Lab instructions:

Dissection teams are to dissect, in parallel, an equine and ruminant thoracic limb, as provided on the day of the lab. Supplement dissection learning with liberal study and review of the limb models available in lab, wet and dry.

Carpus: overview

Recall from osteology and arthrology that the carpus is a construct of 3 major joints and numerous bones (dependent on species).  The 3 joints are the antebrachiocarpal (often referred to as the ‘radiocarpal’ joint in equine), the middle carpal (often the ‘intercarpal’ in equine), and the carpometacarpal joint.  The comparative bones of the carpus for our domestic species can be reviewed here.

Flexion of the carpus occurs at the antebrachiocarpal joint and the middle carpal joint. The synovial cavity of the antebrachiocarpal joint does not communicate with other joints, whereas the middle and carpometacarpal joints always communicate.

Powerful lateral and medial collateral ligaments of the carpus attach on each side to stabilize this complex joint that acts mostly in a dorsopalmar (axial) plane.

Clinical relevance:

Arthrocentesis of the antebrachiocarpal and middle carpal joints is performed dorsally with the carpus held in flexion. The needle is inserted either side of the extensor tendons. Palmarolateral approaches are also possible with the horse bearing weight on the limb. “Carpal hygroma” (aka Capped Knee) is an acquired subcutaneous fluid accumulation over the dorsal carpus that does not typically communicate with the joint spaces (but can!). The term carpal hygroma is loosely used to describe carpal swelling of many causes. The clinical work up must distinguish subcutaneous bursa, from tendon sheath, from joint sac (this is a similar scenario to the “capped hock” of the pelvic limb, where we must confirm the location of the swelling is in a subcutaneous bursa, and not in a tendon sheath or a joint).

Carpal canal

The carpal canal is the tunnel that accommodates the digital flexor tendons and certain vascular and nervous structures (species specific) as they traverse the carpal complex and continue on their way to the metacarpus. The canal is bound dorsally by the relatively thick palmar carpal ligament, which converts the irregular palmar surface of the carpal joint complex into a smooth track for the digital flexor tendons. The canal is bound laterally by the accessory carpal bone and palmaromedially by the flexor retinaculum of the carpus. The flexor retinaculum in ruminants has a superficial and deep part.  Recall that the superficial digital flexor m. in ruminants has superficial and deep bellies, their tendons combining in the distal metacarpal region.  The deep part of the superficial digital flexor m. passes through the carpal canal in ruminants. The superficial part of the superficial digital flexor is sandwiched between the superficial and deep parts of the flexor retinaculum i.e. it lies outside of the canal.  A carpal sheath envelops the flexor tendons.

Metacarpus

Metacarpal bone ligaments – horse

Metacarpals 2 and 4 (medial and lateral splint bones) are attached proximally (i.e. at their base) by a set of interosseous ligaments to metacarpal 3 (cannon bone). Sometimes these ligaments, particularly the dorsal metacarpal ligament, are over-strained and torn, resulting in swelling and pain. Periosteal reaction to the trauma results in a bony ‘spot weld’ between the two metacarpal bones connected by the ligament. This bony callus is often visible. The injury and visible swelling (acute or chronic) is referred to as a “splint”.

Dorsal metacarpal region

From the carpus, moving distally, on the dorsal surface, the tendons of the digital extensors track into the digits. Over the carpus these tendons, along with others, are wrapped in lubricating synovial tendon sheaths.

Palmar metacarpal region

From the carpus, moving distally, on the palmar surface, the tendons of the digital flexors track into the digits. In addition, the accessory ligament of the DDF and the suspensory ligament are important features of this region, with species variation. We will consider these structures next.

3rd Interosseus m. aka Suspensory Ligament

Horse:

The 3rd interosseus m., more frequently referred to as the suspensory ligament, is primarily fibrous in the horse and arises from the palmar distal carpus and proximal metacarpus. In the distal third of the metacarpus the ligament divides into a strong lateral and medial branch. The branches attach to the abaxial surface of the respective proximal sesamoid bone. Lateral and medial extensor branches then continue the suspensory ligament from the sesamoids, passing obliquely to the dorsal pastern to join the common digital extensor tendon. These extensor branches are easily palpable in the live horse, along with the rest of the suspensory ligament.

Ruminants:

The ruminant has two interossei mm. (3 and 4), which are fused along their axial borders. In young calves the interossei are more muscular and they become fibrous with age. In the mid-metacarpus, the interossei give off fibrous branches to the SDF tendons, which assist in the formation of the sleeves for the DDF tendons to pass through. Above the fetlock, each interosseous muscle divides into axial and abaxial tendons to insert on their respective proximal sesamoid bones for each digit. Extensor branches pass dorsally to insert into the extensor tendons of each digit (similar to horse arrangement).

At this point in the dissection there is an equine section and a ruminant section – follow the relevant section for the specimen.

Click here to jump to the ruminant section. 

Fetlock and digit – horse

Ligaments of ergot

We attempted to palpate these structures while the skin was still intact at the start of the dissection.  Shiny, white, fine ligaments, subcutaneous in location, pass obliquely from the fibrous tissue deep to the ergot across to their distal attachments. They pass diagonally across the palmar digital nerves and vessels. The ligaments are about the same size as the nerves, and can be confused with them. The ligaments are a good surgical landmark for making the skin incision to perform a palmar digital neurectomy.

Fetlock joint

Hence swelling of the fetlock joint should be dorsal in location relatively to the suspensory ligament. The digital flexor tendon sheath (see later) and any swelling related to it lies palmar to the suspensory ligament.

Digital flexor tendon sheath and digital ligaments

A synovial sheath, the digital flexor tendon sheath, envelops the DDF and SDF tendons in the distal fourth of the metacarpus and extends distally to the distal pastern region. Synovial effusion of the digital flexor tendon sheath may be referred to as ‘wind puffs’ and this can be observed after a horse has vigorously exercised, secondary to a mild digital tenosynovitis, or effusion may be present on a more chronic basis, once it develops.

The tendons are held in position on the palmar fetlock by the palmar annular ligament of the fetlock. Much like a retinaculum, the palmar annular ligament retains the tendons in the channel created by the proximal sesamoid bones.

Distal to the palmar annular ligament of the fetlock, the flexor tendons are retained by the proximal and distal digital annular ligaments against the palmar pastern. The gap between the proximal and distal digital annular ligaments is the lowest available access site to the digital flexor tendon sheath of the horse on the palmar/plantar distal pastern [palpation of this area is routine and if detect a sponginess it may mean increased synovial fluid is present in the digital sheath].

Back at the fetlock, the SDFT forms a complete sleeve around the DDFT. This is called the proximal manica flexoria. This structure can tear and be a cause of lameness in horses.

In the proximal pastern the SDFT must assume an abaxial position to the DDFT so it can insert on the proximal (Pp) and middle (Pm) phalanges (in the horse, and only the middle phalanx in the ox) while the DDFT continues on its axial path to the distal phalanx (Pd).

Recall the suspensory ligament branches attach to the abaxial surface of the proximal sesamoid bones, and then extensor branches pass dorsally to insert into the common digital extensor tendon. Distally the proximal sesamoid bones are anchored to Pp and Pm by a quartet of (distal) sesamoidean ligaments (straight/superficial, oblique/middle, cruciate/deep, and short sesamoidean ligaments).

At this point there is no more dissection to be done and content is learned on prosections.

The navicular bone is anchored distally by the distal navicular ligament (commonly referred to as the impar ligament) that passes from the distal border of the navicular bone to the flexor surface of the Pd. Medial and lateral collateral (suspensory) ligaments of the navicular bone connect from its proximal border and extremities to the Pp. Located between the DDFT and the flexor surface of the navicular bone is the navicular (podotrochlear) bursa.

Suspensory apparatus – horse

These structures are under significant tension, even when the horse is standing still. This tension is greatly increased during high-speed exercise, which explains why these structures are vulnerable to damage during racing or other types of athletic activity. The straight sesamoidean ligament also crosses the PIPJ, which it helps stabilize during weight-bearing.

In addition to the suspensory apparatus, both the DDF and SDF help support the fetlock. Because of their accessory (check) ligaments and the large amount of fibrous tissue in the muscle bellies, this support is also partly passive. Passive support is particularly important at rest and, after the muscles have fatigued, during exercise.  For optional review, see the TL passive stay apparatus summary in the Appendix to tie all this anatomy together.

Fetlock and digit – artiodactyls

Digital flexor tendons and digit ligaments

The tendons are held in position on the palmar fetlock by the palmar annular ligament of the fetlock (there is one for each digit). Much like a retinaculum, the palmar annular ligament retains the tendons in the channel created by the proximal sesamoid bones.

Digital annular ligaments and manica flexorii are also present in the ruminant, a set for each digit. These need not be identified.

Two ligaments are especially clinically important in the ruminant, the proximal and distal interdigital ligaments, which should be observed on a model and may be dissected too.

The proximal interdigital ligament is a short, thick ligament that connects the axial proximal halves of the proximal phalanges.  It is present in the ox and goat, but not the sheep.  The distal interdigital ligament courses between the middle phalanx of one digit and the distal sesamoid bone of the other digit, forming a cruciate structure, in the ox and goat. In the sheep the distal interdigital ligament courses transversely.  The distal interdigital ligaments provides additional support by preventing spreading of the distal digits.

Lab 7 Appendix

FYI only – Passive stay apparatus of equine thoracic limb

In contrast to the pelvic limb, in which everything centers around locking the patella of one limb and involves shifting the support alternately between limbs, the stay apparatus of the thoracic limbs functions concurrently. When standing squarely, horses support more weight on their thoracic limbs, largely because the head and neck are heavy, and their forward overhang moves the center of gravity of the horse to a level about 38 cm (~16 inches) caudal to the tuber olecrani.

Body weight is transferred to the forelimb primarily at the serrated face of the scapula by way of the attachment of the serratus ventralis. The thoracic part of the serratus ventralis has a thick epimysium, and the muscle itself contains considerable connective tissue, making the transfer largely passive. This point of weight transfer is caudal to the shoulder joint. Flexion (collapse) of the shoulder is prevented by the fibrous band of the biceps, supplemented by a fibrocartilaginous mold over the intermediate tuberosity of the intertubercular groove. Continuation of this fibrous band through the lacertus fibrosus and the extensor carpi radialis tendon pulls a tight, stabilizing band across the dorsum of the carpus, which helps the horse resist buckling forward into flexion.

The digital flexor tendons, with their accessory ligaments, are pulled tightly across the palmar surface of the carpus and provide additional resistance to flexion of this joint complex. As these tendons continue around the fetlock, they help form a fibrous sling for this joint to fall back into. Importantly, the fibro-osseous sling provided by the suspensory apparatus (suspensory ligament, proximal sesamoid bones and the [distal] sesamoidean ligaments) provides great support for the fetlock. The weight-induced traction provided by the extensor branches of the suspensory ligament connecting to the CDE, which inserts on the extensor process of Pd, offsets the flexion pull on the DIPJ produced by the DDF tension.

The only joint subject to question is the elbow. Although the triceps brachii muscle is necessary to set the elbow in its weight- bearing position, it does not appear to do much once everything is set (palpate the “soft” triceps brachii m. in the standing horse). Initial flexion of the elbow joint in the weight-supporting limb is resisted by the following: (1) the digital and carpal flexor muscles, which contain much fibrous tissue, originate primarily from the medial epicondyle of the humerus and thus resist flexion by pulling down on the lever provided by the epicondyle; and (2) the collateral ligaments of the elbow, which pass caudal to the center of rotation of the joint. Thus, initial flexion of the joint is resisted because, at the onset of flexion, these ligaments must be stretched a little as their proximal attachments rotate up and over the center of rotation.