Patello-femoral instability is related to multiple factors, one of them is the morphological variant of patellar articular surface. Wiber’s classification is used to classified anatomical features of patellar articular surface and consist in five morphological types. The aim of this study was to establish intra- and inter-observer reliability of Wiber's magnetic resonance (MRI) criteria.

We selected 25 patients: 11 males and 14 females (mean age 33 years old), that attended to our hospital for anterior knee pain and that underwent to knee MRI. Patient were evaluated with a low intensity magnetic field (0.3 T – Esaote, Genova). Each patient has been examined independently by three trained doctors (two seniors and one junior) who classified each patellar shape in five grades (1 – 2A – 2B – 3 – 4) according to Wiber’s classification to assess inter-observer agreement. Every doctor...

All observers agreed on the same shape for 16 cases. In nine patients the doctors did not agree and in only two cases observers classified patella in three different shapes. Disagreement was mostly related to mistakes between type 2A and type 2B patella’s articular surface. Not mistakes were done for the most serious variants (type 3 and 4). The inter-observed reliability was substantial (0.61<k<0.8; kappa= 0.64), while the intra-observer reliability was excellent (0.81<k<1; k= 0.84).

Our study suggests that Wiber’s classifications is a good and well applicated method to assess patellar morphologic features. Good results are probably due to trained doctors dedicated in musculoskeletal MRI. It could be demonstrated if the same results could be obtained with not dedicated radiologists. The good reliability of inter- and intra-observer evaluation suggests Wiber’s classification as an important skill to evaluate femoro-patellar instability.