When is it acceptable to fuse an ankle in slight equinus?

• A. Good quadriceps strength
• B. Plantarflexion contracture
• C. Obesity
• D. Weak quadriceps muscles

Answer: (D)

The main idea is using a little ankle plantarflexion (slight equinus) after fusion to reduce the knee extensor demand when the quadriceps are not reliable. If the quadriceps are weak, the knee tends to buckle or drift into flexion during stance because active knee extension can’t be sustained. Fusing the ankle in a small plantarflexed position changes the lever arm and the way the ground reaction force acts through the leg, so the knee can stay more extended with less active quadriceps force. In this scenario, the ankle fusion helps create a stable, plantigrade foot without overworking a weak quadriceps.

That’s why weak quadriceps muscles is the best fit. With good quadriceps strength, there’s no need to impose an equinus posture. A plantarflexion contracture would already bias toward too much plantarflexion, making an equinus fusion undesirable. Obesity doesn’t directly dictate this choice, even though it can influence overall treatment planning.