shinfin fins are used as fused ankle fins because they can be fitted a bit higher on your leg to avoid hitting your foot.
Customers with fused ankles generally find it better to wear shinfin fins a little bit higher up the leg than usual, perhaps with the fin tips somewhere near the base of the toes or even a bit higher. Please experiment to find the best height and orientation around the leg. shinfin fins are quite flexible and adaptable to fit at different positions. An angled foot will still cause some drag in the water but shinfin fins will reduce this.