# Mechanical Design of a Bioinspired Compliant Robotic Wrist Rehabilitation Equipment
## 1 Introduction  
The hand, with its 27 bones of the palm skeleton and the fingers, qualifies as the most complicated osteoarticular segment of the human body. This most complex articulation includes the 8 carpal bones displayed in two transversal rows forming the wrist, the 5 metacarpal bones, followed by the 14 phalanxes of the fingers. Obviously a bone system of such complexity is more susceptible to trauma that would inhibit the functions of the hand. Gripping, discriminatory sensitivity, expressivity, and conducting professional tasks can be thus hindered or even cancelled. Any partial or total impairment of the hand is a professional and social emergency that calls for therapeutical methods able to accelerate the recovery of the affected joint. Adequate medical treatment of the effects of trauma sustained by joints of the hand is typically followed by immobilization in a gypsum cast. However, extended repose of the joints leads to muscle hypertrophy or even atrophy, to bone demineralization and dysfunctions of the circulatory apparatus. Consequently, in order to prevent the negative effects of immobilization passive kinesotherapy or excitomotor therapy (the latter in cases of muscle atrophy by denervation) are used [1; 2; 3]. These techniques are part of the general approach to swift patient rehabilitation and thus favor a high functional recovery rate and early return to work life. Motion-based rehabilitation of the hand depends of the location of the trauma and the type of the already applied (surgical or nonsurgical) treatment. The rehabilitation protocols are aimed mainly at pain control and restoring the functionality of the affected area. Such protocols need to be adapted to patients' motor state (muscle tone), sensitivity state (proprioception), and last but not least to their psycho-social and occupational state. The functional reeducation program needs to be initiated approximately 3 days after the trauma has been treated. In order to avoid excessive local straining, the use of customized orthoses is recommended, as well as light physical exercises that consist of compression/elevation active and passive mobilization of the fingers [4]. Subsequently to this stage continuous passive motion (CPM) is applied. This entails the mechanical mobilization of the affected joint without straining the patient's muscles. These motions are designed to impede the generation of fibrous tissue and to reduce joint rigidity. CPM is performed by means of specially conceived equipment that is capable of applying customized optimum rehabilitation motions to the joint. The passive mobilization of the hand joints by means of rehabilitation equipment requires controlling the various parameters of the motion, like the amplitude and speed of the motion and duration and frequency of the exercises. Not to be neglected is the magnitude of the applied forces such as to ensure pain-free rehabilitation exercising. CPM rehabilitation equipment need to allow the adjustment of the quantities mentioned above so that rehabilitation exercises can be adapted to the clinical state of the patient. At present the rehabilitation of the hand joints is performed with the help of equipment the majority of that is actuated electrically. Thus the 6000 Hand CPM OrthoAgility(r) is used for postfracture recovery, reconstructive surgery on bone, cartilage, tendons, and ligaments, and allows patients to achieve a full composite fist of 270\({}^{\circ}\)[5]. Another often-deployed piece of equipment is the Kinetec--8091 Portable Hand CPM designed for rehabilitation after prosthetic replacement of the MCP (MetaCarpoPhalangeal), PIP (Proximal InterPhalangeal) and DIP (Distal InterPhalangeal) joints, and related to rheumatoid/neurological or afterburn stiffness [6]. Designed for postoperative rehabilitation, the Kinetec Maestra(tm) CPM is yet another device that provides a rehabilitation solution for wrist pathology, allowing the achievement of a full composite fist de 255\({}^{\circ}\)[7].