May one use a knee scooter in an area without an eruv if one has a stress fracture and the doctor prescribes using one whenever outside the house (though one may walk without it inside the house, as long as one treads lightly). What if outside is a definite karmelit and one needs the scooter to get to shul? Would it help if one left the scooter outside for all of Shabbat? Thank you!
ב"ה Shalom, First of all you should have a Refuah Shleima. – a complete recovery. Secondly, although I will answer your question, this is something , I suggest you mention to the Rabbi of your community and hear what he has to say on the issue. Now in regard to the question itself. My understanding of a knee scooter, is that it is basically like a wheelchair in function, however the user does not sit in the scooter but rather rests one knee on a seat and uses the healthy leg to push. In any case, in regard to Shabbat there seems to be no difference between a wheelchair and a knee scooter. Rav Moshe Feinstein zt"l was asked this question in regard to a wheelchair, and allowed a person to push himself on his own on a wheelchair. Rav Feinstein zt"l , allowed even other Jews to assist to lift the wheelchair over the curb onto the sidewalk since it is less than 4 Amot. ( אגרות משה או"ח ח"ד סי' צ) In our time, due to many municipal regulations to improve accessibility for the sake of the physically challenged, this may no longer be in issue in many places. Unfortunately, in many places it still is. Similarly, Rav Neuwirth zt"l in (Shmirat Shabbat Kehilchata,; 34,27) allows a person to wheel himself in a place which is not considered an absolute Reshut Harabim. Also, he makes the stipulation not to take anything along in the wheelchair. Leaving the scooter outside, has to be defined. If you leave it outside in order to minimize transfer from one domain – Reshut Hayachid- to another – Karmilit- the scooter would have to be outside any enclosed area which is defined a Reshut Hayachid; such as , the fenced- in yard of the house, or outside a building. Depending upon where you live this may not be practical. So since I can't fully cover all scenarios, when in doubt have actions of transfer done by a non-Jew. (כי בו שבת, סי' ש"א, עמ' קנא) All the best