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.