SCIENTIFIC ARTICLE Minimum joint space width (mJSW) of patellofemoral joint on standing “skyline” radiographs: test-retest reproducibility and comparison with quantitative magnetic resonance imaging (qMRI) Paolo Simoni & Sanaa Jamali & Adelin Albert & Saara Totterman & Edward Schreyer & Jose G. Tamez-Peña & Bruno Beomonte Zobel & Victoria Alvarez Miezentseva & Philippe Gillet Received: 15 May 2013 /Revised: 15 July 2013 /Accepted: 22 July 2013 /Published online: 24 August 2013 # ISS 2013 Abstract Objective To assess the intraobserver, interobserver, and test- retest reproducibility of minimum joint space width (mJSW) measurement of medial and lateral patellofemoral joints on standing “skyline” radiographs and to compare the mJSW of the patellofemoral joint to the mean cartilage thickness calcu- lated by quantitative magnetic resonance imaging (qMRI). Materials and methods A couple of standing “skyline” radio- graphs of the patellofemoral joints and MRI of 55 knees of 28 volunteers (18 females, ten males, mean age, 48.5±16.2 years) were obtained on the same day. The mJSW of the patellofemoral joint was manually measured and Kellgren and Lawrence grade (KLG) was independently assessed by two observers. The mJSW was compared to the mean cartilage thickness of patellofemoral joint calculated by qMRI. Results mJSW of the medial and lateral patellofemoral joint showed an excellent intraobserver agreement (interclass corre- lation (ICC)=0.94 and 0.96), interobserver agreement (ICC= 0.90 and 0.95) and test-retest agreement (ICC=0.92 and 0.96). The mJSW measured on radiographs was correlated to mean cartilage thickness calculated by qMRI (r =0.71, p <0.0001 for the medial PFJ and r =0.81, p <0.0001 for the lateral PFJ). However, there was a lack of concordance between radiographs and qMRI for extreme values of joint width and KLG. Radio- graphs yielded higher joint space measures than qMRI in knees with a normal joint space, while qMRI yielded higher joint space measures than radiographs in knees with joint space narrowing and higher KLG. Conclusions Standing “skyline” radiographs are a reproduc- ible tool for measuring the mJSW of the patellofemoral joint. The mJSW of the patellofemoral joint on radiographs are correlated with, but not concordant with, qMRI measurements. Keywords Patellofemoral joint . Radiography . Magnetic resonance imaging . Articular cartilage . Osteoarthritis knee . Radiography . Adult Introduction Osteoarthritis of the knee is a common disease after the sixth decade of life [1] and an important cause of disability in people over 65 years of age [2]. Symptomatic osteoarthritis of the knee may often be related to patellofemoral disease [3, 4]. Isolated patellofemoral osteoarthritis is not uncommon [3–5] and it may be associated with severe disability requiring a total knee arthroplasty [5]. It has been suggested that “skyline” radio- graphs are the optimum radiological method for assessing P. Simoni (*) : S. Jamali : V. A. Miezentseva Diagnostic Imaging Departement, Domanine du Sart Tilman, CHU de Liège, Bât. 35, 4000 Liège, Belgium e-mail: paolo.simoni@chu.ulg.ac.be A. Albert Biostatistics Departement, Domanine du Sart Tilman, CHU de Liège, Bât. 35, 4000 Liège, Belgium S. Totterman : E. Schreyer : J. G. Tamez-Peña Qmetrics Technologies, 140 Allens Creek Road, Rochester, NY 14618, USA B. B. Zobel Diagnostic Imaging Departement, Campus Bio-Medico University, Via Álvaro del Portillo, 21, 00128 Rome, Italy P. Gillet Orthopaedic surgery Department, Domanine du Sart Tilman, CHU de Liège, Bât. 35, 4000 Liège, Belgium Skeletal Radiol (2013) 42:1573–1582 DOI 10.1007/s00256-013-1701-9