Fig. 1

A flow diagram illustrating the process of selecting dogs for the study. The source population consisted of 14 dog breeds. For each dog, we collected the date of birth and an official canine hip dysplasia (CHD) grade from the Norwegian Kennel Club (NKK) [2]. The NKK uses the Fédération Cynologique Internationale (FCI) grading system for CHD, and the evaluation of lumbosacral transitional vertebra (LTV) was based on standard ventrodorsal FCI radiographs. LTV was classified into four types: Type 0 represents normal lumbosacral anatomy; Type 1 is characterised by an independent spinous process of the first sacral vertebra, separated from the medial sacral crest; Type 2 is a symmetrical form where the transverse processes are partially fused with the sacrum or ilium, but the vertebral body remains separate from the sacrum; and Type 3 is asymmetrical, with one side resembling a lumbar vertebra and the other a sacral wing that articulates with the ilium [2, 8]. Our sample included dogs from two selected breeds, based on their home addresses, including dogs with CHD grades A or B (considered free of CHD) and a balanced representation of LTV types across breeds. The target age range was 4 to 8 years, but dogs outside this range were included if their owners were willing to participate. No additional health information was available beyond the details described above