Comparison of tibial anatomical-mechanical axis angle between predisposed dogs and dogs at low risk for cranial cruciate ligament rupture

- Tibial anatomic-mechanical axis angle (AMA-angle) is described.
- AMA-angle is significantly higher in dogs predisposed to cranial cruciate ligament rupture (CCLR).
- AMA-angle higher than 1.87° predicts a CCLR with a sensitivity of 0.941.
- AMA-angle higher than 1.87°predicts a CCLR with a specificity of 0.965.
- AMA-angle is a more accurate predictor of CCLR than tibial plateau angle, relative tibial tuberosity width and Z-angle.

The purpose of this prospective, radiographic, descriptive study was to compare measurements of tibial anatomical-mechanical axis angle (AMA-angle), tibial plateau angle (TPA), relative tibial tuberosity width (rTTW) and Z-angle from mediolateral radiographs of the tibia between two canine breeds (72 dogs) not predisposed to cranial cruciate ligament rupture (CCLR) and those from a consecutive series of 185 large dogs and 17 West Highland white terriers (WHWT) diagnosed with unilateral, surgically confirmed CCLR. Correlations among these measurements were determined, and levels of inter- and intra-observer variability among and within three observers for each measurement were established using Kendall's coefficient of concordance.Breed had a significant effect on AMA-angle. The median AMA-angle of the subject population of large dogs affected by CCLR was 2.80° (range 1.09°-5.21°); for the WHWT, it was 6.34° (range 5.68°-8.88°); and for the clinically normal dogs, it was 0.74° (range 0.00°-5.40°). In the CCLR group, AMA-angle and TPA were strongly correlated (r = 0.745; p < 0.0001). A receiver operating characteristic (ROC) curve analysis showed that an AMA-angle higher than 1.87° had a sensitivity of 0.941 (95% confidence interval [CI]: 0.898-0.966) and a specificity of 0.965 (95% CI: 0.919-0.987) for predicting CCLR and was more accurate than TPA, rTTW and Z-angle at predicting CCLR (p < 0.0001). Good inter- and intra-observer agreement was found for all measurements. The highly significant difference in AMA-angle found between clinically normal dogs and dogs with CCL injury suggests that AMA-angle magnitude may be a clinically relevant predisposing factor for the development of canine CCLR.