Horses are prone to hoof cracks for various reasons. Cracks can lead to the hoof chipping away in certain areas.
Cracks can be a cosmetic problem that can be easily corrected, but sometimes, they indicate something more serious is happening within the hoof.
Cracks often form in the hoof wall in a vertical direction, following the hoof tubules. They often originate from the bottom hoof surface or, less commonly, the coronary band. At times, a hoof crack may run the entire length of the hoof wall.
Cracks can also run horizontally, but these are less common and often have a different cause than those running vertically.
Hoof cracks can involve just the outer hoof wall or can invade the underlying sensitive structures and laminae.  Depending on the severity, cracks may or may not cause lameness.
Hoof cracks are usually described by their location, length (partial or full), and the presence or absence of bleeding and/or infection. In many cases, the underlying hoof damage is considerably more extensive than what is seen from the outside. 
A horse’s outer hoof comprises three main parts: the wall, sole, and frog.
The hoof wall (seen when the horse is standing) can be divided into three parts: the toe (front), quarters (sides) and heel (back). The sole and frog are visible when the hoof is lifted.
The hoof wall is made of keratin, a tough protein with low moisture content. Keratin is very strong and also insensitive, similar to human fingernails.
Along with the sole and frog, the hoof wall bears the horse’s weight while protecting sensitive structures inside the hoof capsule.
One of the most important structures in the hoof is the laminae, finger-like tissue projections that help attach the hoof wall to the coffin bone. The laminae also help the hoof maintain its structural integrity despite tremendous forces exerted by the horse. 
The hoof wall consists of three distinct layers:
- The stratum externum, or periople, is the thin, outermost layer of the hoof wall
- The stratum medium, or tubular horn, makes up the middle and thickest layer of the hoof wall
- The stratum internum which arises from the lamellar epidermis and, when combined with the laminae, attaches the hoof wall to the coffin bone (distal phalanx) 
Normally, the hoof wall grows at a rate of about 3/8 inch per month. New layers of hoof wall are continuously produced just below the coronary band, where the skin and hoof wall meet. 
Causes of Hoof Cracks
Researchers believe most hoof cracks occur due to foot imbalance, either side to side or front to back (long toe/low heel conformation). 
Other causes of hoof cracks include:
- Improper or neglectful trimming/shoeing
- Hoof disease
- Coronary band defects
- Thin walls
- Poor/improper nutrition
- Dry or overly moist hooves
- Trauma to the hoof 
Some horses, such as Thoroughbreds, may be genetically predisposed to having low heels and a long toe, as well as thin hoof walls. Therefore, they may be more prone to hoof cracks. 
Types of Hoof Cracks
Hoof cracks may be referred to as grass cracks (originating from the toe) or sand cracks (originating from the coronary band).
Horse owners also classify cracks by the part of the hoof they affect: toe, quarter, or heel.
Toe cracks occur in the front third of the hoof. They primarily occur in the front feet since the horse bears more weight in the front. This type of crack can arise if the toe grows too long, placing excess pressure on that area. 
Quarter cracks are vertical cracks that occur at the side of the hoof wall. They usually originate at the top of the hoof, beginning at the coronary band and proceeding toward the bottom of the hoof.
A true quarter crack is a fracture that can lead to instability, inflammation, and infection.  Quarter cracks are more likely to cause lameness than toe cracks.
Quarter cracks can have numerous causes, including:
- Trauma to the coronary band
- Pre-existing damage to the corium from infection
- Abnormal hoof conformation
- Foot imbalances
- Conformational abnormalities affecting the limbs
- Shoes that are too short
- Abnormal landing pattern
- Poor hoof wall quality from genetics, nutrition, or environment 
Like toe cracks, quarter cracks most often occur in the front feet. However, they are occasionally seen in the hind feet as well.  They are often associated with side-to-side foot imbalance or sheared heels. 
Ideally, horses with full-thickness quarter cracks should be taken out of work and allowed time to heal while corrections are made in hoof care.
If the horse must remain in training or competition, repair and stabilization of the crack are necessary before hoof distortions, and soft tissue displacements can be corrected. 
Heel cracks occur at the back of the hoof below the heel bulbs. They are often caused by shoes that shift to the side or are too small to support the entire heel.
Overstriking, where the horse hits the back of a front foot while overreaching from behind, is another common cause of heel cracks.
Like quarter cracks, heel cracks may also cause lameness.
Horizontal cracks in the hoof usually result from trauma to the coronary band or can occur after an abscess has burst at the coronary band.
Cracks Involving the Coronary Band
The coronary band is the junction between the skin of the lower limb and the hoof wall.
Cracks involving the coronary band may cause lameness. The coronary band may be painful to the touch, and the edges may move apart when the horse puts weight on that foot. 
If an infection is present, it must be treated before attempting to stabilize the hoof crack. The horse should also be updated on tetanus vaccination if needed. 
Cracks that involve the coronary band may need hoof stabilization with a bar shoe and restricted exercise. 
Coronary band damage may lead to permanent scarring and long-term hoof complications. The resulting cracks will need constant care and supervision. 
Treating Hoof Cracks
Addressing hoof imbalances and treating infections should be the first priority with hoof cracks in horses. Some complicated cracks may require the removal of affected tissues or stabilization and therapy.
Because most hoof cracks are caused by foot imbalance, correcting the imbalance should help to solve the problem. Routine hoof care will ensure that superficial crack(s) completely grow out and don’t return.
Avoid turnout in excessively wet or dry ground conditions to prevent further hoof damage. Adding moisture to excessively dry feet can help with superficial cracks.
Sometimes, farriers will cut a horizontal groove in the hoof above the crack (often called “grooving”) to stop the upward progression of the crack. However, this treatment may further weaken the hoof and isn’t always recommended. 
Special shoes, such as straight bar, egg bar, z-bar, and heart bar shoes, may be helpful for stabilizing hooves. Sometimes, this form of stabilization is enough to allow the horse to continue performing, even with a significant crack. 
For some types of severe cracks, a skilled farrier can wire or lace the crack together through horizontal holes drilled through the hoof. This is then further stabilized with fibreglass or acrylic patches placed over the crack. A shoe with full-bar clips is also needed if this method is used. 
Sometimes hoof cracks are filled with composite materials to help stabilize the hoof. Many