SIDEBONE

• On each side of the pedal bone is a large,rounded cartilage called the lateral cartilage.
•  This two cartilages support and protect the soft tissues at the back of the foot.They also support the hoof wall in that area.
• Sidebone is a condition in which the lateral cartilages become calcified.These tissues probably calcify because of chronic,repeated concussion,such as frequently exercising on hard surfaces.
• It is more common in horses with narrow,upright feet.It is also common in horses with unbalanced feet,especially those that toe in or toe out.In most cases changes occur in the forefeet.
• In very severe cases sidebone can be diagnosed with palpation;the normally flexible cartilages just above,and to the outside of the heel bulbs become thickened and hard.
• In most cases sidebone can be diagnosed by radiographs.

• Mild or early cases of sidebone may be seen as small islands of calcification.
• Moderate to severe sidebone is seen as calcification of all of the lateral cartilage.The lateral cartilage are blocked by palmar digital nerve block. 

Reference

Lameness

Recognizing and Treating The Horses Most common Ailment

Equine research 

by Christine King and Richard Mansmann