Have you noticed that buono (good) is often translated as "bravo" in Italian? Bravo is a multi-faceted word with a variety of meanings, for instance: skilled, clever or well-behaved: It is of course also used to show our delight at a show or performance (and should always agree in gender and number): Bravo! Brava! Bravi! Brave!
Two good singers = due bravi cantanti / due cantanti bravi
A very good doctor = una dottoressa molto brava
A very good child = un bambino molto bravo/bravissimo
Notice that when buono (singular only) is put before a noun, in strict grammatical terms it should follows the same rule as the indefinite article. In fact, nowadays this doesn't always happen, as buon tends to be used instead:
Un buono studio/Un buon studio
Un buon pranzo
If you have a bad knee don’t say: ho un brutto (ugly) ginocchio or ho un cattivo ginocchio.......
You can’t translate bad literally and need to find a different turn of phrase to describe that or similar illnesses or conditions. If what you mean is that your knee hurts, you could say: “mi fa male il ginocchio”.