Session: 07-08-01: Biomedical Devices, Sensors, and Actuators I

Paper Number: 164044

Rectal Tonometer for Detection of Spinal Cord Injury 

Approximately 18,000 new spinal cord injuries (SCIs) occur each year in the United States, according to data from the National Spinal Cord Injury Statistical Center (NSCISC). The prevalence of people living with SCIs is estimated to be around 294,000 individuals. The most common causes of SCIs are vehicle accidents (38%), falls (32%), violence and gunshot wounds (14%), sports-related injuries (8%), and other causes (4%).

SCIs are diagnosed using structural information from MRI images and functional information from bedside neurological exams, including digital rectal examination (DRE). The DRE is a quick bedside neurological assessment used to evaluate the integrity of the sacral nerve roots (S2-S4) and spinal cord function. While MRI is invaluable for visualizing structural damage, it cannot replace the functional insights provided by a DRE in the acute assessment of SCI.

Despite its benefits, it is surprising that in the age of artificial intelligence (AI), machine learning (ML), and the Internet of Things (IoT), the DRE remains a manually performed exam. This procedure is often conducted multiple times by different hospital services, relying on subjective interpretation at a single point in time. Multiple studies have assessed the accuracy of the DRE for evaluating abnormal anal tone, but no definitive conclusion has been reached. The divided opinions may stem from the examination’s highly subjective nature and lack of standardized methods. Clinicians and doctors correctly recognize anal sphincter pressure changes only 64% of the time, which is not precise enough to serve as a primary screening technique for suspected SCI before MRI imaging.

This paper presents the development of a handheld sensor (tonometer) designed to measure rectal pressure. The device utilizes a hollow elastomeric probe that assesses contractile pressure through changes in the pressure of enclosed air. A unique harmonica-like design allows the structure to exhibit axial rigidity and resistance to buckling while offering sufficient radial deformation upon contact with the rectal wall.

It is well known that polymers can exhibit viscoelastic responses, which lead to hysteresis and are generally undesirable. We have examined multiple polyurethane compositions currently used in additive manufacturing, such as stereolithography (SLA). An optimal composition and processing parameters were determined. To facilitate the accurate representation of rectal tone, a calibration pressure chamber was also developed.

Preliminary results from the calibrated devices on two test subjects indicate sufficient sensitivity and good repeatability of the measurements. Average resting tone and squeeze tone were found to be around 40 mmHg and 120 mmHg, respectively. These values are commensurate with the pressure levels determined by anorectal manometry. A noticeable low-frequency drift was also observed in the data. Possible causes included thermal expansion of the enclosed air and the thermoelastic response of the polyurethane. Subsequent analysis indicated that air thermal expansion led to the observed drift, which was confirmed experimentally using concurrent temperature and pressure measurements. No significant thermal expansion was observed in the polyurethane matrix. Consequently, the device was redesigned to include a temperature compensation circuit.

Presenting Author: Eniko Enikov University of Arizona

Presenting Author Biography: Eniko T. Enikov is Professor in the Department of Aerospace and Mechanical Engineering. He is a faculty member in the BIO5 Institute. Dr. Enikov received his PhD from the Dept. of Mechanical Engineering at the University of Illinois at Chicago, Chicago, IL in 1998 and a Postdoctoral Training at the University of Minnesota, Minneapolis, MN. Since 2000, he has been with the University of Arizona. Dr. Enikov has a wide range of research interests in the field of micro-electromechanical systems (MEMS), nanotechnology, sensors, actuators, biomedical devices, applied control theory, signal processing, and modelling of electro-mechanical systems. He is the author or coauthor of more than 133 scholarly products. He is the editor, author or co-author of: Bona, Francesco, and Eniko T. Enikov, eds. “Microsystems Mechanical Design”, Springer Vienna, 2006; Enikov, Eniko T. "Structures and Materials." The Mechatronics Handbook. CRC Press, Boca Raton, 2001, 2017. Dr Enikov is an inventor or co-inventor on five US patents in the area of micro-technology and biomedical diagnostic devices

Authors:

Eniko Enikov University of Arizona
Rein Anton University of Arizona
Richard Chua University of Arizona
Venkata Sai Siva Reddy Koppula University of Arizona