The navicular bursa is a tiny, fluid-lined sac within the heel of the hoof. It rests between the deep digital flexor tendon and the navicular bone as the tendon passes over it. It helps smooth the deep flexor tendon's action and reduces friction as this tendon passes passes over the bone.

Nails and screws penetrating near the frog or heel can penetrate the deep digital flexor tendon and bursa, damaging and introducing bacteria into these structures. Resulting lameness is usually very severe. In most cases, horses cannot bear weight on the limb. Within hours, swelling may migrate up the limb. These are disastrous injuries. Early diagnosis and aggressive treatment is the only hope for return to soundness.

After the nail or other foreign body has been removed, diagnosis may require a variety of techniques including radiography, ultrasound and sampling of the fluid from the navicular bursa to establish that the bursa is involved.

In the past, this injury was treated with what is known as a "street nail procedure." In this operation, a large central window of the frog and deep digital flexor tendon is surgically removed, to allow proper drainage of the bursa. Today, this is still a possible treatment, but should be considered after other less invasive treatments are considered. In most cases, horses that undergo a street nail procedure will have some degree of chronic lameness.

Any nail puncture can cause non-weight bearing lameness. It is critical for your vet to differentiate a simple abscess from infection of this area because rapid and correct treatment is important.