34th Annual Scientific Meeting proceedings

Radius and ulna fractures are quite common in dogs and represent the third most common limb fracture in dogs with an incidence of about 18%. Approximately 85% of antebrachial fractures occur in the distal third of the diaphysis of the radius and ulna. In relation to the dog breed, these fractures occur more frequently in toy breeds and these breeds have a relatively high complication rate. Various breed-related factors have been identified as possible causes in toy breeds, such as morphological, vascular, bony and mechanical differences to other breeds. Nevertheless, the causal mechanism for the predisposition and the increased complication rate in antebrachial fractures of toy breeds is not yet fully understood.

Therefore, the aim of our study was to research the associated mechanisms by investigating the trabecular structure parameters (BV/TV, Tb.N, Tb.Th, Tb.Sp), Conn. D, DA) and the cortical bone density (Mean Density) for toy breeds and small dogs of other breeds using microcomputed tomography and to identify the differences between both groups.

A total of 70 forearms, originating from toy breeds (n=36) and small dogs of other breeds (n=34) were evaluated by microcomputed tomography.

The results of this study showed a lower bone volume fraction (BV/TV - %), more (Tb.N - mm^-1) but thinner trabeculae (Tb.Th - mm) with greater separation (Tb.Sp - mm) and decreased diaphyseal cortical density (Mean Density - mg HA/ccm) in toy breed dogs. This could thus contribute to the increased risk of antebrachial fractures in toy breeds. Within the group of toy breeds inhomogeneous results were observed, whereby the values of the Chihuahuas deviated. This breed had significantly thinner trabeculae (Tb.Th) with a greater trabecular separation (Tb.Sp), lower bone volume fraction (BV/TV) and significantly lower diaphyseal cortical density (Mean Density). These parameters could therefore indicate an increased antebrachial fracture and complication risk, especially in Chihuahuas.

To investigate whether this phenomenon also occurs at other bone localizations, the osseous structure of the canine carpal joint was analyzed in more detail in another study. Here, we also investigated the bony microarchitecture of the cancellous and cortical bone of the carpus. For this purpose, a total of 80 carpal joints from 20 medium-sized dogs and 20 toy breeds were examined and compared with each other using microcomputed tomography. The same structural parameters were examined and compared as in the radius and ulna study In addition, the cortical structure was classified using a three-staged scoring system.

It was shown that all carpal bones have a cancellous structure. In addition, the microstructure differs significantly between the groups, without one group being mechanically superior. The evaluation implies that the second carpal bone appears to be very stable. The formation of the cortex differs massively between the groups, with the toy breeds having only a very thin, partially interrupted bone lamella, whereas the medium-sized dogs have a normal cortex. Within the group of toy breeds inhomogeneous results were observed, whereby the values of the Chihuahuas deviated. This breed had significantly fewer (Tb.N) and thinner trabeculae (Tb.Th) with a greater trabecular separation (Tb.Sp), lower bone volume fraction (BV/TV) and higher bone surface (BS/BV). This indicate a decreased stability of the Chihuahua`s carpal bones.

In summary, the results of the two studies show that the bony structural parameters of the radius and ulna as well as the carpal bones differ between the toy breeds and the other dogs examined. Within the toy breeds, the Chihuahua stands out with its different values in both studies.

One possible reason for these different bone parameters between Chihuahuas and other toy breeds could be that dogs of this breed are increasingly carried in the arms or in a bag, which puts less strain on the bones. Whether this or other causes are responsible for these different trabecular and cortical structural parameters needs to be investigated in further studies.