Comparison of Insall-Salvati Index and Its Modification in Normal Dogs from Four Different Body Weight Groups.

Objective:  The objectives of this study were to record the values of Insall-Salvati (IS) index and its modification in dogs classified into four different body weight groups and to determine whether these index values were influenced by sex.

Study Design:  Bilateral mediolateral radiographs of 86 dogs were obtained. The dogs were classified into four different body weight groups: small, medium, large and giant. The patellar length (PL), caudal and cranial patellar ligament lengths (LLca and LLcr), stifle angle and superimposition of the femoral condyles were measured on the radiographs. The IS index and its modification (LLcr:PL) were calculated by dividing LLca or LLcr by the PL respectively. Spearman's correlation test and analysis of variance were used for statistical analyses.

Results:  The sex of the dog and the presence of enthesophytes at the apex of the patella were two influential factors for the IS and LLcr:PL values. The mean IS values were 1.60 ± 0.28 in small dogs, 1.73 ± 0.27 in medium dogs, 1.82 ± 0.21 in large dogs and 1.97 ± 0.13 in giant dogs. The mean LLcr:PL for the same groups were 2.05 ± 0.23, 2.13 ± 0.19, 2.23 ± 0.21 and 2.27 ± 0.16 respectively.

Conclusion:  The reference intervals for the IS index were 1.04 to 2.16 in the small dogs, 1.19 to 2.27 in the medium dogs, 1.40 to 2.24 in the large dogs and 1.71 to 2.23 in the giant dogs. The reference intervals for the LLcr:PL index were 1.59 to 2.51 in the small dogs, 1.75 to 2.51 in the medium dogs, 1.81 to 2.65 in the large dogs and 1.95 to 2.59 in the giant dogs. Dogs with values exceeding these reference intervals have patella alta and those with values below these intervals have patella baja.