Answered by Ustadh Tabraze Azam
I had a knee operation a few years back. I would sit on a chair and prostrate by leaning slightly forward because I was unable to bend my knee. Was it valid?
Answer: Wa alaikum assalam wa rahmatullah,
Yes, your prayers and your makeups were valid as long as you were not able, actually or effectively, to prostrate on the ground. This is because you are only responsible to the extent of your ability.
[Razi, Tuhfat al-Muluk]
قال في التحفة: ((ويقضي المريض فائتة الصحّة على حسب حاله)) اهـ.
And Allah Most High alone knows best.
Checked & Approved by Shaykh Faraz Rabbani