To determine whether the Q-angle, measured in a defined and reproducible manner, correlates with the TT-TG distance in patients with patellar instability. The Q-angle represents the angle between the vector of action of the quadriceps and patellar tendon. The normal angle is 14+/−3° in males and 17+/−3° in females. An increased Q-angle is associated with an increased risk of patellar instability, although there is disagreement on its reliability and validity. It can be affected by the anatomical points used to record the measurement, the position of the limb and whether the quadriceps are relaxed or contracted. TT-TG is ascertained by axial CT scanning, with a value exceeding 20mm associated with patellar instability. Q-angles were measured in patients presenting to the patella clinic who had previously undergone Lyon protocol CT scanning for patellar instability. Patients were positioned supine with both feet in neutral rotation taped to a specially designed wooden board (the same position used for CT scanning). The anatomical landmarks were the anterior superior iliac spine, the centre of the patella and the centre of the tibial tuberosity. Both knees were measured with the quadriceps relaxed (relaxed Q-angle) and contracted (contracted Q-angle). Thirty-four knees were measured, 24 pathological and 10 non-pathological. Pearson moment correlation demonstrated a significant correlation between relaxed Q-angle and TT-TG in all knees (R=-0.377; p=0.028). In pathological knees, contracted Q-angle also demonstrated a significant correlation with TT-TG but to a lesser extent than relaxed Q-angle (R=-0.428; p=0.037, R=-0.578; p=0.003 respectively). Linear regression analysis demonstrated relaxed Q-angle as a significant predictor of TT-TG distance in pathological knees. Contracted Q-angle was not significant.Statement of purpose
Methods and results
Ultrasonography for rotator cuff disease is a cheap and non-invasive investigation. Our study investigates the tendon specific pathologies leading to misinterpretation of ultrasound findings and their implication for the surgical management. On hundred and five consecutive patients who had an ultrasound scan by a single musculoskeletal radiologist and then underwent shoulder arthroscopy by a single shoulder surgeon for rotator cuff pathologies were included. Surpraspinatus Tendon (SST): There was a sensitivity of 90%. The relatively low positive predictive value (76%) and specificity (42.5%) were influenced by a high number of false positives. This was a mixed group of 23 cases, in which ultrasonography had described either a full-thickness (FTT) or partial-thickness (PTT) tear when arthroscopy did not show any evidence for a cuff tear. Seven of these cases were described as FTT with dimensions less than 1 cm and in ten cases the radiologist described a “possible sub-centimetre tear”. Subscapularis Tendon (SSC): There was a specificity of 100%. The poor negative predictive value (78%) and sensitivity (26%) were caused by a high number of false negatives. Further analysis of the 20 “false negative” patients showed four FTT and sixteen PTT. All partial thickness tears involved the superior fibres of the subscapularis tendon. Our results confirm that USG is a reliable investigation in larger full thickness tears, particularly of the superior rotator cuff (SST). The reliability is significantly reduced in sub-centimetre tears and partial thickness tears, particularly of the subscapularis tendon. Associated tendon pathologies like intra-tendinous calcifications and intra-substance tears make an accurate diagnosis even more difficult and add to the tendency to ‘over-diagnose’ tears of the rotator cuff with use of ultrasonography. The shoulder surgeon should be aware of the potential misinterpretation of ultrasonography findings and be prepared to adjust the surgical procedure accordingly.
