Previous Chapter
Electronic Syllabi Home
Table of contents
Next chapter


Chapter 4. THORACIC LIMB, DOG

Identify the following features in the skeleton and locate them in live dog and in radiographs.

 

4.1 SKELETON

4.1.1Scapula: spine, acromion, neck, supraglenoid tubercle, and glenoid cavity (articular).
 
4.1.2 Humerus: head (articular), neck, major tubercle (locate this in live dog and radiograph; important landmark in bone pinning.), minor tubercle, intertubercular groove, deltoid crest, brachial sulcus, condyle (articular), medial and lateral epicondyles, medial and lateral olecranon fossa; supratrochlear foramen (one of the factors contributing towards the susceptibility of distal humerus to fracture.
 
4.1.3 Radius: Head proximal extremity. Notice that shaft (body) is slightly convex dorsally and medially ; styloid process (medial).
 
4.1.4 Ulna: Olecranon tuber, Processus anconeus (Reference should be made to the condition of ununited anconeal process or elbow dysplasia coronoid process shaft or body styloid process (lateral)
 
4.1.5 Manus:
Carpus: proximal row (medial to lateral) radial, intermediate, ulnar, and accessory
distal row (medial to lateral) CI, CII, CIII, CIV
Metacarpals: (from medial to lateral) Digits I - V (Digit 1 non-functional) Each functional digit consists of proximal, middle and distal phalanges (PI, PII & PIII) and palmar sesamoids. The axis of the limb
passes between digits III & IV. Palmar sesamoids, 2 per metacarpophalangeal joint = 8 (+ the palmar sesamoid of digit 1).
Dorsal sesamoids, 1 per metacarpophalangeal joint = 4 (sometimes remaining cartilaginous.)
 
Identify all the above on skeleton and radiographs.
 
Regarding fracture of the distal extremity of the humerus, see dissected elbow joint, articulated bones, radiographs, and slides for the following:
The articular condyle of the humerus (consisting of lateral and medial parts) is supported by the lateral and medial epicondyles between which is the supratrochlear foramen. More weight appears to be borne by the lateral part of the condyle than the medial: also the lateral part is straight whereas the medial is oblique. The lateral epicondyle is a narrow column of bone curving out from the main shaft and separated from the main axis of the shaft by the supratrochlear foramen, whereas the medial epicondyle is strong and in the direct line of axis of the shaft. But the upward thrust of the radius is transmitted to the humerus mainly by the lateral part of the condyle and the weak lateral epicondyle.
Luxation of elbow joint occurs while the joint is fully flexed (the processus anconeus of the ulna being clear of the olecranon fossa) and when the joint is subjected to a twist. As to whether it is a medial or lateral luxation, it is customary to describe the position of the distal bone in relation to the proximal one. Luxation is usually lateral.
 
4.1.6 Muscles
A more complete knowledge of position of muscles in dog is required than in horse because of greater frequency of remediable dislocations and fractures in dog.
There is no 'mechanism' in the dog enabling it to stand for long periods as in the horse, but corresponding structures are there,now actively controlled instead of being passively 'mechanically' tensed. Instead of the interosseuss of the horse, there are the mm. interossei (interosseous muscles) one caudal to each main metacarpal each having the same distal attachments as in the horse i.e., to (palmar) sesamoids, and then prolonged to extensor tendon.
Observe the suspensory ability of the metacarpophalangeal joints compared with that of the corresponding joint in the horse. Note that the active suspensory action exerted by the tonus of the interosseous muscles is assisted by the fact that the metacarpophalangeal joints are supported by the metacarpal pad; similarly, interphalangeal joints are supported by the digital pads.
Except for the fact that the dog has more digits than the horse, the tendons of the deep and superficial flexors behave in the same way as in the horse and, therefore, perform the same function. There is nothing representing the check ligaments of the horse, but there are certain other structures helping to compensate for their absence. Where the superficial flexor tendon passes over the carpus (not through carpal groove as in horse), it receives a fibrous band from the accessory carpal bone. Note this and, at the same time, note the abductor of the fifth digit which arises, laterally, from the accessory carpal bone and whose tendon may be cut surgically in cases of fracture of this bone. The quite powerful tendon inserted on the upper part of the accessory carpal bone is the flexor carpi ulnaris. The ulnaris lateralis of the dog inserts on the fifth metacarpal not on the accessory carpal bone as in the horse.
Identify muscles of forearm, arm, shoulder joint, and shoulder girdle. Note position of superficial cervical lymph nodes.
 

 

4.2 Vessels

 

4.2.1. Arterial:-
The position of the main arterial trunk should be followed out in the dissected limbs.
The axillary artery turns round the first rib, reaches the axilla and medial aspect of shoulder joint, and runs down as brachial on the medial aspect of arm in the groove between the biceps and the medial head of triceps; then as it approaches the elbow joint, it inclines forward, runs under the short muscle called pronator teres and, a short distance below the level of the elbow joint, it changes its name to median artery. The median artery lies a short distance behind the medial border of the radius and comes fairly near the surface as it reaches the medial aspect of the deep flexor tendon just above the carpus: it maintains this vulnerable position until, at the flexor aspect of the carpus, it runs between the superficial and deep flexor tendons and is still in this position in the paw, until it ends about middle metacarpal region by dividing into the palmar common digital arteries. These unite with the palmar metacarpals, the union being situated in each case between the metacarpophaiangeal joints. Each one of these now divides into 2 proper digital arteries. .
Another large artery in the forearm (next in size to the median artery) is the caudal interosseous artery (a branch of the common interosseous branch of the brachial). It is deeply situated but susceptible to damage in fracture of shaft of radius and/or ulna. It runs down the caudal aspect close to the shafts of radius and ulna (and covered by the muscle which stretches between the shafts here, the pronator quadratus muscle). It supplies most of the blood to the palmar metacarpal arteries mentioned above. In other words, the caudal interosseous and the median supply most of the blood to the palmar aspect of the paw.
 
4.2.2 Venous:-
There are satellite veins to the main arteries. The large subcutaneous vein of the thoracic limb of the dog is the cephalic. It drains much of the blood from the palmar aspect of the paw and inclines forward across the medial border of the radius and then reaches the cranial aspect of the forearm at which point (just above the carpus) it receives the accessory cephalic vein which drains the dorsal aspect of the paw. The cephalic vein maintains its cranial position in the forearm (site of venipuncture) till it reaches the flexor aspect of the elbow, where it forms an anastomosis with the brachial vein. From this point, the cephalic inclines more laterally and runs up in the groove between the brachiocephalic and brachialis muscles (see craniolateral approach to shaft of humerus) to join the axillary vein, but also sending two tributaries to the jugular vein.
Alongside the cephalic vein in the forearm are an artery and nerves (the artery is the small cranial superficial antebrachial artery which supplies blood to dorsal aspect of paw: the nerves are two branches of the radial N. and are sensory to the dorsal aspect of the paw.)
 
4.2.3 Nerves:
Brachial plexus - from ventral branches of last 3 cervical and first 2 thoracic Nn. (C6,7,8 & T1,2).
 
Suprascapular N. C6 and sometimes 7 as in horse turns round cranial border of scapula and supplies supra- and infraspinatus muscles.
 
Axillary N. C7 and sometimes 6 & 8) passes out behind caudal border of scapula near the neck and supplies true flexors of shoulder joint. (teres major, teres minor, deltoid).
 
Musculocutaneous N. C 7,8) Note position in arm, especially as regards shaft of humerus. Craniomedial to shaft of humerus, between brachial artery and biceps muscle. Supplies biceps and brachialis muscles (the flexors of the elbow joint).
 
Radial N. (C7,8 & T1,2) behaves like that of horse with one notable difference: in dog, radial N. is sensory to dorsaI aspect of paw (whereas, in horse, the lowest it gets is as cutaneous fibres on lateral aspect of forearm to carpus. As in horse, it is vulnerable at first rib, also having similar course it is susceptible to compression or stretching with, because of similar muscular distribution, similar symptoms. Follow its course of distribution on dissected limbs. From brachial plexus it plunges in between long and medial heads of triceps; keep finger on it at its point of disappearance and, at same time, turn limb so that lateral aspect is in view; now reflect sectioned lateral head of triceps and identify radial N. lying on brachialis muscle At this point the radial N. is dividing into deep and superficial branches. The deep branch is going almost straight down to supply the extensor muscles (extensors of carpus and digits). The superficial branch is gradually dividing into two fine nerves which lie alongside the cephalic vein in the forearm and end up as sensory to dorsal aspect of paw (all of dorsal aspect except lateral aspect of digit V which receives a branch from the ulnar N.)
To summate: - the radial N. in the dog supplies the extensors of elbow, carpus and digits, and is sensory to almost all the dorsal aspect of paw.
Median and Ulnar Nn. (C8 & T1,2) From brachial plexus these two run at first alongside each other, between brachial artery and vein on medial aspect of arm; then more than half-way down the arm they separate. The Median N. follows the brachial artery, then the median artery (in other words, the main arterial trunk) right down to the palmar aspect of paw where, between the superficial and deep digital flexor tendons, it ends by dividing into branches which, along with the terminal branches of ulnar N., are sensory to the palmar aspect of paw. The ulnar N., after deviating from the median in the arm, runs back along the medial head of triceps and the medial epicondyle of humerus and so into the forearm where it lies, mostly, between the two heads of the flexor carpi ulnaris which inserts on the accessory carpal bone. The ulnar N. then runs down medial to accessory carpal bone and divides into branches which are motor and sensory to the palmar aspect of paw. A branch of the ulnar N. in addition supplies the lateral dorsal aspect of the digit V.
Precis of motor innervation is the same as for horse limb . knowledge of cutaneous nerve distribution is necessary for testing integrity of nerve reflexes.
 

4.3 Reduction of Fractures: Shaft of Humerus, Radius, and Ulna

4.3.1Humerus
4.3.1.1Closed manipulation (i.e. with minimal incision of skin and no reflection of tissues for the purpose of inserting an intramedullary pin). Identify major tubercle and go medial to the cranial point of this.
 
4.3.1.2 Open reduction i.e. involving skin incision sometimes quite extensive and reflection of tissues.
(a) For more proximal part of shaft, the craniolateral aspect: reflect the brachiocephalic muscle cranially (communicating veins encountered). In reaching the bone, the brachialis muscle will be encountered lying in the brachial sulcus.
(b) For more distal part of shaft, the craniolateral aspect: on incising skin, look for cephalic vein. On incising cranial border of triceps look for the radial N. which winds round distal part of brachialis. Reflect radial N. & m. brachialis caudally (a certain difficulty is explained by fact that brachialis inserts medially).
(c) For more distal part of shaft, the medial approach may be used - the bone is accessible and not so curved as on lateral aspect, but there are more vessels and nerves on the meaial aspect. On incising between biceps and medial head of triceps, watch for branches and tributaries of the brachial vessels and also nerves in the cranial-to-caudal order of musculocutaneous, median and ulnar.
 
4.3.2 Radius
 
4.3.2.1 Open Reduction, if necessary:
(a) head (proximal extremity): craniolateral approach, radial N. being the most important structure to avoid damaging.
(b) proximal shaft: craniolateral approach. Go between common and lateral digital extensors. Branches of radial N. may be encountered (if incision is made too cranially, cephalic vein will be encountered).
(c) distal shaft: craniomedial approach. On incising skin, locate cephalic vein and ease it away from bone. Small medial vessels lie on caudal edge of bone (perhaps these vessels ruptured by fracture).
(d) distal extremity of radius or carpal joint: dorsal (cranial) approach. Avoid accessory cephalic vein on more distal part of incision.
4.3.3 Ulna
If open reduction is necessary (since olecranon tuber is so accessible to pinning by closed reduction):-
caudal approach. Between ulnaris lateralis (ext. carp. uln.) and flexor carpi ulnaris. The cat ulna is more suitable than dog's for intramedullary pinning, being straighter and relatively thicker.
Accessory carpal bone: lateral approach.
Metacarpus and Phalanges: if open reduction necessary, dorsal approach.

4.4 CLINICAL CONSIDERATIONS:

The following are some surgical considerations relevant :
Nerve paralyses
Sites of joint puncture (surgical)
Fractures of distal humerus
Ununited anconeal process
Cephalic venipuncture
Common surgical approaches to fracture sites
Surgical approaches to joints
 
Cutaneous innervation
 
Cutaneous innervation-canine
Teres minor and association structures