Background: The anterior interval of the knee has been defined as the space between the infrapatellar fat pad and patellar tendon anteriorly, and the anterior border of the tibia and the transverse meniscal ligament posteriorly. Investigation of the normal kinematics of this region is necessary as we begin to appreciate the significant impact that pathologic processes of the anterior interval have on the knee.
Methods: Non-weight bearing and weight bearing dynamic MRIs of 20 healthy knees were evaluated at 30 degrees intervals from 0 degrees to 120 degrees flexion. The angle subtended by the patellar tendon and the anterior tibia was measured at each interval of flexion by three independent observers. The amount of angular change over each interval of flexion was also evaluated and the differences between the relative weight bearing conditions were statistically evaluated.
Findings: The angle formed by the anterior tibia and the patellar tendon decreases with knee flexion (45.2 degrees (SD 10.1 degrees ) at full extension vs. 1.2 degrees (SD 2.1 degrees ) at full flexion). The average patellar tendon-tibial angle excursion was significantly reduced with full-weight bearing, 43.1 degrees (SD 11.2 degrees ) from 0 degrees to 120 degrees of flexion, compared to non-weight bearing, 30.9 degrees (SD 6.1 degrees ) over the same range of motion (P<0.001). Full-weight bearing decreased the angle excursion by 28% compared to non-weight bearing.
Interpretation: The observed changes in the anterior interval are influenced by multiple factors including load, knee architecture, tendon elasticity and tibio-femoral and patello-femoral kinematics. The impact of load on the mechanics of the anterior interval is most pronounced between 0 degrees and 30 degrees of flexion.
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