The sealed osteons of cortical diaphyseal bone. Early observations revisited with scanning electron microscopy.

The frequency, structure, mode of formation and significance of
sealed osteons remain unsettled. Sealed osteons have been reported as an
unusual finding in the cortical bone of experimental animals: we extended
the observation to human cortical bone studied with SEM. Tibial bone
specimens from three patients who sustained a traumatic below-the-knee
amputation were used in the study. The observed total mean density of
osteons was 19.25/mm2 and the percentage of sealed and partially sealed
osteons was 4.2% and 1.7% respectively. The material sealing the central
canal showed an X-ray microanalysis spectrum with the same Ca/P ratio
as the peripheral lamellae and a lower carbon signal. The morphology
suggested a reactivation of bone apposition triggered by exclusion of the
occluded canal from blood flow rather than a physiological evolution of
the closing process of secondary osteons. This presupposes collapse and
degeneration of the central vessel before the osteoblasts resting on the
inner surface of the canal could start to lay down new bone matrix. This
explanation is consistent with a dynamic model of intracortical blood flow.