The study population consisted of puppies with DIs that are associated with the development of osteoarthritic changes characteristic of CHD in the CFJ. The frequency of CFJ osteoarthritis at maturity increases in proportion with DI scores ≥ 3.0.35 Distraction index scores used in our investigation were derived from radiographs obtained at the youngest age at which the scores are reported to be prognostic, although that age varies among breeds.5
The population of dogs used in our study had a mean DI score of 0.67, indicating that most puppies were likely to develop CHD. As such, these puppies were appropriate for use in evaluating the relationship between GRFs and CFJ laxity, although by strict definition, they did not have radiographically evident degenerative joint changes that were characteristic of the disease at the time of the study.
Normal and pathologic patterns in canine gait variables have been well documented, but most of the gait alterations reported are attributable to pain.16,18,20,21,30
Changes in GRFs that are specifically related to pathologic changes in CFJs of dogs and humans include reduced y- and z-plane peak force and impulse as well as maximum loading and unloading rates.16,20,21
Mean values for z-plane peak force and impulse in the hind limbs were higher than those reported21,33
in dogs with CHD, whereas mean loading and unloading rates were lower, although direct comparisons were difficult given the differences in gait velocity used between studies. Values for y- and z-plane fore- and hind limb GRFs in our study were comparable to those published29
for clinically normal dogs with similar stance times, except for hind limb craniocaudal propulsion peak force, which was complicated by high individual variability in the present study. Fore- and hind limb x-plane GRFs were < 6% of body weight and primarily directed laterally, comparable to values in clinically normal dogs.33
Similarly, forelimbs bore 60% of body weight, whereas hind limbs bore 40% of body weight in our puppies, similar to findings in clinically normal dogs.31
On the basis of this information, values for x-, y-, and z-plane peak force and impulse in our study were consistent with those reported in mature dogs unaffected by orthopedic disease. Differences from reported reference range values in mean loading and unloading rates in our study may have arisen from the fact that the values were calculated over the entire loading and unloading phases, rather than at intervals, a method that more accurately reflects limb function.32
Despite this fact, both rates were higher in the forelimbs, as expected, and values between limbs had limited asymmetry, making it unlikely that the differences reflected pathologic gait changes.
Values for asymmetry varied widely for the variables evaluated. In general, values for z-plane and stance time asymmetry were least pronounced, whereas values for y-plane asymmetries were most pronounced, findings that were in accordance with those from previous studies.15,37
Values for y-plane asymmetries in our study were slightly higher than those of a previous study15
in clinically normal dogs that were evaluated at the same velocity; however, the small differences do not necessarily indicate that there were gait abnormalities. Also notable in our study was the fact that asymmetry values in the fore- and hind limbs were comparable in the y-plane, whereas values for asymmetry were highest in the x- and z-planes for the fore-and hind limbs, respectively. Results of a previous study15
revealed comparable values for asymmetry between fore- and hind limbs in z-forces, whereas values of y-plane asymmetry were higher in the hind limb. According to the formula used in the present study, SI values are close to 1.0 if differences within a gait variable are small, compared with its absolute values, and vice versa. Values for SI that are significantly different from 1.0 may occur if values for a given gait variable are close to 0, if the differences in values between right and left limbs are large, or both.37
The limits of a normal degree of asymmetry vary among gait variables, and asymmetries in gait should be interpreted with caution. Our findings in the gaits of immature dogs were in agreement with those from a study15
of mature dogs. Z-plane and stance time asymmetry indices appear to be the most applicable for canine gait analysis.
In the present study, a range of canine gait variables with associated SEMs and detection powers were described for future reference. Intertrial variation may have contributed to some variability in measured values because the dogs were immature; in humans, intertrial variation is known to be more important in children than in adults.38
Every effort was made to control such variation by limiting the velocity and acceleration at which the trials were conducted and by evaluating all trials for consistency and validity. It is possible that variation could have been reduced by limiting the velocity and acceleration ranges further, but although the puppies were all the same age, their sizes varied widely. The ranges were chosen to include comfortable trot rates to avoid introduction of variation from artificially altering the gait. Gait constraints are not recommended in human gait assessments because they prohibit evaluation of representative gait kinematics and kinetics.39
Puppies were allowed to trot at their preferred velocities so that the most representative GRFs would be recorded. Had the velocity range been restricted, the overall variability in GRF may have been reduced, resulting in higher statistical power. However, had only the puppies that trotted comfortably within a limited velocity range been included, a number of individuals would have been excluded from the study, reducing the range of CFJ laxity. It was determined that study objectives were best met by inclusion of the widest range of CFJ laxities; therefore, data from all puppies were included in analyses.
To our knowledge, these data are the first obtained from evaluation of immature animals. Neuromuscular function and coordination may not have been fully developed and may have resulted in some of the gait inconsistencies observed. The stages of gait development in children have been extensively evaluated kinetically and kinematically, and mediolateral stability is the most important factor in gait development in clinically normal infants.40
Ground reaction forces in the x-plane are considered too variable to be useful for gait analysis.41,31
It is therefore difficult to assess whether animal development contributed to the high variation and low sensitivity of the x-plane GRFs in our study. Our results support that x-plane GRFs have little value in gait evaluation in mature or immature dogs.
In normal ambulation, the primary function of the canine forelimb is deceleration, whereas that of the hind limb is propulsion.31
from clinically normal dogs at a similar trot velocity were 57% of stance time spent in breaking and 43% spent in propulsion in the forelimb, compared with 30% of stance time in breaking and 70% of stance time in propulsion in the hind limb. Although values for hind limbs in our study were comparable (24% braking, 76% propulsion), the values in forelimbs (83% braking, 17% propulsion) were proportionately different. There were large trial variations in the propulsion and braking GRFs, as indicated by the SEMs. This has not been reported as a problem in the evaluation of mature dogs and may have been related to the young age of the dogs in our study. Although every attempt was made to ensure consistency between trials, there were some unavoidable obstacles that are inherent to the study of young animals. This does not diminish the importance of findings from such investigations, but consideration must be given to the age of the subjects. Further studies are necessary to assess whether there are differences in individual variation in braking and propulsion as dogs mature.
Stable joints are necessary for normal gait.26–28
To our knowledge, gait alteration related to joint laxity that is not complicated by arthrosis has not been previously investigated in dogs. A wide range of CFJ laxity, from slightly greater than normal to very severe,35
was evaluated against GRF gait variables for detection of potential associations in our study. It is important to distinguish between active laxity and DI passive laxity when considering that no associations between DIs and GRFs were detected in this study. Other methods of imaging the CFJ may be more representative of active CFJ laxity, and such laxity may be more closely correlated with GRF values.42
Although CFJs in our dogs had moderate to severe degrees of passive laxity, it is possible that dogs actively stabilize the joint or otherwise compensate for instability by altering joint angles, pelvic tilt, and muscle group activation during ambulation.1
Determining whether such mechanisms are used would require the acquisition of kinematic and electromyographic data and could serve as the basis for future studies.
Values for GRFs in the present study were consistent with those derived from evaluations of clinically normal adult dogs, supporting the finding that gait variables and passive CFJ laxity appear to be unrelated in young dogs. The results also validate the use of gait analysis in young dogs as a method of objective evaluation. This information is essential to future investigations of CHD, specifically in the role played by joint laxity in the pathophysiologic features of hip dysplasia and associated changes in gait.