IEEE TRANSACTIONS ON REHABILITATION ENGINEERING, VOL. 8, NO. 1, MARCH 2000 35 Measurement of External Pressures Generated by Nerve Cuff Electrodes Frank A. Cuoco, Jr., and Dominique M. Durand Abstract—When external pressures are applied to a peripheral nerve, tissue damage can occur via compression and blood flow occlusion, resulting in degeneration and demyelination of axons. Although many types of nerve electrodes have been designed to avoid or minimize this pressure during stimulation of the nerve or recording of its activity, the measurement of the pressure exerted by these cuffs has not been reported. Currently, only theoretical models are used to predict nerve cuff electrode pressures. We have developed a nerve cuff electrode pressure sensor to measure ex- ternal pressures exerted by peripheral nerve cuff electrodes. The sensor has a high sensitivity, linear response with little hysteresis and reproducible output. Pressure measurements have been ob- tained for split-ring and spiral cuff electrodes. The measurements obtained are in agreement with theoretical predictions. Moreover, they indicate that the pressures exerted by cuffs currently used for stimulation generate only a small amount of pressure, which is below the pressure required to occlude blood flow in nerves. The re- sults also suggest that this new sensor can provide reliable measure- ment of external pressures exerted by nerve electrodes and would be an important tool for comparing various nerve cuff electrode designs. I. INTRODUCTION N ERVE cuff electrodes (NCE’s) are utilized for functional stimulation of muscle groups via electrical activation of peripheral nerves which innervate them [6], [9], [21], [22]. Ad- vantages of using NCE’s over surface or intramuscular elec- trodes include 1) lower threshold currents, decreasing power consumption and the probability of electrically induced tissue damage; 2) remote location of the electrode, reducing mechan- ical disturbance caused by muscle contraction; and 3) greater functional selectivity using fewer electrodes, minimizing the number of electrodes implanted and surgical procedures [6], [18]. These cuff electrodes have also been used with great suc- cess to record neural activity in peripheral nerves [4], [23]–[25]. One problem associated with the use of NCE’s is mechan- ically induced neural damage [1]–[8], [10]–[12]. For safety reasons, early NCE models were designed with inner diameters much larger than the outer diameters of the corresponding nerves they were being used to stimulate. However, the loose- ness of these cuffs did not allow for selective stimulation of nerve fascicles. Newly designed cylindrically shaped cuffs are placed snugly around the peripheral nerve to ensure good Manuscript received October 16, 1998; revised December 7, 1999. This work was supported by a Whitaker Foundation graduate fellowship and by the NIH under Grant NS32845-05. The authors are with the Department of Biomedical Engineering, Neural En- gineering Center, Case Western Reserve University, Cleveland, OH 44106 USA (e-mail: dxd6@po.cwru.edu; cuocof@msb.edu). Publisher Item Identifier S 1063-6528(00)01753-5. electrical contact for stimulation. They are fixed in place using suture or another closing mechanism. During surgical implan- tation, mechanical disturbance of the nerve can cause tissue swelling, increasing the nerve diameter by up to one-third of its original size. If nerve swelling exceeds the dimensions allowed by the cuff, the cuff will begin to exert external compressive forces on the nerve. Large external pressures have been shown to cause neural damage leading to demyelination and degeneration of axons [13]–[17], [19]. Nerve compression studies by Powell et al.[14] have shown that demyelination of axons can occur at external pressures as low as 10 mmHg (13.57 cm H O), while signifi- cant axonal damage including degeneration occurs at pressures greater than 80 mmHg (108.6 cm H O). Zochodne et al.[16] examined the effects of acute nerve crush injury and suggested that nerve damage, due to mechanical injury of nerve fibers, will occur after only 30 s of nerve crush. Rydevik et al.[13] examined the effects of compression on intraneural blood flow and found that venular flow was impaired at pressures as low as 20 to 30 mm Hg (27–41 cm H O). Arteriolar and interfascicular capil- lary flow was retarded and completely stopped at pressures of 40 to 50 mm Hg (54–68 cm H O) and 60 to 80 mm Hg (81–109 cm H O), respectively. This decrease in blood flow may lead to ischemia and play a role in the degeneration of axons. Therefore, it is clear that NCE’s should not exert more than 20 mm Hg (27 cm H O) of pressure after implantation [13]. In order to design safe nerve cuff electrodes, it is important to determine the pres- sures than these cuffs can exert. To date, only theoretical models [6] have been used to pre- dict external pressures applied by NCE’s, and no experimental measurements of these pressures have been made. The goal of this study is to design a method of measuring the pressures ex- erted by NCE’s experimentally and to measure the pressures for various cuff designs. The relations between the diameter of the cuff and nerve diameter will be measured and compared to theoretical calculations. These results will be useful 1) to pre- dict pressures and postimplant blood flow interference for NCE designs, 2) to quantitatively compare pressures exerted by dif- ferent NCE’s, 3) to compare results between theoretical and ex- perimental models for NCE’s, and 4) to aid in new NCE designs capable of minimizing pressures exerted on nerves. These re- sults have been presented in abstract form [26]. II. METHODS A. Theoretical Pressure Analysis for Nerve Cuff Electrodes Theoretical models to determine external pressures exerted by two commonly used NCE’s, the split ring and spiral cuffs, 1063–6528/00$10.00 © 2000 IEEE