The influence of knee extension and slight flexion on MRI measurements of patellofemoral instability

Objective: To determine the effect of knee position from full extension to slight flexion on cross-sectional MRI measurements for assessing patellofemoral instability. Methods: 25 healthy volunteers (12 men and 13 women;age range: 19-27 years;average age: 24.61 ± 2.52 years) underwent 3T MRI of both knees in full extension and 20° of flexion. The trochlear facet asymmetry, lateral trochlear inclination, trochlear depth, patellar tilt angle, lateral patellar displacement and tibial tuberosity-trochlear groove d1 stance (TTTG) of each knee were measured on MRI by two observers. Paired sample t test was used to compare the six measurements between full extension and 20 ° of flexion positions. Results: In slight knee flexion, measurements of the patellar tilt angle (9.42° ± 4.61° by observer 1, 8.43 ° ± 4.42° by observer 2) were significantly (both P < 0.001) smaller than that of full knee extension (12.91° ± 5.32°, 12.32° ± 5.53°). Differences in measurements of lateral patellar displacement in flexion (-3.92 ± 2.51 mm, -5.03 ± 2.72 mm) and extension (-2.81 ± 3.22 mm, -3.92 ± 3.33 mm) were also significant (P = 0.006, P = 0.012) as were differences (both P < 0.001) in TTTG measurements between flexion (6.93 ± 3.32 mm, 7.43 ± 4.73 mm) and extension (15.12 ± 2.93 mm, 15.93 ± 3.24 mm) for both observers. There were no significant differences (P > 0.05) in measurements of trochlear facet asymmetry in flexion (73.92% ± 9.21%, 61.93% ± 10.32%) and extension (72.41% ± 12.22%, 62.82% ± 12.83%), lateral trochlear inclination in flexion (19.72° ± 4.21°, 20.23° ± 4.82°) and extension (20.71° ± 4.22°, 20.62° ± 3.73°), or trochlear depth between flexion (6.72 ± 1.11 mm, 5.94 ± 1.03 mm) and extension (6.81 ± 1.12 mm, 5.93 ± 1.24 mm) for both observers. Conclusion: Varying degrees of flexion have significant influence on MRI measurements of patellar tilt angle, lateral patellar displacement, and TTTG without affecting measurements of trochlear facet asymmetry, lateral trochlear inclination, and trochlear depth in healthy volunteers. (authors)