Anatomy

The fibularis (peroneus) tertius is an entirely tendonous muscle that lies between the long digital extensor and the cranialis tibialis muscles, which cover the craniolateral aspect of the tibia. The fibularis tertius originates from the extensor fossa of the femur. Distally the fibularis tertius divides into branches that enfold the tendon of insertion of the tibialis cranialis and insert on the dorsoproximal aspect of the third metatarsal bone, the calcaneus, and the third and fourth tarsal bones. The tendon is an important part of the reciprocal apparatus of the hindlimb, which coordinates flexion of the stifle and hock.

The fibularis tertius is the most echogenic structure on the craniolateral aspect of the crus and is identified readily by ultrasonography as a well-demarcated hyperechogenic structure relative to the surrounding muscles (Figure 80-1).

Fig. 80-1 Transverse ultrasonographic images of the craniolateral aspect of the midcrus of 7-year-old horse with left hindlimb lameness of 2 months’ duration. The fibularis tertius is the most echogenic structure in the center of the image of the right (R) hindlimb (solid arrow). Compare this with the image of the left (L) hindlimb, in which the fibularis tertius is markedly hypoechoic (open arrow). Note also that the overlying muscle is increased in echogenicity compared with the right hindlimb.

History and Clinical Signs

Rupture of the fibularis tertius invariably is caused by trauma resulting in hyperextension of the limb; for example, a horse trying to jump out of a stable and getting one hindlimb caught on the top of the stable door. This usually results in rupture of the tendon in the middle of the crus but occasionally farther distally. Alternatively, rupture may be caused by a laceration on the dorsal aspect of the tarsus, resulting in transection of the tendon. Occasionally, partial tearing of the tendon occurs, usually at the level of the tarsocrural joint, with prominent swelling. Occasionally the reciprocal apparatus is partially but not totally disrupted. Avulsion injuries of the origin of the tendon rarely occur in young foals. Injury close to the origin is unusual in mature horses, occurring in two of 25 adult horses with fibularis tertius injury.¹

The clinical signs are pathognomonic, because rupture of this tendon allows the hock to extend while the stifle is flexed. When standing at rest, the horse may appear clinically normal, although with acute injury careful palpation may reveal some muscle swelling on the craniolateral aspect of the crus or farther distally. When the horse walks, it should be viewed carefully from behind and from the side. The hock may extend more than usual. The tendons of gastrocnemius and the superficial digital flexor muscles may appear unusually flaccid, and a dimple is seen on the caudal aspect of the crus about one handbreadth proximal to the tuber calcanei. At the trot the horse appears severely lame, with apparent delayed protraction of the limb because of overextension of the hock.

If the limb is picked up and pulled backward, the hock can be extended gradually and “clunks” into complete extension while the stifle remains flexed. A characteristic dimple appears in the contour of the caudal distal aspect of the crus (Figure 80-2). If rupture is only partial, or if lameness is chronic and some repair has taken place, clinical signs may be less severe and the diagnosis less obvious.

Fig. 80-2 A horse with rupture of the fibularis tertius. The hock can be extended while the stifle is flexed. Note also the characteristic dimple in the contour of the caudodistal aspect of the crus. Clinical signs developed after the horse had attempted to jump out of its stable and had got hung up on the door. The horse made a complete recovery.

Presumably, strain of this tendon can occur, resulting in lameness, but I have no experience of this, and to my knowledge this condition has not been documented.

Diagnosis

The diagnosis of rupture of the fibularis tertius is based on the pathognomonic clinical signs. The site of rupture can be identified with ultrasonography (see Figure 80-1). The normally echogenic structure is not clearly identifiable and may be replaced by a region that is hypoechoic relative to the surrounding muscles. In horses with chronic injuries the surrounding muscles may become hypertrophied. Usually no associated radiological abnormalities are apparent in adult horses, although avulsion fracture of the origin has been described in foals.

Treatment

Confinement to box rest for 3 months, followed by a slow resumption of work, usually results in total resolution of clinical signs. Most horses are able to return to full athletic function without recurrence of clinical signs; however, injury may recur if work is resumed prematurely. Healing should be monitored ultrasonographically. Fifteen of 21 horses (71%) retuned to full athletic function, with a mean convalescent period of 41 weeks.¹ Performance horses were less likely to return to their former activity than pleasure horses. Site and cause of injury (laceration or trauma) did not influence the outcome, but the presence of other injuries adversely influenced the prognosis. Delayed recognition of the clinical signs and failure to confine the horse may result in a chronic lesion, which fails to heal satisfactorily. However, compensatory hypertrophy and/or fibrosis of surrounding muscles may permit functional recovery.^1,2