title: Radiographic grading of the patellofemoral joint is more accurate in skyline compared to lateral views.
abstract: BACKGROUND
  The patellofemoral joint is perhaps the most commonly involved compartment in knee osteoarthritis (OA). Radiographic studies are routinely ordered and are seen as the first-line investigations for patellofemoral osteoarthritis (PFOA). The Kellgren-Lawrence (K&L) grading scale is often used to quantify the severity of radiographic OA. In this study, we aim to determine the correlation of the K&L grading scale on both the skyline and lateral views with arthroscopic visualization of articular cartilage damage.
  
  
  METHODS
  All patients with clinical and radiographic features of PFOA who underwent knee arthroscopy by a single surgeon from 2006 to 2010 in our institution were reviewed. The study group consisted of 66 patients with PFOA. All patients had skyline and lateral radiographs of the knee taken before surgery. All patients had arthroscopic evidence of PFOA. Blinded investigators graded the radiographs according to the K&L grading scale. At arthroscopy, the patellofemoral joint was graded according to the Outerbridge classification. Correlation and statistical analysis of the radiographic and arthroscopic grade was carried out.
  
  
  RESULTS
  The general trend shows that the higher the radiographic K&L grading, the greater the severity of articular cartilage degeneration on arthroscopy. However, an increasing K&L grade accounts for only 39.7% and 28.4% of the variation of severity of arthroscopically-determined articular cartilage degeneration on skyline and lateral views respectively. Interestingly, on both views, better correlation with arthroscopic findings was seen in early (K&L grades 1 and 2) PFOA. Skyline views were superior to lateral views in terms of specificity, PPV, NPV and accuracy in predicting early OA.
  
  
  CONCLUSIONS
  Skyline radiographs are more accurate than lateral radiographs in prediction of severity of PFOA.
authors.affiliation: Department of Orthopaedic Surgery, Singapore General Hospital, Outram Road, Singapore 169608.
authors.forename: Hwee-Yee Christian
authors.initials: HY
authors.lastname: Heng
authors.affiliation: Department of Orthopaedic Surgery, Singapore General Hospital, Outram Road, Singapore 169608.
authors.forename: Hamid Rahmatullah
authors.initials: HR
authors.lastname: Bin Abd Razak
authors.affiliation: Department of Orthopaedic Surgery, Singapore General Hospital, Outram Road, Singapore 169608.
authors.forename: Amit Kanta
authors.initials: AK
authors.lastname: Mitra
keywords: Kellgren-Lawrence (K&L); arthroscopy; patellofemoral osteoarthritis (PFOA); radiographic