Interpreting Soft Tissue Parameters and Lesions

2002 - 15th Annual Bluegrass Laminitis Symposium Notes

Interpreting Soft Tissue Parameters and Lesions - The Influence of Trimming and Shoeing the Normal and Pathological Foot

Written and presented January 2002 by R.F. (Ric) Redden, DVM

Developing a detailed, methodical radiographic protocol is the first step in learning the healthy range of soft-tissue parameters of the equine foot, in addition to the commonly found soft-tissue image variations. Radiography should be considered a discovery exercise, as we are seeking information that will confirm, aid in the diagnosis of pathology or reveal parameters or lesions that may suggest a diagnosis. Farriers need DP and true lateral views to fully assess anterior-posterior balance, medial-lateral balance, sole depth and the palmar angle. Simple imbalance can lead to soreness and lameness that can be quickly alleviated with therapeutic trimming and shoeing. This information allows farriers to assist the attending veterinarian with a large variety of foot problems.

Knowing your equipment and how to get the most from it allows the radiologist to make finite adjustments that meet the demands of the exam.

The Lateral View In order to take a true lateral radiograph, the primary beam should be centered 3/4 to 1 inch over the positioning block or at the level of palmar surface of PIII. The true lateral will offer information on sole depth, HL zone, CE, palmar angle and digital breakover.

Sole depth Sole depth is measured from the apex of PIII to the ground surface. Breed and other factors influence the depth of the sole. Centering the beam close to the palmar surface offers accurate measurement of the distance between the palmar surface of the shoe or ground surface. Your positioning block also needs an opaque marker (wire embedded in the face of the block works well). This marker clearly reveals the ground surface of unshod feet. The shoe becomes the marker for all shod horses. When taking a true lateral radiograph, notice that the film will have the branches of the shoe superimposed; therefore, you will only see one branch of the shoe.

The barefoot horse will seldom have air density between the sole and the opaque marker, as the sole is a natural load bearing surface. Often though the shod horse will have air dense space between the sole and the foot surface of the shoe. This space is present for several reasons, each of which is considered vital to full assessment. Strong feet with adequate mass and ideal conformation have a natural cup that often produces a large air space between sole and shoe. Trimming the feet also greatly influences the sole depth and cup of sole. When comparing changes in progressive film, it is necessary that the film be comparative.

Measuring the sole depth on every lateral film will help you develop an eye for the range of normal or at least what you normally find on the feet you examine. There is a difference. Like all other parameters, if you only measure the pathological cases the range of normal will escape you, diminishing your ability to distinguish subtle pathological lesions. Mature five inch feet on most light breed horses will have a sole depth of 15-20 mm. Fifteen millimeters is the minimum depth.

Venogram studies on many sound feet with good conformation indicate that the sole corium normally occupies approximately 10-12 mm of the solar depth. The concave surface of the sole and/or coffin bone is not visible on the lateral projection; therefore we are seeing the depth of sole directly beneath the perimeter of PIII. Less than 15 mm of sole depth is considered inside the comfort zone, as there is not adequate room for the normal circulatory solar plexus. Race horses and event horses that often have 10 mm or less are generally foot sore horses with collapsed heels and extremely thin walls. Regardless of the concomitant pathology, their feet are severely compromised on a good day. Sever lack of foot mass is the primary diagnosis. Excessive wear can be a contributing factor for thin-soled, barefoot horses, while unshod horses that paw continuously when tied can quickly remove several millimeters of sole and toe. This wear pattern will be obvious to the trained eye.

I use my film to confirm a diagnosis, while including evidence found through a thorough physical exam relative to the history. White line disease and other common problems certainly have pathognomonic lesions, but a good history and even better exam will often become quite valuable.

The acute stage of laminitis can be more difficult to diagnosis and assess due to similar pain-producing syndromes; therefore, astute attention is required when examining the foot, the film and the history. Baseline film made prior to your examination can be invaluable especially if positioning and film detail are relatively comparative. Pre-purchase film taken months before can be invaluable, but keep in mind that the purpose of the pre-purchase exam may have been quite different than yours. Beam positioning and detail may vary considerably forcing you to read between the lines. Comparative films are just that, they have identical positioning and detail. I like to first identify beam location and make a written note of the dissimilarities concerning detail and content then pass my opinion concerning the similarities. Having a very strict, personal protocol offers me significant advantages for all comparative films.

Horn-Lamellar Zone (HL) Horn-Lamellar zone is one of the most valuable parameters for assessing lamellar and horn health. This zone is greatly influenced by beam positioning. To alleviate distortion, a consistent, perpendicular beam-film relationship must exist and be centered at the area of greatest interest. I like my primary beam to strike the foot at the approximate level of the palmar surface. This offers accurate measure of the depth of sole, thickness of the lower HL zone, and palmar angle. The upper HL zone is slightly distorted, but with consistent positioning this distortion remains constant.

Most light breed horses will have an HL zone measuring 15 mm. When taking this measurement, I use two points of reference: the area just beneath the extensor process and the apex of the coffin bone, measured perpendicular to the wall marker (opaque paste). Exceptions to the normal 15mm HL zone include Standardbreds that generally measure 18-20 mm. Aged, heavy Thoroughbred brood stock and stallions will often measure 20 mm or more. Warmbloods and others with 5 inch wide feet will also range from 20- 22 mm. Most often the front and hind feet will be very similar, within 1-2 mm. Weanlings and yearlings will vary greatly depending on their stage of development. Normally the upper measurement will be greater than the lower by 1-2 mm. Adult horses that have had their toe backed up hard will have a smaller zone at the apex. Radiographically, there is a striking difference on a backed up toe.

The opaque zone seen on the radiograph delineates the junction of sensitive and non-sensitive laminae. It also clearly reveals lamellar thickness within the lamellar space, in addition to horn depth. This normal soft tissue structure is an invaluable aid when distinguishing rotational changes that have occurred within the horn wall (e.g. white line disease and occasional club foot) verses capsular rotation that occurs due to laminitis. Law suites often hinge on which side of the zone is beyond the range of normal.

Subsequent films taken after your baseline views may reveal subtle but significant swelling of this zone. With laminitis these changes are very significant and often can be visualized long before other more obvious signs of rotation and/or distal displacement have occurred. Clinical cases that have 10 mm of acute lamellar swelling as a rule have a grave prognosis. Success with these cases is dependent on swift reperfusion of the digit.

Chronic thickening of the lamellar zone must not be confused with acute swelling seen in acute cases. These are two totally different problems often can only be distinguished with a venogram.

Palmar Angle This angle is measured by drawing lines along the palmar surface of the wings of PIII and the ground surface. The angle where these two lines intersect is the palmar angle.

I measure at the wings because they better represent the body of the bone, and represent the area of the digit designed to carry load and absorb tremendous energy. The palmar surface at the apex is often in a more proximal plane. In addition, the sharp, fragile border of the apex is often distorted due to pathological conditions that cause excessive loading and trauma. Bending or lysing can be found with club feet, laminitis, white line disease, flat, shallow feet and a variety of flexor deformities.

Measuring the angles can be arbitrary and non-specific as there is seldom a straight line to follow. I am concerned with large increments of angle change; 5-10 degrees or15-20 degrees, as smaller more detailed changes are difficult to measure and do not add to the big picture.

As a farrier, I use this angle to help design all therapeutic shoes. This angle can range from a negative 10 degrees (severe caudal rotation), which is frequently found in rear feet with very steep coronary band angles, to 45 degrees in severely rotated, chronic laminitic cases. Ponies and other breeds with very upright hoof capsules often have relatively thick soles with deep cups. These feet may be 15-20 degrees in their normal state.

Coupling the palmar angle with sole depth and HL zones, we can find a direct correlation to the events that precede many pathological conditions. Pathological conditions that involve the apex, anterior lamellar zone and horn wall can be treated successfully by shifting load away from diseased areas to the healthier heel zone. I often use the palmar angle to facilitate this mechanical advantage. Utilizing the simple mechanical shift of load enhances the healing environment and diminishes the painful response. This offers the podiatry team a meaningful, reliable means of treating a variety of foot ailments.

The normal palmar angle is certainly not written in stone, as it varies considerably among breeds as well as within breeds due to unique characteristics of foot conformation, environment, training demands, etc. As a rule, light breed horses with acceptable, sound hoof conformation will have a natural 3-5 degree palmar angle in the front and slightly higher in the hind feet. The coffin bone angle is normally 50-51 degrees in most feet . The exception are club feet and extremely low angle hoof capsules. These feet will have a hoof angle somewhere between 53-56 degrees. This is not a standard, just a reflection of a collection of commonly found normal feet.

Mustangs that I have radiographed in northern Nevada were quite flat and void of a palmar angle. A stark difference can be found with the internal structures of these feet verses the domestic foot. Domestic feet with a flat (zero) palmar surface have a crushed digital cushion, normally thin soles, thin walls and an assortment of heel related soreness. I am often asked, "What is the ideal palmar angle?" I really cannot say without a reference to breed, sport and conformation of each individual case.

Digital Breakover To measure digital breakover, drop a vertical line from the apex of PIII to the ground surface. Measure forward to where the shoe or bare foot no longer touches the ground. This distance is the digital breakover.

This point is where the hoof and shoe pivot forward as the heel leaves the ground. Prior to the heel leaving the ground, tremendous forces are executed around the apex of PIII. The coffin bone is actually suspended within a very flexible soft tissue, blood filled cushion. This very flexible, but yet durable, strong attachment is compressed in areas and stretched in others before the heel begins to lift. Therefore, I prefer to reference the point of digital breakover at a level compatible to internal pivot of the digit instead of a line carried along the face of PIII to the ground surface. This point of reference is dependent on sole depth and/or shoe thickness which play a role in the forces applied to the true point of digital pivot or breakover.

The length of digital breakover becomes a valuable tool for the podiatrist team because manipulating this distance drastically changes the tension or resistance of the deep flexor tendon and other supporting ligaments and tendons.

The pure lateral soft tissue detail film with wall and ground marker is a most valuable blueprint for the podiatrist team. Measuring the HL zone, sole depth, palmar angle and digital breakover, coupled with a working knowledge of soft tissue lesions associated with the four parameters, lays the ground work for the discovery and planning phases that should precede any and all therapeutic protocols. Without these parameters, I would have to resort to external manipulation as the only means to shift load or relieve other areas of compression and tension.