Quarter cracks can be a serious problem in the horse, because when you have a full hoof-wall crack the sensitive lamina will become inflamed from trauma and/or infection. This causes pain in the form of lameness, and takes the horse out of service. Fortunately, most of the time this is a temporary problem.
A quarter crack is the result of the lamina that comprise the hoof wall, and run vertically, separating completely through to the underlying sensitive lamina. The hoof wall is made up of keratin tubules that are basically dead tissue, just like hair. The hoof wall grows down from the hairline or coronary band, and grows at a rate of about half an inch per month. The underlying sensitive lamina are rich in nerve endings, and any inflammation to that structure is painful. The quarters, or heel area, of the hoof wall take most of the concussive force of weight-bearing during the landing phase of a horse's stride.
Factors that predispose horses to quarter cracks include conformation of the legs and feet, use of the horse, environment and old scars through the coronary band. I've seen more quarter cracks in horses with abnormally steep hoof walls than in more normally conformed horses.
I've seen this problem in "thin-walled" hooves more often than the more "rugged" or thick-walled hooves. Also, the more athletically demanding job the horse has, the more stress is placed on this structure. As for the environmental factor,
I see far more hoof cracks in horses that are constantly going from wet to dry conditions. This scenario leaches out the natural oils and protective barriers of the hoof wall that help maintain integrity. Horses that have old scars, commonly wire cuts, through the coronary band will always draw an uneven line down the hoof wall under that scar. Luckily, with proper hoof care, this situation can be successfully managed the majority of the time and the horse used normally.
Treating a quarter crack starts with trying to relieve the inflammation in the sensitive lamina. Things like soaking the foot in hot water with Epsom salts, and applying a bar shoe to minimize expansion and contraction with weight-bearing are starters. More severe quarter-crack solutions require surgically cutting out the crack with a hoof knife or Dremel drill to better treat infection and avoid further "pinching" trauma on the sensitive lamina. There are many ways to try and stabilize the hoof wall to encourage the trimmed crack to "grow out"-lacing the crack together, filling the defect with a bonding material and special shoeing, to name a few.
The range of significance of quarter cracks is quite wide, and each should be treated accordingly. I tend to try the more conservative methods first and adjust to the situation as it evolves. Fortunately, most quarter cracks are a temporary problem.STW