Smashed cartilage where? My old mare had damaged cartilage in the fetlock joint where the connon bone had 'smashed' into the long pastern, she had keyhole surgery to remove the damaged cartilage and spent a long time in recovery. She seems to have made a full recovery but TBH I never really jumped her again afterwards as didnt want to risk the leg!
One of the vets I worked with horse had an arthroscopy to tidy his stifle up after doing this and he returned back to full work and was in his mid teens when he did it. Length of recovery all depends on how severe the damage was in the first place. We did stifle ops regularly to tidy up joints and all returned to normal work.
On the extreme side.
I had a welsh that went loopy in a trailer on the motorway and ended upside down on her back having put her hind legs through the front window ( spent the night at work in colic box under obs ) next day trotted up slightly lame after flexion but put it down to bruising so took her home in horse ambulance and stupid creature tried to do it again but ended up wedged with her back against the rear ramp.
The next day she was 10/10 lame with huge stifle effusion, back to work in horse ambulance in sling doped up so she could barely stand, turns out she had completely snapped her medial and collateral ligaments and her cruciate ligaments. We think she had probably torn them the first time and made them go completly the second time.
Her prognosis was very poor, 18mths box rest, 6 mths barn rest then she may be sound enough to go in a field, wouldnt ever be completly sound and due to severe issues with being alone ( used to have to get field friend in too to do anything with her ) I had her put down. My mares injury was the most serious my work had ever seen ( we pm'd her and her stifle was literally obliterated) and have never seen anything like it since. We never did find out what caused her to flip, she was an experienced traveller and had company ( luckily the other horse was unharmed and stayed calm throughout all the thrashing about.