Magnetic resonance measurements of the deviation of the angle of force generated by contraction of the quadriceps muscle in dogs with congenital patellar luxation.

Objective: To measure the quadriceps angle (Q-angle) in dogs with congenital patellar luxation using magnetic resonance (MR) methods.

Study Design: Prospective clinical study.

Animals: Thirty-eight client-owned dogs.

Methods: Thirty-eight dogs were examined and placed into the following groups based on the degree of patellar instability: normal, grade I, grade II, and grade III. MR images of 37 pelvic limbs without patellar instability, 33 pelvic limbs with patellar luxation, and 6 limbs with cranial cruciate ligament (CrCL) rupture were made. The Q-angle was calculated using trigonometric methods based on MR images. Limbs with patellar luxation were compared with normal stifles and stifle with other disorders.

Results: The average Q-angle of the normal group was 10.5 degrees (24.9 degrees to -2.0 degrees ). The grade I group had an average Q-angle of 12.2 degrees (28.8 degrees to 2 degrees ), the grade II group 24.3 degrees (44.6 degrees to 7.7 degrees ), and the grade III group 36.6 degrees (51.4 degrees to 15.6 degrees ). The average Q-angle of limbs with an isolated CrCL rupture was 19.3 degrees (34.7 degrees to 3.9 degrees ).

Conclusion: MR images can be used to make exact calculations of the Q-angle.

Clinical Relevance: MR images can be used to quantify the degree of patellar luxation.