ANATOMY OF THE COMMON CALCANEAL TENDON IN HORSE (EQUUS CABALLUS)

Paweł Szaro¹, Grzegorz Witkowski², Bartłomiej J. Bartyzel³, Bogdan Ciszek^2
1 Department of Descriptive and Clinical Anatomy and 1^st Department of Radiology, Medical University of Warsaw
² Department of Descriptive and Clinical Anatomy, Medical University of Warsaw
³ Department of Morphological Sciences, Warsaw University of Life Sciences - SGGW, Poland

ABSTRACT

A detailed knowledge of the internal structure of the tendon may turn out to be helpful in better diagnosis and possible successful reconstruction of the ruptured common calcaneal tendon.

The aim of the study was to relate individual tendons fibers contributing to the formation of the examined tendon (the common calcanel tendon) with its layers and parts. We examined six separated pelvic limbs of the horse previously fixed in 10% formaldehyde. The layer-by-layer method was used during dissection. We separated individual muscles and then analyzed the internal structure of the full-length common calcaneal tendon between the muscle bellies and the attachment to the tuber calcanei.

The flexor digitorum superficialis constitutes the most superficial layer of the tendon. The fascicle composed of fibers from the soleus and the lateral head of the gastrocnemius forms the deeper layer and it contributes to formation of the medial border of the common calcaneal tendon. Deeper to the latterly mentioned the fascicle of the medial head of the gastrocnemius can be identified. The tarsal tendons of the biceps femoris and the semitendinosus represent the deepest layer of the tendon.

The common calcaneal tendon it is built of the fascicles from individual muscles and that rotation around the long axis of the tendon can be observed.

Key words: tendon, horse, common calcaneal tendon, rotation, fascicles.

INTRODUCTION

The injures of the horse common calcaneal tendon need to be diagnose and treat. The knowledge of the internal anatomy of the tendon is necessary in order to understand its biomechanics and pathology. The function of the muscle is influenced by its architecture [2, 10] including architecture of the tendon.

The common calcaneal tendon is attached to the tuber calcanei. In the horse it is composed of the following tendinous components: gastrocnemius, soleus, biceps femoris, semitendinosus and flexor digitorum superficialis muscles [14]. The distribution of the individual components is unknown. The pattern of the common calcaneal tendon was firstly described in animals. The three separate fascicles of the common calcaneal tendon were observed and distinguished while dissecting the pelvic limb in beavers [9].

The injuries of the Achilles tendon, both direct and indirect, can occur while racing or bucking and the tendon is prone to luxation (especially lateral) of the superficial flexor tendon because it runs over the tuber calcanei and it is partially attached by means of the fascial attachments [17]. Radiographs, ultrasounds or even magnetic resonance [1, 4, 7] (which is challenging in horse) can be used to make an accurate diagnosis enabling better therapy and clinical outcome [8].

The surgical treatment of the injured common calcaneal tendon, which can be performed, should not be only a simple restoration of the continuation of the tendon but also a restoration of the internal structure of the tendon to restore the function of the tendon.

MATERIALS AND METHODS

Six, previously fixed in 10% formaldehyde, pelvic limbs of the horse coming from the collection of the Warsaw Agricultural University were examined. The age of the examined horses ranged from 26 to 29 years (average age was 27,5 years).

The layer-by-layer method was used under dissection. The first step was to dissect the superficial digital tendon from the gastrocnemius. Tendinous attachments to the tuber calcanei were cut and the tendon was placed laterally to visualize the gastrocnemius muscle. The second stage was to dissect the gastrocnemius muscle, and then tendinous fascicles from the medial and the lateral head were indentified and dissected down to the tuber calcanei. Afterwards, the soleus muscle was identified and its tendon was separated from other components of the common calcaneal tendon. At the end, the deep (or ventral) part of the tendon was examined by separating the calcaneal slips of the biceps femoris and the semitendinosus muscles. As a result of this procedure, we studied tendinous fibers, which form fascicles running from the muscle belly down to the attachment on the tuber calcanei. After this procedure calcaneal bursa was indentified and opened. During the dissection, macroscopic and digital magnification photographs were taken.

RESULTS

The common calcaneal tendon in the horse begins caudal to the stifle joint (or articulatio genus) and it is a complex of partially connected tendons, which are attached to the tuber calcanei where the tendon ends (Fig. 3). The flexor digitorum superficialis constitutes the most superficial layer, which is partially attached to the medial and the lateral side of the tuber calcanei (Fig. 1). The next layer composed of the fascicles of the soleus muscle and the lateral head of the gastrocnemius is formed on the medial side (Fig. 2, Fig. 3). Ventral or deeper to the latterly mentioned structure there is the fascicle of the medial head of the gastrocnemius located (Fig. 2). The deepest layer is medially composed of flat tarsal tendons coming from the biceps femoris and laterally by the semitendinosus. (Fig.1 and Fig.2). During the inspection of the fascicles of the common calcaneal tendon rotation of the fascicle can be seen (Fig.3).

Figure 1. The right common calcaneal tendon in the horse. Dorsal view. Directions and centimeter scale are given on the figure. 1-the medial head of the gastrocnemius muscle and its fascicle (1a) , 2-the lateral head of the gastrocnemius muscle and its fascicle (2a), 3-the flexor digitorum superficialis muscle and its tendon (3a), 4-the soleus muscle and its fascicle (4a), 5-the tarsal tendon of the semitendinosus muscle, 6-the tarsal tendon of the biceps femoris muscle, 7-tuber calcanei, * and ** the attachments of the flexor digitorum superficialis muscle to the tuber calcanei.

Figure 2. The right calcaneal tendon of the horse - dorsal oblique view, lateral oblique view, cranial oblique view. Directions and centimeter scale are given on the figure. The tendon of the flexor digitorum superficialis was cut off. Dashed lines mark the borders between the fascicles. 1-the fascicle of the lateral head of the gastrocnemius muscle, 2,3-the fascicle of the medial head of the gastrocnemius muscle (2-medial band, 3-lateral band), 4-the fascicle of the soleus muscle, 5-the tarsal tendon of the semitendinosus muscle, 6-tuber calcanei, 7- the lateral head of the gastrocnemius (muscle belly), 8-the soleus muscle (muscle belly), 9-the medial head of the gastrocnemius muscle, 10 - the tarsal tendon of the biceps femoris muscle (partially covered by the tarsal tendon of the semimembranosus muscle).

Figure 3. The left common calcaneal tendon in the horse. Posterior view. The flexor digitorum superficialis cut off from the tuber calcanei and reflexted. Directions and centimeter scale are given on the figure. 1-the fascicle of the medial head of the gastrocnemius muscle, 2-the fascicle of the lateral head of the gastrocnemius muscle; 3-the fascicle of the soleus muscle, 4- the flexor digitorum superficialis tendon, 5-the tuber calcanei, 6- continuation of the flexor digitorum superficialis tendon

The fascicles are twisted around the long axis of the tendon (Fig. 3) in the following order: the fascicles which start on the cranial and the medial side run in the caudal, lateral and dorsal (superficial) direction and those starting on the cranial and the lateral side run towards the caudal, ventral and medial direction (Fig. 4).

Figure 4. Horizontal cross section through the common calcaneal tendon in horse just above the tuber calcanei. Directions are given on the figure.1-the fascicle of the lateral head of the gastrocnemius muscle, 2-the fascicle of the medial head of the gastrocnemius muscle; 3-the fascicle of the soleus muscle, 4- the tarsal tendon of biceps femoris muscle, 5- the tarsal tendon of semimembranosus muscle, 6- the flexor digitorum superficialis tendon

DISCUSSION

Not all animals have the common calcaneal tendon. A gorilla is said not to have the common calcaneal tendon because the tendons of the gastrocnemius, the soleus and the plantaris muscles are entirely separated from each other [20]. The criterion for the belonging of a tendon to the common calcaneal tendon is an insertion to the tuber calcanei and its connection to other tendons above the common insertion (Fig. 4). In the horse all tendons are connected, but they can be separated using an anatomical dissection. The flexor digitorum superficialis is separated from the gastrocnemius tendon by the bursa (Fig. 1, Fig. 2, Fig. 3). A similar arrangement of the human calcaneal tendon or the Achilles tendon can be seen both during dissection [18] and on the radiological imaging obtained from magnetic resonance [15] and ultrasound examination [3]. In the human and in some animals (e.g. horses) individual fascicles of the Achilles tendon can be ruptured separately, which is called a partial tear [6, 18, 19]. The inflammation of bursae can affect the individual fascicles such as gastrocnemius fascicles, which causes lameness exacerbated by hind limb flexion [5, 11, 16, 17, 20]. The correct anatomical identification of tendons and burase in the region of the common calcanel tendon is essential for better clinical outcome [6, 7, 12, 13]. The rapture of the tendon of the flexor digitorum superficialis in the horse is an equivalent of a partial rapture of the Achilles tendon in the human.

Comparing the common calcaneal tendons of the horse and the Achilles tendon in the human, more distinct rotation of the fascicles can be observed in the horse. One of the reasons explaining this fact can be that a horse is an odd-toed ungulate mammal, whereas a human is a plantigrade animal. The fixation of the autopodium as well as the structure of the stay apparatus in digitigrade require a stronger construction of the tendon, which involves not only a greater rotation of the fibers but also an accessory element such as tarsal tendons. The rotation pattern of the fascicles in the common calcaneal tendon is in accordance with the general pattern of the organization of the common calcaneal tendon in the beaver given by Parsons [9].

CONCLUSIONS

The common calcaneal tendon of the horse is not a homogenous structure but it is composed of twisted fascicles (Fig. 4), which continue from the muscle belly down to the tuber calcanei. The fascicles can be dissected and separated.  The fascicles in the common calcaneal tendon in the horse, in comparison with the human or beaver tendon, are more wrapped around their long axis and more complex in their structure. The complexity of the common calcaneal tendon in the horse is possibly related to the type of locomotion.

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Acceped for print: 21.12.2011

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Paweł Szaro
Department of Descriptive and Clinical Anatomy and
1^st Department of Radiology, Medical University of Warsaw
ul. Chałubińskiego 5, 02-004 Warszawa
Telefon: +48 694 836 501
Fax: +48 22 629 52 83
email: pszaro@wp.pl

Grzegorz Witkowski
Department of Descriptive and Clinical Anatomy, Medical University of Warsaw
ul. Chałubińskiego 5, 02-004 Warszawa, Polska

Bartłomiej J. Bartyzel
Department of Morphological Sciences,
Warsaw University of Life Sciences - SGGW, Poland
Nowoursynowska 159, 02–776 Warsaw, Poland
Fax: +48 22 59-362-18
email: bartlomiej_bartyzel@sggw.pl

Bogdan Ciszek
Department of Descriptive and Clinical Anatomy, Medical University of Warsaw
ul. Chałubińskiego 5, 02-004 Warszawa

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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.

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