Ah, the well-turned pastern: slender, shapely, yet incredibly strong, the bearer of thousands of pounds of pressure relentlessly imposed during the million steps of a lifetime. So much rides on the pastern, and so little goes wrong with it, at least compared to the fetlock above and the ever-challenged hoof below. Accidents, including fractures, cuts and abrasions, do afflict the area, for sure, and strains and pulls of tendons and suspensory ligaments crisscrossing the pastern do occur. But only two abnormalities--ringbone and sidebone--pop up on the pastern often enough to have earned labels in common stable parlance.
Both "bones" are evidenced by visible irregularities on the pastern, yet neither is always or even often the cause of lameness. Sidebone, especially, is usually more a matter of abnormal appearance than of altered function. Ringbone, on the other hand, does disable horses, sometimes temporarily, sometimes permanently.
Your greatest ally in deciding the seriousness of a pastern irregularity is the horse himself: You can bet that no matter how awful the landscape there may look to you, if the horse isn't limping, there's no cause for panic. Even if he is "off," the lumpy pastern may well be blameless in the lameness, an unfortunate concurrence with another painful place in the lower leg. By familiarizing yourself with the normal pastern landscape and function, you'll be prepared to determine which lower-leg lumps are harbingers of trouble and which are merely blemishes.
The two bones called phalanxes or phalanges that make up the pastern are equivalent to your two longest finger bones. Your third phalanx resides within your fingertip and the horse's within his hoof, where it's called the coffin or pedal bone. So little movement takes place in the pastern joint connecting the long and short pastern ones that casual observation would lead you to believe a single bone links the fetlock and hoof. Yet the joint is visible just below a pair of dimplelike depressions on the inside and outside of the pastern two or more inches above the hoof. This pair of hollow spots is one of the "good" irregularities you'll find on the normal pastern. The joint connecting the short pastern bone and the coffin bone is not visible and barely palpable because it's just within the hoof capsule and overlaid, on its sides, by thick cartilage pads (collateral cartilages).
Flexibility is the primary feature of your own joined phalanxes, allowing you to curl your fingers to grasp and manipulate. In contrast, the equine design requires rigidity between the upper two bones so the pastern can act as a stiff strut and firm anchor for the soft-tissue "straps" that hold the leg bones in line. Because of the tendons' pulleylike action, which wouldn't be possible without an immobile pastern joint, the fetlock and the coffin joints rotate in concert with each other and the larger hinges up the leg during the touchdown, support, liftoff and airborne portions of each stride.
The tendons and ligaments on a weight-bearing pastern are difficult to discern because they hug the bone surfaces so tightly. If you lift your horse's leg and manipulate his hoof while palpating the pastern with your other hand, you should be able to feel the play of the extensor branches of the suspensory ligament angling forward over the sides of the pastern just below the fetlock. The deep digital flexor tendon lies along the rear aspect of the pastern joint: The one small window of opportunity for palpating it is in the vaulted arch formed by the superficial flexor tendon where it encircles the deep tendon just above the cleft between the heel bulbs. The extensor tendon, the strap that advances the in-flight foot to its landing position, runs down the front of the pastern toward the tip of the toe. On fine-skinned horses, these supporting structures are discernible not as bulges or lumpiness but as clean-edged ridges slanting across the bones.
The joining of hoof wall to skin is accomplished at the coronary band or coronet, the raised, rather hard area encircling the foot from heel to heel. Just inside the coronet, on the sides and toward the heels, are the cartilage pads overlying the coffin joint. These collateral cartilages, so called for their location on each side of the hoof, are partly within the hoof wall and partly above it. In addition to serving to smooth the transition between the slender column of the pastern bones and the wide "mouth" of the hoof capsule, these cartilages contribute to shock absorption and circulation. Rigid enough to protect the blood vessels and nerves passing through them to the hoof's interior, these cartilages are just sufficiently flexible to participate in the hoof's expansion and contraction during weight bearing and flight. Normal collateral cartilages are readily visible and palpable as smooth, somewhat "giving" bulges that are wider and higher near the heels and taper toward the toe. Draft horses and other individuals with blocky, upright pasterns may have collateral cartilages that, even when healthy and functioning normally, are quite prominent.