Medial patellar luxation grade IV according to the Singleton system can be classified into skeletally immature and skeletally matured types

Yukari Nagahiro et al.
Am J Vet Res. 2023 May 29;84(7):ajvr.23.02.0020. doi: 10.2460/ajvr.23.02.0020.

Objective: To determine the signalment and musculoskeletal morphology of small-breed dogs affected by medial patellar luxation (MPL) grade IV based on the age of the CT scan.

Animals: 40 small-breed dogs (54 limbs) with MPL grade IV.

Procedures: Dogs that had undergone corrective surgery for MPL grade IV and had performed CT of the hind limb before surgery were included. Signalment (age, body weight, sex, laterality, and breed) and concomitant cranial cruciate ligament rupture (CrCLR) were recorded. Femoral inclination angle, anatomical lateral distal femoral angle (aLDFA), femoral torsion angle, quadriceps muscle length to femoral length ratio (QML/FL), and patellar ligament length to patellar length were obtained by CT images. The dogs were categorized into 2 groups based on their age at the time of the CT scan, the skeletally immature group and the skeletally matured group. Signalment and group were included in the multiple regression analysis to determine the factors associated with each measurement parameter. A logistic regression analysis was conducted to determine the risk of CrCL concomitant with age.

Results: The multiple regression model demonstrated that the group was associated with the value of aLDFA and QML/FL. aLDFA was higher, and QML/FL was lower in group SI than in group SM. CrCLR was present in 5/54 limbs (9.2%), with a mean age of 70.8 months and it was associated with increasing age.

Clinical relevance: In Singleton's classification, dogs classified as grade IV can be categorized into 2 groups based on musculoskeletal morphology and pathophysiology: the skeletally immature and skeletally matured types.