**4. Joint contact areas and force distribution within the elbow joint**

 The mean body weight distribution between fore- and hindlimbs is approximately 60% : 40% in dogs [ 56 , 94 ]. A large study investigating 123 different breeds found

## *Biomechanics of the Canine Elbow Joint DOI: http://dx.doi.org/10.5772/intechopen.99569*

that the grand mean proportion of mass was 60.4% on the forelimbs (range: 47.6 to 74.4%) [94]. Only sex was shown to be a significant factor altering that ratio, with females being below the mean value throughout different breeds [94]. Another study comparing kinematic and kinetic data of orthopedic healthy Labrador retrievers and German shepherds reported that Labrador retrievers carry a higher percentage of the weight on their forelimbs compared to the German shepherd (69% vs. 62%, p < 0.001) [66]. If this breed specific mechanical overload plays a role in the pathogenesis of DED and contributes to the high rate of Labrador retrievers with developmental elbow disease, in particular MCD, is not known.

Within the elbow joint load and forces are not homogenously distributed throughout the whole joint surface. It was believed that the radial joint surface is the main weight bearing surface of the radio-ulnar joint. However, more recent studies have shown, that the radius takes 51 to 52% of load [73, 75, 91]. Therefore the ulna plays a more important role in weight bearing than previously assumed. Despite an overall equal load and force distribution between the radius and the ulna, not every part of the joint surface represents an active joint contact area. Within the combined radioulnar joint surface three distinct contact areas can be found: the craniolateral aspect of anconeal process, the joint surface of the radial head, and the medial coronoid process [7, 24, 73]. There is no particular contact at the medial aspect of the anconeal process and the center of the trochlear notch (**Figure 2**). The latter one might be explained by the slight physiological humero-ulnar incongruence leading to a bicentrical contact

#### **Figure 2.**

*Colored animation of the in vivo humero-ulnar joint contact pattern at the ulnar joint surface at the beginning of weight bearing in a healthy canine elbow joint (red: Humero-ulnar contact). Joint contact is present along the medial coronoid process and the lateral and proximal aspect of the trochlear notch. The radius is not shown in this animation.*

pattern [6, 7, 9, 73, 95]. When the elbow joint is loaded the force applied by the humeral condyle is distributed along the anconeal process and the coronoid region. With increasing load the concave ulnar notch is stretched and these pressure forces are partially transformed to traction forces [8, 95–97]. Therefore this physiological incongruence leads to a more even stress distribution within the humero-ulnar joint. In human elbow joints the proximal and distal contact area confluent when high loads are acting onto the ulnar joint surface [98]. This load dependent change in contact pattern has not been described in canine elbows so far [7].

The presence of these three contact areas within the elbow joint is further supported by increased subchondral bone density measurements at these anatomic areas [95, 99]. Bone is a dynamic tissue which has the ability to remodel in response to mechanical load (Wolff's law) [100]. Therefore, increased bone density can be found in areas with increased load. Increased subchondral bone densities are present at the disto-medial and cranial aspect of the humeral trochlea and in the olecranon fossa, the anconeal and medial coronoid processes of the ulna and the cranio-medial region of the joint surface of the radius [95]. The same study showed a significant age-dependent increase in the subchondral bone density of the joint surfaces of all three bones, representing continuous adaption of the bone to mechanical stress with increasing age [95].

Though increased loading of the ulnar joint surface does not result in confluence of the bicentric contact pattern, other factors can influence the joint contact patterns of the humero-ulnar and humero-radial joint surfaces. An in vitro study investigated the influence of positive radio-ulnar incongruence (short radius) on joint contact patterns. Presence of a positive RUI leads to a shift of the contact area at the medial coronoid process towards the cranio-lateral aspect of the coronoid process and reduction of the anconeal contact area [93]. Other in vitro studies show similar results. After induction of a 1.9 mm positive RUI medial compartment contact area decreases significantly while the lateral contact area increases. Likewise the mean contact pressure and peak contact pressure increase within the medial compartment and decrease in the lateral part [91, 92]. Therefore, presence of a static positive RUI has to be assumed as an important factor in the disease process of developmental elbow disease and a correlation between the severity of cartilage damage and static RUI has been shown in affected elbows [76, 77, 101]. In vivo evaluation of the ulnar joint contact pattern during the walk in a dog with positive static RUI before and after bi-oblique dynamic proximal ulnar osteotomy (DPUO) confirmed the results of different in vitro studies [24]. Following DPUO positive static RUI decreased, leading to a significant increase of the contact area at the medial coronoid process and to a shift of the contact area from the cranio-lateral aspect (tip and radial incisure) towards the medial aspect and the base of the medial coronoid process (**Figure 3**) [24]. This positive effect of different forms of ulnar and humeral osteotomies onto humero-radio-ulnar contact and force distribution has previously been shown in vitro [75, 91, 92]. Whether a static RUI changes the kinematic pattern of humero-radial, humero-ulnar or radio-ulnar motion and by that the intra articular contact areas and pressure distribution or has a purely mechanical influence without dynamic changes has not been investigated so far.

Further, joint contact areas change during the regular locomotion. Pronation leads to reduction of the contact area in the medial and to a lesser amount in the lateral compartment of the radio-ulnar joint surface. The effect of pronation is further influenced by the elbow joint angle, with significant reduction of the medial contact area by 23% at 135 degree of flexion, what represents the average flexion angle during

#### **Figure 3.**

*Humero-ulnar joint contact pattern at the ulnar joint surface at the beginning of weight bearing in a canine elbow joint with MCD (red: Contact area). (A) Contact pattern before bi-oblique DPUO; focal concentration of joint contact at the medial coronoid process (MCP) and slight contact at the medial and lateral aspect of the anconeal process is present. (B) Contact pattern 12 weeks postoperative; joint contact is more homogenously distributed throughout the ulnar joint surface and the craniolateral aspect of the MCP is even not in contact with the corresponding humeral trochlea [24].*

the stance phase [73]. A reduced contact area will result in increased pressure when the same load is applied to the joint. Further, pronation of the antebrachium leads to a shift of the peak contact pressure towards the apex of the medial coronoid process. Otherwise supination of the antebrachium leads to caudal displacement of the peak contact pressure on the medial coronoid process [73, 75]. This might explain that dogs with medial coronoid disease show a more supinated stance to release pressure from the apex of the medial coronoid [60]. Moreover, flexion and extension, the main motion pattern during the normal locomotion, influence the intra articular pressure distribution. Flexion increases peak pressure at the medial radio-ulnar joint compartment and extension decreases pressure [73]. It is assumed that this change is due to dynamic changes within the radio-ulnar joint surface in healthy canine elbows [72, 73]. In a cadaveric study extension of the elbow joint induced lowering of the radius and ulna, however more pronounced in the ulna (3.8 mm) compared to the radius (1.9 mm). This corresponds to findings of the in vivo investigation of the radioulnar joint cup conformation in healthy elbow joints during the walk, where a negative RUI (short ulna) was induced during weight bearing [72]. This lowering of the ulna relative to the radius might protect the medial coronoid process from mechanical overload during locomotion in healthy canine elbows. In contrast, altered radio-ulnar kinematics preventing elevation of the radius might lead to continuous excessive mechanical overload and subsequent joint pathologies.

Considering the changes of intra articular contact areas and pressure distribution as a function of limb position might explain the typical clinical signs in dogs with developmental elbow disease. Affected dogs stand with the elbow slightly abducted and the antebrachium in slight external rotation (supination) [102]. Furthermore, the elbow joint is more rapidly extended during the swing phase and kept in a more extended position during weight bearing [60]. This motion pattern aims to reduce the contact and pressure at the medial coronoid process, where most commonly lesions attributed to developmental elbow disease occur [90, 103].
