Beräkning av bendensitet och benvolym vid datortomografisk helkroppsundersökning av katt : jämförelse av två metoder för beräkning samt inverkan av storlek, ålder, kön och osteoartrit

Sammanfattning: Bone volume and bone density may be affected by disease. Measurement of bone volume and bone density in cats could possibly be used as a tool in diagnosing osteoarthritis (OA), lameness and other diseases expected to affect bone density and bone volume. Measurement of bone volume is also useful when evaluating body composition. However, at present there is a lack of validated methods for measuring bone volume and bone density in cats using computed tomography. Additionally, knowledge of normal variation in bone volume and bone density is scarce. This study had three main aims. The first was to compare two methods (one time consuming manual and one fast automatic) for measuring bone volume and bone density using computed tomography and based on Hounsfield units as an estimation of bone density. The second was to examine whether factors such as sex, age and bone volume affects bone density in healthy cats. The third was to examine if bone density and bone volume differs between cats with radiological signs of moderate to severe OA in right elbow joint compared to cats without OA. The study material consisted of whole body computed tomography scans of 75 cats. The study was divided into three parts. Part I compared two methods (one manual and one automatic) for measuring bone volume and bone density of the whole body as well as the right front leg only in all 75 cats. Part II investigated whether bone density in healthy cats without OA (n = 15) is affected by sex, age or bone volume. In part III, bone volume and bone density were compared between a group of cats with right elbow joint OA (n = 9) to cats without OA (same individuals as in part II). In part II and III of this study, only measurements made by the manual measurement were used since it was considered the reference method. The two methods tested in part I had very good correlation (P <0.0005; R2 = 99.90-100.00 %). There were significant, systematic differences between all of the means of the differences of the methods and for measurements of whole body volume, the difference was multiplicative. In part II, the “healthy” non-OA group consisted of 15 cats. The subgroups (sex, age and bone volume) contained 5 to 10 individuals. Results showed that, in healthy individuals, only whole body bone volume had a significant influence on bone density (P = 0.035). In part III, cats with OA showed significantly higher bone volume and bone density, both when measuring whole body (bone volume P = 0.017, bone density P = 0.018) and right front leg separately (bone volume P = 0.042, bone density P = 0.049) compared to “healthy” non-OA cats. In conclusion, there are systematic differences between the two methods but both methods can be used, as long as measurement values from the automatic method are converted using a regression equation to corresponding values for the manual method. Despite small subgroups in part II there was a significant difference in bone density between individuals with large bone volume and individuals with small bone volume when measuring the whole body but not when measuring right front leg. Cats with right elbow OA had both higher bone volume and higher bone density compared to cats without OA. Further studies are needed to investigate the causes behind the findings in this study.