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The mean-square radius of gyration (S2) was determined by small-angle X-ray scattering and light scattering for 20 samples of atactic oligo-and poly(methyl methacrylate)s (a-PMMA), each with the fraction of racemic diads f, = 0.79, in the range of weight-average molecular weight M" from 4.02 x 102 to 2.83 X 106 in acetonitrile at 44.0 °C (0). The ratio (S2)/xw as a function of the weight-average degree of polymerization, xw, exhibits unusual behavior; it passes through a maximum at xw ca 50 before reaching its asymptotic value for large xw. First, a comparison is made of the experimental data with the theoretical values on the basis of three types of the rotational isomeric state model, and it is shown that none of them can explain the observed maximum. Then, it is shown that the helical wormlike (HW) chain theory may well explain the data with the parameter values X"*k0 = 4.

Background Although a wide variety of acetabular deformities in developmental dysplasia of the hip (DDH) have been reported, the morphologic features of the entire pelvis in DDH are not well characterized and their correlation with acetabular deformity is unknown. Questions/purposes We determined whether there was a rotational deformity of the entire innominate bone, and if so, whether it related to acetabular version and coverage. Patients and Methods We examined the morphologic features of the pelvis using CT for 50 patients with DDH (82 hips). Forty normal hips were used as controls. The innominate rotation angle was determined at three levels in the axial plane. The acetabular sector angle served as an indicator of acetabular coverage of the femoral head. We evaluated the association between innominate rotation angles and acetabular version and coverage. Results We observed greater internal rotation of the innominate bone in patients with DDH than in the control subjects. Internal rotation of the innominate bone was associated with increased acetabular anteversion angle and acetabular inclination angle. In hips with acetabular retroversion (nine of 82 hips; 11.0 %), the entire innominate bone was externally rotated, compared with hips with acetabular anteversion. Internal rotation of the innominate bone also was associated with decreased anterior and superior acetabular coverage. Conclusion Our observations suggest structural abnormalities exist throughout the pelvis in DDH, and the morphologic abnormalities of the acetabulum are not caused solely by local dysplasia around the hip, but are influenced by the morphologic features of the entire pelvis.

In patients with developmental dysplasia of the hip, acetabular retroversion results from relatively deficient coverage by the posterior portion of the acetabulum. Developmental dysplasia with acetabular retroversion is associated with an earlier onset of pain than is developmental dysplasia with anteversion, suggesting a correlation between deficiency of the posterior acetabular wall and the earlier onset of pain.