How to Use a Digital Extension Device in Lameness Examinations.

1. Introduction

Many clinicians use dorsal digital extension tests when doing a lameness examination of the forelimbs.

Traditionally, the limb under examination is placed on the extremity of a long board, while an assistant picks up the contra-lateral limb. The opposite end of the board is then slowly raised to extend the digital joints. This tenses the deep digital flexor tendon (DDFT), its inferior check ligament, and the collateral and impart ligaments of the distal sesamoid (DS) as the palmar angle of the DDFT around the navicular bursa and the DS increases.

Apart from practical problems like bending or breaking boards, pinched fingers, and slipping of the subject’s foot, evaluation of positive or negative responses are only standardized, if at all, with a personal protocol, (e.g., by always using the same board, carefully placing the foot at its extremity, and deciding on a height to be reached at the opposite end like the examiner’s knee) [1].

Horses show when they are getting close to maximum tolerated extension by the shivering of their cleidobrachial and extensor carpi muscles and the shifting of their weight onto their hindlimbs, which declines their cannon bone backwards. In rare cases, the heels will start lifting off the board. Maximum tolerance is, therefore, determined by slowly raising the board until the horse shows any of these signs, specifically the weight shifting, which is easily perceived by the clinician. This indicates that the horse is about to jump off the board.

Elevating the foot sideways, either laterally or medially, with the use of a board is sometimes done by clinicians to evaluate tolerance to lateral or medial elevation. This may be useful in the diagnosis of arthrosis (arthritis in the digital joints) and painful collateral ligaments of these joints or of the DS as well as show sources of subcondral pain with a lateral or medial location in the joints and lesions of the distal part of the DDFT within its lateral or medial lobes. These structures are often injured, which we are more aware of now thanks to magnetic resonance imaging and sophisticated ultrasound studies that have widened our understanding of "navicular syndrome" [2].

It is interesting to note that lateral or medial elevation of the foot does not only cause movement, but it also pinches and tenses in the frontal plane (side to side) of the proximal interphalangeal joint (PIP) and especially, the distal interphalangeal joint (DIP). There is also torsion in the horizontal plane with a strong rotational movement of the distal phalanx (PIII) relative to the intermediate phalanx (PII) and the DS. This movement has been studied in vitro and described with the name of colateromotion [3].

From the above, it can be concluded that digital extension tests are certainly not very specific as to the exact site of pain or lesion and until now, not even very sensitive; however, this certainly varies with the experience of the clinician. This does not necessarily diminish the practical value of digital extension tests, especially in regard to some treatment options like therapeutic farriery. It is, for example, likely that a horse with a (perceived) intolerance to dorsal extension of the digit will benefit from specific farriery techniques that facilitate dorsal break-over [4].

2. Materials and Methods

To increase the ease of use and specifically, the objectivity and sensitivity of the digital extension test by making it quantifiable, the author developed a graduated digital extension device [DED] [a].

The DED consists of three round plates of which the middle one attaches to a 4 X 4 X 120-cm-long handle. At its opposite extremity, the handle has a rubber-covered grip, and close to this, a protractor with a needle incorporates a spirit level. The top plate is covered with rubber to prevent slipping and has a diameter of 21 cm. A set of internal ball bearings permits the middle plate and its attached handle to rotate relative to the top and bottom plates. This makes it possible to perform a dorsal extension and a lateral and medial elevation on the same limb without having to replace the foot on the DED; after each measurement, the handle is lowered and turned to the next position. The sides of the plates are beveled at a 45° angle; this reduces the diameter of the ground surface of the bottom plate to 16.5 cm, which makes lifting easier for the clinician (Fig. 1-Fig. 3).

Figure 1. Dorsal extension.

Figure 2. Lateral elevation.

Figure 3. Medial elevation.

When reaching the maximum degree of extension that the horse tolerates (dorsally) or when the opposite side of the foot is starting to lift off the plate (lateral and medial elevation), the needle is placed at water level where it stays through friction. This allows the clinician to read the measurement on the protractor after the handle is lowered again. The protractor has a cc milled scale of 0 - 60° (Fig. 4). For ease of transportation (the entire DED is 1.40 m), the handle can be disassembled in the middle, resulting in two parts of equal (0.70 m) length.

Figure 4. Protractor with spirit level.

3. Results

To judge the clinical significance of a given extension value in a lame horse, normal ranges in sound horses have to be established first. A recent study of 250 sound horses of different breeds, correlating extension/ elevation values with dorsal hoof angle, height, weight, breed, age, sex, and use gave the following results: dorsal extension (mean [CI 95%] = 43.18±0.93°), lateral elevation (mean = 18.83±0.26°), and medial elevation (mean = 19.79±0.32°). The SDs were 7.46°, 2.12°, and 2.53°, respectively [b].

All horses measured were recently trimmed or shod to eliminate excessive hoof growth as a variable. Repeatability of results was tested by having a subgroup of 50 horses measured again by different testers.

Repeatability of results was tested by having a subgroup of 50 horses measured again by different testers.

The capacity for dorsal extension of the digit in the sound horse is related to conformation and is more variable than the lateral and medial values. Horses with long and oblique pasterns with lowheeled feet (low palmar angles of PIII), like Thoroughbreds, have a larger capacity for dorsal extension than horses with short upright pasterns and large palmar angles (e.g., most Quarter Horses).

In lame horses with well-matched front feet, a disparity of more than a few degrees in dorsal extension tolerance is significant, and when present, the lower value correlates well with the lame limb. In horses with mismatched front feet (one steep and one flat), the steeper one has less capacity for dorsal extension (tighter DDFT). Different values are not necessarily abnormal; however, anything <30° should be suspect, even in upright feet. Lateral and medial elevation values are amazingly uniform across breeds and conformation types.

Lateral values average nearly 1° less than medial values. This is probably explained by the fact that the test is done on the weight-bearing limb, which the horse tends to place under the center line of its body; this may slightly close the lateral joint spaces.

Anything <17° on lateral and <18° on medial elevation is considered suspect by the author. The exception being when the low value goes together with an increased elevation value on the opposite side of the same foot. In this case, it is probably a case of mistaken trimming on that particular foot. If, for example, a foot has a lateral elevation value of 16° (the point at which the medial side has started to lift off the plate) but a medial elevation of 23 - 24°, it is probably a case of lowering the outside wall, relative to the inside hoof wall, by about 3 - 4° to get normal values and clinical improvement. To put it another way, if the sum of the lateral and medial elevation values is ≥35° and one of them is decidedly lower than the other, latero-medial joint movement can be improved by trimming on the side of the lower value. However, if the sum is lower than this, the capacity for latero-medial joint movement is considered to be compromised.

4. Discussion

Digital extension test values do not pinpoint a specific lesion, but nevertheless, they have great clinical value. They are extremely useful in shoeing prescriptions when there is more than one lesion found with diagnostic imaging, and they also give a rational approach to lateral medial balancing of the foot.

Table 1 summarizes therapeutic shoeing techniques for DIP-PIP arthrosis and podotrocleosis in relation to diagnostic and clinical findings (including DED tests). The surface that the horse works on must be taken into account.

When diagnostic imaging reveals more than one lesion in the same limb that would require, theoretically, opposite therapeutic shoeing prescriptions, the DED lets the horse "talk" for itself. The DED facilitates break-over in the direction of the lower tolerance until extension/elevation is established. For example, if there are reasons to suspect both a suspensory ligament (SL) desmitis, and a form of podotrocleosis, the horse will show reduced tolerance to dorsal extension if the latter is clinically more significant then the first. Applying, for example, a blunt-toe egg-bar shoe is only justified in such a patient if this is indeed the case, because this shoe will increase fetlock extension on penetrable ground.

On hindlimbs, DED tests, especially dorsal ones, are hard if not impossible to perform; however, in the case of proximal SL desmitis, it can be useful to perform a standing, weight-bearing, flexion test with the DED. The affected hindlimb is placed on the DED, and the opposite one is picked up with the handle pointing caudally. When raising the handle, a normal horse will flex its DIP joint all the way until it slides off the DED (do not go >40° because of risks). If there is pain from the SL, horses will show intolerance to this weight-bearing flexion test, sometimes already in the low 20°s, as DIP flexion increases fetlock extension. These patients will probably not be helped with plantar extensions like egg bars.

The author has seen the DED used in other creative ways by colleagues. One example is a dynamic test where the horse is trotted off in a straight line after a timed extension test (e.g., 30 s at close to the maximum tolerated dorsal extension). Another example is a weight-bearing, flexor tendons ultrasound examination performed while raising and lowering the extension device to check for adhesions.

Although the author developed the DED, he has no commercial interests in its distribution or sale.

Footnotes

1. Colleoni srl, Via Boscaccio, 40 21012 Cassano Magnano VA, Italy.
2. Castelijns CG, Castelijns HH, Del Castillo AL. Quantification of the normal ranges of tolerated digital extension and lateromedial elevation in the horse. Equine Vet J 2007. In press.