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Dear Editor,

We present a design ([Fig. 1A]) for a three-dimensional (3D) printed two-point discrimination (2PD) tool[1] that has many of the features of commercially available 2PD tools but can be manufactured for relatively low cost using a 3D printer or laser cutter.

Fig. 1 Two-point discrimination tool optimized for different means of production and use. (A) Three-dimensional printable version ranging from 2 m to 16 mm separation in 2 mm steps suitable for use in clinical environments. (https://www.thingiverse.com/thing:5360547). (B) Design of laser cuttable version ranging from 1 to 25 mm separation in nonlinear steps suitable for educational settings where lower density sensory areas may be tested (as used by Medical and Veterinary students in The Department of Physiology, Development and Neuroscience at The University of Cambridge, UK). Low power etching pathway is shown in blue, the red line indicates the path for cutting. (https://www.thingiverse.com/thing:3378130). Both designs are available for download on Thingiverse.

Sensation is a critical hand function, and testing of digital sensory function is an important part of clinical evaluation by hand surgeons. 2PD is an established test of the minimal separation that a patient can distinguish two discrete points of contact touching the cutaneous zone of interest. The test was first described in 1834 by E.H. Weber in De Tactu, where the points of a compass were used.[2] It is easy to perform and remains popular despite criticisms regarding its subjectivity and intra- and interobserver variability.[3] Fingertip 2PD testing is the accepted means of evaluating sensory impairment in occupational health assessments.[4] Values less than 6 mm are classified as 0% impairment, from 6 to 15 mm as 50% impairment, and more than 15 mm as 100% impairment. Easy access to a tool for measuring 2PD is thus important for classification of impairments in patients in a range of environments from clinical settings to occupational health assessments of work-related injuries

Popular commercially available 2PD tools include those similar to the MacKinnon-Dellon Disk-Criminator (US Neurologicals, Liberty Bay Marina, Washington, United States), and perform favorably compared to the use of a paperclip and an aesthesiometer.[5] Commercially available devices are, however, costly and accidental damage of the finger points can render them inaccurate or nonfunctional. Many tools come in sets with multiple ranges to allow users to measure 2PD in 1 mm increments, but this adds to inconvenience. The use of bent paper clips for 2PD testing is also popular but can be inaccurate given the spring-like nature of the metal and requires calibration with a separate ruler.

Our tools are single discs featuring multiple pairs of points with a range of separations from 2 to 16 mm in 2 mm increments ([Fig. 1A]) and from 1 to 25 mm in rising increments ([Fig. 1B]) both with the separation distances indicated. The first model ([Fig. 1A]) has been optimized for 3D printing and the second model ([Fig. 1B]) for laser-cutting. Both versions of the tool can be used for testing of 2PD over a wide range of separations with little compromise on accuracy. The 3D printed tool is 1 mm thick and can be produced using about 3 g of polylactic acid filament on consumer-grade 3D printers at low cost or laser-cut from appropriately thick material (e.g., acrylic or acetate sheets). This tool can be produced locally at both scale and low cost, whether printed or cut, making it ideal for use in educational environments (e.g., teaching hospitals, universities, colleges, and even schools) by hand surgeons, clinicians, educators, and students even in resource-poor settings. Critically, the low cost and small size mean that physicians, surgeons, emergency care workers, and even patients could carry them for assessment or triage without concern for cost or fragility. The reliability of this tool as compared to other commercially available tools has, however, not been formally studied. Therefore, when this tool is being used in clinical and research settings, each printed tool should be assessed for accuracy by ensuring the distance between points is accurate using standardized equipment.

In conclusion, we present our design for a 2PD testing tool that is low cost, portable, covers a wide range in a single instrument, and is easily produced locally using consumer-grade 3D printers or low-end laser-cutters (e.g., K40).

Authors' Contributions

CMC wrote the first draft of the manuscript. CMC, JIE, and CS reviewed and edited the manuscript and approved the final version of the manuscript.

Publication History

Article published online:
13 March 2023

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