Nerve Tensiometer for Spondyloptosis Surgery M. Matinfar 1 , R.J. Webster III 2 , L. Ford 3 , I. Iordichata 1 , C.C. Edwards II 3 and R.H. Taylor 1 1 Dept. of Computer Science, Johns Hopkins University, Baltimore, MD 2 Dept. of Mechanical Engineering, Vanderbilt University, Nashville, TN 3 Maryland Spine Center, Baltimore, MD Abstract— Spondyloptosis is a type of severe spinal deformity that develops during childhood and progresses to adulthood. Spondyloptosis causes back pain, limited range of mobility, leg weakness and severe hamstring tightness. Correction of spondyloptosis requires careful preoperative analysis and special surgical techniques to ensure optimal nerve root stretch; both under-stretch and over-stretch pose a danger to the patient. In this paper, we describe a tensiometer useful for predicting impending nerve injury during spondyloptosis surgery. I. INTRODUCTION We hypothesize that measuring tension in nerve roots will improve the safety of spondyloptosis correction surgery. Spondyloptosis is a spinal deformity wherein the lumbar spine shifts forward relative to sacrum and can become completely dislocated, falling into the pelvis (see Fig. 1). Predisposing factors include congenital malformation of the spine at the lumbo-sacral junction, or a L5 stress fracture during childhood. Fig 1. Spondyloptosis; the misalignment of the S1 and L5 vertebrae. Correction of spondyloptosis requires careful preoperative analysis and extensive experience since there is no way to objectively quantify excessive L4 and L5 root stretch. Gradual Posterior Instrumented Reduction (GPIR) can achieve full correction of spondyloptosis in fewer surgical sessions compared to alternative methods [1], but it does not remove the requirement of nerve elongation during corrective surgery. Thus, we propose to augment the GPIR procedure with a tensiometer for objectively monitoring nerve root stretch during surgery. The tensiometer is able to measure both the force and the displacement of the nerve root. We expect this information to be useful in determining nerve root elongation limits. Currently, such limits are set using heuristics based on patient age, duration of deformity, L-S kyphosis, lumbar flexibility, and the number of previous surgeries [1,2,4]. Surgical experience has indicated that safe levels of nerve elongation range from 2 to 5 cm. II. ELECTRICAL AND MECHANICAL DESIGN The purpose of tensiometer is to measure both the force and the displacement of a nerve root during surgery. The mechanical design consists of a load cell to measure forces, a linear stage to measure displacement and a cabling and hook subsystem to transfer the force. The linear stage is attached to a passive arm to ensure that the hook is perpendicular to the nerve root cross section, as shown in Fig. 2. The lockable arm is a standard passive positioning arm that provides the necessary degrees of freedom to enable the surgical team to place the hook directly above the nerve, with the load cell aligned toward the nerve. The load cell (Entran ELFS) has a range of 5lbs and a resolution of 42.2 mV/lb. The linear stage consists of a micrometer stage (Daedal Inc. MX25) with a maximum travel of 25 mm. Passive arm Load Cell Micormeter Stage Fig. 2. The nerve tensiometer during the animal experiment The electrical board of the tensiometer is an instrumentation amplifier. Electrical screening was accomplished using an oscilloscope for the experiments reported in Sec. 3, but a computer interface and software are available for future experiments. The interface board and computer software will provide real-time intraoperative data processing. III. ANIMAL EXPERIMENT RESULTS An animal experiment was performed at the Minimally Invasive Surgery Training Center (MISTC) at Johns Hopkins