Correção: Her kness no longer can support her overweight

Bom, estava querendo formar uma frase sobre o joelho da minha avó, que já não consegue suportar o excesso de peso dela. A construção está correta?

Her kness no longer can support her overweight.
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Her knees no longer can support her weight.
My grandma is overweight and her knees can´t support her additional weight.
My grandma is overweight and her knees can´t carry the heavy load put on them.

These are ways to express that, but sentences #2 and #3 can be seen as impolite to many, so it would be used in limited contexts (perhaps explaining to a doctor).
Sentence #1 shows more empathy, I think.

As for the word choice: I don´t think one can say the knees support/can´t support overwheight, they would support weight on them.
Or something overweight on them, like an overweight body, for example.