SLISHMAN SPLINT _TM

The Slishman Splint is exceptionally simple to use.  But because the inner workings are not visible, and because its true mechanical advantage is not obvious, we have included this page for the curious.  A basic understanding of pulley physics is all you will need to understand the ski pole version of our splint.

For any pulley system the underlying question is "what is the mechanical advantage?" In the case of our splint, the answer is 3:1.. or 4:1.. depending on your perspective. The answer depends on which portion of the splint is fixed and which portion moves.  If the distal pole tip is fixed, meaning the middle and inner poles are prevented from moving, the advantage is 4:1.  This model most accurately represents the advantage when used for femoral traction and for shoulder reduction. However, if the handle end is fixed, meaning the outer pole is prevented from moving, then the advantage is 3:1.

Your gut reaction may be "that's crazy." But it's just a matter of perspective.  A simpler pulley model may be instructive here. Attach one end of a rope to a weight. Pass the rope through a pulley which hangs from a tree branch. Now pull on the rope.  The advantage you gain in trying to lift the weight is only 1:1, since the pulley only changes the direction of the rope. You essentially lift the weight with no help at all from the pulley.  Now suppose you use a weight that is heavier than you. The weight will remain stationary. However, your mechanical advantage in bending the tree branch is 2:1, since the pulley is allowing you and the weight to combine forces to pull the branch down. You will have to pull the rope 2 feet for every 1 foot that the pulley and branch bend downwards, which is the essence of a 2:1 mechanical advantage.

So to relate the tree example to the splint,  fixing the handle end of the pole is similar to using a pulley to lift a weight off the ground.  You have less mechanical advantage.  Fixing the distal end of the pole is analogous to using a pulley to bend the tree branch.. more advantage.  In reality, however, our splint is a bit more complex since both ends actually move.  Through experience in using the splint, it appears that the handle end of the pole does move more than the distal end.  But since both ends do move, we can only say that the mechanical advantage lies somewhere between 3:1 and 4:1.

For the femur-only splint the mechanical advantage lies between 2:1 and 3:1, which is more than enough advantage. Initial prototypes had between a 7 and 8:1 advantage which was sufficient to rip limbs in half.