Development of the cross sectional area of flexor tendons in the metacarpal region of 2-year-old horses of different breeds

2Ludwig-Maximilian-Universitat Munchen, Klinik fur Pferde, Munchen Summary: This study examined the development of the CSA of the superficial and deep digital flexor tendon (SDFT and DDFT) and of the accessory ligament of the DDFT (AL-DDFT) in both forelimbs of six 2-year-old Thoroughbreds (TB), ten 2-year-old Standardbreds (SB), five 2-year-old Quarterhorses (QH) and ten 2-year-old German Warmblood Riding horses (WB) during one year with the hypothesis that it would be different among breeds. The initial CSA measurements were done in the yearling QH in November and thereafter in March, July and November of their 2-year-old season, and in the horses of all other breeds the measurements were in January, April, August and December of their 2-year-old year. The CSA was measured with ultrasound at four cm intervals, starting four cm distal from the distal border of the accessory carpal bone down to the metacarpophalangeal joint. The TB and SB were in race training, the WB were halter trained and the 2-year-old QH were schooled for western riding. For the SDFT, breed, CSA at specific distances distal to the distal border of the accessory carpal bone, (p 0.05). The overall CSA from all measurements points was smallest in QH, largest in TB and WB, with SB having a CSA measurement between the other breeds. The overall CSA of SDFT in TB did not change during the observation period. However, in QH and WB there were significant decreases in the CSA, and these decreases varied between the CSA measured at specific distances distal to the distal border of the accessory carpal bone. In addition, in SB there were significant increases in the overall CSA measurement of the SDFT, followed by a decrease from August to December with CSA returning to initial values (p 0.05). The overall CSA from all specific distances distal to the distal border of the accessory carpal bone of DDFT was smaller in QH than in the horses of the other breeds studied (p 0.05 among all). The QH also did not have any change in the CSA of AL-DDFT during the observation year at any specific distance distal to the distal border of the accessory carpal bone or between limbs (p>0.05). In comparison, SB CSA of AL-DDFT increased at the 8cm measuring point in the left forelimb between January and August and then decreased below the initial value by December (p<0.01). There were no changes of CSA of AL-DDFT in the left forelimb of TB, while in the right forelimb CSA increased continuously at 4cm and 8cm distal to the distal border of the accessory carpal bone between January and August and then returned to the levels measured in January (p<0.05). In WB CSA of AL-DDFT decreased continuously during the year of observation at all measurement points in both forelimbs (p<0.01 at least). In conclusion, the development of the CSA of flexor tendons in 2-year-old horses varied markedly among breeds and during the year of observation. It remains to be proven whether the changes are due to genetics only or the management and especially the physical training of the horses played a role.