Is there an Association Between Distal Phalanx Angles and Deep Digital Flexor Tendon Lesions?

There were no significant differences in the distal phalanx angles between horses of mixed breeds with and without deep digital flexor tendonitis in the digit. However, Thoroughbred horses with deep digital flexor tendon (DDFT) lesions had a trend toward more acute distal phalanx angles than clinically sound Thoroughbred horses.

1. Introduction

It has recently been recognized that deep digital flexor tendonitis is a relatively common cause of foot pain that could lead to lameness [1,2], but the etiopathogenesis is poorly understood at this time. Horses that jump may be at greater risk than horses used for other purposes [1]. Pathological changes at the insertion of the deep digital flexor tendon (DDFT) have been noted in horses with weak heel conformation [3], suggesting that foot conformation may be a predisposing factor for injury.

The DDFT has a close relationship with the navicular bone, and in the propulsion phase of the stride, the DDFT bends over the middle scutum (the fibrocartilaginous insertion of the straight sesamoidean ligament on the middle phalanx) and comes into full contact with the bone. Tension in the DDFT is maximal, and active muscle contraction and the elasticity in the tendon and in its accessory ligament result in extension of the distal interphalangeal (DIP) joint [4]. At the beginning of the swing phase of the stride, the tension in the DDFT contributes passively to induce flexion of the interphalangeal joints.

Currently, there is limited information about the orientation of the distal phalanx within the hoof capsule and its relationship with gross conformation of the foot. The normal angle of the solar margin of the distal phalanx with the horizontal is 2-10° [5], and it has been suggested that this angle may be smaller in horses with low heels and long toes [6].

A recent study in 31 sound crossbred horses investigated the relationship between foot conformation and the force applied to the navicular bone by the DDFT [7]. There was no relationship between conformation of the heels as assessed radiographically and the theoretical forces applied to the navicular bone by the DDFT; however, there was a negative correlation between the force applied to the navicular bone and the angle of the solar margin of the distal phalanx and the horizontal. It was concluded that an increase in this angle by 1° would decrease the DDFT force, and therefore, its force on the navicular bone, by 4%.

Concurrent lesions of the DDFT and the navicular bone have been well documented by post-mortem studies [8,9]. In horses with navicular disease, a previous in vivo study suggested that the force and stress exerted on the navicular bone by the DDFT was doubled in the early stance phase compared with control horses [10].

It was therefore hypothesized that horses with deep digital flexor tendonitis would have a more acute angle of the distal phalanx to the horizontal compared with horses free from lameness. The objectives of this study were to determine the repeatability of measurements of various angles of the distal phalanx and hoof capsule and to determine whether there was any relationship between these measurements and the prevalence of DDFT injury. The relationship between age, height, body weight, and prevalence of injury were also assessed.

2. Materials and Methods

Patient Selection

The horses in the control group (Group A) comprised mature horses that underwent a pre-purchase examination performed by one author (SJD) that were free from lameness or any major conformational abnormality that precluded recommendation for purchase, and that had undergone radiographic examination of the front feet (n = 25) and a group of research horses free from lameness (n = 9).

The horses in Group B underwent a comprehensive lameness examination between January 2001 and December 2003 that included magnetic resonance imaging and were determined to have a lesion of the DDFT contributing to lameness [1,2]. Some of these horses had abnormalities of more than one structure within the digit, including the navicular bone. Horses with evidence of concurrent chronic laminitis were excluded.

The age, breed, sex, height at the withers, and body weight were recorded for each horse.

Radiographic Examination

All radiographic examinations were performed after the shoes were removed and a standard foot preparation performed [6] The foot to be examined was placed on a 20-cm-high block, and the X-ray beam was centered ~1 cm distal to the coronary band, midway between the dorsal and palmar aspects of the hoof. Lateromedial (LM) radiographic views were obtained [a] using single emulsion mammography film [b], high definition screens, and a parallel grid (8:1) at 71 kV and 28 mA, with a film focal distance of 105 cm.

Selection of Radiographs

A preliminary study indicated that slight obliquity of a LM radiograph resulted in unacceptably high coefficients of variance (CVs) for the angles measured (see below). Therefore, only true LM projections were accepted for inclusion in the study. The radiographs had to fulfill the following criteria:

• The condyles of the middle phalanx were parallel and no more than 5 mm apart at any point.
• The bases of the lateral and medial solar borders of the distal phalanx were superimposed.
• The palmar aspects of the extensor process of the distal phalanx were superimposed.
• The navicular bone was clearly defined and cube shaped.
• There was a distinct trabecular pattern in the spongiosa of the navicular bone with a sharply defined flexor cortex.

Measurement of Radiographs

Radiographs were evaluated by one observer (SSS) without knowledge of the group designation. Radiographs were fixed onto a horizontal viewing box. A clear film was overlaid, and lines were drawn with a fine tipped pen. The weight-bearing surface was marked by a horizontal line along the distal aspect of the hoof, marked by the nail heads in the block on which the hoof was placed. Lines were drawn to measure the following angles (Fig. 1):

• The dorsal aspect of the distal phalanx to the horizontal (Angle P)
• The dorsal hoof wall to the horizontal (Angle W)
• The concave, parietal solar surface of the distal phalanx to the horizontal (Angle S)
• The solar border of the distal phalanx to the horizontal (Angle B)

The results were related to body weight, height, and breed.

Figure 1. Measurements from radiographs: angle P (dorsal border of distal phalanx to ground), angle W (dorsal surface of hoof wall to ground), angle S (concave solar surface of distal phalanx to ground), and angle B (solar border of distal phalanx to ground).

Data Analysis

The CVs for 10 repeated measurements of angles P, W, S, and B were calculated. The CVs for angles P, W, and S ranged from 0% to 5% (mean = 2.0%). There was greater variation in the measurements of angle B (CV = 0-28%; mean = 15.5%). Slight obliquity of a LM radiograph resulted in marked increases in the CV; thus, only radiographs fulfilling all criteria above were included.

The variation in age, height, and weight between the groups was tested using independent samples t-tests. Variation of the angles P, W, S, and B of the distal phalanx between the groups was analyzed using Mann-Whitney U tests. A Spearman rank correlation was used to test for an association between age, height, and weight and the angles of the distal phalanx. All tests had a confidence level of 95%.

3. Results

Forty radiographs from Group A, obtained as part of a pre-purchase examination, were rejected (despite being of good diagnostic quality, they did not fulfill the criteria for a perfect LM projection). One hundred and eight radiographs were rejected from Group B for similar reasons. All radiographs from the research horses were included, but, in some, two or three attempts were required to obtain a perfect image. Ultimately, there were 34 horses in Group A and 42 horses in Group B. The horses were Thoroughbred, Thoroughbred cross, warmblood, pony, and other breeds (Table 1).

Measurements were obtained from 76 radiographs (Table 2). Bilateral forelimb radiographs of 16 horses were used, and unilateral forelimb radiographs were used from 60 horses. The age, height, weight, and angle S of the distal phalanx of all the horses were normally distributed. However, angles P, W, and B were non-parametric.

There was no significant difference in age between the groups. There was no significant difference in height between Groups A (158 ± 15.4 cm) and B (163 ± 7.3 cm); however, horses in Group B (574 ± 63.5 kg) were significantly (p = 0.01) heavier than horses in Group A (499 ± 100 kg). Group B had a significantly greater weight-to-height ratio than horses in Group A (p = 0.003).

There was greater variability in angle B compared with the other angles. There were no significant differences in any of the measured angles between the groups (Table 2). There was a strong correlation between angles P and W and angles P and S in both groups (p < 0.02); there was no correlation between angles S and B in either group. In Group B only, there was also correlations between angles P and B, angles W and S, and angles W and B.

When the angles of the Thoroughbred horses alone were compared, there was no significant difference between the groups. However, there was a trend (p = 0.08) toward angle S being more acute in Thoroughbred horses in Group B (16.0 ± 3.4°) than in those in Group A (18.3 ± 3.2°).

4. Discussion

The hypothesis that a more acute distal phalanx angle to the ground may pre-dispose a horse to deep digital flexor tendonitis was unproven. However, if the Thoroughbred horses were considered alone, there was a strong trend toward horses with DDFT lesions (Group B) having a more acute angle of the concave solar border of the distal phalanx (angle S) to the ground than normal horses (Group A). The concave solar surface of the distal phalanx (angle S) is immediately distal to the region of insertion of the DDFT and thus, reduction in angle S would increase tensile force on the DDFT [7].

The interpretation of the data in this study is confounded by the overrepresentation of ponies in Group A (Table 1). The natural variation in foot shape must also be considered, because Warmblood breeds tend to have a more upright foot conformation than Thoroughbreds. The lack of correlation between angles W and S, angles P and B, angles W and B, and angles S and B in Group A would tend to suggest that there may also be some variability of shape of the distal phalanx within the hoof capsule. Horses with chronic lameness may develop a more narrow and upright hoof capsule, which is likely to be mirrored by alteration in angles of the distal phalanx; this factor could potentially mask differences between the groups.

A large number of radiographs were rejected from the study because of slight obliquity, despite being of good diagnostic quality. Variations in distal limb conformation, e.g., toe in and distortions in hoof capsule shape, make it extremely difficult to obtain a true LM projection in all horses fulfilling the criteria prescribed for this study. No previous study has used such strict criteria for selection of radiographs for measurement, nor has this range of angle measurements previously been documented. The need for accurate measurements is reflected by the much smaller variability of angle W measured in this study compared with a standard deviation of >10% in the results of another study [11]. Nonetheless, the results of this study are in broad agreement with previous studies [11,12], although the angles of the hoof wall and the dorsal aspect of the distal phalanx were larger than those reported for American Thoroughbred racehorses [13]. In the current study, the dorsal hoof wall was not always straight; in some horses, there were dips and ridges, and in others, the hoof wall was slightly concave distally. The proximal half of the dorsal hoof wall was therefore used to obtain angle W. Despite the strict criteria for radiograph selection, the standard deviation for measurements S and B was much greater than those for P and W, and this may also confound interpretation of the results.

Horses in Group B were significantly heavier than horses in Group A, and the weight-to-height ratio of horses in Group B was significantly greater than the weight-to-height ratio of horses in Group A. There was an association between DDFT cross-sectional area and weight but not height [14]. Therefore, horses in Group B, with a greater weight than horses in group A, would be expected to have a greater DDFT cross-sectional area. However, there might still be an increase in strain on the DDFT with increased weight if the increase in cross-sectional area is not proportional to weight.

In conclusion, in this small study, no significant difference was found between the angles of the distal phalanx in horses with and without DDFT lesions when horses of mixed breeds were assessed. However, when Thoroughbred horses were considered separately, horses with DDFT lesions had a trend toward a more acute angle of the distal phalanx, which may increase strain in the DDFT. A difference in the angle of the distal phalanx between horses with and without DDFT lesions may be masked by differences in the angle of the distal phalanx to the ground between breeds and by different shapes of the distal phalanx.

Footnotes

1. Siemens Polydoros 100 x-ray machine, Siemens Medical Solutions, Bracknell, Berkshire, RG12 8FZ UK.
2. Konika Films, Malmesbury, Wiltshire, TW13 7AD UK.