Abstract
Biodynamic studies on normal hip joint have seldom referred to shelf hypoplasia. In the present study, hip joint resultant forces caused by changes in shelf angle (45°, 50°, 55°, 60°) and femoral neck body angle (110°, 130°, 150°) were subjected to our biodynamically comparative investigation in two-dimensional photoelastic experiments. The following results were obtained: 1) Median loading at a shelf angle of 45° (normal) or 50° to normal femoral bone (collodiaphyseal angle 130°) produced no observable changes in either stress distribution or stress concentration, but at angles of 55° and 60°, stress concentration acting on the shelf lateral border showed an increase in hip joint stress relative to the increase in shelf angle. 2) It was observed that stress distribution and stress concentration resulted in no particular change in cartilage degeneration by progress of shelf hypoplasia, even if the cartilage thickness was reduced to half. However stress concentration reached a value 2.5 times the former value in the case of reduced cartilage defect. 3) Changes in hip joint resultant forces due to changes in femoral collodiaphyseal angles (±10%) showed decreases in resultant forces due to decreased collodiaphyseal angle and increases due to increased collodiaphyseal angle (±10%) . Observation of natural courses in patients with shelf hypoplasia indicated an increase in shelf angle and a decrease in thickness due to cartilage degeneration. Rotational acetabular osteotomy for these patients resulted in stress concentration passing to a region of low cartilage degeneration with a decreased stress value. This was confirmed clinically to be in good accord with the results of the present study. Trochanterosteotomy for decreased femoral collodiaphyseal angle resulted in decreased hip joint resultant forces.
