RUPTURE OF THE THIRD PERONEAL MUSCLE IN THE HORSE

Paweł Dukacz¹, Karolina Barszcz², Małgorzata Dzierzęcka^2
1 Horse Health Center, Veterinary Clinic, Nasielsk, Poland
² Department of Morphological Sciences, Faculty of Veterinary Medicine, Warsaw University of Life Sciences - SGGW, Poland

ABSTRACT

The third peroneal muscle (m. peroneus tertius s. m. fibularis tertius) is an exclusively tendinous muscle in the horse. It is important component of the reciprocal apparatus, coordinating the flexion and extension of the stifle and hock. The clinical signs associated with this injury are characteristic in appearance: the hock does not flex as the pelvic limb moves forward. During passively flexing the limb, the hock can be moved independently from the stifle. The cause of the rupture is usually trauma. The rupture occurre in the midbody of the third peroneal muscle, at the insertion and at the origin. The location of the rupture can be determined with the use of ultrasonography and radiography. The prognosis of this injury depends on the location and degree of the rupture. The aim of the study was to describe ethiology, diagnosis and treatment of the rupture of the third peroneal muscle in 4-year-old gelded Polish Halfbred Horse.

Key words: third peroneal muscle, rupture, horse.

INTRODUCTION

In horses, the third peroneal muscle (m. peroneus tertius s. m. fibularis tertius) is almost entirely tendinous, with practically no characteristics of a muscle remaining. It is located on the extensor surface of the crus, overlapping the cranial tibial muscle, and covered by the belly of the long digital extensor muscle. It originates in the extensor fossa of femur, and its final insertions are located on the dorsal surface of the tarsus and the proximal end of the metatarsus [4].

The third peroneal muscle and the common calcaneal tendon play a crucial role in the equine pelvic limb stay apparatus, creating a functional interdependency between the stifle and the tarsal (hock) joint. In the species, the flexion of the stifle is coupled with the flexion of the hock. Thus, in a normally functioning pelvic limb, the extension of one of these joints causes the simultaneous extension of the other. If, however, the tendon of the third peroneal muscle is ruptured, the two joints become functionally independent, and thus, extension of the hock with a flexed stifle is possible [7, 8].

The symptom is very characteristic, as it is present in over 85% of horses with third peroneal tendon rupture. In literature, few papers are available on the rupture of the third peroneal tendon in horses [3, 7, 8]. Literature on the avulsion of the muscle's insertions in other ungulate species is also sparse. One such case in a reticulated giraffe has been described [10].

CASE HISTORY

A 4-year-old gelded Polish Halfbred Horse, used recreationally, was released into a paddock with other horses. It exhibited no symptoms of musculoskeletal system injury. When the animal has spent several hours in the paddock, with no supervision, abnormal gait was observed by the owner.

A generalized physical examination showed the patient to be in good condition. No skin lesions, such as wounds or abrasions, were present. A focused examination of the musculoskeletal system at rest showed that the horse had normal posture and put weight on all limbs normally.  However, an enlargement of soft tissue around the zeugopodium (crus) of the left pelvic limb was observed, in the approximate mid-point, cranial side. An examination of the horse in motion showed distinct swinging leg lameness in the left pelvic limb. The hoof lifting movement was impaired, which caused the dorsal surface of the keratinous wall of the hoof to come into contact with the ground while walking. A distinct extension of the tarsocrural joint was also observed. The crus formed a nearly straight line with the metatarsus, with the stifle joint flexed (Fig. 1, 2).

Palpation of the limb revealed increased temperature and tenderness of the crus area. When walking backwards and when the left pelvic limb was lifted, overextension of the hock joint was observed, along with the dimpling and laxity of the common calcaneal tendon (Fig. 3). Subsequently, an ultrasound examination was performed. The ultrasound scanner used was the SonoScape A6V model (SN 16243130) with the L761V linear probe, frequency range 11 Hz–5 MHz. The ultrasound examination of the injured limb revealed an enlargement of the third peroneal muscle outline, with a hypoechoic center, indicating injury to the muscle. Other structures were normal. An ultrasound examination of the other pelvic limb was performed for comparison (Fig. 4, 5).

An injury to the third peroneal muscle of the left pelvic limb was diagnosed, and two months of complete stall rest were recommended. Simultaneously, for one week after the injury, a topical anti-thrombotic, anti-inflammatory and anti-edematous agent containing heparinoid (Hirudoid gel) was applied to the affected area of the pelvic limb. Additionally, a general non-steroidal anti-inflammatory drug, Flunimeg 50 mg/ml (active ingredient: flunixin, 50 mg/ml) was administered intravenously, once daily for three days directly following the injury, dosed 1.1 mg of flunixin per kg bodyweight. One week after the injury, heating ointments were introduced. In follow-up examinations, performed one month and two months after the injury, the lameness was less distinct. However, in an ultrasound examination performed one month after the injury, no significant improvement in the injured muscle was shown.

Such clear improvement was first seen in the ultrasound examination performed 3 months after the injury (Fig. 6, 7). Palpation revealed the affected area of the crus to be only slightly enlarged, and lameness was minimal. The horse no longer dragged the hoof on the ground in gait. In the limb lifting test, a minimal laxity of the common calcaneal tendon was observed, compared to the healthy pelvic limb. In the backward step test, the hock joint was not overextended. Ultrasound imaging only showed a slight enlargement of the third peroneal muscle outline. The image was hyperechoic and similar to the normal image of the other pelvic limb.

DISCUSSION AND CONCLUSION

The rupture of the third peroneal muscle is a fairly rare injury in horses [5, 11]. In differential diagnosis, fractures of the femur and of the crural bones should be considered, as these injuries also produce the limb dragging symptom associated with the third peroneal muscle rupture [2]. In the third peroneal muscle rupture, the symptom results from impaired function of the tarsocrural joint flexors, accompanied by a greater activity of their antagonistic muscles, i.e. the hock joint extensors [6]. The characteristic clinical appearance of simultaneous stifle flexion and hock extension was observed in 23 out of 27 horses. The most common cause of the injury, present in 21 horses, was blunt force trauma with no laceration. The site of rupture was determined by ultrasonography and radiography. In 11 horses, the rupture occurred in the midbody of the tendon, and in the same number it occurred at the site of insertion. The least common location, in just 2 horses, was at the origin of the muscle. In the remaining 3 horses, the rupture site was not identified. In the cases where tendon rupture occurred at the origin, concurrent avulsion fracture of the lateral femoral condyle was observed. This also resulted in the rupture of the long digital extensor muscle. According to the authors, in the studied group of 27 horses, the site of the third peroneal muscle rupture had no influence on the horses' return to exercise [8]. Own research confirmed that the signs of the third peroneal muscle rupture in horses are not as clear at rest as they are in motion, because the horse bears weight properly on the pelvic limb. As in the case described above, other authors also reported that the common calcaneal tendon is visibly loosened and characteristically dimpled [3, 7, 8].

Data from own observations and from literature indicate that horses with diagnosed the third peroneal muscle rupture require, above all, a significant reduction of exercise [1, 8]. The best solution is a minimum of 2–3 months of stall rest. After 2–3 months, gradual reintroduction of exercise is beneficial, as confirmed both by own observations and by other authors [12].

A horse with diagnosed the third peroneal muscle injury requires regular orthopedic follow-ups with ultrasound examinations [9]. In the convalescence period, the follow-up examinations should be performed once a month at a minimum. Ultrasound images show fibrous tissue formations at the site of the injury. Another beneficial practice directly following the injury, i.e. in the first week, approximately, is the use of topical anti-thrombotic, anti-inflammatory and anti-edematous agents, and of general anti-inflammatory drugs, as confirmed by own observation. After approximately one week from the injury, heating ointments proved effective. In the case described here, as well as in most horses with the third peroneal muscle rupture, most scar tissue formed during the 60–90 days following the rupture, which is strictly related to the extent of the injury. One must remember, however, that in approximately 20% of horses with this type of tendon rupture and the extent of the injury prevents full recovery, leading to persistent lameness [8].

REFERENCES

Responses to this article, comments are invited and should be submitted within three months of the publication of the article. If accepted for publication, they will be published in the chapter headed 'Discussions' and hyperlinked to the article.