White Line Disease
Excessive distraction of the laminar interface may result in "delamination". This condition is often called "white line disease", and represents a separation between the insensitive laminae of the hoof wall and the sensitive laminae of the underlying tissue. The underlying tissue is directly attached to the third phalanx (often referred to as the coffin bone or pedal bone). The separation of these tissues results in the creation of a space within the laminar interface (i.e. white line) of the hoof. The presence of this space invites contamination with soil, gravel, as well as infectious agents (bacteria and fungi). Since the bloodstream does not have direct access to this space, the horse is unable to deliver white blood cells into this area to fight off infection. Therefore, the space becomes an ideal location for bacterial/ fungal proliferation, tissue compromise, and chronic/ recurring infection.
Most horse owners will treat this problem with topical disinfectants (like bleach), topical antimicrobial/ antifungal medications, and even systemic antibiotics in an attempt to resolve ongoing infection. It is very important to realize, however, that the contamination/ infection of the laminar interface (white line) is not the primary problem. The infection is merely a consequence of having laminar separation. Soil, gravel, and infectious agents will not invade a space that does not exist. The problem, therefore, is not the infection itself but the fact that it has a place to occur. Therefore, when we treat white line disease with Epsom salt soaks and/or antimicrobial therapy we are really addressing a secondary problem.
In order for treatment of white line disease to be successful in the long run we must eliminate the primary problem which is delamination (laminar separation).
How do we treat laminar separation (delamination)?
In most cases, delamination occurs as a result of abnormal biomechanical forces within the foot structure. Separation within the laminar tissues occurs as a result of excessive distractive forces between them (i.e. the laminar interface is being "pulled apart"). Distraction across the laminar interface is often secondary to downward or rotational movement of the third phalanx (P3) within the hoof capsule. We refer to this as "mechanical rotation" of P3. It occurs very slowly and over a long period of time so there is often no associated inflammation and pain (it is therefore different than acute laminitis/ founder). The rotational movement of the third phalanx, in turn, is a consequence of excessive tension of the deep digital flexor tendon (DDFT), which inserts on the underside of P3. Constant tension in the DDFT "pulls" the third phalanx in a downward direction.
To re-summarize, infection (white line disease) occurs as a result of delamination (the creation of the space) which may occur (in part) as a result of mechanical P3 rotation which in turn occurs as result of excessive DDFT tension.
Therefore, excessive DDFT tension can be considered one of the potential primary problems. Consequently, treatment (in our opinion) should involve normalizing and/or reducing DDFT tension. This is usually done conservatively by elevating the heels relative to the toes either through trimming or the application of wedged pads and shoes. Elevating the heels relative to the toes transfers weightbearing tension from the flexor tendons to the suspensory apparatus. Reducing DDFT tension can stabilize P3 within the hoof capsule and allow the basement membrane to produce hoof wall from the hairline all the way to the ground surface without making adjustments in orientation. Consequently, we are left with straight, intact laminar interface from top to bottom. There is no interlaminar space available for contamination/ infection. On average, it takes about 10-12 months for the wall to completely grow out and the delamination to resolve.
Therefore, successful treatment for white line disease should include a plan to address DDFT tension. You can read more about DDFT tension HERE.