# Novel Instrument for Clinical Evaluations of Active Extraocular Muscle Tension
## 1 Introduction  
Strabismus is a condition in which the visual axes of the eyes are misaligned, resulting in ocular misalignment and symptomatic diplopia. The alignment of the visual axis is controlled by the six extraocular muscles (EOMs) attached to the eyeball in the superior, inferior, lateral, medial, superior oblique, and inferior oblique positions. Contemporary hypotheses regarding the etiology of strabismus have given rise to the \"sensory versus motor\" theory. In this context, an abnormal binocular sensory system is characterized by the absence of binocular fixation with normal visual acuity in each eye, the presenceof strabismus, diplopia, abnormal retinal correspondence, and impaired stereopsis. An abnormal motor system is characterized by the abnormal full duction and version as well as abnormal speed saccades [1; 2; 3]. In this regard, abnormal EOM tension leads to an imbalance in the eyeball that results in a disconjugate gaze. Strabismus is not merely a cosmetic problem because, if untreated in childhood, the turned eye could lead to amblyopia. In addition, ocular misalignment can cause double vision, visual confusion, and abnormal head posture in adults [4; 5]. Typically, strabismus surgeries are planned based on measuring the degree of squint in prism diopters, rather than assessing muscle tension. Nevertheless, postoperative challenges, like overcorrection and undercorrection, are common occurrences [6; 7; 8]. Strabismus is often treated by surgically adjusting the tension on the EOM [9]. Knowledge of EOM tension is, therefore, important for the diagnosis of and surgical planning for strabismus [10; 11]. The forced duction test (FDT) or force generation test (FGT) have traditionally been carried out in typical clinical situations to evaluate EOM tension. The passive EOM tension is evaluated using the FDT, in which the clinician grips and moves the eyeball using forceps to perceive the mechanical restriction of the EOM with the eyeball facing away from the direction of the applied force [12; 13]. On the other hand, active EOM tension can be evaluated using the FGT, which checks for EOM weakness. The clinician fastens a cotton-tipped swab above the limbus of the cornea in order to prevent the eyeball from moving. The patient then tries to move the eyeball toward the direction of the cotton-tipped swab [12]. For example, in a patient with lateral rectus (LR) paresis, the eye is held at the temporal limbus with a cotton swap and is adducted. The patient is then asked to abduct the eye, and the force generated (by the LR) is estimated. The conventional FDT and FGT methods are simple and easy for clinical evaluations of EOM tension. However, these clinical tests are often performed based on the subjective and qualitative perceived assessments of tension by the hand of the clinician. The results of the FDT and FGT are, therefore, subjective and highly dependent on the experience and skill of the clinician [14]. To overcome these limitations, several trials have been conducted to measure EOM tension by using force transducers or strain gauges [15; 16]. The authors previously designed a simple and compact device that integrated forceps, load cells, and tilting sensors [11]. It provided quantitative and continuous measurements of passive EOM tension and could be applied to the treatment of patients with intermittent exotropia [17; 18; 19]. [PERSON] et al. measured active EOM tension generated by saccadic eyeball movement by implanting a force transducer in the EOMs [15]. [PERSON] et al. measured active isometric EOM tension by connecting strain gauge probes to detached EOMs [16]. Although there are several currently available quantitative instruments and recording systems for measuring active EOM tension, most are restricted to use in the research laboratory setting because they are expensive, complicated to operate, or too complex for use in the clinic or operating room.