A Lightweight Dynamic Hand Orthosis With Sequential Joint Flexion Movement for Postoperative Rehabilitation of Flexor Tendon Repair Surgery

During the postoperative hand rehabilitation period, it is recommended that the repaired flexor tendons be continuously glided with sufficient tendon excursion and carefully managed protection to prevent adhesion with adjacent tissues. Thus, finger joints should be passively mobilized through a wide range of motion (ROM) with physiotherapy. During passive mobilization, sequential flexion of the metacarpophalangeal (MCP) joint followed by the proximal interphalangeal (PIP) joint is recommended for maximizing tendon excursion. This paper presents a lightweight device for postoperative flexor tendon rehabilitation that uses a single motor to achieve sequential joint flexion movement. The device consists of an orthosis, a cable, and a single motor. The degree of spatial stiffness and cable path of the orthosis were designed to apply a flexion moment to the MCP joint prior to the PIP joint. The device was tested on both healthy individuals and a patient who had undergone flexor tendon repair surgery, and both flexion and extension movement could be achieved with a wide ROM and sequential joint flexion movement using a single motor.

as cuts [1], and these individuals undergo tendon repair surgery to regain their impaired hand function.During the postoperative rehabilitation period, the repaired flexor tendons should be continuously glided with sufficient tendon excursion and carefully managed protection to prevent adhesion with surrounding tissues [2], [3], [4].Due to the absence of a gold standard rehabilitation protocol, multiple rehabilitation protocols, including early active and passive mobilizations, are utilized [2], [4].Patients should voluntarily move their fingers for active mobilization; however, excessive load applied to the repaired tendons through careless movement can cause re-rupture [4], [5].Thus, passive mobilization, such as Duran-type regimens [6], in which fingers are passively and repeatedly moved by skilled physiotherapists, is currently utilized.
Because physiotherapists should devote a significant amount of time to passive mobilization, postoperative flexor tendon rehabilitation devices have been developed to reduce their workload.To prevent adhesion and re-rupture, the devices should provide at least 3 mm [2] of tendon excursion in repaired tendons without subjecting them to excessive load.Thus, the devices should assist finger joints with a wide range of motion (ROM) while preventing hyperextension.A wide ROM can be achieved by flexing finger joints with various inter-joint coordination, and sequential joint flexion movement may be particularly beneficial for physiotherapy due to maximized tendon excursion.This is because degrees of metacarpophalangeal (MCP) joint flexion can be maximized without contacts made between fingertips and palm when finger joints flex in a sequential manner, starting with MCP joint movement followed by proximal interphalangeal (PIP) joint movement [7], [8].Furthermore, four-finger mobilization can be advantageous, as tendon excursion is greater when the index, middle, ring, and little fingers are moved simultaneously [9].
While exoskeletal rehabilitation devices consisting of multiple exoskeletal links and motors have been developed to assist with passive mobilization [10], [11], [12], these devices are less convenient to use in daily life for physiotherapy.The links are mounted on the dorsal aspect, and assistive torque generated by motors is transmitted to the links and finger joints via a multi-bar or cable-driven mechanism.The devices that utilize multiple actuators for each finger [10], [11], [12] can flex fingers simultaneously with various inter-joint coordination and joint velocity, thereby promoting tendon healing [13].However, the use of multiple links and actuators can result in poor compactness due to their weight, making the devices less convenient to use.Under-actuated exoskeletal devices [14], [15], [16], [17], [18], [19], [20], [21], [22] were developed to reduce the weight; however, they cannot achieve sequential joint flexion movement because they focused on assisting the simultaneous movement in the MCP and PIP joints with a single actuator.
While soft robotic gloves have been developed for convenient use [23], [24], [25], [26], [27], there is a safety concern regarding hyperextension during assistance [28].The gloves are made of soft materials, such as fabric or silicone, and the actuators are remotely mounted on a desk or worn on another part of the body to reduce the external weight applied to the hand.With the remotely placed actuators, passive mobilization is assisted by a cable-driven mechanism or pneumatic actuation.However, the soft structures cannot prevent hyperextension during assisted finger joint extension [28] and cannot assist in sequential joint flexion movement when a single actuator is utilized for each finger.
This paper presents the development of a lightweight postoperative flexor tendon rehabilitation device that enables four-finger mobilization and sequential joint flexion movement through a single motor and cable, while avoiding hyperextension.The degree of spatial stiffness and cable path of the orthosis were designed to enable the sequential joint flexion movement via the single motor, consequently achieving lightweightness by minimizing the number of required actuators.Additionally, a user interface was provided to ease the adjustment of exercise parameters, such as joint velocity.The device was evaluated on healthy individuals and a patient who had undergone flexor tendon repair surgery.The rest of this study is organized as follows: Section II presents the design of the device.Section III presents the experimental results.Section IV discusses the significance and implications of the results, and Section V concludes the paper.

II. MATERIALS AND METHODS
Based on related previous research and interviews with clinicians, necessary functional requirements of the device for physiotherapy were determined as follows: a. Targeted finger movement: The device was designed to facilitate high tendon excursion of the flexor digitorum superficialis (FDS), thus targeting assistance of transition from an open hand posture to a straight fist posture [29] with sequential joint flexion movement [7], [8] and four-finger mobilization [9] (Fig. 1a).
b. ROM requirement: According to previous research [2], at least 3 mm tendon excursion should be provided to prevent adhesion.Thus, the device should assist a total ROM in the MCP and PIP joints at least 46.2 • [7], which will result in a 3 mm FDS tendon excursion during flexion.
c. Lightweightness: The device should be fit on the hand and be lightweight (<500 g [30]) for convenient use.Thus, minimal number of actuators should be used for assistance.

A. Working Principle of the Device
The proposed flexor tendon rehabilitation device (Fig. 1b) primarily consists of an orthosis, a small-sized motor (10∅, 64:1, 315172, MAXON Inc., Sachseln, Switzerland), and a cable (Power pro, Shimano Inc., Caringbah, Australia).In order to achieve the targeted functional requirement, only a single motor was used with the orthosis.The orthosis is located at the dorsal aspect of the hand and forearm, and the motor is mounted on the forearm and connected to the orthosis by the cable.Under applied tension on the cable, the orthosis is flexed to apply flexion force on the four fingers for sequential joint flexion movement, and it is extended by its elasticity to apply extension force on the fingers under released tension.To apply tension on the cable, the device adopts a twisted string actuation (TSA) mechanism to reduce the weight of an actuation module [31].Tension is applied (Tensioned) by twisting the cable using the motor and released (Untensioned) by untwisting the cable.Note that the device is designed to be able to flex even stiff fingers.In the literature [32], [33], [34], a required torque for maximal flexion of the MCP and PIP joints ranged from 0.02 Nm to 0.3 Nm.With the determined cable path, cable tension required to maintain the straight fist posture against the maximal resistive finger joint torques was Fig. 2. The working principle of the device: (a) The orthosis comprises "Three-in-one" joints (j 1 , j 2 ), which consist of an elastic film and beams.The elastic film is attached to the beams at proximal and distal aspects, allowing the joints to function as hinge joints ("hinge") with elasticity that returns them to their extended posture ("elasticity").Additionally, the range of motion of the joints is limited by the beams ("joint limit"), (b) Assisted finger and orthosis movements.The cable is routed at the routing positions of the orthosis.When tension is released (Untensioned), the joints and fingers are extended by the elasticity of the orthosis.When tension is applied (Tensioned), flexion and extension moments are respectively applied to j 1 and j 2 , resulting in j 1 and MCP joint flexion.After j 1 flexion, the flexion moment applies to j 2 to generate j 2 and PIP joint flexion.calculated as 200 N. Therefore, a motor capable of applying cable tension of 400 N [31], considering a safety factor of two, is utilized for the device.
The orthosis is flexed and extended with respect to its Threein-one joints ( j 1 , j 2 ) under flexion and extension moments.The Three-in-one joints are designed through adjustment of degrees of stiffness spatially distributed at the orthosis and consist of an elastic film and rigid beams (Fig. 2a) made with a flexible material (TPU; Hyvision System, Inc., Seongnam, South Korea) using a three-dimensional (3D) printer (Cubicon Single, Hyvision System, Inc., Seongnam, South Korea).The Three-in-one joints can serve three mechanical functions, namely "hinge", "joint limit", and "elasticity".The elastic film of the Three-in-one joints is rectangular-shaped and is folded with respect to its center axis under a moment-free condition, as shown in Fig. 2a.The outer surfaces of the elastic film are respectively attached to the proximal and distal beams, making the elastic film serves as a hinge joint between the beams ("hinge") under moments.Specifically, the beam at the distal aspect is flexed with respect to the joint ( j 1 and j 2 flexion) under an external flexion moment and returned to its initial extended posture ( j 1 and j 2 extension) by the extension moment generated by the elasticity of the elastic film ("elasticity") when the flexion moment is removed.Moreover, the ROM of the rigid beams is restricted by blocking that occurs between the beams ("joint limit").Specifically, the j 1 and j 2 can flex up to 90 • , and further extension of j 1 and j 2 from the initial extended posture is restricted.There are two Three-in-one joints within the orthosis to assist MCP and PIP joint movements.
With the motor and orthosis, the sequential joint flexion movement is achieved by adjusting cable path where the cable is routed at the orthosis (Fig. 2b).The cable path is designed to apply a flexion moment on j 1 prior to j 2 under applied tension, and the details are described in the Section II-C.Initially, when the tension is released, the j 1 , j 2 , and finger joints are extended due to the elasticity of the orthosis.When the tension is applied, flexion and extension moments are respectively applied to the j 1 and j 2 , generating j 1 and MCP joint flexion while limiting the j 2 and PIP joint movements.After the j 1 and MCP joint flexion, the flexion moment begins to apply to j 2 , generating the j 2 and PIP joint flexion.The j 1 , j 2 , MCP, and PIP joints are extended by the extension moment generated by the elasticity when the tension is released.

B. Orthosis Design
The orthosis comprises rigid (Splint) and bendable (Beam) structures (Fig. 3a).The Splint is rigidly designed using Geomagic Freefrom software (3D Systems, Inc., Rock Hill, SC, USA) based on a 3D scan of the hand of each individual.The 3D scanning was performed using a 3D scanner (Sense2, 3D Systems Inc., Rock Hill, SC, USA) while the users fully extended their thumb and flexed their wrist to 10 • .The Splint fixes the thumb in the fully extended posture to prevent it from blocking flexion of other fingers.The wrist is fixed in a 10 • flexed posture because it has been reported that immobilizing a wrist in a slightly flexed posture can be applied to postoperative flexor tendon rehabilitation [35].
The Beam, which comprises the elastic films and beams, is designed based on measured anthropometric dimensions, taking variations in hand size into considerations (Fig. 3a).The base of the Beam is positioned at the MCP joint of the thumb, and the longitudinal length of each segment (denoted as proximal, middle, and distal) is determined based on the measurement results of the middle finger, which is the longest among the fingers.Specifically, the length of the proximal segment is equal to the sum of the distance between the base and the distal edge of the metacarpal (L 1 ), and half of the thickness (t) of the Beam.The length of the middle and distal segments corresponds to the sum of the length of the proximal phalanx (L 2 ) and the thickness of the Beam, and the sum of the length of the middle phalanx (L 3 ) and half of the thickness of the Beam, respectively.The determined locations of j 1 and j 2 allow the orthosis to assist in achieving full ROM in finger joints during flexion, as shown in Fig 3a .Due to different joint locations of fingers and the device, the occurrence of slippage between fingers and the device is necessary during flexion.Additionally, the proximal and middle segments have a constant thickness that ensures the orthosis weighs less than 500 g.The thickness of the distal segment gradually increases along its length to allow for a maximal PIP joint angle of 103.98 • [36] when j 2 is maximally flexed at 90 • .
A minimal thickness that the 3D printer can fabricate was used for the elastic films.The elasticity of these films should passively extend the fingers; however, joint stiffness may differ between individuals [37] and can vary over time.Excessive elasticity is beneficial for finger extension; however, it should be avoided to prevent unnecessarily high assistive force for flexion, which can increase the size of the motor.Thus, a minimal thickness is used for the elastic films of the Beam, and additional elasticity required for extension is achieved by applying rubber bands to adjust the elasticity for extension against varying finger joint stiffness (Fig. 3b).Note that the number of rubber bands used at j 1 and j 2 is denoted as n 1 and n 2 , respectively.Moreover, because full extension of the MCP joint is considered to apply a high load on flexor tendons [4], j 1 is designed to be slightly flexed in a tension-free condition by adjusting α in Fig. 3b to restrict full extension of the MCP joint when the tension is released.The α was set to 40 • [4]  for j 1 and 0 • for j 2 .
The fingers are tightened onto the Beam using fabric straps to apply extension force to the fingers under j 1 and j 2 extension moments.Because the length of the middle and little fingers is significantly dissimilar, two different fabric straps are used to tighten the fingers at regions above their PIP joint (Fig. 3c).Additionally, the wrist is tightened onto the Splint using a fabric strap.
The routing structures are simultaneously 3D printed with the orthosis.The cable is then routed through a hole in the routing structures (Fig. 3d).In addition, a rigid rod is placed at a hooking position to enable the routed cable to be hooked when j 1 and j 2 flexion occur.The location of routing and hooking positions are determined to enable sequential joint flexion movement.

C. Routing and Hooking Positions
To achieve j 1 flexion prior to j 2 flexion under various conditions of joint stiffness at j 1 and j 2 , which are generated by rubber bands and fingers, the cable should initially apply j 1 flexion and j 2 extension moments under applied tension.The routing positions, denoted as r 1 and r 2 in Fig. 4, were adjusted by a geometrical model to apply j 1 flexion and j 2 extension moments until j 1 flex to 90 • [36], and then begin to apply j 2 flexion moment after j 1 flexion reaches 90 • .
The r 1 was determined by locations of the r 2 , j 1 , and j 2 , where the r 2 was fixed at the end of the Beam for modeling simplicity.For the determination, the lines were respectively defined as follows: The other line was defined by the 90 • (i.e., ϕ in Fig. 4) rotated position of the r 2 and j 2 with respect the j 1 , as follows: Authorized licensed use limited to the terms of the applicable license agreement with IEEE.Restrictions apply.The r 1 should be positioned within the shaded area in Fig. 4 generated by ←→ l 1 and ←→ l 2 to apply j 1 flexion moment while loading j 2 extension moment.The r 1 should be also positioned on ←→ l 3 to begin applying j 2 flexion moment when j 1 flexion reaches 90 • .Thus, the r 1 should be located between the points of P 1 and P 2 , which respectively represent the intersecting points of The location of the r 1 was numerically found using MATLAB (2018a, MathWorks Inc., Natick, MA, USA) as follows: The k denotes the unit vector of the z axis in Fig. 4.Among the obtained r 1 values, the one closest to P 2 was used because it provides increase in the j 1 flexion moment arm.The hooking position was assigned below j 2 and between ←→ l 3 and the edge of the Beam to enable the cable to apply j 1 flexion moment during j 2 flexion.In the geometrical model, it was not possible for j 2 flexion to occur when j 1 flexion reaches at 90 • due to the j 2 flexion moment arm being zero.However, it was considered that an increase in tension cause a deformation in the hole of the routing structure at r 1 , which alter the cable path to provide a j 2 flexion moment arm for j 2 flexion when j 1 flexion reaches 90 • .

D. Device Control
The electrical components of the device consist of a graphical user interface (GUI) and a motor controller, which includes a motor driver (446023, MAXON Inc., Sachseln, Switzerland) and an Arduino Uno (Fig. 5a).The GUI includes an LCD display (NOKIA 5110, NOKIA Inc., Espoo, Finland) and three mechanical buttons that users can utilize to adjust exercise parameters such as the number of exercise repetitions (N ), velocity, and ROM.The velocity and ROM are adjusted by regulating the motor rotation speed (dψ/dt) and rotation angle (ψ).
The device provides users with an interface to select three options of targeted rotation angle.During the setup stage, the maximum targeted rotation angle is set when the fingers are considered to be in a straight fist posture.The other targeted rotation angles are set within this maximum value.Additionally, the maximum targeted rotation angle can be adjusted by users using a user interface, allowing the orthosis to provide users with a wide ROM under varying finger joint stiffness.Excessive flexion of the orthosis, caused by either a highly increased maximum targeted rotation angle or reduced finger joint stiffness, can be restricted by "joint limit" of Threein-one joints within the orthosis.
The motor is controlled to rotate at a constant speed until the targeted rotation angle is reached.The device offers users three options for the targeted rotation speed: minimum, medium, and maximum.During the setup stage, these speeds are respectively set to enable the motor to reach the maximum targeted rotation angle in approximately 45 s, 30 s, and 15 s.
The motor rotates according to the pre-set exercise parameters (Fig. 5b).Once the exercise is started via the GUI, the motor begins to rotate from the initial rotation angle to the targeted rotation angle at the selected rotation speed for finger flexion.The initial rotation angle is the motor angle when the cable is completely untwisted.When the motor angle reaches the targeted rotation angle, it is held for 5 s to maintain the flexion posture.Subsequently, the motor rotates in reverse at the selected rotation speed for finger extension.Once the motor angle reaches the initial rotation angle, it is held for 5 s to maintain the extension posture.The exercise is repeated N times, as pre-set by the user.Users can select new exercise parameters after N repetitions.Additionally, there is an emergency button to allow users to stop the exercise whenever they wish.

A. Subject Recruitment
Three healthy subjects (S1-3; demographics listed in Table I) who had not previously suffered from flexor tendon Authorized licensed use limited to the terms of the applicable license agreement with IEEE.Restrictions apply.injuries in their hands, and one subject (P1; demographics listed in Table I) who had suffered from a flexor tendon injury, were recruited to evaluate the device.The orthosis was custom-made based on individual hand measurements.For P1, hand measurements were taken during hospitalization before tendon repair surgery, and the orthosis was fabricated during a two-week immobilization period immediately following the surgery.During the immobilization period, all fingers were fixed in an extended posture using a static brace.The experiment was conducted after the immobilization period ended.The study was approved by the institutional review board at the Seoul National University Bundang Hospital (B-2006-616-005, 2020/07/22), and written consent was obtained from each subject after the experiment was explained.

Orthosis Movement:
The capability of the orthosis to achieve sequential joint flexion movement at j 1 and j 2 before donning was evaluated.Reflective markers were attached to the lateral side of the beam segments, as shown in Fig. 6, for measuring the movement.The orthosis designed for S1 was used for the evaluation.The j 1 and j 2 flexion angles (θ 1 , θ 2 ) were calculated based on 3D trajectories of markers captured by the motion capture system (OptiTrack V120: Trio, NaturalPoint, Inc., Corvallis, OR, USA) until j 2 flexed to  approximately 90 • for a single trial.Thereafter, the j 1 flexion angle value (θ 1B ) when j 2 flexion initiates was obtained.
2. Finger Movement: The capability of the device to achieve a straight fist posture with sequential joint flexion movement was evaluated.Reflective markers were attached to the lateral side of the index finger at the fingertip, distal interphalangeal (DIP) joint, PIP joint, and middle of the proximal phalanx, as shown in Fig. 6, for the measuring the movement.The joint angles (θ MC P , θ P I P ) of the MCP and PIP joints were calculated based on the 3D trajectories of markers measured during a single trial, until the PIP joint was flexed to approximately 90 • for safety [36].Thereafter, the MCP joint angle value when the PIP joint flexion initiates was obtained.Moreover, to confirm sequential joint flexion moments created by a single cable, flexion moments at the MCP and PIP joints were measured with S1 by placing force-sensing resistors (FSR) (FSR01CE, OHMITE Manufacturing Co., Warrenville, IL, United States) on the dorsal aspects of the middle and proximal phalanges, while measuring cable tension using a tension sensor (DBCM, CASSCALE Co., Seoul, South Korea).
3. Range of Motion and Questionnaire: Based on the obtained joint angles (θ MC P , θ P I P ), the sum of the ROM in the MCP and PIP joints was calculated.It was confirmed whether the device can provide the total ROM in the finger joints at least the 46.2 • , which is considered to generate a tendon excursion of 3mm minimally required to prevent adhesion [2], [7].Additionally, all subjects were asked to answer a questionnaire comprising 1) the weight of the orthosis is  n 2 ) applied at the joints.When tension is applied, j 1 flexion occurs from state A to B, followed by j 2 flexion occurring from state B to C. When tension is released, j 1 and j 2 extensions occur from state C to A. Note that state A represents the instant when tension is completely released, state B represents the instant right after significant j 1 flexion movement, and state C represents the instant right after significant j 2 flexion movement, (c) The value of θ 1 when j 2 flexion initiates (θ 1B ), is plotted for different values of n 1 /n 2 .The θ 1B decreases as the value of n 1 /n 2 increases.manageable and 2) the orthosis is comfortable while using.These questions were determined based on the orthotics and prosthetics user's survey (OPUS) [38].The satisfaction of the subjects was ranked on a scale of one to five, with a higher value indicating greater satisfaction.

C. Orthosis Movement
Before donning, the orthosis was flexed in a sequential manner during actuation, with j 1 flexion followed by j 2 flexion.In Fig. 7a, "Experimental" represents the measured trajectory of the marker attached to the tip of the distal beam segment, while "Predicted" represents the predicted trajectory of this marker.The predicted trajectory was calculated by assuming the rigidity of the Beam and that j 2 flexion is initiated when j 1 flexes to 90 • .In the "Experimental", j 1 flexion occurred prior to j 2 flexion; however, j 2 flexion was initiated when j 1 flexed to 78.8 • , resulting in a significant trajectory difference between the "Experimental" and "Predicted" results after j 2 flexion was initiated.The same two rubber bands were applied at j 1 and j 2 in this experiment.Additionally, different number of rubbers bands applied to j 1 and j 2 to evaluate the capability of achieving sequential joint flexion movement.
Sequential joint flexion movement was achieved with different numbers of rubber bands applied to j 1 and j 2 .However, j 2 flexion occurred before j 1 flexed to 90 • .In the condition of n 1 < n 2 (Fig. 7b), two and six rubber bands were respectively applied to j 1 and j 2 .When the tension was applied to the cable, j 1 flexion significantly occurred from state A to B, as shown in Fig. 7b.Thereafter, j 2 flexion was initiated when j 1 flexed to 85.8 • , and j 2 flexion significantly occurred from state B to C. During j 2 flexion, j 1 flexion also occurred by applied j 1 flexion moment until a blockage occurs between proximal and middle beam segments.When the tension is released, j 1 and j 2 were then extended when the tension was released.Note that state A represents the beam state when the tension is completely released, while state B and C represent the beam states when significant flexion of j 1 and j 2 is terminated, respectively.In the condition of n 1 > n 2 (Fig. 7b), six and two rubber bands were respectively applied to j 1 and j 2 .The j 2 flexion was initiated when j 1 flexed to 67.7 • in this condition.
To investigate the reason for the early initiation of j 2 flexion, the value of θ 1B , which represents the j 1 flexion angle at the onset of j 2 flexion, was measured for different values of n 1 /n 2 .Because achieving sequential joint flexion movement required that j 2 be in full extension under a tension-free condition, a minimum of two rubber bands required to ensure full extension were applied to j 2 .Varying number of rubber bands was applied to j 1 , and j 2 flexion began when θ 1B was 72.4 • ± 8.4 • for various n 1 /n 2 conditions, as shown in Fig. 7c.Moreover, θ 1B tended to decrease with an increasing value of n 1 /n 2 .A higher tension may be required to flex j 1 with an increased n 1 /n 2 , which could easily cause deformation of the hole of the routing structure (r 1 ) to generate a j 2 flexion moment arm, leading to earlier initiation of j 2 flexion with an increased n 1 /n 2 .Nevertheless, the obtained θ 1B values were comparable to the MCP joint angle of the index finger at a straight fist posture (70.83 • [36]).Therefore, sequential finger joint flexion was considered achievable with the orthosis after donning.
The repetitive flexion movement of the orthosis was measured to assess its robustness against stretching caused by flexion (Fig. 8).The motor rotated to execute 10 3 cycles of flexion within the orthosis for evaluation.It was confirmed that the orthosis could endure 10 3 flexion cycles at both j 1 and j 2 without any observed breakage within the structure, resulting in a consistent path for the beam angles between the first and last cycles of flexion.Minor variations in the path may result from the softening of the orthosis material and rubber bands due to stretching.The orthosis demonstrated its ability to withstand a high number of flexion cycles, as no significant changes in the path of the beam angles were observed during the 10 3 cycles of flexion.The same two rubber bands were applied to both the j 1 and j 2 in this experiment.

D. Finger Movement
The device successfully achieved sequential joint flexion movement for all subjects.Rubber bands were applied to the Authorized licensed use limited to the terms of the applicable license agreement with IEEE.Restrictions apply.orthosis to ensure full extension of j 1 and j 2 immediately after donning, which extended the finger joints via elasticity before applying tension.When tension was applied, the MCP joint flexion phase movement from state A to B occurred first, followed by the PIP joint flexion phase movement from state B to C, as shown in Fig. 9.During PIP joint flexion, MCP joint flexion also occurred due to the applied j 1 flexion moment until a blockage occurs between proximal and middle beam segments.Note that slippage between the device and fingers during flexion, resulting from their different joint locations, was facilitated by placing low-frictional films between the device and fingers, allowing the fingers to flex as the device moved.The MCP joint angle when PIP joint flexion initiated  II), a capability that previous under-actuated devices cannot achieve.Because P1 felt pain around the surgical site during PIP joint extension, the fabric straps were loosened until P1 no longer felt pain, resulting in a small degree of PIP joint ROM for P1 compared to S1-3.Once the pain subsides, further extending the PIP joints in a tension-free condition would increase the degree of PIP joint ROM for P1.The device also enabled the fingers to achieve a straight fist posture.The maximally flexed joint angles (69.2 • and 81.8 • on average) were comparable to the joint angles of the index finger in a straight fist posture, which were respectively 70.83 • and 72.4 • [8], [36] for the MCP and PIP joints, respectively.
Additionally, it was confirmed that the flexion moment was applied to the MCP joint prior to the PIP joint during the flexion from an open hand posture to a straight fist posture (Fig. 10).The flexion moments at the MCP and PIP joints (τ MC P ,τ P I P ) were calculated by multiplying force values (F MC P , F P I P ) measured by FSR sensors and moment arms tracked by the motion capture system.Note that reductions in cable tension and τ MC P were observed at the onset of PIP joint flexion due to a sudden decrease in joint stiffness at j 2 initially restricted by the blocking of beams under an extension moment.These reductions could be prevented by implementing tension control on the motor.

E. Range of Motion and Questionnaire
The device provided a sufficient ROM for all subjects.When tension was released, the finger joints extended, and when tension was applied, they flexed.With this assistance, the MCP joint angle increased from 35.7 • ± 4.1 • to 69.2 • ± 3.5 • , and the PIP joint angle increased from 29.2 • ± 23.9 • to 81.8 • ± 3.9 • .The total ROM in the finger joints was calculated as 86.0 • ± 20.2 • , demonstrating that the ROM provided was high enough to prevent adhesion.Additionally, the subjects reported that the device is sufficiently lightweight (score: 4.3 ± 0.5) and comfortable while using (score: 4.0 ± 0.8).

IV. DISCUSSION
In this study, a lightweight postoperative flexor tendon rehabilitation device that provides sequential joint flexion movement with four-finger mobilization via a single motor and a cable is presented.The degree of stiffness spatially distributed at the orthosis and its cable path where the cable is routed were designed to achieve the sequential joint flexion movement for finger joints with only a single motor.The device was applied to a hand, and its capability to assist with sequential joint flexion movement was evaluated.The experimental results demonstrated that the device can assist with sequential joint flexion movement with sufficient ROM.Furthermore, the device worn on the hand weighed only 342 g, including the orthosis and the motor, which is light enough for daily use.
The flexor tendon rehabilitation devices should assist finger joints with a wide ROM to prevent adhesion [2], while also being lightweight for convenient use.Although a wide ROM can be achieved by using each motor to mobilize individual finger joints [10], [11], [12], minimizing the number of actuators is necessary for lightweightness.While assistance of multiple finger joints can be accomplished by a single motor with exoskeletal links [14], [16], [17], [18], [19], [21], [22], these links are heavy and require significant space on the dorsal aspect.Alternatively, employing a bendable and lightweight orthosis with a cable-driven mechanism may enable devices to achieve both a wide ROM and lightweightness [39].Additionally, a TSA mechanism that provides a high contraction force-to-torque ratio can be employed to reduce the weight of an actuation module [40].However, this mechanism has a limited contraction stroke, allowing the cable to contract by a maximum of 30 % of its untwisted length [40], which can limit finger flexion during assistance.Thus, the motor should be located based on the measurement results of the required contraction stroke for finger flexion to provide sufficient untwisted length.
While orthosis positioning at the palmar aspect can mobilize fingers, finger flexion can be limited due to the structure positioned at the palm.The orthosis can be designed to cover the palmar aspect to transfer assistive flexion force to finger joints via cable or exoskeletal links; however, this results in limiting degrees of finger flexion by the structure positioned at the palm.Therefore, the proposed device avoids positioning structures at the palmar aspect, allowing for further finger flexion than the exoskeletal orthosis [15] and well-known Kleinert splint [41].Additionally, flexing the PIP and DIP joints before the MCP joint can limit MCP joint flexion, which restrict the tendon excursion [7], [8].This is because even a slight increase in MCP joint flexion when the PIP and DIP joints are already flexed can cause the fingertips to make contact with the palm [8].When the fingertips make contact with the palm, joint compression increases, which is undesirable for rehabilitation.Thus, the proposed device was designed to flex the MCP joint prior to the PIP joint using a single motor through a structural design for lightweightness, rather than employing an individual motor for each finger joint.Previous under-actuated devices [14], [15], [16], [17], [18], [19], [20], [21], [22], [25], [26], [27] cannot achieve sequential joint flexion movement because they focused on assisting the simultaneous movement in the MCP and PIP joints.On the other hand, it was experimentally shown that the proposed device can sequentially flex finger joints into a straight fist Authorized licensed use limited to the terms of the applicable license agreement with IEEE.Restrictions apply.
posture without fingertips contact, regardless of the relative stiffness of the joints, possibly maximizing ROM in finger joints for postoperative rehabilitation.
The degree of finger joint stiffness can vary over time after postoperative immobilization [42], and the orthosis should provide the assistive force for passive mobilization against the varying joint stiffness.While the finger joints can be extended by the structure having fixed elasticity [39], the elasticity may not be sufficient to achieve finger extension against the increased joint stiffness.Thus, the varying finger joint stiffness should be accounted for when designing a dynamic hand orthosis that employs an elastic structure for passive assistance.While dielectric elastomer transducers [43] can be used to achieve the passive assistance against these finger joints by their voltage-dependent variable stiffness, the usage of structures with easily adjustable elasticity, such as rubber bands and springs [44], would be more beneficial for the lightweightness of the orthosis.
The proposed spatial stiffness and cable path design scheme can be employed to design lightweight rehabilitation devices that are required to achieve joint stretching and recovery monitoring.If multiple body joints can be mobilized in sequence via a single motor and a cable, quantitative measurement of characteristics of each joint, such as stiffness, become possible using a single tension sensor.This may enable both the stretching and recovery monitoring for multiple joints by a single actuator and sensor.The proposed design scheme can be applied to design lightweight devices for rehabilitating stiff fingers [42] and feet with plantar fasciitis [45].
While this paper presents an orthosis that enables sequential joint flexion for only two joints, the degree of stiffness spatially distributed at the orthosis can be designed to generate additional joints with an adjusted cable path to apply flexion moment at the additional joints in sequence.Additionally, different orthosis design may be required to flex fingers from the distal to proximal joints in sequence.The joint of the orthosis that assists PIP joint should be positioned closer to the PIP joint to achieve sufficient degrees of PIP joint flexion before MCP joint flexion.
To be practical in clinics, postoperative tendon rehabilitation devices must provide both comfort during wear and the ability to assist with bi-directional finger movement.While lightweight devices, such as the Kleinert splint [41], comprising only a static brace and rubber bands, can assist with passive flexion after voluntary extension, patients may struggle to perform voluntary movements due to increased joint stiffness and pain at the surgical site.Therefore, incorporating actuators to achieve passive bi-directional movements is required to encourage rehabilitation, which would be more beneficial in preventing adhesion.Improving comfort by using a minimal number of actuators is necessary, and this can be achieved through a structural design of the orthosis in terms of spatial stiffness and cable path.
The device has limitations in its ability to assist with finger joint movements.Physiotherapists are able to flex the four fingers of patients to their joint limits simultaneously for a high ROM.However, the device is unable to achieve full flexion of the four fingers simultaneously due to different joint positions in each finger.Nonetheless, the device is considered useful in physiotherapy due to its lightweightness and ability to aid finger joints with sufficient ROM to prevent adhesion.Moreover, there is a limitation in that the device was evaluated on a limited number of subjects, comprising a single patient and three healthy subjects who exhibited differences in both gender and age.Nevertheless, it is believed that the device can be applied to patients for rehabilitation, regardless of their gender and age, as the device can be designed to fit various hand sizes and ensure sequential joint flexion movement in finger joints, regardless of the relative stiffness of the joints.Additionally, an easily replaceable cable module will be necessary because a cable can break after multiple cycles of twisting [31].While cable breaks do not pose safety issues such as cuts or skin damage, they prevent the passive mobilization from being assisted, which can lead to inconvenience during use.Cable friction at the routing and hooking positions could also contribute to a decrease in the lifetime of the cable.While cable friction, measured as 33.3% of the applied tension, may not hinder the device from achieving the targeted movement with the determined cable path, it could damage the cable and necessitate an increase in the required assistive force.

V. CONCLUSION
In this study, we present a novel design of a lightweight dynamic hand orthosis that uses a single motor and cable to provide four-finger mobilization and sequential joint flexion movement in the MCP and PIP joints.Using two motors for the MCP and PIP joints allows sequential flexion of the MCP joint followed by the PIP joint; however, our primary focus was to minimize the number of actuators required for sequential joint flexion movement to achieve lightweightness.This sequential joint flexion movement was achieved using a single motor through a structural design within the orthosis, allowing the cable to apply flexion moment to the MCP joint prior to the PIP joint under applied tension.Experimental results showed that the MCP joint can be flexed before the PIP joint, possibly maximizing tendon excursion during assistance for postoperative rehabilitation.The lightweight orthosis, achieved through incorporating spatial stiffness and cable path designs, successfully satisfied both functional requirements and lightweightness goals.

Fig. 1 .
Fig. 1.The passive mobilization achieved by the proposed device: (a) Targeted finger movement, in which the finger joints are sequentially flexed starting with the MCP joint and followed by the PIP joint, (b) Assisted finger movement, in which the finger joints are sequentially flexed when tension is applied to the cable (Tensioned), and extended by the elasticity of the orthosis when tension is released (Untensioned).

Fig. 3 .
Fig. 3.The orthosis design.(a) The orthosis comprises moving (Beam) and supporting (Splint) structures.The longitudinal length of the Beam segments is determined based on measured anthropometric dimensions of the middle finger, (b) Rubber bands are applied at j 1 and j 2 , located on the outer surface of the Beam for extension.The number of rubber bands is denoted as n 1 and n 2 , respectively.Additionally, the slope (α) of the Beam segment is adjusted for j 1 to restrict full extension of the MCP joint, (c) Fabric straps are located at regions above the PIP joints, (d) The routing and hooking structures are located on the Beam.The cable is inserted through the hole of the routing structures, and the rod is stuck at the hooking position.The routed cable can be hooked by the rod during flexion.

Fig. 4 .
Fig. 4. Determining cable path to achieve sequential joint flexion movement.The routing position (r 2 ) is fixed at the tip of the Beam for modeling simplicity.The other routing position (r 1 ) is determined using the lines ( ←→ l 1 ,

Fig. 5 .
Fig. 5.The electrical components and control algorithm used for the device.(a) Electrical components are used to adjust the exercise parameters and control the motor.(b) Active flexion is achieved by rotating the motor based on the selected exercise parameters, comprising number of exercise repetitions (N), rotation speed (dψ/dt), and rotation angle (ψ).The motor rotation angle increases according to the selected rotation speed until it reaches the selected rotation angle.Extension is achieved by reversing motor rotation with the selected rotation speed until the tension is completely released.

Fig. 6 .
Fig.6.The experimental setup used to measure the beam angles (θ 1 , θ 2 ) and finger joint angles (θ MCP , θ PIP ).Reflective markers are attached and captured using a motion capture system.

Fig. 7 .
Fig. 7. Movement of the orthosis under applied tension.(a) Comparison of the measured path of the marker at the tip ('Experimental') with the predicted path of this marker ('Predicted').(b) The path of the beam angles (θ 1 , θ 2 ) is shown for different numbers of rubber bands (n 1 ,n2 ) applied at the joints.When tension is applied, j 1 flexion occurs from state A to B, followed by j 2 flexion occurring from state B to C. When tension is released, j 1 and j 2 extensions occur from state C to A. Note that state A represents the instant when tension is completely released, state B represents the instant right after significant j 1 flexion movement, and state C represents the instant right after significant j 2 flexion movement, (c) The value of θ 1 when j 2 flexion initiates (θ 1B ), is plotted for different values of n 1 /n 2 .The θ 1B decreases as the value of n 1 /n 2 increases.

Fig. 9 .
Fig. 9. Movement of the finger during assistance.(a) The averaged ROM across subjects from state A to B is shown.Note that state A refers to the instant when the tension is completely released, and state B refers to the instant right after the significant j 1 flexion movement.The state changes from A to B during the actuation.(b) The averaged ROM across subjects from state B to C is shown.Note that state C refers to the instant right after the significant j 2 flexion movement.The state changes from B to C during the actuation.(c) Individual joint angle data during assistance.

Fig. 10 .
Fig. 10.Measured cable tension and flexion moments at the finger joints during flexion.The flexion moment was applied to the MCP joint prior to the PIP joint, enabling sequential joint flexion movement.