Development of biochemical heterogeneity of articular cartilage: influences of age and exercise

The objective of this study was to document the development of biochemical heterogeneity from birth to maturity in equine articular cartilage, and to test the hypothesis that the amount of exercise during early life may influence this process. Neonatal foals showed no biochemical heterogeneity whatsoever, in contrast to a clear biochemical heterogeneity in mature horses. The process of formation of site differences was almost completed in exercised foals age 5 months, but was delayed in those deprived of exercise. For some collagen-related parameters, this delay was not compensated for after an additional 6 month period of moderate exercise. It is concluded that the functional adaptation of articular cartilage, as reflected in the formation of biochemical heterogeneity in the horse, occurs for the most part during the first 5 months postpartum. A certain level of exercise seems essential for this process and withholding exercise in early life, may result in a delay in the adaptation of the cartilage.     

Contact areas and pressure distribution on the proximal articular surface of the proximal phalanx under sagittal plane loading

The objective of this study was to map topographically contact areas and pressure distributions on the proximal articular surface (PAS) of the proximal phalanx (PI) under various clinically relevant loading conditions. Left and right forelimbs of 13 mature horses were transected halfway down the radius and loaded in a position mimicking the weightbearing attitude close to the midstance phase. Five loads were used which corresponded with loads that can be expected in different gaits or during athletic performance (stance: 1800 N, walk: 3600 N, trot: 5400 N, gallop: 10,500 N and jumping: 12,000 N). Contact areas and pressure distributions at the PAS of PI were determined using a methylene blue dye staining technique and 2 pressure sensitive films (low pressure: range 2.5-10 MPa and medium pressure: range 10-50 MPa). The contact area of PI was positively correlated (r = 0.86; P<0.01) with the applied load. The contact area increased from 63% at 1800 N to 95% at 12,000 N and gradually shifted to include more of the edges of the articular surface, but especially the dorsal articular margin of PI. Pressure distribution patterns were similar under the different loading conditions. Pressure was less at the palmar margin and in the central depression and highest at the dorsal articular margin. With increasing load, the highest peak pressures were measured at sites of the dorsal articular margin that are not loaded in the standing or walking horse. The results of this study suggest that the frequent occurrence of osteochondral lesions at the dorsal articular margin of PI is caused by the combination of the intermittent character and the high absolute values of loads at this site as they occur during athletic performance.     

Relationship between the forces acting on the horse's back and the movements of rider and horse while walking on a treadmill

Reasons for performing study: The exact relationship between the saddle pressure pattern during one stride cycle and the movements of horse and rider at the walk are poorly understood and have never been investigated in detail. Hypothesis: The movements of rider and horse account for the force distribution pattern under the saddle. Method: Vertical ground reaction forces (GRF), kinematics of horse and rider as well as saddle forces (FS) were measured synchronously in 7 high level dressage horses while being ridden on an instrumented treadmill at walk. Discrete values of the total saddle forces (FStot) were determined for each stride and related to kinematics and GRF. The pressure sensitive mat was divided into halves and sixths to assess the force distribution over the horse's back in more detail. Differences were tested using a one sample t test (P<0.05). Results: FStot of all the horses showed 3 peaks (P1‐P3) and 3 minima (M1‐M3) in each half‐cycle, which were systematically related to the footfall sequence of the walk. Looking at the halves of the mat, force curves were 50% phase‐shifted. The analysis of the FS of the 6 sections showed a clear association to the rider's and horse's movements. Conclusion: The saddle force distribution during an entire stride cycle has a distinct pattern although the force fluctuations of the FStot are small. The forces in the front thirds were clearly related to the movement of the front limbs, those in the mid part to the lateral flexion of the horse's spine and the loading of the hind part was mainly influenced by the axial rotation and lateral bending of the back. Potential relevance: These data can be used as a reference for comparing different types of saddle fit.     

Influence of intensity and changes of physical activity on bone mineral density of immature equine subchondral bone

Reasons for performing study: Subchondral bone provides structural support to overlying articular cartilage and plays an important biomechanical role in osteochondral diseases. Mechanical features of bone correlate strongly with bone mineral density, which is directed by the loading conditions to which the tissue is subjected. Objective: To investigate the influence of physical activity levels on subchondral bone mineral density (sBMD) in foals during early development. Methods: Three groups of foals were subjected to different physical activity levels from birth until age 5 months. A proportion of these foals were subjected to euthanasia at 5 months while remaining foals were subjected to similar physical activity levels for 6 months until euthanasia at 11 months. Osteochondral specimens were collected for measurement of sBMD with peripheral quantitative computed tomography at 2 differently loaded anatomical sites of the proximal phalangeal bone at 1, 2, 3, 4 and 5 mm depth from the osteochondral junction. Results: Growth significantly increased sBMD but by a different amount depending on anatomical location and physical activity level. Significantly higher sBMD was found at the habitually loaded central area in comparison to the intermittently peak loaded marginal site. Exercise increased sBMD throughout the whole depth of analysed tissue, but changes were generally more obvious at a depth of 2 mm. Interestingly, foals subjected to additional sprint training preserved the exercise‐induced sBMD increase at the habitually loaded central area during the 6 months of the second phase of the study. Conclusions: Habitual low‐intensity loading elicits a greater response in sBMD in quantitative terms than high‐intensity low‐frequency loading at the sites investigated in this study. Future sBMD may be influenced by means of well‐tailored exercise regimens at young age. Potential relevance: Specific physical activity levels during early development may potentially reduce the prevalence of osteochondral injury later in life.