Hi Harriet,
The patient likely has some lateral ankle instability, especially if walking without orthotics (due to the foot dropping into PF/inversion). A posterior leaf spring AFO will prevent that classic foot drop position by maintaining the ankle in dorsiflexion with neutral eversion/inversion. In most cases, this prepares the foot and ankle well for initial contact and effectively provides that lateral stability. The posterior leaf spring AFO has the advantages listed above in terms of ease of donning, weight, cost, and flexibility of footwear, so it is best to start with this option.
If the patient had spasticity, contracture, or weakness beyond this nerve distribution, a more aggressive AFO may be warranted.
Chrissy