How Venograms Can Guide Shoe Selection For Laminitic Horses

Imaging technique can provide valuable information prior to P3 displacement

By Amy Rucker, DVM

Terms For Measurements On Lateral Radiographs: Distance In Millimeters (mm)

Horn-Lamellar Zone (HL): Width of the dorsal hoof wall when measured perpendicular to the face of P3. Half the width is horn (epidermis — diseased in white-line cases) and half is lamellae (dermis, which contains the blood vessels — diseased in laminitis.) A healthy 2-year-old Thoroughbred foot has a HL measuring 15 mm at the proximal and distal aspect of the face of P3. This number will be larger with certain breeds (warmbloods, draft horses), or increases with disease.

Coronary-Extensor Distance (CE): Vertical distance from the level of the coronary band to the top of the extensor process of P3. If a horse is a “sinker,” this number may increase in a matter of days as P3 descends in the hoof capsule. The CE may also slowly increase in cases of chronic laminitis due to collapse of the P3 suspension and resultant capsule distortion.

Forces On The Distal Phalanx: Laminitis destroys the attachment between P3 and the hoof capsule. The forces of the deep digital flexor tendon continue to pull the apex of P3 away from the dorsal wall and down onto the solar dermis. This compresses the blood supply of the sole, reducing sole growth. Further collapse of the foot results in alterations in wall growth, bone disease, abscesses/seromas, etc.

Sole Depth (SD): Depth of sole measured directly below the apex/tip of P3. If the foot is strong, I’ll include the measurement of SD + cup. On radiographs the “cup” of the foot appears as a black space distal to the sole. I encourage sole growth: the heavier the sole, the greater protection for P3. I also correlate increases in SD with health. (If the load is controlled within a foot, and the dermis has a good blood supply, it will grow sole.) There is a wide range of “normal” in SD, but while a thin sole may be common, it is not healthy. Although many horses race with 10 mm SD, that foot isn’t optimal. In a foot with a heavy sole, SD will approach 20 mm. A foot with a heavy sole will have a deep sulcus around the frog (even at the apex) and usually has a cup and a strong wall.

CE: Coronary-Extensor Distance –13 mm. HL: Horn-Lamellar Zone – 19-20 mm. SD: Sole Depth – 17 mm. DB: Digital Breakover – 23 mm. BA: Bone Angle – 45 degrees. PA: Palmar Angle (measured to low medial wing of P3) – 3 degrees. TSA: Tendon Surface Angle – 30 degrees. Example Horse: 14-year-old warmblood gelding. Chronic lameness due to rim fractures on distal border of P3 when adopted 7 years ago. Poor wall quality, difficult to maintain sole depth and mild arthritis. Rotational limb deformity (toes out) and crushed medial heel.

Normal Venogram: A: Terminal arch of palmar digital vessels. B: SLVB - Sublamellar vascular bed. C: Coronary plexus. D: Circumflex vessels (with sole papillae). E: Heel vessels. F: LCJ – Lamellar/Circumflex junction.

*Digital Breakover (DB): Distance measured by dropping a line from the apex of P3 to the ground surface (DB = 0) and then measuring anterior to the front of the toe (the place the foot is last in contact with the ground as it moves forward). DB is manipulated by trimming the foot (rocker toe) or shoeing techniques (rocker or bevel toe, rocker shoe, etc.). DB may be negative if the breakover is behind the apex of P3. We manipulate/reduce DB to reduce the lever-arm forces of the toe on the dorsal wall and lamellae, as well as the joints, DDFT, and navicular apparatus.

*Palmar Angle (PA): Angle between the palmar surface of P3 and the ground. The PA may be positive (for example, a clubfoot), zero (palmar surface of P3 is parallel to the ground) or negative (wings of P3 are “lower” than the apex in a low-heel, long-toe foot). Note: Increasing the PA will decrease the strain of the DDFT, reducing the pull of P3 away from the dorsal wall and down onto the sole dermis.

Venogram: Folded lamellar-circumflex junction, loss of sole papillae. Shoeing: Trim heel parallel to palmar surface of P3, elevate PA to 20 degrees using Modified Ultimate shoe, reduce DB to directly below the apex of P3 (or farther back if a low-heeled foot), place putty within the RMU shoe but have no contact with the sole below apex of P3. Once the foot has grown a considerable amount of sole (SD at least 20 mm) and the venogram has returned to a normal appearance (including having sole papillae), the bottom wedge may be removed from the Ultimate shoe or the foot is shod in a rocker rail.

Venogram: Folded LCJ, loss of sole papillae, distortion of coronary plexus, apex of P3 is below the folded LCJ. Shoeing: If this is an acute case (within 48 hours) with a treatable initial cause, consider shoeing with Modified Ultimate as described in the case at left, but plan to repeat venogram in 48 to 72 hours to assess response to treatment. If this is a chronic case, consider shoeing and performing a deep flexor tenotomy.

Bone Angle (BA): Angle of P3 when measuring from its dorsal face to palmar surface. On one horse, a front foot may measure 45 degrees while the off club measures 60.

*Tendon Surface Angle (TSA): Angle measured from the palmar-distal border of the navicular bone to the ground. Changing this angle changes the forces between the DDFT and the navicular bone.

*These are things we alter with our trimming and shoeing.

Venogram: Distal displacement (sinker), uniform widening of SLVB with loss of contrast below P3. As the sinker progresses, contrast is greatly reduced below the coronary band (but often remains in the heel and may or may not be present in the terminal arch).

Treatment: Consider shoeing and performing deep digital flexor tenotomy. Shoeing for a deep digital flexor tenotomy: Trim heel parallel to palmar (bottom) surface P3, with shoe 20 mm below palmar surface. Apply support putty to heel, frog and sole but do not cover sole directly below apex P3. Glue shoe (5 degree tenotomy rail) to foot using Equilox. Place shod foot in 20-degree elevated-heel shoe (modified Ultimate) during surgery to soften the DDFT. Remove the modified Ultimate after cutting deep flexor tendon and before closing skin to make certain entire tendon has been transected.

Principles Of Treatment:

1. Increasing the PA reduces tension of the deep digital flexor tendon. If disease is severe, the tension is eliminated by cutting the DDFT.
2. Reducing DB reduces forces on the diseased lamellae in the dorsal wall. (This is also true with medial and lateral breakover.)
3. Shoeing goals include restoring digital alignment and applying forces perpendicular to the bottom (palmar) surface of the coffin bone (P3).
4. Our trimming often increases the weight-bearing surface of the heel, which is relatively healthy compared to the anterior half of the foot. (Trimming moves the heels back toward the heel bulbs.) However, if we cut off the heels, we add the height back with our shoe apparatus. NEVER lower the PA, because this increases the tension of the DDFT.

Initiate treatment of the laminitic horse as soon as possible (ideally within the first 48 hours of the development of clinical signs). This is the greatest opportunity for healing and recovery. Many things influence our outcome, including age of the horse (young horses heal better), foot’s health before it developed laminitis, cause of laminitis (can it be cured or controlled?), owner resources (mental, financial physical), insurance company, the desired use of horse, etc.

The only thing we can influence is the amount of experience, knowledge and skill of the veterinarian/farrier team.

Before there is any measurable displacement of P3 on radiographs, changes are apparent on the venogram. Pathology in the foot results in compression of veins, which appears as a reduction of contrast compared to a normal pattern. Dermal/epidermal separation creates a widened appearance to the sublamellar vascular bed in the lamellar dermis. Other alterations in the normal pattern are created by collapse of P3 within the capsule.

Venograms should be used to determine if a case is mild or severe. Serial venograms determine response to treatment.

Chronic laminitis: Venogram: little contrast below the apex of P3, LCJ remodeled around remodeled apex of P3, feathering of contrast into dorsal SLVB. Chronic laminitis with increased CE, HL widened, poor sole depth, and remodeling of the apex of P3. Often lame due to unstable attachment-P3 to hoof capsule. Shoeing: Use a rocker rail by trimming the heel parallel to the wings of P3, rockering the shoe to maintain the original PA (DO NOT cut off heel and lower PA!), and reducing breakover to behind the apex of P3. The center of the balance point of the shoe is directly below the center of the coffin joint. (On a clubfoot, it may be slightly forward, below the extensor process of P3.) In this example, the PA is high — around 20 degrees — and there is considerable bone damage. If the bone damage is minimal and the PA is lower, you may shoe with a full rocker or rockered shoe (withhout the rail).