Original Research Comparison of Quadriceps Angle Measurements Using Short-Arm and Long-Arm Goniometers: Correlation With MRI Christine E. Draper, PhD, Kelvin T. L. Chew, MD, Roberta Wang, MD, Fabio Jennings, MD, Garry E. Gold, MD, Michael Fredericson, MD Objective: To compare the reliability of quadriceps-angle (Q-angle) measurements per- formed using a short-arm goniometer and a long-arm goniometer and to assess the accuracy of goniometer-based Q-angle measurements compared with anatomic Q angles derived from magnetic resonance imaging (MRI). Design: An intra- and interobserver reliability study. Setting: University hospital. Participants: Eighteen healthy subjects with no history of knee pain, trauma, or prior surgery were examined. Methods: Two physicians, blinded to subject identity, measured Q angles on both knees of all subjects using 2 goniometers: (1) a short-arm goniometer and (2) a long-arm goniometer. Q angles were derived from axial MRIs of the subjects’ hip and knees. Main Outcome Measurements: The intra- and interobserver reliabilities of each goniometer were assessed using the intraclass correlation coefficient (ICC). The com- parison between clinical and MRI-based Q angles was assessed by using the ICC and a paired t-test. Results: Intra- and interobserver reliabilities of the long-arm goniometer (intraobserver ICC, 0.92; interobserver ICC, 0.88) were better than those of the short-arm goniometer (intraobserver ICC, 0.78; interobserver ICC, 0.56). Although both goniometers measured Q angles that were moderately correlated to the MRI-based measurements (ICC, 0.40), the clinical Q angles were underestimated compared with the MRI-based anatomic Q angles (P  .05). Conclusion: The results of this study suggest that, although reproducible Q-angle measurements can be performed using standardized patient positioning and a long-arm goniometer, methods to improve the accuracy of clinical Q-angle measurements are needed. PM R 2011;3:111-116 INTRODUCTION The quadriceps angle (Q angle) is an estimate of the line of action of the quadriceps muscle and reflects the combined pull of the quadriceps and the patellar tendon on the patella. It is defined as the angle formed between the line that joins the anterior superior iliac spine (ASIS) and the center of the patella, and the line that joins the tibial tuberosity and the center of the patella [1]. The Q angle is frequently used in the clinic to diagnose and treat lower-limb injuries [2]. In a prospective study, runners with Q angles 20° were almost twice as likely to be injured compared with those with lower Q angles [3]. This highlights the value of the Q-angle measurement in the clinic and the need for accurate and repeatable measurements. The Q angle is traditionally measured with the patient supine and the knee extended or slightly flexed. Most physicians perform this measurement in the clinic using a standard pocket goniometer, which results in measurements that are highly dependent upon the physical examination technique [4]. Q-angle measurements are particularly sensitive to error because the measurement necessitates defining the center of the patella. It has been reported that the center of the patella must be defined to within C.E.D. Department of Radiology, Stanford Uni- versity, Stanford, CA; James H. Clark Center, Rm S-355 MC 5450, 318 Campus Dr, Stan- ford CA 94305-5450. Address correspon- dence to: C.E.D.; e-mail: cdraper@stanford. edu Disclosure: nothing to disclose K.T.L.C. Changi Sports Medicine Centre, Changi General Hospital, Singapore Disclosure: nothing to disclose R.W. Department of Physical Medicine and Rehabilitation, Santa Clara Valley Medical Center, San Jose, CA Disclosure: nothing to disclose F.J. Rheumatology Division, Sao Paulo Fed- eral University, Sao Paulo, Brazil Disclosure: nothing to disclose G.E.G. Department of Radiology, Stanford Uni- versity, Stanford, CA Disclosure: 2A, Zimmer Arthrocare; 7B, GE Healthcare; 8B NIH, SCBT/MR Coulter Foun- dation M.F. Department of Orthopedics, Stanford Uni- versity, Stanford, CA Disclosure: 2A, Cool Systems Inc. Disclosure Key can be found on the Table of Contents and at www.pmrjournal.org Financial support provided by the NIH (EB0002524, EB005790). Submitted for publication June 23, 2010; accepted October 12, 2010. PM&R © 2011 by the American Academy of Physical Medicine and Rehabilitation 1934-1482/11/$36.00 Vol. 3, 111-116, February 2011 Printed in U.S.A. DOI: 10.1016/j.pmrj.2010.10.020 111