Initial Concept

This was the original design to replace the thumb test. The device sits between the cut face of the bone and the surgeon's thumb. This allowed the surgeons to provide the force applied to the bone. As force was applied the spring would compress and display the force it experienced on the side.

First Prototype

The device was revised to use a spring as the source of force rather than a surgeon's thumb. This ensures that the indenter will apply the same force in every measurement. This removes the surgeon as a source of potential inconsistency. The indenting tip is also constrained by the housing, limiting the depth the indenter can penetrate the bone. The device operates by pulling the handle back compressing the spring. The operator then turns the handle to catch the indenter guide on a shelf milled into the housing. The device is placed against the bone and the surgeon turns the handles to release the indenter from the locked position.

Current Prototype

The design was refined to use a button release. The indenter tip is now a interchangeable part, seperate from the indenting rod, simplifying the manufacturing. To operate the device the surgeon pulls the handle back until the device locks. The surgeon then holds the device against the bone and presses the button. This process can be done with one hand, a noticeable improvement over previous models.

Fig. 1: Test Set-up

To determine the stregth of spring required for the device to indent bone, drop testing was done using the saw bones sent to the team by Exactech. The test set up is shown above in Fig. 1. The blue box represents the sawbone block. The cyclinder represents a 50 in. pipe. And the red pyramid represents a weight with a pyramid shape. The cyclinder was placed on top of the sawbone and weights of different sizes were dropped through it. The cylinders purpose was to ensure the weight would fall the same distance and the in the same orientation each trial.

Fig. 2: Initial Drop Testing

During the testing the data was recorded. The team took note of the mass of each weight, the height from which it was dropped and the indentation depth each collision created. For the first analysis the same weight and height was used in each trial. The graph in Fig. 2 above compares the indentation depth of 3 samples with diffferent densities measured in pounds per cubic foot or PCF. The results showed a linear relationship between PCF and indentation depth.

Fig. 3: Flatpoint Drop Tesing

The next phase of testing was done with a 3D printed flatpoint indenter tip with a diameter of 0.1 in. instead of the weights used previously. The same test set-up was used, and the height was kept constant but the weight used was varied, with the purp[ose of indenting each block with varied potential energies. Each of the blocks was indented with 0.5J, 1.0J and 1.5J of potential energy. The results showed the largest difference in indentation depth when the samples were exposed to 1J of potential energy. This information was then used to choose the device's spring coefficient and compression.