If you'll excuse me, I'd like to share a more educated answer.
There are three reasons for inaccurate gains:
1) The external transmitter is not placed correctly onto the receiver underneath your skin.
You get a stethoscope to use on your leg when distracting to hear the Fitbone motor. When the transmitter is not positioned correctly, you can hear that the nail doesn't do a full rotation per electrical pulse. The transmitter also recognizes weak turns and does not register it on the machine. For example, you might hear 9 pulses, and the machine reads 5.
2) Muscle resistance
Known issue with motorized internal nails, ISKD, Fitbone, Precise
3) Defective nail/Defective Transmitter
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The Precise nail has the same issues.
1) You may not achieve the full amount lengthened per cycle if the ERC machine is not positioned directly above the magnets inside the nail. The ERC machine has a penetration depth of 3 inches. You may have inaccurate gains if you place the ERC machine in the wrong position or do not apply enough pressure to the thigh to be within the 3 inch range, given all the muscles surrounding the femur.
2) Muscle resistance
Check out Plzen's diary, at one point he was doing 2mm distractions with PreciseII but didn't gain that amount.
3) Defective nail/Defective ERC Machine
Here's a study done on the Precise I:
http://www.ncbi.nlm.nih.gov/pubmed/24758320The reliability of the PRECICE system is comparable to that of other intramedullary lengthening devices such as the ISKD. The motorized external remote controller and its application by the patients is a weak point of the system and needs strict supervision.
Given both options, I would go with the Precise II. I think it is the better and more accurate nail overall. I have heard slightly more people complain about Fitbone gains vs Precise II gains, though it is hard to determine the exact reasons for failure or the amount missing per mm turned.