ANSWER: You do NOT have to be totally disabled to collect disability retirement. You are considered disabled if you are unable to render "useful and efficient service." If you are unable to perform satisfactorily a single (just one and any one will do) "critical element" of your current government job on account of an illness or injury, then you cannot render "useful and efficient service" and are eligible for disability retirement. It does not matter that you may be able to perform all of the other elements of that job. If you can't perform that single "critical element," then you are by definition disabled.
To some this sounds too good to be true; particularly to some physicians who are accustomed to thinking of disability in terms of wheelchairs, oxygen tents, and life support machines. They often fail to grasp the fundamental point that Congress has chosen to define disability for government employees in far more liberal terms than disability is defined in other areas of the law. As a consequence of their misunderstanding, many doctors are reluctant to go all out for their patients who do not meet the more traditional definition of disability. One result is that their patients end up being denied a benefit to which they have a legal right. One of my major tasks in representing government employees seeking disability retirement is to educate physicians and others on this critical point so that they can, in turn, really go to bat for their patients.
Example: Andy suffers from the chronic pain of a slipped disc whenever he is required to sit for even short intervals. Andy's desk job requires long stretches of sitting. Andy has no trouble playing golf or dancing till dawn, but he cannot sit without pain. Because Andy can not render "useful and efficient service," he is disabled from his government job even though he can play golf and dance. If he meets the other requirements, he is entitled to disability retirement.