DESIGN OF A BODY-POWERED FINGER PROSTHESIS FROM MEDICAL IMAGES AND KINEMATIC ANALYSIS

With the advancement of technology, many prosthesis were developed to improve the life standard of a person who lost one or more limb. One of the most common limb loss is finger lose. Many finger prosthesis were developed to imitate the functionality of natural human fingers. But most of them are not personalised (does not fit perfectly to the user) and/or not easy to apply. Some finger prosthesis needs surgical operation to apply. The aim of this study is to design and manufacture accessible and sustainable finger prosthesis that is easy to apply and personalised. In this study, hand skeleton structure was modelled from computerized tomography images. Distal and middle phalanx bones were removed to show finger loss. Personalised finger prosthesis that fits perfectly to the user was designed on the hand skeleton structure. By using kinematic analysis, finger prosthesis movement capabilities are determined and general mathematic model was developed to be used in other patients. Before production, newly designed finger prosthesis movement was analysed and model was optimised with the result of the analysis. Finally, the gripping ability of the prosthesis that was produced and its similarity to the natural finger mechanical structure were confirmed by experiments and measurements.


Introduction
According to the Healthcare Cost and Utilization Project data, finger losses make up 80% of the total upper limb amputation. In addition to physical ability limitation, finger loss also causes mental problems due to visual differences from others [1]. Many studies focus on regaining the physical ability [1][2][3][4] but overlook that prosthesis must fit perfectly to the user in order to accomplish regaining the physical ability.
Finger prosthesis can be classified under 3 main headings; non-functional fingers prosthesis (only aesthetics concern), external powered fingers prosthesis and body powered fingers prosthesis [3,4]. Body powered and external powered prosthesis are functional and can imitate natural finger movements. Most of the powered prostheses were designed to grasp an object with two or more fingers. Tendon-driven systems are usually used at finger prosthesis however these systems are not suitable for natural hand movement. Finger joints do not always start rotation at the same time. On the other hand, mechanic drive system provide smoother movement. Finger movement of the mechanical drive system is very similar to natural hand movements. For these reasons, mechanic system was used in this study.
Understanding hand anatomy is a significant requirement to make a functional finger prosthesis. Moving parts of the hand skeleton structure is phalanx bones. Distal phalanx (DP), middle phalanx (MP) and proximal phalanx (PP) lengths and anatomic structures are different from each other as seen in Figure 1.

Fig 1 Human hand anatomy
In this study DP and MP bones are modelled from computerized tomography (CT) images. Finger prosthesis was designed based on kinematic analysis. Optimised finger prosthesis design was manufactured by using a 3D printer while gripping capabilities of manufactured finger prosthesis was tested by placing a hand.

Material and Method
This study aim to design personalised, easy to apply finger prosthesis. CT images (taken to corresponding author) were used in order to make design personalised. Distal and middle phalanx bones are removed at the solid modelling stage to simulate finger loss. It's not possible to get solid model directly from medical images. Therefore different image processing programs were used. Design stages of the finger prosthesis were shown in Figure 2. Advantage of using mechanic drive system is that all joints rotate at the same time but with different ratios. In this design, movement of the MCP joint, make the PIP and DIP joints move simultaneously. Patient's proximal phalanx connect to finger prosthesis linking bar and the finger prosthesis moved using proximal phalanx.
Image processing stage is required to personalise the prosthesis design. Prosthesis finger parts measurements were determined according to bone models. Therefore, bone models must be accurate to successfully measure prosthesis finger parts. Results of kinematic analysis were used to determine joint rotation rates. After the determination of joint rotation rates, finger prosthesis design was completed and assembled to bone models. Movement analysis of assembled hand model was done and finger prosthesis was optimised for better grasp and perfect fixation.

Prosthesis design
3D solid model transferred to SolidWork program for prosthesis design. In order to simulate finger loses, index finger (distal and middle phalanxes bones) was removed from hand model. CT images of the author, who does not have any finger loss, were used for this study. Therefore, finger parts were removed to simulate target group of this study. Reason of removing finger parts, CT images were used for this study belong to corresponding author who don't have any finger loses. To simulate target group this revision has done.
The patient is assumed to has index finger loss. For this reason, ring finger measurements were used for index finger prosthesis design. After the completion of design, movement capabilities were analyzed at the assembly section of the SolidWorks program. Finger prosthesis design and assembly with hand model can be seen in Figure   5.

Kinematic Analysis
The designed prosthesis model provides a mechanical system that can be used in place of the lost limb for people who have finger loss. Applying newly designed prosthesis is painless and easy, do not required surgical operation.
Designed prosthesis have 4 moving connections and 3 main joints. 2 fingers were modelled at this study (index and middle finger). Up to 4 independent finger can be designed if needed.
The system was designed as 4 bar mechanism. This mechanism can be used as basic and cross (X) 4 bar. While X bar system is transferring reverse rotation to joints, basic system provides same direction rotation.
X bar mechanism was used between MCP -PIP and basic bar mechanism was used between PIP -DIP joints. Figure 6 shows X and basic bar mechanism at designed prosthesis.

Fig 6 Cross (x) and basic bar mechanism
Kinematics and dynamic analysis must be solved together. But with the help of kinematic analysis result, it will not be necessary to repeat the same analysis every time. Determining joint rotation rates will be quite easy using these results. The link structure of the 3-pieced prosthetic finger which is shown in Figure 7 was placed to x -y coordinate system.  Table 1 for each moving part of the prosthetic finger. Links were assumed rigid hence the link part twist was ignored.
In order to the designed prosthesis to replace the fingers and give a realistic sense of use, it must also follow the path followed by human fingers during grip. With dynamic analysis joint rotation rates must be determined. In this section, grip capabilities of the human hand and designed finger prosthesis were compared. Figure 8 shows the path of the human finger follow while gripping a ball.
= sin + sin( Parameters , ve are phalanx lengths, , ve are joint angles. Phalanx lengths determine from bone models (from ring finger), joint angles will be determine at this stage. The route of the joint follows depends on gripping object diameter. At this point, object diameter also must be certain. Before the final decision of the joint rotation rate, for different gripping object diameters joint rotations must be analysed.

Result
Joint rotation is change according to gripping object size. Figure 9 shows different rotation angles for different gripping object diameter. After kinematic and dynamic analyses were made, finger prosthesis design was optimized. Parts of the designed finger prosthesis were manufactured by using 3D printer as shown in Figure 10

Discussion
In this study, a prosthetic model, which is mechanically operated (controlled by muscles), has been developed to minimize disadvantages of the limb loss (finger loss). By using a 3D printer and making a personal design from participant's anthropometric measurements; economic, easy to manufacture, and sustainable prosthesis can be produced. Another positive feature of this design, this prosthesis is not required surgical operation The analyses in this study were carried out to fully grasp cylindrical objects with a minimum diameter of 20mm, it was assumed that smaller diameter objects could be grasped with fingertips. The prosthesis was designed to allow easy assembly and replacement of the fingers. It is possible to change prosthesis fingers parts and joint ratios 10 thus finger parts can be designed and manufactured for gripping different sized objects. This adaptable design increase the life quality of the patient.
Many studies have been done on hand prosthesis design. In these studies, general hand measurements were used, personalized design methods were ignored. This may cause these studies to be inaccurate. Compatibility of the prosthetic and hand is very important both in scientific studies and in daily use. In this study, the factors that cause inaccurate results have been eliminated by making a personal design. As a result of the analysis and the formulas produced, the production of perfectly compatible prostheses has been easy and accessible [11][12][13][14][15][16][17][18].
In future studies, prosthesis models that include all fingers can be designed. The Disadvantage of this study, the proximal phalanx is needed for the movement of the prosthesis. It is not suitable for those who suffer from complete finger loss. To eliminate this disadvantage different prosthesis designs can be made.
The design is envisaged to be used on human subjects with limb loss, but difficulties have been encountered in finding the participants. For the development of the model, the sense of touch can also be taken as feedback with the pressure sensors placed on the fingertips.

Conclusion
In this study, personalized, easy to manufactured, economic and accessible finger prosthesis was carried out. In this context finger prosthesis modelled from CT images and model manufactured from 3D printer. Kinematic and dynamic analysis have done to provide design perfect fit to hand and different sized object increased grasp capability.

Declarations
Author contributions All the authors have equally contributed.
Funding None.

Compliance with ethical standards
Conflict of interest The authors declare that they have no competing interests.

Research involving human and/or animals rights
Hand photos and CT images are belong to corresponding author (Cagatay TASDEMIRCI), has consent.