The term "corrective shoeing" is often overused and misunderstood. It sometimes implies that the farrier can correct some conformational faults of feet and legs. In reality, very little can be done to change the way a horse is built. Trying to fix a horse's conformation problems through corrective shoeing will put more stress on other parts of the limb.
No horse's limbs are perfectly symmetrical and balanced. A fine line exists between acceptable and poor conformation: it depends on how a horse is put together, how he handles his feet and legs, and how he is used. If he can manage to run, jump, cut cattle, or finish a hundred-mile endurance race and stay sound, you don't need to worry about "correcting" his faults. You can, however, make small corrections with each trimming and shoeing, to keep the feet as balanced as possible to prevent limb interference.
Most farriers strive to keep the feet balanced and only in a few cases try "corrective work". True corrections are generally done by trimming, not shoeing, because true corrections are only effective on foals.
CORRECTING A FOAL Many small conformation problems can be corrected or kept from becoming more serious with regular careful trimming when the horse is a foal. Often corrective trimming is simply balancing the foot. Without proper trimming, a leg slightly out of line may get worse as a foal grows. A crooked foot or leg that toes in or out will produce uneven wear on a hoof, starting a vicious cycle— the more the foot wears unevenly, the more crooked the foot or leg becomes.
The optimal time to attempt actual corrective trimming is the first four months of a foal's life and definitely no later than 12 months. After 7 months, leg bones are not as malleable, and once bones are no longer growing, there's nothing you can do to correct a leg permanently. However, overcorrecting in young horses can be harmful. Lowering a foot too much on one side may create pinching of the growth plate directly above it in the pastern or fetlock joint in a still-growing horse.
Overcorrection can also cause problems father up the leg (whether a horse is young or mature) because changing the foot puts the leg off balance and violates the horse's conformational integrity. Corrections are best done frequently, in small increments.
Also keep in mind that many young foals toe out at first due to lack of muscle development. These foals generally straighten on their own as they grow and fill out. If you try to correct them, they will become crooked later due to interference with their bone growth.
A foal that toes in or out because of the bone rotation of the fetlock joint or the entire leg can't be corrected with foot trimming. You must look at the whole leg to determine what should be done with the crooked foot. Knowing what types of deviations can be helped by trimming is very important because corrections sometimes can ultimately hinder or injure a horse.
Source: Understanding Equine Hoof Care: Your Guide to Horse Health Care and Management by Heather Smith Thomas