You might try modifying your Mares fins to a scoop fin design. Your avanti quattro fins have a dead spot just beyond the toes, which increases the load on the knee. I've worked on this concept for decades, and the scoop design is more efficient than a straight blade, or even the Quatro design. You need to replace the center of the fin with a flexible interior, which at the toe is straight across, but then goes to a full semicircle design at the end of the fin. This "scoops" the water out the back of the fin, and reduces a lot of the strain on the knee. You can see the difference on the Voit Viking fins I modified in the 1970s. You can see the number of fins I've evaluated over the years (and that photo is not complete). If you need more information, PM me.
SeaRat