Well, according to the non cannon expanded universe, to use the force at its fullest a force user must have all his body parts. If you lose your leg or arm, you cannot use the force at the same extent. That why Darth Vader could not become a true Sith lord, even though he have the title of Darth.
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I guess that's why general grievous couldn't use the force at all. Since he had very little of his body left
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Grioveus wasn't a force user, he was a rebel leader who count douku happen to found when he was dying. He got turned into a robot. There had been some non force users that can use a lightsaber. The lightsaber is just a weapon preferred by Jedi.