## 6 Discussion
### ISP Probe Considerations in Practice  
Since measuring vocal fold collision pressures during bilateral vocal fold phonation has proven to be challenging in vivo [7; 9]. Positioning a pressure probe against one vocal fold in a normal larynx can result in very disordered phonation due to tissue anterior and posterior to the probe vibrating in a chaotic fashion, which also disrupts vibration (and pressure contact measurements) on the contralateral side due to the irregular glottal closure characteristics. These issues are mitigated in a select group of patients with laryngeal cancer who have undergone a conservative endoscopic treatment that allows for subsequent voice restoration [41; 42; 43]. Following this surgical procedure (a partial laryngectomy), one intact vocal fold comes in contact with a surgically reconstructed, non-vibrating scar band on the contralateral side that is medialized to promote glottal closure. The scar band replaces the vocal fold that has been surgically removed to treat the cancer. Many of these patients have perceptually normal conversational voices compared with the pre-surgical condition of a glottal insufficiency producing an excessively inefficient and breathy voice. In these patients, the ISP probe would rest against a patient's non-vibrating vocal fold to yield stable and reliable collision pressure measurements of the medial surface of the contralateral vocal fold, under the assumption that proper anesthetic of the laryngeal tissues would not alter basic phonatory mechanics. Figure 12 illustrates the ISP probe configuration in which the clinician holds a left-handed ISP probe with pressure sensing elements facing the left vocal fold of the patients with a right unilateral cordectomy. In the photograph, the clinician's right hand holds a transoral rigid endoscope coupled to a laryngeal endoscopic imaging system to visualize the vibrating vocal fold in real time. This kind of transoral laryngeal procedure, which is performed using topical anesthesia of the upper airway, has been a routine part of the laryngeal surgeon's practice for several decades. As with any office-based procedure that places an instrument in proximity to the vocal cords (injections, biopsies, etc. ), the surgeon must be skilled with office-based laryngeal techniques. The risk of mild irritation/abrasion of tissue during the pressure measurements is minimized by using a cannula that does not have any sharp edges. Figure 12: Illustration of a clinician holding a left-handed ISP probe that is designed to measure the left vocal fold collision pressure. The clinician’s right hand operates a standard camera sensor for standard video or video stroboscopic imaging. In practice, the following guidelines are recommended for maintaining appropriate positioning of the ISP probe to accurately capture vocal fold collision characteristics:  
1. Endoscopic visualization of the larynx during phonation via videostroboscopy or high-speed videoendoscopy for general placement of the ISP probe such that the proximal sensor is positioned intraglottally and the distal sensor subglottally. 2. In patients with a unilateral cordectomy, placement of the ISP probe on the non-vibrating flap of tissue to maintain a flat surface on which the contralateral, functioning vocal fold can come into contact. 3. Real-time monitoring of both pressure sensor waveforms to verify that the two waveforms are distinct from each other. In particular, the proximal pressure sensor would be in the strike zone during phonation when the intraglottal pressure waveform exhibits: 1. An impulsive peak in the direction of increasing pressure at the start of the closed phase; 2. A rounded peak following the impulsive peak during the open phase; 3. A minimum value reaching a zero or negative value during the open phase. It may be necessary to ask the subject to produce a sustained vowel while the ISP probe is swept in the superior-inferior dimension due to challenges in precise positioning.