Completely digitally fabricated custom functional finger prosthesis

Abstract

The loss of a finger in any capacity as a result of trauma has a significant influence on the patient’s everyday life, as well as their psychological and physical health. Multiple conventional techniques have been reported in the literature, mostly offering psychological and cosmetic benefits to such individuals. However, there is a paucity of literature for functional finger prosthesis. This case report describes rehabilitation of an amputated index finger using an innovative digital workflow, thereby making it impression-free, cast-free, accurate, less time-consuming, and above all functionally viable. Digital technology was used for designing, and fabrication of this prosthesis was done using three-dimensional (3-D) printing. When compared to traditional prostheses, this 3-D–printed prosthesis was functional, allowing the patient to conduct everyday activities and providing the patient’s confidence a psychological boost.

INTRODUCTION

The most prevalent kind of partial hand amputations includes complete and partial finger amputations, with trauma being the most common cause, as well as congenital abnormalities or absence. Prosthetic rehabilitation of the amputated finger has been shown to help psychosocial characteristics of a patient by providing reduced hypersensitivity and increased strength in grip.[1] Despite the fact that these prostheses give psychological and esthetic benefits, they generally lack the functioning of a normal finger.

Fabrication of a traditional prosthesis is extremely time-consuming and complex as it requires customization and sculpting to fit each individual.[2] All of these issues can be avoided with the use of computer-aided design (CAD)/computer-aided manufacturing and rapid prototyping technologies. Additive manufacturing technologies can either be used as adjuncts or can have a completely digital workflow.[3]

When compared to traditional approaches, these technologies drastically shorten the time needed to design and fabricate such prostheses.[4] In the literature, improved accuracy and efficiency of socket fabrication have been documented using CAD combined with standard tessellation language (STL) files and additive manufacturing.[1] A combination of these technologies can also be used for remote manufacturing of prosthesis if the necessary designs and measurements are obtained.[5]

Although there are a few accounts of functional prosthesis in the literature, there is no literature on characterized functioning three-dimensional (3-D)–printed finger prosthetics, to the best of the authors’ knowledge. This case report describes the usage of a novel innovative cast-free 3-D–printed prosthesis to replace a traumatically severed finger, as well as the fabrication procedure.

CASE REPORT

A 65-year-old male patient was referred to the department of prosthodontics, with a chief complaint of a partially missing index finger in his right hand. Case history revealed that the patient had lost the distal and middle phalanges, at the proximal interphalangeal joint, 2 years back, due to an acquired traumatic injury. The finger was severed after it became entangled in machinery during work and was never reattached due to a delay in treatment. On inspection, it was observed that wound healing appeared to be complete with no signs of infection or inflammation in the surrounding tissues [Figure 1]. The patient had no prior experience with a prosthesis for the same. After evaluation, the patient was explained about the option of a 3-D–printed functional finger prosthesis and the patient consented for the same.

Figure 1.

Prerehabilitation photographs, dorsal view (a), palmar view (b), and close up of the finger stump (c)

This cast-free procedure requires specific measurements of the finger stump to be made. The residual stump, in this case, was 44 mm (mm) in length and had a diameter of 25 mm, which was used to design the finger socket of the prosthesis. This finger socket is responsible for the retention of the prosthesis on the stump. Measurements of the opposite index finger were taken for customization of the middle (26 mm) and distal (24 mm) components of the prosthesis [Figure 2]. Measurement of linkage length is critical because it is this component that acts as an anchoring point and initiates the motion of other components. These dimensions were then utilized to alter the stock CAD (Knick’s Prosthetic Finger version 3.5.5) [Figure 3].[6] The stock CAD consists of eight total components, namely, knuckle plugs (4 in number), finger socket, tip knuckle, tip cover, middle bumper, middle segment, base knuckle, and wrist linkage [Figure 4]. An open-source software OpenSCAD system (http://www.openscad.org) was used to customize these individual components, based on patient’s measurements, following which digital models were rendered and files of customized components were exported in STL format. After this, the customized data set was sent to the laboratory, where it was prepared for final 3-D printing using software (Chitubox v1.4.0; Shenzhen CBD Technology Co. Ltd.; China). The final printing was done using a photopolymer resin (Phrozen TR300 Ultra-High Temp Resin, Phrozen, Hsinchu, Taiwan) and a Digital Light Processing 3-D Printer (Phrozen Shuffle, Phrozen, Hsinchu, Taiwan). Additional components included elastic cords (2 mm in diameter) and nylon string (2 mm in diameter). These individual components were then assembled, according to the designers’ instructions [Figure 5].[7]

Figure 2.

Measurements of the finger stump (a-c). Measurement of linkage length (d), middle (e), and distal phalanx (f)

Figure 3.

Stock CAD Model (Knick’s Prosthetic Finger version 3.5.5)

Figure 4.

Three-dimensional–printed components. (From left to right) Top row: Two pairs of knuckle plugs, finger socket, and tip knuckle. Middle row: Tip cover, middle bumper, middle segment, and base knuckle. Bottom row: Linkage

Figure 5.

Assembled finger prosthesis

The prosthesis was then evaluated for fit and function. Demonstration of various movements the prosthesis could simulate was given to the patient [Figure 6], following which characterization of the prosthesis was done.

Figure 6.

Trial of prosthesis (a). Demonstrations of movement (b) and prosthesis function (c)

Covering the complete prosthesis with a silicone over-sleeve for esthetics would have restricted mobility and impaired the prosthetic’s functioning. Hence, this type of prosthesis is best paired with sectional silicone sleeves. It was decided to fabricate silicone sleeves (Silicone A-2186 Platinum Silicone Elastomers, Factor II, Lakeside, USA) for nonfunctional components, i.e., middle segment and the tip cover. The sleeves were fabricated using 1.5 mm thick wax sheets adapted to above-mentioned components. These sheets were then flasked using type II gypsum product (Kaldent, Kalabhai Pvt. Ltd., Mumbai, India) and mold was created using lost-wax technique. The silicone base material was then mixed with catalyst in a ratio of 10:1 by weight and thixotropic agent (Factor II) was also added to thicken the mixture. Intrinsic staining (KT-699, Silicone Coloring Kit, Factor II, Lakeside, USA) of silicone was done to match the patient’s skin color. Shade matching was done to match the shade of the adjacent middle finger and thumb. This colored silicone was packed into the mold in layers and the flask was closed.

Following this, the silicone was then processed at 100°C for 30 min according to the manufacturer’s instructions. After processing, the silicone sleeves were retrieved and adapted to the corresponding components. A custom nail was fabricated and fixed in position using cyanoacrylate adhesive (Fevi-Kwik, Pidilite Industries, Mumbai, India). To provide components that were not covered in silicone with a more appealing appearance, characterization was done with color-matched acrylic paints (Kokuyo Camlin Industrial Ltd., Mumbai, India).

The final characterized prosthesis was then placed over the patient’s stump, and the linkage was attached to a bracelet that was fastened around the patient’s wrist [Figure 7] along with instructions to practice flexion of the wrist while trying to hold onto an object. Furthermore, the patient was instructed to remove the prosthesis during bedtime so that it does not get damaged during sleep. On the 3^rd day of follow-up, the patient reported having been satisfied with the esthetics along with no difficulties in operating and controlling the finger. The patient was instructed to report to the department for monthly follow-ups or in case of damage to the prosthesis or stump.

Figure 7.

Final prosthesis fit after customization of three-dimensional–printed components and silicone sheath application (a). Customized three-dimensional–printed prosthesis in function (b)

DISCUSSION

Dexterous people suffer tremendously in terms of esthetics and usefulness when a limb is lost. Rehabilitation gets easier when just the distal phalanx is affected, and near-normal functioning can be restored with a suitable prosthesis.[8] However, it has been revealed in surveys, that 30%–50% of upper limb amputees do not use their prosthesis regularly due to poor esthetics, low functioning, and controllability.[9] This is where a functional prosthesis can help. The patient can then satisfactorily perform daily life activities such as holding a pen and picking up objects with ease.

To the best of the authors’ knowledge, a total of three functional finger prostheses have been reported in the literature.[1,10] First of which was reported by Pattanaik and Pattanaik, who incorporated a pliable hairpin into modified conventional silicone finger prosthesis. However, the functioning in their prosthesis was restricted to only two fixed positions, opened or closed position, with the patient having to adjust the pin manually. Furthermore, fabrication of the same is technique sensitive.[10] Young et al., conducted a case study comparing commercially available finger prosthesis (MCP-Driver™ finger prosthesis, NAKED Prosthetics Inc., Olympia, WA, USA) and a locally 3-D–printed finger prosthesis (LPF). They compared both the prostheses on various parameters such as gross manual dexterity, satisfaction, and upper extremity functional status. It was observed that patient satisfaction was greater with LPF, whereas gross manual dexterity and upper extremity functional status were closely matched. The design of LPF fabricated by the author was quite similar to the prosthesis described in this case report, except for the joint between the middle and distal segments of the prosthesis, which was fixed at 30° downward angulation.[1]

The approach described in this case report uses digital technologies to create customized finger prostheses without the need for an impression. Thus, the process is much more rapid when compared to the conventional method. Because of hinges that act between the tip knuckle and the middle segment, as well as the middle segment and the base knuckle, the functional prosthesis presented in this case study provides full functioning, identical to an anatomical finger. Hence, the range of motion achieved by this prosthesis is much greater when compared to LPF.

However, no prosthesis is completely perfect and limitations of the prosthesis described in this case report lie in the esthetics. Since restoring functionality is the prime objective of this prosthesis, a complete silicone over-sleeve cannot be fabricated, as it was found to hinder smooth functioning of the prosthesis. Hence, a decision to process silicone in sections was taken. Newer, thinner sheath materials will provide a scope for further enhancement in this regard.

To activate the finger, the patient has to move their finger stump in a downward direction and or flex their wrist, which would result in flexion of the finger prosthesis. Relaxation of the wrist results in the prosthesis attaining an extension position. This particular prosthesis has a tension-driven voluntary closing mechanism, which is activated upon flexion of the stump and/or the wrist. The movement is activated when the thread attached to the linkage detects a tug. Now, since the linkage is anchored to a bracelet tied to the wrist, it transmits pulling forces to the string attachment and sets the prosthesis into motion. Initially, the string attached to linkage tugs onto the tip knuckle, which rotates to almost 30°, after which the base knuckle also gets activated. Base knuckle gets activated because the force vector through the prosthesis changes from a vertical to a more angled one. After the above-mentioned stage, continued pull on the string owing to movement of the stump and/or wrist will result in increased rotation in both knuckles. As a result, the prosthesis is now rendered functional. Because of opposing tension generated by elastics positioned between the tip knuckle and middle segment, as well as the base knuckle and middle segment, when tension from the string is removed by relaxing the stump and the wrist, the prosthesis returns to its original position, thus completing the full cycle.

CONCLUSION

Functional disability caused by loss of finger can be overcome by this prosthesis to a large extent as this prosthesis can replicate flexion and extension like an anatomical finger. It provides the patient with psychological and functional advantages. Another advantage is the time saved in making an impression and casts for the same. This functional prosthesis can only be used in patients with missing middle phalanx. If the middle phalanx is present this prosthesis cannot be fabricated. However, in conducive cases, this prosthesis can be very helpful.

Declaration of patient consent

The authors certify that they have obtained all appropriate patient consent forms. In the form, the patient has given his consent for his images and other clinical information to be reported in the journal. The patient understands that his name and initials will not be published and due efforts will be made to conceal identity, but anonymity cannot be guaranteed.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.