We compared ventrodorsal hip and laterolateral lumbar spine projections in the radiological diagnosis of LTV in German shepherd dogs, and scrutinized the radiographic and CT features of the LTV.
The incidence of LTV is dependent on the definition. In the present study, dogs with eight lumbar vertebrae as the only abnormal finding were classified as to having a LTV, which contrasts with the previous studies. It has argued that L8 is a clinically irrelevant finding [6
] and hence the diagnosis can reliably be based on ventrodorsal hip radiographs. However, in our study all ten dogs with eight lumbar vertebrae had a short and caudally positioned last presacral vertebra (L8). The angle and disc space of the lumbosacral junction in these dogs resembled the normal condition, but the position of the L8 relative to the ilium was near the position of the normal S1 vertebra. Additionally, L7 was positioned more cranially in relation to the ilium (Figure ) and the relative length of L6/L7 was smaller in these dogs. Our findings support those in a study of the vertebral canal between dogs with numerical vertebral variation, in which 11 of 20 dogs with L8 vertebra were German shepherd dogs. In that study, the widest diameter of vertebral canal was at the same level in dogs with seven and eight lumbar vertebra, if L8 was assumed as S1 in the latter group [9
The incidence of LTV was 40% in our study, which was markedly higher than in other studies [6
]. In our study, separation of the S1 spinous process from the median crest of the sacrum was classified as LTV, which could explain the high incidence, since the aforementioned radiographic sign was found in 62 dogs (27%) in our study. In a recent study [8
], in which separation of the S1 spinous process was classified as LTV, the incidence was also quite high (29%).
In our study, the relative length of L6/L7 was similar in dogs with separation of the S1 spinous process from the median crest as the only abnormality and dogs with LTV, suggesting similar morphology. From the results, we can also conclude that in all three LTV types the relative length of L6/L7 tended to be smaller than in the dogs with normal lumbosacral junctions; i.e. dogs with all types of LTV have a longer L7 in comparison to L6 than dogs with normal lumbosacral junctions. Separation of the S1 spinous process from the median crest as the only abnormality can be genetically a mild form of LTV, and selection against the trait could decrease the incidence of more serious forms of LTV in German shepherd dogs.
The laterolateral projection made the diagnosis of L8 possible. A statistically significant increase in the diagnostic accuracy of LTV was detected when the laterolateral lumbar spine projection was included in the protocol in addition to the ventrodorsal projection. It would have been be possible to diagnose a short L8 with abnormally small transverse processes from the ventrodorsal projection used in hip dysplasia screening, but this would have been difficult if the second to the last lumbar vertebra was not included in the radiograph for comparison. This is seldom the case, because only the last lumbar vertebra is consistently seen in hip radiographs made according to the protocol of Fédération Cynologique Internationale (FCI) [12
Some of the radiographic signs, such as separation of the S1 spinous process from the median crest of the sacrum and separation of the sacral vertebrae were visible in only one of the two projections. Every missed case in the ventrodorsal projection had eight lumbar vertebrae. Surprisingly, five of six dogs with symmetrical intermediate or lumbar-type LTV had a normal sacrum based on a laterolateral radiograph. The existence of abnormal transverse processes was ensured with CT in two of these dogs (dog 3 and dog 9 in Table ). In CT images, a deep indentation between the first and second spinous process of the median crest (Figure ) was seen in five dogs. In ventrodorsal radiographs this was seen as separation of the S1 spinous process from the median crest of the sacrum (Figure ). We showed that variation in the radiographic findings of LTV was wide even in the rather small number of cases. The dogs were classified into intermediate (type 2) or sacral (type 3) types, based on visibility of the transverse process in the ventrodorsal radiographic projection. This classification, based on the appearance of the tip of the transverse process, was factitious, since the visibility of the transverse processes of the LTV in radiographs was influenced by the projection and superimposition of the ilium. In Switzerland, the Swiss Dysplasia Committee recently introduced a four-scale grading, in which type 0 is a normal lumbosacral area, type 1 is a sacrum with S1 separated from the median crest of the sacrum, type 2 is a symmetrical LTV, and type 3 is an asymmetrical LTV [13
]. This grading was not published when we planned our study, but it seems reasonable, since it does not attempt to classify the LTV, based on the appearance of the transverse processes, which can lead to erroneous classification, as was seen in our study.
A limitation in our study was the lack of radiographs of the total spine, since it can be argued that a transitional vertebra in the thoracolumbar junction can cause the extra vertebra. However, the markedly caudal position of L8 speaks for an LTV. Another limitation was the low number of dogs; however, the results were statistically significant.