Functional Sensibility Assessment. Part I: Develop a Reliable Apparatus to Assess Momentary Pinch Force Control Haw-Yen Chiu, 1 Hsiu-Yun Hsu, 2,3 Li-Chieh Kuo, 4 Jer-Hao Chang, 4 Fong-Chin Su 3 1 Section of Plastic Surgery, Department of Surgery, National Cheng Kung University, Tainan, Taiwan, 2 Department of Physical Medicine and Rehabilitation, National Cheng Kung University, Tainan, Taiwan, 3 Institute of Biomedical Engineering, National Cheng Kung University, 1 University Road, Tainan, 701 Taiwan, 4 Department of Occupational Therapy, National Cheng Kung University, Tainan, Taiwan Received 28 May 2008; accepted 22 December 2008 Published online 4 February 2009 in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/jor.20859 ABSTRACT: A precise magnitude and timing control of pinch performance is based on accurate feed-forward and feedback control mechanisms. Ratio of peak pinch force and maximum load force during a functional performance is a sensitive parameter to reflect the ability to scale pinch force output according to actual loads. A pinch apparatus was constructed to detect momentary pinch force modulation of 20 subjects with normal hand sensation. The results indicated high intra-class correlation coefficient and small coefficient of variation of the detected force ratio among three repeated tests, which represented that the stability test of the measured response confirmed the feasibility of this apparatus. The force ratio for a 480 g object with a steel surface ranged between 1.77 and 1.98. Normal subjects were able to scale and contribute pinch force precisely to a pinch-holding-up test. This study may provide clinicians a reliable apparatus and method to analyze the recovery of functional sensibility in patients with nerve injuries. ß 2009 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 27:1116–1121, 2009 Keywords: sensation; momentary pinch force control; functional sensibility; feedback control; pinch apparatus The biomechanical advantage of an opposed thumb combined with the mechanoreceptors in the glabrous skin of grasping digits provide the capacity of optimizing and maintaining precision grip. Through a well- integrated sensori-motor system, humans coordinate the force to maintain an optimal pinch and grasp stability in fine manipulative activities. When we lift an object, the acceleration requires an increased tangential force and a parallel increased pinch force to maintain pinch or grasp stability. 1 The information regarding the magnitude and timing of force control is shown in many daily activities; to perform those activities, accurate feed-forward and feedback control mechanisms are required. 2 Previous researchers dem- onstrated that impaired control resulted in less precise adjustments in the friction force between the object and finger pulps owing to poor digital sensation by the local anesthesia on the digits or a long-term deprivation of digital sensation. 3–5 Any sensory deficit might contrib- ute to permanent impairment, so that complete loss of palmar sensation of these parts is considered a 50% deficit of functional hand capacity. 6 However, few well- controlled works or apparati exist to investigate how sensory function contributes to the impairment of hand function. Evaluation of the hand with diminished sensibility presents a challenge due to the complexity of sensibility; to assess it precisely is time-consuming. The common sensory evaluations include the static and dynamic two-point discrimination tests and Semmes–Weinstein monofilament test. 7,8 However, classical sensibility tests do not reflect the true functional sensation of the injured hand. 9–11 Cutaneous sensory message is closely linked to motor areas, 12 and sensory feedback from the thumb and fingers may influence the coordination of precision grip. 13,14 Consequently, the capacity to perceive normal and tangential force with self-initiated lifting of an object is a model for predictive sensori-motor control. 1 The ratio between the peak grip force output and the peak load force input is a very sensitive parameter, defined as the grip efficiency. 15 The close coupling between grip and load forces during grasping-holding-moving of objects shows that the major role of the cutaneous mechanor- eceptors is in economical grip force scaling. 15–18 There- fore, a detailed examination of pinch force regulations according to the load force change is an objective method to evaluate functional sensation. Sensory information from the digital pulp provides the enhanced cortical activation necessary for scaling the pinch force during dexterous manipulations. Effects of aging or injury decrease cutaneous sensibility function, so that a strategic response to insufficient sensory input is to exert excess pinch force to prevent objects from accidentally slipping. 19–22 However, the pinch force control of handling smooth-and-hard objects in subjects with normal hand sensation is rarely studied. It is important to establish a database of the peak pinch force to maximum load force ratio in healthy young adults to compare with patients or older adults. Furthermore, little is known about the comparison of fingertip force control between males and females and the dominance effect in hand manipulation. Our first goal was to develop a reliable apparatus to detect the efficiency of pinch force adjustments due to momentum-induced load fluctuation during discrete vertical movement in healthy young adults. Our second goal was to develop a normal force ratio database when handling a smooth-and-hard object to investigate effects of hand dominance and gender. METHODS Pinch Device The pinch apparatus was adapted from Nowak’s design. 5 The specification was a cuboid in consideration of hand 1116 JOURNAL OF ORTHOPAEDIC RESEARCH AUGUST 2009 Correspondence to: Fong-Chin Su (T: þ886-6-2760665; F: þ886-6- 2343270; E-mail: fcsu@mail.ncku.edu.tw) ß 2009 Orthopaedic Research Society. Published by Wiley Periodicals, Inc.