Fig. 4.

Histologic images corresponding to magnetic resonance images at eight weeks from the same femoral head illustrated in Figure 3, E and F, are shown. The figures are oriented in the same coronal plane as the magnetic resonance imaging scans. A: Photomicrograph of a coronal plane histologic section of the entire femoral head and physis (epiphysis) and the adjacent neck (metaphysis) are shown. The medial part of the head is at the right. The articular and epiphyseal cartilage thickness is increased from the normal (compare with Fig. E-1, B). The fibrovascular invasion (*) from a lateral point of entrance has passed in an arc-like fashion across the femoral head into the medial segment. Dense intramembranous bone formation (+) is seen at the upper and lower margins of the fibrovascular tissue. Endochondral bone formation resumes at the undersurface of the continuous articular-epiphyseal cartilage. This orientation corresponds to the magnetic resonance images in Figure 3, E and F, with the fibrovascular tissue of high signal intensity, the dense intramembranous bone of low signal intensity, and the endochondral tissue of intermediate signal intensity. Tissue within the long rectangular box is seen at higher magnification in B, from the smaller box at left in C, and from the box at the upper right in D (paraffin-embedded tissue, 1% toluidine blue stain). B: The prominent band of fibrovascular tissue is seen in the middle portion of the histologic section along with a large transversely oriented vessel. Below this accumulation, tissue differentiation by means of the intramembranous mechanism has formed woven bone on which lamellar bone is being synthesized. Endochondral bone has not formed at this site. Above the fibrovascular tissue, there is also new intramembranous bone synthesis and, above that, the endochondral bone sequence from the epiphyseal cartilage has reestablished itself. The intact physis is seen at bottom (paraffin-embedded tissue, 1% toluidine blue stain). C: Higher-magnification photomicrograph shows intramembranous woven bone formation, at left, from osteoblasts, which have differentiated from the fibrovascular tissue invasion. Osteoblasts rim the surface of the woven bone (paraffin-embedded section, 1% toluidine blue stain). D: Higher-magnification photomicrograph of tissue response at the upper right region of the medial part of the femoral head showing continuation and/or resumption of endochondral bone formation (paraffin-embedded section, 1% toluidine blue stain).