Measurement Guide

June 26, 2016

[PDF Version, 1.5mb: Finger Prosthetic Measurement Guide rev_0_4.pdf]

For measurements, you’ll need some tape (medical is best, but you could get by with other kinds), scissors, a marker, and a measurement device.  Digital calipers are best, but a measuring tape or ruler will do.  Be as accurate as possible – all measurements are in millimeters, and decimals are ok too.

 

Socket Width 

This is the single most important measurement for comfort.  Too small will never work, but too large can be compensated for by wrapping medical tape on the stump. There are two settings that control this in the model, as seen in the following diagram.

image4 Step 1:

Wrap the finger remnant / stump with medical tape, not tightly, but just barely snug enough to avoid bunching.   Only wrap areas that you expect to be fully covered by the socket.

 

image5Step 2: 

Very carefully cut the tape off with scissors, in as straight of a line as possible.

If you have a hard time with this part, try laying a piece of string lengthwise down your finger before taping it, which gives a hold to pull the tape away from the skin.

image6Step 3: 

Stick the tape smoothly to a flat surface, and measure lengthwise (in millimeters) at the widest point – this gives you the circumference.  Now divide this by pi (3.14) in order to get the diameter.  In my case:  62mm / 3.14 = 19.74mm

The model accepts values for both the bottom and top of the socket, typically the top is a couple mm smaller, which helps for a snug fit.

Parameters: socket_width_top, socket_width_bottom

 

image7Socket Scallop Depth

Measure from shallowest part of remnant for the socket scallop depth, the cutouts between fingers.  Socket depth values for top and bottom should be longer, which increases stability.

Parameter: socket_depth_scallop

 

image8Socket Depth Top

Measure from the front of the first knuckle, to the tip of the remnant.  As long as possible without extending to a point where it rubs or binds.

Parameter: socket_depth_top

 

image9Socket Depth Bottom

This should be shorter the socket_depth_top, to prevent pinching as you make a fist.  Close finger as much as possible, and measure from inside to the tip of remnant.

Parameter: socket_ depth _bottom

Socket Best Practices

You may want to print multiple sockets with different thicknesses, as this can vary depending on healing, and swelling, even throughout a single day.  They are the easiest part to snap on and off, so it’s super easy to swap them out.   I also like to print them with different values for socket_depth_top, as I’ve found that this point takes a fair amount of pressure and can get irritated or calloused.  By using a variety, it prevents the strain from always hitting the same spot.

Socket can be trimmed easily if needed, as well as smoothed or sanded with a dremel sanding drum – so if it’s too long somewhere – just hack at it a bit, and then measure it once you get it to the right length, for next time.

Finger Length

For finger length measurements, it is ideal if there is an intact digit (substitute finger) on the other hand for comparison.  If not, you may have to guess more for these values, e.g. most people have similar length index and ring fingers, both of which are slightly shorter than middle finger, or by measuring someone else with similarly sized hands.

Middle Section Length

image10Step 1:

Mark at middle of each knuckle with a marker, while the substitute finger is bent into a fist.

 

image11Step 2:

Straighten the finger, and measure between the lines in mm.

Parameter middle_section_length.

 

image12Tip Length

This one is really easy now – use the same mark you already made on the last knuckle, and measure to the tip of the finger.

Parameter: tip_length

Base Extra Length

The base section (the part that connects to the socket, and sits at the end of the stump), is designed to sit as flush as possible.  My amputation was just under my knuckle, so keeping the base short helped so that my knuckles line up well when flexing or making a fist.  If your remnant is shorter, you may need to add extra length to the base to keep the knuckles aligned.

Parameter: base_extra_length

 Wrist Linkage

Almost done!  The wrist linkage length varies depending on how tight of a bracelet you use, and how much slack is in the tendon (fishing line).

To start with, measure the back of hand with your palm flat against a table, wrist bent, and arm sticking up.

Measure from the bend of the wrist, to the middle of the first knuckle in question.

Parameter: linkage_length

Other considerations

Write down your measurements in the form on the next page, which will serve as a handy reference for using the customizer or OpenSCAD.  Also – take lots of pictures – of every measurement!  This helps with any confusion later, and if you’re not making it yourself, the pictures will be a guide for whomever is building it.  The parameters are roughly in order of importance – the socket settings are the most important for comfortable fit, where the finger length is just to keep it in proportion with your hands.

There is also a single knuckle mode – for folks missing only the first finger section, or for missing thumbs.  In this case there is no middle section, but the other measurements should be about the same.

There are LOTS of other settings (hundreds!).  Some I’ve exposed in the customizer as advanced settings, which let you control the clearances between parts – each printer is different, so if parts are sloppy, or too tight to fit, try increasing clearance.  If you really get fancy, there are even more settings that can only be seen if you open the source code in OpenSCAD.

If you’re sending your measurements to us or someone else to build it for you, also indicate any color preferences.   We’ll do our best, but please be understanding that it is expensive to keep a large inventory, so there may be limited choices.  Generally the finger looks best as two-tone, with one color for the plastic structure, and another for the flexible parts.