FORELIMB

Chapter 4 THE FORELIMB

As with the previous two chapters, any part of the limb may be involved in surface trauma with bruising and hemorrhage requiring cleaning, disinfection and debridement. Luckily, most of the important structures are on the medial aspect of the limbs and are therefore protected, in most instances, from such damage.

There is no bony connection of the shoulder to the main vertebral skeleton so the shoulder can be over-abducted from the body leading to trauma to the brachial plexus and damage to axillary vessels. This can occur when inexperienced people lift dogs by the front legs without supporting the weight of the body. In a complete brachial plexus paralysis there is loss of skin sensation over the lateral shoulder region. In radial paralysis, there is loss of skin sensation only over the cranial antebrachium and dorsum of the manus. There is obviously also loss of motor supply to the extensors of the elbow, carpus and digits. Radial paralysis is indicated by dragging of the limb with the dorsum of the manus in contact with the ground. This results in severe dysfunction, as the dog is unable to stabilize the elbow joint and the lower limb. As well as being damaged in the brachial plexus, the radial nerve can also be damaged at the point where it emerges from its passage (in a spiral) down the brachial groove. It can be damaged here in a fracture or when the repair is made. The supraspinatus and infraspinatus muscles may atrophy when there is damage to the suprascapular nerve and increased prominence of the scapular spine is then seen. In working dogs there are conditions involving infraspinatus contracture (with an associated atrophy) possibly secondary to trauma. This results in dogs swinging their legs in a circumferential arc and requires surgical correction by tenotomy. Many bony landmarks are palpable in the forelimb, notably the dorsal border of the scapula, the greater tubercle of the humerus, deltoid tuberosity, olecranon, medial surface of radius and accessory bone of the carpus, spine of scapula, and acromion.

The scapula may be subject to trauma with fracture of the neck or acromion process. The greater tubercle of the humerus and the acromion are palpable and are used to assess the normal position of the shoulder joint. The shoulder joint is often a site of osteochondritis dissecans (OCD), inflammatory changes imposed on the underlying condition of osteochondrosis, which is a degenerative condition of cartilage. Fragments of cartilage may separate and cause pain within the joint. They sometimes wedge under the tendon of the subscapular muscle or within the synovial sheath of the biceps. These flaps often originate from the caudal part of the head of the humerus. They may also sit in a large caudal pouch where often they do not cause any pain. Attached pieces of cartilage may also cause lameness. When operating for shoulder OCD, the caudolateral approach tends to be favored. In this a curved incision is made from about the mid-point of the scapula, over the acromion and extending about one-third of the way down the humerus. The acromial and spinal heads of the deltoid are then retracted cranially and caudally respectively, followed by cranial retraction of the teres minor and internal rotation of the limb.

The body of the humerus is ‘S’-shaped and for this reason repair of fractures is difficult as intramedullary pins are difficult to insert. You should always use as big a pin as possible to fill the shaft of the humerus but in this case a smaller pin has to be used so that it will traverse down the curved medullary cavity. Fractures of the body of the humerus tend to be in middle third or distal in position with approximately one-third involving the distal articular surface. This is due to the massive thickness of the bone proximally and the relative thinning distally, i.e. under pressure the fracture is at the weaker points. They also tend to be spiral fractures. Humeral fractures tend to present with a dropped elbow and with the paw resting on its dorsal surface. In young animals the fractures tend to be Salter Harris type but in the older dog the ‘Y’ fracture of the distal humerus is more common. No structures pass through the supracondylar foramen (covered by connective tissue), whereas in the cat, the brachial artery and median nerve pass through it. The groove immediately medial to the greater tubercle has a great use in that it is used to introduce an intramedullary pin which can be driven distally to effect immobilization of a fracture of the body of the humerus. In Springer spaniels lateral or medial condylar fractures also occur due to failure of ossification in this site.

Essentially there are three sites for intramedullary pinning of the humerus. Proximal fractures are repaired by separating the caudal edge of the M. cleidobrachialis from the cranial edge of the M. triceps (lateral head). A mid-shaft fracture is approached by dividing the caudal edge of the M. cleidobrachialis from the cranial edge of the brachialis muscle. A distal fracture is approached by dissecting between the caudal edge of the M. brachialis and the cranial edge of the M. triceps. This is the point at which the radial nerve is exposed to trauma. Intramedullary pins are rarely used in isolation but are frequently combined with an external fixator or coerciage wires. The pin is generally inserted in a normograde manner starting laterally to the ridge of the greater tuberosity and anchoring it in the medial condyle.

The elbow joint can be a site of several important clinical conditions. The anconeal process of the ulna fits into the olecranon fossa of the humerus and therefore the joint can only dislocate when the joint is in full flexion or when fracture of the humeral epicondyle occurs (repaired by surgery and use of screws). This usually occurs in traumatic accidents. It can also be put back in place with full flexion of the joint. The anconeal process has occasionally a separate centre of ossification from that of the olecranon. In this case, the anconeal process detaches following trauma or fails to unite and is pulled off by M. triceps and may have to be removed surgically. Caudolateral arthrotomy of the elbow is used to remove an anconeal process that is disunited. A curved incision is made over the distal third of the humerus to the proximal third of the radius. The pronator teres and the flexor carpi radialis are then separated to expose the joint capsule.

Medial arthrotomy of the elbow joint is used to treat osteochondritis of the elbow joint but not the underlying osteochondrosis cause, and for removal of a fragmented coronoid process which can also be screwed back into place. This primarily affects large breeds of dogs such as St Bernards. An incision is made over the medial humeral epicondyle.

The styloid process of the ulna fractures easily in the dog. It is important clinically to recognize the sesamoid bones as normal features and not bone chips, and to remember that there is a dorsal sesamoid at the level of the metacarpophalangeal joint. The accessory carpal bone has two centres of ossification and it may therefore be vulnerable to dislocation. It is important to repair these types of fractures as they lead to instability of the carpus which will not resolve without repair. Fracture of the accessory carpal bone is often seen in racing greyhounds. Hyperextension injuries of the carpus are relatively common in medium and large breeds of dog and require carpal arthrodesis.

It is possible for rupture of all the forelimb muscle bellies, especially in working dogs or racing dogs like greyhounds. These heal with a lot of scar tissue. Dogs may also rupture the tendinous support structures. For example, tearing of the flexor retinaculum holding the deep digital flexor in place in the carpal canal. Clinically, remember that all extensors of the carpus and digits arise from the lateral epicondyle of the humerus and all flexors from the medial epicondyle of the humerus.

The other major site of damage in the forelimb may be at the inter-tubercular site in the humerus where bursitis may occur. The tendon sheath of the M. biceps slides through this groove and is held in place by the transverse ligament of the humerus.

The cephalic vein has considerable importance in clinical practice for collection of blood samples, administration of supportive therapy (intravenous drips) and for intravenous anesthesia and for euthanasia. The cephalic vein is also in close proximity to the medial and lateral branches of the superficial ramus of the radial nerve.

The superficial lymph nodes associated with the front leg are the superficial cervical which takes most of the drainage from the foot, carpus and lateral humeral and shoulder regions and the axillary lymph node which drains the axillary region and part of the lateral chest wall. Neither is normally palpable and the former is by far the most important.

Fig. 4.1 Surface features of the neck and forelimb: left lateral view. The palpable bony prominences of the neck and forelimb are shown. In addition the trachea is clearly palpable on the underside of the neck. ‘Soft’ structures identifiable include the mandibular lymph nodes at the cranial end of the neck and the superficial cervical nodes in front of the scapula at the caudal end. A pulse may be detected in the common carotid artery in the neck, but also from the brachial/median artery in the cubital fossa on the flexor surface of the elbow joint. The external jugular vein may be raised by pressure in the jugular fossa: the cephalic vein may also be raised in the antebrachium by pressure immediately distal to the elbow joint. Fig. 4.1A at the foot of the page shows the main topographical regions that are recognized for descriptive purposes. The boundaries of regions in the neck are a trifle arbitrary because of the absence of discernible underlying features. The four main topographical regions of the forelimb are based on internal osteological components; the subsidiary topographical regions are related to the underlying joints between segments.

Fig. 4.1A Topographical regions of the neck and forelimb: left lateral view. 1 Dorsal Neck Region. 2 Lateral Neck Region. 3 Parotid Region. 4 Pharyngeal Region. 5 Laryngeal Region. 6 Tracheal Region. 7 Scapular Region. 8 Shoulder Joint Region. 9 Axillary Region. 10 Brachial Region. 11 Tricipital Region. 12 Cubital Region. 13 Olecranon Region. 14 Antebrachial Region. 15 Carpal Region. 16 Metacarpal Region. 17 Phalangeal (Digital) Region.

Fig. 4.2 Skeleton of the neck and forelimb: left lateral view. The palpable bony features shown in the surface view are colored green in the accompanying drawing. Additional areas of bone are deeply palpable through overlying musculature. These include the cranial border of the scapula, the humeral shaft proximal to the elbow joint, the distal ends of both radius and ulna approaching the carpus, and much of the carpal, metacarpal and phalangeal bones in the forepaw. Adjacent bones of the vertebral column and ribcage are included in the picture to give the approximate position of the forelimb skeleton in relation to the trunk in the normal standing posture. The position of the olecranon process (‘point of the elbow’) in relation to the chest is important since it may be used as a reference point for determining the position of thoracic organs. It should be noted that any features of the third to seventh cervical vertebrae are palpable only with difficulty even in thin dogs.

Fig. 4.3 Surface features of the forelimb: cranial view. The major palpable bony features already noticed in the lateral view of the limb are shown again as reference points and are indicated in the drawing. In addition bony prominences palpable on the medial aspect of the limb are shown. The triangular jugular fossa is a visible depression at the base of the neck lateral to the sternal manubrium. Careful palpation within the fossa may identify the first rib where it borders the thoracic inlet. A second triangular depression, the cubital fossa, is visible and palpable cranial to the elbow joint and within it the median nerve and pulsations in the brachial/median artery may be felt.

Fig. 4.3A Topographical regions of the forelimb: cranial view. 1 Scapular Region. 2 Shoulder Joint Region. 3 Brachial Region. 4 Cubital Region. 5 Antebrachial Region. 6 Carpal Region. 7 Metacarpal Region. 8 Phalangeal (Digital) Region. 9 Axillary Region. 10 Ventral Neck Region. 11 Lateral Neck Region. 12 Dorsal Neck Region.

Fig. 4.4 Skeleton of the forelimb: cranial view. The major palpable bony features shown in the surface view are colored in this picture and are illustrated in the accompanying drawing. The complete absence of bony continuity between forelimb and trunk skeleton is now apparent. A clavicle, which in many mammals unites the lower end of the scapula with the manubrial region of the sternum, is only represented in a dog by a tendinous intersection within the brachiocephalic muscle. The entirely muscular ‘joint’ between the forelimb and trunk is based upon the deeply positioned and therefore impalpable ventral serrate muscle passing from the upper end of the scapula down onto the ribs and cervical vertebrae. This main weight-bearing muscle is assisted by other muscles such as the pectorals, brachiocephalic, latissimus dorsi, trapezius, and rhomboid, a number of which are palpable around the shoulder region and on the chest.

Fig. 4.5 Radiograph of the shoulder joint: left lateral view. The major osteological features of the shoulder joint are shown in this radiograph. The relatively shallow glenoid fossa in the scapula contrasts with the considerably greater articular surface of the humeral head. However, the depth of the glenoid cavity is increased to some extent by a surrounding lip of fibrocartilage (radiolucent), although the overall articular area is still only about one half the area of the head.

Fig. 4.6 Radiographs of the elbow joint: craniocaudal and lateral views. The major osteological features of the elbow joint are shown. The trochlea of the humeral condyle fits closely into the deep trochlear notch of the ulna providing its very stable hinge-like action. When extended (b) the anconeal process of the olecranon is located within the olecranon fossa stabilizing the joint; when flexed (a) the process is withdrawn and stability is reduced. When a paw is raised and the elbow flexed some measure of rotation occurs at the proximal radioulnar joint, apparent as a limited amount of pronation and supination of the paw. In the craniocaudal view (c) the two main subdivisions of the elbow joint are apparent – the humeroradial joint, a weight-transferring component between forearm and brachium, and the humeroulnar joint, more specifically concerned with movement and stability.

Fig. 4.7 Surface features of the shoulder and brachium: left lateral view. Palpable and visible features are indicated.

Fig. 4.8 Skeleton of the shoulder and brachium: left lateral view. The features colored green correspond to the palpable bony features shown in the previous figure (Fig. 4.7).

Fig. 4.9 Skeleton of the shoulder and brachium: left medial view. The features colored green are bony features which may be palpable in the live dog.

Fig. 4.10 Superficial fascia of the shoulder and brachium: left lateral view. Only the skin has been removed. The superficial fascia of the shoulder and brachium is continuous with the cervical fascia cranially, and with the trunk fascia caudally. This fascia contains a myriad of small blood vessels and nerves. The deep fascia of the lateral shoulder and brachium (fascia omobrachialis lateralis), which is also continuous with that of the neck and trunk, has firm attachments to the spine of the scapula and to the crest of the greater tubercle. Fascia omobrachialis lateralis, together with fascia omobrachialis medialis, are continuous with the fascia antebrachii which covers the muscles of the forearm as a closely applied tube.

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Tags: Color Atlas of Veterinary Anatomy Volume 3

Jul 8, 2016 | Posted by admin in SUGERY, ORTHOPEDICS & ANESTHESIA | Comments Off

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