test and retest measurements due to placement of the vibrator and BC vibrator pressure.

Why accuracy of bone-conduction is so important for quality evaluation

Bone conduction testing is one of the most powerful differential diagnosis tests in diagnostic pure-tone audiometry. The results of BC test compared with air conduction test results indicate whether the individual’s hearing loss is primarily sensorineural or conductive, or a combination of the two.

BC testing influences the diagnosis and treatment plan of a patient directly and profoundly. Hearing healthcare professionals are very aware of the importance of audiometric results - measures are taken to ensure the accuracy of the results before sending them to other health professionals. BC is paramount in a patient’s diagnosis and management. Choice of medical treatment, surgery, hearing assistive devices, and the need for further diagnostic assessments are based heavily on the audiogram.  

One such point to make is the air bone gap. Perhaps one of the most important audiogram features. By distinguishing between conductive and sensorineural hearing loss, air-bone gap influences patient care. A false air-bone gap can result in inappropriate surgical or medical treatment. A missed air-bone gap can delay medical management which may lead to a much more complicated hearing loss.

As stated above, BC testing is essential for differentiation of the type of hearing loss and decision of treatment and management. Therefore, it is essential to ensure that the clinician’s BC assessments are reliable and accurate.

Before we go into details around factors influencing BC results, I think it is important to note that with the constant and incredible evolution of medical devices, we must first  address the use of immittance and otoacoustic emissions (OAEs) as a substitute for BC testing. We should not perform a audiological procedure that does not add any value to the patient’s diagnosis and management. If immittance findings (Tympanometry and acoustic reflexes) are completely normal and, more especially, the OAEs are within normal limits - then BC testing will add no clinical value to the patient or the clinician. However, for patients who have abnormal middle ear status/functionality, BC testing should definitely be conducted. Why? In the case of middle ear functionality being abnormal, the patient is very likely to fail the OAE, however, not necessarily because of the cochlear’s status, but because of the middle ear abnormality. In developing countries or clinics that are just starting up, where additional audiological procedures such as the OAEs and Immittance testing are not readily available, it is always advised to perform BC in the event where air conduction indicates a hearing loss.  

Now, let us go back to the matter at hand..

Issues around placement of BC vibrator

Clinically, numerous factors can influence the BC threshold testing. However, in this article, we will be discussing one significant factor - the vibrator placement.  The vibrator placement is an important factor for obtaining accurate and reliable BC thresholds.

Various authors have examined the effects of vibrator placement on the measurements of BC thresholds (Dirks, 1964; Studebaker, 1962; Dean, 1930). The teeth and the vertex of the skull have been examined as possible sites for BC vibrator placement, however, the forehead and mastoid placement are the most popular sites for BC testing.

Vast research has been done in order to find the clinically optimal placement of the vibrator that would yield reliable and accurate BC test results. Let us look deeper into the examination of BC responses from the mastoid and frontal bone vibrator placement.

Forehead BC

Forehead bone-conduction vibrator placement

Forehead vibrator placement has been shown to have less test-retest variability and higher inter-subject reliability (Dirks, 1964). This means that the forehead placement is superior due to fewer variations produced by vibrator to skull pressure and less artifact produced by middle ear sound conduction mechanism abnormalities that may be caused by middle ear abnormalities. A practical forehead vibrator placement advantage is the fact that it is very stable and does not move when placed on the patient. In contrast, BC vibrator placed on the mastoid can shift very easily, in some rare cases even pop off the mastoid during testing.

Mastoid BC

Mastoid bone-conduction vibrator placement

The mastoid placement is an unfavorable position (Bárány, 1938; Békésy 1932) because (i) shifts of the vibrator placed on the mastoid causes larger threshold shifts as compared to similar shifts on the forehead. As a result, inaccurate bone conduction thresholds may be recorded due to ‘wrong’ vibrator placement. (ii) Inter-subjective variations in underlying tissue and skin are much greater at the mastoid than at the forehead. (iii) The middle ear affects the BC threshold more when measured at the mastoid than at the forehead. In spite of several advantages of forehead vibrator placement, BC testing through mastoid vibrator placement has been proved to have a wider dynamic range than on the forehead. This is because there is more attenuation from the forehead to the middle ear than from the mastoid to the middle ear.

View this article for more information on the masking in audiology.

Why does the KUDUwave™ employ forehead vibrator placement?

In order to avoid shortcomings of the vibrator placement on the mastoid - several authors have advocated for bone-conduction through vibrator placement on the forehead (Békésy 1932; Hood, 1957; Link & Zwislocki, 1951).

BC testing with insert earphones (occluded), additional circumaural earcup attenuation, and a bone-vibrator placed on the forehead provides equivalent thresholds compared to mastoid placement BC testing (Swanepoel, 2011). Of interest is the improvement of forehead placement BC thresholds when both ears are occluded (Studebaker, 1963). Occluded BC thresholds are obtained while both ears are covered with circumaural headphones or blocked with insert earphones. When the ears are occluded, a 'stronger' BC signal reaches the cochlear as compared to unoccluded ears. This ‘boost’ effect that takes place is called occlusion effect. As a result, occluded BC thresholds are 'better' (lower) than unoccluded BC thresholds. The occlusion effect is always accounted for in the bone-conduction results during masking

The KUDUwave™ diagnostic audiometer allows for BC testing outside a soundbooth through the combination of testing with insert earphones (occluded) and circumaural earphones (Swanepoel, 2015).

With that being said, examination of the lowest BC threshold that can be achieved using either mastoid or forehead placement should be done. Inspection of data revealed that bone conduction thresholds measured through forehead placement of the vibrator and the mastoid placement of the vibrator are within typical test-retest variability (Swanepoel, 2011; Studebaker, 1963).

There has been a long standing controversy about the best placement of the bone oscillator - if it is on the mastoid or the forehead, and it has not been definitively resolved. Both BC thresholds measured through vibrator placement on the forehead or on the mastoid have their benefits and limitations. However, both placements are within each other’s test-retest reliability limits.

 Find Out More About KUDUwave Audiometers Thinking out the booth

 

 

Hlolo Ramatsoma

Written by Hlolo Ramatsoma

Hlolo is a clinical, research & support Audiologist at eMoyo. He is involved in many parts of the business, from consulting to R&D to supporting and training customers. He earned his BSc in Audiology from the University of Cape Town and is an experienced clinical audiologist specialized in ototoxicity monitoring, product specialist and training audiologist.

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