Investigation into the Morphology of the Canine Sacrum and its Relationships with Selected Structures of the Vertebral Column and the Hind Limb

Abstract

In dogs, the sacrum consists of three fused vertebrae (Evans & De Lahunta, 2013) and recently the population of greyhounds in Victoria, Australia have shown a remarkably high incidence of sacrocaudal fusion in which the sacrum consists of four fused vertebra (Oheida, Philip, Yen, & Davies, 2016). Scientific data and the available literature regarding how the occurrence of sacrocaudal fusion might influence the morphology of other parts of the locomotory system in dogs is not clear enough to explain why sacrocaudal fusion appears to be so prevalent in the Victorian greyhound population. This study set out to explore the hypothesis that there are measurable differences in the morphology of the sacrum (S. Weight, S. Length, and S. Width) between greyhounds with standard (3 fused vertebrae) and those with different types of fused sacra (4 fused vertebrae, Type B, C, and D). Also, it was hypothesised that there is an association between the sex and body mass of the greyhounds and morphology of the sacrum (S. Weight, S. length, and S. Width). In addition, this study included an exploration into the influence of sacrocaudal fusion on the morphology of related anatomical structures within the spine such as the L.7 vertebra, and selected bones in hind limbs such as the calcaneus, talus, and patella.

The sacra were collected from 171 greyhounds and classified using two systems. For the first system, sacra were classified based on the number of fused vertebrae and the type of fusion into four types: Type A (standard sacra), Type B (complete fusion between the transverse processes and between the bodies of the S3 and Ca1 vertebrae), Type C (fusion only between the transverse processes of the S3 and Ca1 vertebrae), and Type D (fusion only between the bodies of the S3 and Ca1 vertebrae). For the second classification system, sacra were classified based on the presence, reduction, or absence of the median sacral crest between the spinous processes of the S1 and S2 vertebrae into the following three types: Type F (a complete fusion or crest), Type R (reduction in the height of the crest between successive spinous processes), and Type N (absence of the crest). The length, width, and weight of sacra, calcanei, tali and patellae were recorded and compared between the greyhounds with standard and fused sacra. Also, the length of the L.7 vertebra and the angle of the lumbosacral junction and the angle of the spinous process of the 1st sacral vertebra were compared between the greyhounds with standard and fused sacra. There were variable numbers of the population in various chapters because some bones were too difficult to measure or because of breakage of bones during processing. The exact sample numbers are indicated in each table illustrating the results.

The results of the studies showed that there was an increase in the convexity of the curvature of the pelvic surface of the sacrum and an increase in the roughness of the articular surface of the sacral wing in association with the occurrence of sacrocaudal fusion. Sacrocaudal fusion was found in 71 out of 171 greyhounds (41%). Overall, the mean weight, length, and width of the fused sacra were found to be significantly greater (P < 0.01) than standard sacra. Among the fused sacra, 13.5% (23/71) were Type B, 21.1% (36/71) Type C, and 6.4% (11/71) Type D. Type B sacra (complete fusion) were the heaviest (P <0.001), broadest (P <0.001), and longest (P <0.001) sacra compared to Type A, C or D sacra. Regarding standard and different types of fused sacra, there was no association between the sex, body mass and the occurrence of sacrocaudal fusion. In addition, different scenarios were suggested for the evolution of sacrocaudal fusion in the greyhound. The first scenario was termed “Continuous Dependent Sacrocaudal Fusion”, in which it was proposed that fusion occurred first between the S3 and Ca1 vertebral bodies, followed by the occurrence of fusion (Type C), with a final result of complete fusion (Type B). The second scenario was termed “Continuous Independent Sacrocaudal Fusion”, in which the sacrum might change from Type A to either Type C or D, and then further evolve into Type B (complete fusion). The third proposed scenario was termed “Non-continuous Independent Sacrocaudal Fusion”, in which a sacrum may evolve directly from Type A (standard) to be Type B, C or D without going through the possible sequences assumed in the first or second scenarios.

The reduction (R) or absence (N) of the median sacral crest between the spinous processes of the S1 and S2 vertebrae, which has been observed in association with the occurrence of sacrocaudal fusion, allowed us to suggest this new classification of sacra in greyhounds. This classification divided sacra into three different types (F, R, and N) and the percentage of each was found to be significantly different between standard and fused sacra (P <0.001) with the fused sacra more commonly showing the N type, and not associated with sex or body mass. In addition, the angle of the spinous process of the 1st sacral vertebrae of sacra with median crest type N was statistically significantly less (more upright) than those in sacra with median crest type F (P <0.042). Investigating the morphology of the L.7 vertebra as an anatomical structure articulating directly with the sacrum and lumbosacral junction, showed that the L.7 length was significantly (P ≤ .008) longer and the angle of the lumbosacral junction in greyhounds with fused sacra was significantly increased (P <0.028) in dogs with sacrocaudal fusion compared with those with standard sacra and this variation was not associated with sex (P <0.765) or body mass (P <0.802).

The results showed that the right and left calcanei, tali, and patellae in greyhounds with standard and fused sacra were anatomically similar. Among all greyhounds, asymmetry between some of measurements was found, including in the width of the calcaneus (P <0.009) and the talus (P <0.025) and the length of the calcaneus (P <0.001). Measurements were higher in those bones from left hind limbs. Studying specifically the greyhounds with standard sacra also showed asymmetry in the length of the calcaneus (P <0.008) and again those calcanei from left hind limbs (L.C.L) were longer than those from right hind limbs (R.C.L). For greyhounds with fused sacra, asymmetry was found in the width of the talus (P <0.024) and again those of left hind limbs were wider than those from right hind limbs. Regarding the influence of sacrocaudal fusion on bones, there were no significant differences in the mean of measurements in bones between greyhounds with standard and those with fused sacra except for the mass of the right (M.R.C, P <0.003) and left (M.L.C, P <0.006) calcaneus. Also, the measurements of these limb bones from greyhounds with fused sacra were heavier than those with standard sacra.

This thesis provides quantitative data about the occurrence of sacrocaudal fusion and is the first study to investigate the morphological anatomy of the two types of sacrum: standard or fused, as well as selected bones in the vertebral column and hind limbs of greyhounds, such as the L.7 vertebra, calcaneus, talus, and patella.

In summary, sacrocaudal fusion in greyhounds was associated with a change in the morphology of the sacrum, which included an increased angle of the spinous process in the 1st sacral vertebra, a reduction or absence of the median sacral crest between the spinous processes of the S1 and S2 vertebrae, and changes in related structures, such as an increase in the length of the L.7 vertebra and angle of the lumbosacral junction. Furthermore, it seems that sacrocaudal fusion has a type of influence on certain hind limbs bones such as the mass of the right and left calcanei.

These variations in the anatomical features of the sacrum between dogs with or without sacrocaudal fusion may influence the effect of forces applied to the sacral region and also the incidence of injuries.

In conclusion understanding the morphological variations of different types of sacra, in addition to the data provided by this thesis, is establishing a solid base for further research, which might help to explain why sacrocaudal fusion is so common in the Victorian greyhound racing population.

Overall, this work provides an anatomical foundation for understanding the function of the sacrum in greyhounds and highlights how investigation into the sacral anatomy and related structures of the greyhound body may further the current understanding of dog locomotion.