**3.2 The healthy elbow joint**

Most studies on elbow joint kinematics are based on video-kinematographic analysis and have investigated the motion of the elbow only in the sagittal plane [43–45, 47–53, 55–59, 62, 63, 65, 68, 69]. Caron et al. were the first to describe the real 3D kinematics of the canine forelimb of healthy Labrador retrievers and dogs with medial coronoid disease using video-kinematographic analysis [60]. Another study evaluated the 3D motion of orthopedic healthy canine forelimbs using videokinematography and compared that data to fluoroscopically gained motion analysis, which was additionally calculated in one of the dogs [28].

One complete gait cycle consists of the swing and the stance phase. The swing phase starts when the paw breaks contact with the ground and ends with first ground contact of the paw. The time between initial ground contact and paw lift is defined as the stance phase. The ratio between swing and stance phase depends from the gait pattern and the dog's velocity [28, 29, 70, 71]. At the walk the swing phase of the forelimb accounts for 39 to 43% of the whole gait cycle [60] and increases to approximately 50% to two thirds of the whole gait cycle during the trot, depending from the trotting speed [28, 43, 45, 58, 62, 64, 66]. During running the swing phase is further prolonged and accounts for approximately 75% of the gait cycle [62]. Conversely, with increasing speed the stance phase decreases [45, 70, 71].

The sagittal plane range of motion of the elbow joint (flexion-extension) is between 48.1 degrees and 70 degrees during one complete gait cycle when the dog is moving on a flat surface (**Table 2**), with the majority of motion occurring during the swing phase [28, 43–45, 47–50, 52, 53, 56–61, 63–67]. Range of motion is influenced by different parameters like breed, limb and body segment length, gait, velocity, exercise, age, contralateral limb amputation and concurrent orthopedic disease. With increasing speed of the gait the range of motion of joints increases [29, 45, 57, 62, 66, 68, 69]. Obese dogs show an increased range of motion as well, especially during the stance phase [57]. However, increasing age leads to an decrease in total range of motion, even in orthopedic healthy dogs [64]. Further, different exercises like descending stairs, uphill and downhill walking influence the range of motion, with descending stairs, obstacle exercises and uphill walking increasing the range of motion, while downhill walking decreases the amount of sagittal motion in the elbow [52, 65].



#### **Table 2.**

*Summary of the values for range of motion in sagittal plane and flexion and extension angles of the canine elbow joint from different kinematic studies. All values are expressed in degrees and were calculated, if necessary, based on data of each study to allow comparison between studies. 180 degrees represent maximum extension and 0 degrees maximum flexion.*

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

The stance phase is mainly characterized by continuous extension of the elbow joint until lifting of the paw from the ground. Some studies have shown flexion of the elbow joint just after weight bearing [43, 45, 47, 53, 58, 60, 64], resulting in two peaks of extension during the gait cycle. The first peak of extension occurs during the late swing phase and the initiation of ground contact and a second peak occurs at the end of the stance phase. The amount of this flexion differs between studies by several degrees. Further, this movement has not been described using fluoroscopic kinematography, what represents the gold standard of kinematic gait analysis [28]. This might be due to breed and inter-individual differences in the gait, due to the different techniques used for kinematic analysis or due to a soft tissue artifact, which occurs with skin mounted markers, and does not represent the in vivo motion of the bony cubital joint, but the movement pattern of the complete limb including the soft tissues [28, 32, 33]. Maximum extension of the elbow joint is reached at the end of the stance phase and is followed by continuous flexion during the swing phase. The peak flexion of the elbow joint is reached at approximately the middle of the swing phase and is followed by continuous extension of the elbow joint as a preparation for paw strike [53, 60, 64].

Besides flexion and extension, which represent the main motion pattern of the elbow joint, supination and pronation of the antebrachium and abduction and adduction of the humerus and antebrachium occur during the regular locomotion. In healthy Labrador retrievers the antebrachium is positioned in mild supination at the initial stance phase and shows minimal pronation during the remainder stance phase with a mean supination of the antebrachium of 3 ± 9 degrees [60]. In healthy Beagle the forelimb is placed onto the ground in mild pronation and is kept in this position during two thirds of the stance phase and then externally rotated during the last third of stance [28]. During the initial swing phase the antebrachium is supinated and maximum supination (mean 19 ± 9 degrees) occurs at the middle of the swing phase, together with maximum flexion of the elbow joint, in healthy Labrador retrievers [60]. In orthopedic sound Beagle a similar motion pattern is present during the swing phase, with supination of the antebrachium occurring during the first third of the swing phase [28]. Prior to foot strike rapid pronation of the antebrachium occurs and the limb is placed on the ground in a slightly supinated position in Labrador retrievers and slight pronation in Beagle [28, 60].

Three dimensional micromotion of the humerus, radius and ulna relative to each other was measured in different studies using marker based fluoroscopic kinematographic analysis [22–24, 54, 72]. Results of these studies show that the bones of the antebrachium have a complex motion pattern and radius and ulna cannot be seen as one single object. At the walk and the trot an axial movement between radius and ulna occurs in healthy and MCD affected elbows [22, 54]. In healthy canine elbow joints the radius shows an mean axial movement of 0.7 (SD 0.31) mm to 0.8 mm in relation to the ulna. This axial motion was detected in different mid to large breed dogs, like Fox hounds, Australian shepherd, Labrador retriever, Eurasian, German shepherd, Bernese mountain dog and mixed breeds [22, 54]. After the initiation of ground contact the radius moves proximally and remains in a slightly elevated position relative to the ulna, resulting in a dynamic negative radio-ulnar incongruence (RUI) [22, 72]. These results correspond with data from an in vitro study, which investigated the effects of limb loading and flexion and extension onto the radio-ulnar joint conformation and intra articular contact areas and which showed, that elbow extension leads to a relative lowering of the ulna in relation to the radius [73]. Extension is the main

motion of the elbow during the weight bearing phase and therefore the induction of a dynamic negative RUI might be seen as a adaption to joint loading [72]. Further, internal and external rotation between the radius and ulna occurs during the walk. Prior to foot strike the radius is in an externally rotated position relative to the ulna und shows internal rotation during the first third of the stance phase. Mean range of motion of the in vivo internal-external radial rotation is 11.4 (SD 2.0) degrees during the initial weight bearing phase [74]. No data exist investigating the in vivo radioulnar movement during the later stance phase and the swing. Therefore, the in vivo motion of the antebrachial bones and the dynamic changes within the radio-ulnar joint during the complete gait cycle are still unknown.

The in vivo humero-ulnar micromotion has only been investigated in one study so far [23]. Movement between the humerus and the ulna is characterized by flexion and extension, but rotational movement of the humerus relative to the ulna takes also place during locomotion [23]. At the walk the humerus shows an relative external rotation of 2.9 (SD 1.1) degrees during the first third of the stance phase in healthy humero-ulnar joints [23, 28]. These data imply that the elbow joint is not completely restricted to sagittal motion only. One study, investigating the 3D kinematics of the whole canine forelimb showed, that at the moment of ground contact the humerus is in an internally rotated position, which is slightly less at the trot compared to the walk (mean segment angle, walk: −34 degrees; trot: −25 degrees) [28]. During the walk the humerus shows internal and external rotation and only external rotation during the trot throughout the complete stance and swing phase, with a net external rotational movement during the stance phase [28]. This external rotational motion of the humerus is contrary to the internal rotation (pronation) of the antebrachium, which occurs prior to paw strike and is maintained during the stance [28, 60].

### **3.3 The dysplastic elbow joint**

When kinematics of the diseased canine elbow joint are evaluated two different types of changes in the kinematic pattern have to be differentiated. First, changes attributed to pain and lameness, i.e. altered kinematics as a result of the disease. Second, changes in elbow joint kinematics, which represent a causative factor of the disease process.

Due to pain, caused by different joint pathologies in the elbow with DED, multiple adaptive mechanisms occur in the affected forelimb. Decreases in stance time, angular displacement and net joint moments can all be seen in the diseased elbow joint [51].

A reduced range of motion in the sagittal plane (flexion-extension) is present in dogs with MCD [51, 59, 60]. In particular flexion of the joint is decreased and the elbow kept in a more extended position during the gait. In Labrador retrievers with MCD a faster extension of the cubital joint occurs during late swing phase and the elbow is more extended by 9 degrees (mean) during initial ground contact and the early stance phase compared to orthopedically healthy elbows [60]. This more extended gait is a compensating mechanism and aims to reduce pressure at the medial joint compartment [7, 73, 75]. At the end of the stance and beginning of swing phase the elbow joint is more rapidly flexed in affected dogs. However, no active push off occurs at the end of the stance phase indicating that the affected limb is pulled off the ground by the proximal musculature [51]. Reduction in active push off aims to reduce the pressure acting on the joint surface. The elbow is held 16 degrees more externally rotated during the end of swing and initial stance phase and the antebrachium is

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

in average 2 degrees more abducted throughout the gait cycle and 9 degrees more supinated during the paw strike and early stance phase [60]. These changes have to be assumed as compensating mechanisms as well. Supination leads to caudal displacement of the peak pressure at the medial ulnar joint surface and by that to a release of pressure and potentially pain at the diseased medial coronoid process. Besides the Labrador retriever a more extended elbow joint is present in other breeds with MCD, e.g. Rottweiler, Staffordshire Bullterrier, Airdale terrier, Golden retriever, Polish Lowland sheepdog, German wirehaired pointer, Belgian malinois, Irish setter and mixed breed dogs [51, 59, 60]. Therefore, these changes in the kinematic pattern represent a general secondary adaption to intra articular pathologies and the corresponding pain in canine elbow joints with MCD.

Primary changes in the kinematics of the radius, ulna and humerus are assumed to play an role in the pathogenesis of MCD. Altered kinematics in the proximal radioulnar joint, were suggested by different researchers to be one potential factor influencing the development of MCD [76–90]. One proposed mechanism was an increased axial translation of the radius relative to the ulna leading to an dynamic radio-ulnar incongruence. Translational movement between the radius and ulna occurs in elbows with and without MCD in vivo [22, 54], with no significant difference in the total amount of movement between both groups [22]. Therefore, increased axial movement between the radius and ulna and induction of a dynamic RUI under weight bearing conditions could be excluded as an primary factor. However, the direction of radial motion is different between normal and diseased joints, with a negative RUI being induced during the initial stance phase in healthy elbows and no significant change in the radio-ulnar joint conformation in MCD affected joints [72]. Based on the results of that study dogs with a static RUI are not able to compensate the radioulnar step formation by radio-ulnar translation and dogs with MCD, but without a static RUI, do not show the same amount of negative dynamic RUI as measured in healthy canine elbow joints [72]. The induction of a negative radio-ulnar step during weight bearing might be a protective mechanism in healthy canine elbow joints. Lowering of the ulna or elevation of the radius during extension of the elbow joint was previously described in vitro and leads to a decrease of intra articular pressure at the medial joint compartment [73]. The inability of the diseased canine elbow joint to adjust the radio-ulnar joint conformation during loading might be one potential biomechanical factor in the pathogenesis of MCD. Especially in dogs without a measurable static incongruence, which account for 40% of all patients with MCD [76], the insufficient adaption to intra articular joint loads can lead to mechanical overload at one distinct joint compartment. Increased radio-ulnar rotation was proposed as another potential cause of mechanical overload along the radial incisure of the medial coronoid process and subsequent cartilage and bone damage [82, 87–90]. The only study comparing in vivo radio-ulnar rotational movement in healthy joints to joints with MCD showed no significant difference in the total amount of radial rotation and in the motion pattern of the radius [74]. The radius starts in an externally rotated position during the late swing phase just before paw strike and rotates internally in relation to the ulna during the early weight bearing phase. At approximately 30 to 40% of the stance phase the radius shows an external rotation again. Values of total rotational movement and internal/external movement of the radius show no significant difference between normal and affected elbow (internal radial rotation, healthy: 5.7 [SD 2.1] degrees; MCD: 5.3 [SD 2.6] degrees; p = 0.1727; external radial rotation, healthy: - 5.8 [SD: 1.3] degrees; MCD: - 4.5 [1.7] degrees; p = 0.7705; total rotation, healthy: 11.4 [SD: 2.0] degrees; MCD: 9.8 [SD: 3.2]; p = 0.2904) [74]. Absence of

#### **Figure 1.**

 *Image sequence of the in vivo humero-ulnar joint motion during the late swing phase (f0), at the moment of weight bearing (f30) and the first third of the stance phase (f60 – f150). (A) Healthy joint; (B) MCD affected joint; relative external rotation of the humerus occurs just after ground contact, when the joint gets loaded. External rotation of the condyle leads to a craniolateral shift of the trochlea, impinging on the lateral aspect of the medial coronoid process [ 23 ].* 

increased radio-ulnar rotational motion does not exclude an biomechanical overload along the lateral aspect of the medial coronoid process of the ulna caused by interaction with the radial head. An abaxial attachment of the tendon of the biceps brachii muscle at the ulna was detected in dogs with MCD [ 90 ]. The pull of the biceps brachii muscle on the ulna could potentially lead to increased pressure between the medial coronoid and the radial head without altering the kinematics. However, no studies have investigated the forces acting between radius and ulna and compared these data between healthy and MCD affected dogs.

 Another significant difference can be seen in the humero-ulnar rotational movement between healthy and MCD affected joints. Increased external rotation of the humeral condyle in relation to the ulna occurs at the first third of the stance phase in cubital joints with MCD (humeral rotation, healthy: 2.9 [SD 1.1] degrees; MCD: 5.3 [SD 2.0] degrees; p = 0.0229) [ 23 ]. This rotation of the humeral condyle leads to compression of the joint space between the medial coronoid process and the humeral trochlea, and might potentially lead to mechanical overload at the coronoid process and consequently to cartilage and subchondral bone damage ( **Figure 1** ). Therefore, increased humero-ulnar rotation has to be considered as one dynamic factor in the pathogenesis of MCD. If this increased humero-ulnar rotational movement is caused by soft tissue laxity, like in the dysplastic hip joint, altered muscle function or due to bony differences altering the joint function has not been investigated so far. The influence of a static positive radio-ulnar incongruence onto the contact areas and pressure distribution within the humero-ulnar joint is known [ 91 – 93 ]. However, the literature is lacking kinematic analysis investigating the influence of a static RUI on elbow joint motion, particular the humero-radio-ulnar micromotion. In the cited study on humero-ulnar kinematics the MCD group consisted of dogs with and without a static positive RUI [ 23 ]. Due to the small sample size no correlation could be found between the presence of static RUI and the amount of humeral rotational motion. Therefore, the influence of this significant bony deformity on the kinematics of the elbow joint remains unknown.
