Patellofemoral Contact Pressures After Patellar Distalization: A Biomechanical Study.

Abstract

To measure the patellofemoral contact pressure in early flexion after a tibial tubercle distalization osteotomy. Ten matched-pair fresh-frozen cadaveric knees were studied. The average Blackburne-Peel ratio of the native knees was 0.91. The knees were placed on a testing rig, with a fixed femur and tibia mobile through 90° of flexion. Individual quadriceps heads and the iliotibial band were separated and loaded with 205 N in anatomic directions using a weighted pulley system. A straight tubercle distalization osteotomy of 1 cm was performed and fixed with screws, with and without a lateral release. Patellofemoral contact pressures were measured at 0°, 10°, 20°, 30°, 45°, 60°, and 90° of flexion using pressure-sensitive films on the medial trochlea and lateral trochlea. Contact force, area, and pressure were measured in the following states: (1) in the native knee, (2) after distalization, and (3) after distalization with lateral release. The average Blackburne-Peel ratio after distalization was 0.64. Tibial tubercle distalization resulted in a 6-fold increase in mean contact pressure at 0° (0.15 MPa vs 0.90 MPa, P < .001) and a 55% increase at 10° of flexion (0.70 MPa vs 1.09 MPa, P = .02). Mean contact pressure was similar from 20° to 90° of flexion (P > .1). After distalization, the total contact area was significantly higher at 0° of flexion (17.7 mm² vs 58.4 mm², P = .02). Lateral release after distalization did not significantly change contact pressure (P > .21). Our results suggest that patella baja, as a result of excessive patellar distalization, can cause increased patellofemoral contact pressures during early flexion at 0° and 10°. No changes were seen in contact pressure from 20° to 90°. Care should be taken to prevent excessive distalization of the patella to avoid patella baja and increased patellofemoral contact pressures during early flexion.