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how your dogs elbows work
How does your dogs elbow bones work
Elbows  (Anatomy)
The elbow joint is a complex joint, because 3 bones: Humerus, Radius and Ulna are integrated. It can be divided into 3 different joints: the humero-radial and humero-ulnar joint, and aditionally the proximal radio-ulnar joint. The first two joints are in form of a condylar joint and the last is built as a trochoid joint. The result of this combination is that two axes are nearly vertical to one another, and problems are to be expected. The articular surface at the distal end of the Humerus is carried laterally by the Capitulum and medially by the Trochlea humeri. It shows medially the distal rim of the trochlea and axially a ridge. So, one can say that there are normally less side movements possible like in a hinge joint. The articular surface of the Ulna, the concave Incisura trochlearis (Viehmann et al., 1999), and the Fovea capitis of the Radius bear the body weight in dogs and cats. Special interest must be given to a) the proximal end of the c-shaped and into two parts divided Incisura trochlearis, which begins with the sharp Processus anconaeus and b) to the distal end with the elongated Processus coronoideus medialis ulnae.
How it works
High ranges of motion (VOLLMERHAUS et al., 1994) can be diagnosed by movements in clinical examination. Extension – Flexion: 100 – 140° Rotations: 50 (70- 80°) Additionally, normal gait movements display a slight adduction and abduction. For chondrodystrophic breeds, we know a normal abduction of ca. 15° occurs in the forearm of the elbow joint.
DEVELOPMENT The articular surfaces of the elbow joint must be built correctly, because a hinge jount (ginglymus) does not allow laxity. This means that the development of the 3 bones: Humerus, Radius and Ulna have to grow simultanously within the first 6 months after birth. And from here, the first and severe clinical problems can arise. These 3 bones are long bones or Ossa longa. But their growing is very different: Knowledge about the development of the articulating bone ends (Vollmerhaus and Roos, 1980; Roos et al, 1981) gives a basis to understand some malformations, fractures of the related bones and of some specific lines in Xrays of young dogs (Waaibl and Brunnberg, 2003) In general, long bones develop with primary and secondary ossification centers. a) primary ossification centers appear in dogs prenatally from 40th day of pregnancy and are the bases of diaphyses (Evans, 1993). b) secondary ossification centers appear in dogs (or cats) postnatally in the 1st to the 4th month) and are the bases of epiphyses or apophyses. Humerus: The Epiphysis distalis humeri bears 2 ossification centers, medial to that of the Trochlea and lateral to that of the Capitulum. Both fuse together in the 3th month and if not, fractures may occur are to attend (Brunnberg et al., 2001). The epiphysial cartilage to the diaphysis disappear in the 5th or 6th month. This distal epiphysis produces only 40 % of the length growth of the humerus. In the 2nd month the apophysial ossification center appears of the Epicondylus medialis which fuses with the distal epiphysis in the 5th month, (Vollmerhaus et al. 1981; Brunnberg et al. 1985). Radius: The ossification center of the Epiphysis proximalis radii appears at the end of the 1st or beginning of the 2nd month postnatal. The closure of the cartilage to the radial diaphyis is from the 8th to 10th months, which is much later than that in the distal end of the Humerus. This epiphysial cartlage is less than 1mm thick and like a round cap over the diaphysis (Roos et al., 1981). Ulna: The proximal end of the Ulna is without an epiphysis, but at the top there is the Tuber olecrani, an apophysis, which appears in the end of the 1st or beginning of the 2nd month. The closure of the flat cartilage with the diaphysis is in the 7th to the 9th months. Much more interesting is the existence of an ossification center in the Processus anconaeus, which perhaps could be a special kind of an epiphysis. But until now, we were not able to finally decide about this ossification center, which appears in the end of the 3rd month postnatal and the closure should be finished in the 4th month. Later closures are suspected for incorrect development. Then, this special processus becomes a clinical problem, because it forms the proximal limit of the length (and the form) of the Incisura trochlearis. To understand problems in the develping elbow joint, one has to regard the distal ends of the radius and ulna, too. Because of the fact that the proximal epiphysis of the Radius has the a potential of growth of only 30 to 40 %, its distal epiphysis must produce 70 to 60 % of the length growth till the 9th month after birth (Riser, 1985). In contrary to this, the Ulna is kown as an one-epiphyseal bone. That means there exists only 1 epiphysis at the distal end, which produces 85 % of the bone length and the rest of 15 % is a general growth of the diaphysis and the olecranon. How can these 3 bones grow simultaniously? It is a miracle and so problems are expected in step constructions of the articular surfaces. As an example, you know that problems in the (distal) epiphysial cartilage of the ulna in the 3rd or 4th month result in a Radius curvus and lesions at the Procesuus anconeaus and perhaps at the medial coronoid process. The special function of the elbow joint with pronation and supination belongs to special muscles. The pronation is made by the M. pronator teres with help of the M. biceps brachii and the M. brachialis, which cooperate at the medial side. As an antagonist works the M. supinator, which is connected to ligaments of the joint. The innervation of the elbow joint is realized by 4 macroscopic nerves: N. radialis, N. musculocutaneus, N. medianus and N. ulanris. In addition there are microscopic fibers from the periosteum and the passing nerves (Stasyk and Gasse, 1999). Special interest must be given to approaches to the joint. There are the courses of the nerves and blood vessels to consider. Most of the bigger vessels are hidden cranially and medially (Frewein et al., 1984) and less lie at the lateral and caudal side of the canine elbow joint.
LUXATIONS Aside from the rare congenital form, luxation of the elbow joint almost always results from trauma. Lateral luxation is the most common presentation, followed by medial and then caudal luxations. The Monteggia fracture is a special form involving radial head luxation and ulnar fracture, and always requires surgical intervention. Internal fixation is performed on the ulna and the radius is repositioned. Severe lameness accompanies elbow luxation. The limb is flexed at the elbow with the humerus adducted, while the antebrachium is medially rotated and abducted. The limb does not bear weight. Passive manipulation is extremely painful and the palpable bony landmarks are displaced. The diagnosis can be confirmed on mediolateral and caudocranial radiographics projection. Most often these luxations can be treated by closed reduction under general anaesthesia. Surgical intervention is required for luxations older than 48 hours and/or with bony involvement (fractures fragments) and in cases in which spontaneous reluxation occurs. DISORDERS OF SKELETAL DEVELOPMENT (ELBOW DYSPLASIA) UNUNITED ANCONEAL PROCESS The growth plate of the anconeal process closes at the age of 16 to 20 weeks, if this fails to occur then the anconeal process is considered ununited. Delayed growth of the long axis of the ulna (short ulna syndrome) irritates the anconeal growth plate and prevents its closure. Fast growing, large breed dogs are most often affected, especially German Shepherds. In 1/3 of the cases the disease is bilateral. Signs include mixed lameness, abduction of the paw and antebrachium, and adduction and internal rotation of the elbow. In addition to lameness and characteristic limb positioning the elbow is swollen (palpable laterally) and painful on extension. The tentative diagnosis can be confirmed by radiographic examination of craniocaudal and mediolateral projections. The flexed lateral projection is decisive, especially in immature dogs. It is important to avoid an incorrect interpretation because the growth plates of the radius, ulna and humerus are not yet closed. OSTEOCHONDRITIS DISSECANS OF THE HUMERAL TROCHLEA OCD is caused by a disturbance of cartilage growth and a failure of normal cartilage development. The cartilage thickness exceeds the capacity of the deeper layer to be supplied by diffusion, leading to malnutrition and necrosis.The humeral trochlea is the predilection site in the elbow joint. Rapidly growing large breed dogs are most often affected, especially Rottweilers and Golden Retrievers.
L. Brunnberg DVM, F. Forterre ECVS DVM Small Animal Clinic Free University of Berlin, Germany H. Waibl DVM Anatomical Institute Tierärztliche Hochschule Hanover, Germany

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