Variability of Measurement of Patellofemoral Indices with Knee Flexion and Quadriceps Contraction: An MRI-Based Anatomical Study

Abstract

PurposeThe purpose of this study was to investigate the impact of varying knee flexion and quadriceps activity on patellofemoral indices measured on magnetic resonance imaging (MRI).Materials and MethodsMRI of the knee was performed in 20 patients for indications other than patellar or patellofemoral pathology. Axial and sagittal sequences were performed in full extension of the knee with the quadriceps relaxed, full extension of the knee with the quadriceps contracted, 30° flexion of the knee with the quadriceps relaxed, and 30° flexion with the quadriceps contracted. Bisect offset, patella tilt angle, Insall-Salvati ratio and Caton-Deschamps index were measured.ResultsWith the knee flexed to 30° and quadriceps relaxed, the mean values of patellar tilt angle, bisect offset, Insall-Salvati ratio and Caton-Deschamps index were all within normal limits. With the knee extended and quadriceps contracted, the mean patellar tilt angle (normal value, <15°) was 14.6° and the bisect offset (normal value, <65%) was 65%, while the Caton-Deschamps index was 1.34 (normal range, 0.6 to 1.3). With the knee extended and quadriceps relaxed, the mean Caton-Deschamps index was 1.31.ConclusionsMRI scanning of the knee in extension with the quadriceps contracted leads to elevated patellofemoral indices. MRI taken with the knee in 30° of flexion allows more reliable assessment of the patellofemoral joint and minimises the confounding effect of quadriceps contraction.

A Sagittal Patellar Angle Linear Equation Reflecting Patellofemoral Kinematics: Evaluation of Patellar Height at any Degree of Knee Flexion Angle

“…One limitation of the present study is that we evaluated the data without identifying weight characteristics, and we acknowledged that may have some influence on the accuracy of the outcome. However, Laugharne et al 21 reported no significant difference with relaxed or flexed position. Second, we found the measured and calculated sagittal angles of the patella were not completely consistent both in the original literature and our study.…”

“…Quadriceps force may alter with different knee flexion angles and subsequently influence the measurement results. Laugharne et al 21 reported the difference under quadriceps in activation or not was negligible in patellar height measurement, however, this is still in dispute.…”

Patella Height as Defined by Patella–Trochlear Articulation on MRI Scan—A Systemic Review of Current Evidence

“…In fact, Laugharne et al showed that even though knee extension and quadriceps contraction lead to the highest Insall-Salvati and Caton-Deschamps indices, these were not significantly different to relaxed and flexed positions. 34 On the contrary, Yiannakopoulos et al utilizing plane radiographs reported that in normal asymptomatic knees, the patella height changes significantly between nonweight bearing and weight bearing utilizing indirect indices. 35 Similarly, quadriceps activation led to significant elevation in modified Insall-Salvati and Blackburne-Peel indices on radiographs in another comparative study.…”

Comparative US-MRI evaluation of the Insall–Salvati index

“…In 1971, Insall-Salvati (IS) index was proposed as the ratio of the patella tendon length (length of the posterior surface of the tendon from the lower pole of the patella to its insertion on the tibia) to the length of the patella (greatest pole-to-pole length) [4]. This index is generally measured on lateral view of knee X-ray or on sagittal plane of MRI in a 30° flexed knee; the normal range is considered between 0.8 and 1.2 (patella baja <0.8; patella alta >1.2) [5,6].…”