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Clinical Commentary Peroneus tertius anatomy and lesions J-M. DENOIX CIRALE - Ecole Nationale Vétérinaire de Maisons-Alfort,14430 Goustranville, France. Keywords: horse; peroneus tertius; reciprocal apparatus

Comparative anatomy and congenital In man the PT is a small muscle inserting on the distal third abnormalities of the fibula and attaching distally on the fifth metatarsal bone; it is very variable and sometimes missing (Williams et al. The peroneus tertius (PT) belongs to the cranial muscles of the 1989). Usually, there is no PT in carnivores, such as dog and crus. In ungulates (equine, ruminant and porcine species) the cat, or in rabbits (Barone 1980). PT and long digital extensor muscle have a common proximal The paper by Caldwell et al. (2007) describes a very rare tendon attaching in the extensor fossa of the distal femur (Fig 1). The PT distal tendons insert on the proximal metatarsus and distal tarsus. In ruminants and pigs the muscle a) body is strong and is the most superficial structure of the cranial crural muscles (Barone 1980; Dyce et al. 2002). The active contraction of the muscle body induces flexion of the tarsus and especially of the tarsocrural joint. In equine species, there is a complete atrophy of the muscle body and the PT intermediate part is only made of a flat fibrous structure, approximately 20 mm wide and 7 mm thick.

b)

Fig 2: Functional anatomy of the peroneus tertius (PT) at high Fig 1: Dissected specimen showing the anatomy of the speed. a) Beginning of the swing phase: flexion of peroneus tertius located between the long digital extensor (a) the stifle and extension of the hock. The PT is elongated and and cranial tibial (b) muscles. 1 = proximal tendon; 2 = body; the common calcanean tendon is relaxed (arrow-head). 3 = crural extensor retinaculum; 4 = sleeve for the cranial tibial b) Intermediate part of the swing phase: the flexion angles of tendon; 5 = dorsal branch; 6 = lateral branch and tarsal extensor the stifle and hock are similar, illustrating the high velocity of retinaculum; 7 = metatarsal extensor retinaculum. the hock flexion. EQUINE VETERINARY EDUCATION / AE / september 2007 417

case of multiple congenital abnormalities. The most common apparatus achieving solidarisation of the femorotibial and teratological malformation involving the equine limbs is the tarsocrural joints during flexion (Barone 1980; Dyce et al. presence of a complete 2nd or 4th metacarpal bone with or 2002). On the horse standing or during slow gaits, flexion of without an atrophied digit (Lesbre 1927). In the case the stifle is accompanied with a similar flexion angle of the presented in this paper, the distal femur did not present any extensor fossa; the long digital extensor muscle was not a) b) inserted on the femur, but on a deep fascia and proximal tibia. These anatomical features represent the normal human disposition. In a number of congenital malformations the abnormality observed in one species reproduces the normal anatomy of others (Lesbre 1927).

Functional anatomy of the peroneus tertius

Because of its insertions on the distal femur and proximal metatarsus the normal PT is an essential part of the reciprocal

Fig 5: Avulsion fracture from the femoral extensor fossa (arrow-heads) of the attachment of the common proximal tendon of the peroneus tertius (PT) and long digital extensor (LDE) muscle in a 9-year-old Selle-Français gelding. a) Lateromedial radiographic projection of the right stifle showing avulsion fragments in the extensor fossa and sulcus (arrow-heads) and the lytic appearence of the extensor fossa of the femur (arrow). b) Longitudinal ultrasound scan of the proximal PT and LDE tendon. In order to fit with the orientation of the radiographic image, proximal is to the top and distal is to the bottom. The tendon is thickened and presents a heterogenous echogenicity compatible with a chronic lesion; the extensor fossa is very irregular and presents many enthesophytes. 1 = common tendon of the peroneus tertius and long digital extensor muscle; 2 = extensor fossa of the Fig 3: Transverse ultrasound scans at the craniolateral aspect of femur; 3 = lateral tibial condyle; 4 = lateral meniscus. the crus of the left and right hindlimbs of a mare having presented a rupture of the right peroneus tertius (PT) 3 months a) b) ago. Left image: left crus with a normal PT. Right image: right crus with severe enlargement and heterogenous echogenicity of the PT (arrow-heads). 1 = peroneus tertius; 2 = long digital extensor muscle; 3 = cranial tibial muscle; 4 = tibia.

Fig 6: a) Lateromedial radiographic projection of the left hock showing severe lysis (arrow-heads) and enthesophyte production (arrow) of the dorsal aspect of the proximal third metatarsal bone. A small fracture line can be seen proximally. b) Longitudinal ultrasound scan of the PT inserting on the third metatarsal bone. In order to fit with the orientation of the Fig 4: Lateromedial radiographic projection of the left stifle in a radiographic image, proximal is to the top and distal is to the 6-month-old filly. Avulsion fracture from the femoral extensor bottom. The tendon is thickened and presents hypoechogenic fossa (arrow-heads) of the attachment of the common zones. The dorsal aspect of the third metatarsal bone presents proximal tendon of the peroneus tertius and long digital a big enthesophyte (arrow). 1 = dorsal branch of the PT; extensor muscle. 2 = third metatarsal bone; 3 = distal tarsal bones; 4 = talus. 418 EQUINE VETERINARY EDUCATION / AE / september 2007

hock. Coupling of the stifle and hock is different at high (Fig 5). Although bone remodelling at the distal insertion of speeds. Because of the velocity of the cranial retraction (first the PT over the proximal metatarsus is occasionally found, part of the swing phase) of the hindlimb and inertia of the severe distal metatarsal enthesopathy of this structure is an distal limb (metatarsus and digit), flexion of the stifle precedes uncommon condition (Fig 6). In some cases, an uptake can be flexion of the hock (Fig 2a). Therefore, at the beginning of the seen over the fourth tarsal bone on nuclear scintigrams; this swing phase at high speeds, the PT is stretched and stores finding is compatible with a tarsal enthesopathy of the PT. passive energy; its elasticity improves the velocity of tarsal flexion during the following (intermediate) part of the swing References phase (Fig 2b). Inducing a reduction of weight of the limb and improving the velocity of tarsal flexion during the swing phase, Barone, R. (1980) Anatomie Comparée des Mammifères Domestiques, the equine tendinous PT is a wonderful illustration of the Tome 2: Arthrologie et Myologie. Vigot, Paris. adaptation of horses to move at high speeds. Caldwell, F., Tudor, R. and Neuwirth, L. (2007) Agenesis of the peroneus tertius in a foal. Equine vet. Educ. 19, 413-415. Acquired conditions Denoix, J-M. (1998) Joints and miscellaneous tendons and ligaments. In: Equine Diagnostic Ultrasonography. Eds: N.W. Rantanen and There are several acquired conditions involving the PT in young A.O. McKinnon, Williams & Wilkins, Baltimore. pp 475-514. and mature horses. The most common is the rupture of Dyce, K.M., Sack, W.O. and Wensing, C.J.G. (2002) Textbook of the intermediate part inducing pathognomonic clinical Veterinary Anatomy, 3rd edn., W.B. Saunders, Philadelphia. manifestations, e.g. defect of hock flexion while the stifle pp 509-521. flexes and concomitant relaxation of the common calcanean Dyson, S.J. (2003) Other soft tissue injuries. In: Diagnosis and tendon. The diagnosis is based on these clinical signs and Management of Lameness in the Horse. Eds: M.W. Ross and S.J. ultrasonographic findings (Fig 3) (Denoix 1998; Dyson 2003). Dyson, W.B. Saunders, St. Louis. pp 708-711. Pulling the leg caudally to demonstrate the abnormal hock Lesbre, F.X. (1927) Traité de Tératologie de l’Homme et des Animaux hyperextension is not appropriate as this can worsen the lesion Domestiques, Vigot, Paris. or deteriorate the healing process. Wamsley, J.P. (2003) The stifle. In: Diagnosis and Management of Avulsion fractures of the origin of the PT in the extensor Lameness in the Horse, Eds: M.W. Ross and S.J. Dyson, W.B. fossa is reported in foals (Wamsley 2003) (Fig 4). Severe Saunders, St. Louis. pp 455-470. proximal enthesopathy of the PT can be seen in adult horses Williams, P.L., Warwick, R., Dyson, M. and Bannister, L.H. (1989) In: following forceful hyperextension of the distal hindlimb Gray’s Anatomy, 37th edn., Churchill Livingstone, Edinburgh. p 647.