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The Journal of Bone and Joint Surgery, Vol 75, Issue 10 1508-1523, Copyright © 1993 by Journal of Bone and Joint Surgery, Inc
Healing and remodeling of articular incongruities in a rabbit fracture model
A Llinas, HA McKellop, GJ Marshall, F Sharpe, M Kirchen and A Sarmiento
J. Vernon Luck, Sr., M.D., Bone and Connective Tissue Laboratory of Los Angeles Orthopaedic Hospital, California.
Healing of 0.5 or 1.0-millimeter step-off defects associated with displaced
intra-articular fractures of the medial femoral condyle was examined in
fifty-four adult New Zealand White rabbits. The rabbits were treated with
either immobilization for three weeks, intermittent active motion, or
continuous passive motion for seven days. At twelve weeks, the healing and
remodeling of the step-off defects were examined with use of
contact-pressure maps on pressure-sensitive film, light microscopy (with
hematoxylin and eosin or safranin-O staining), and scanning electron
microscopy. Macroscopically, the sharp profile that had been present
initially with both sizes of step-off defect had rounded off; however,
there was less residual incongruity with the 0.5-millimeter step-offs than
with the 1.0-millimeter step-offs. Among step-off defects of the same size,
the method of treatment had no discernible effect on the macroscopic
appearance of the surface of the joint. With fresh step-offs (the control
group), the contact pressure of the cartilage on the elevated side was
approximately three times greater than that at a distance from the
step-off. On the depressed side, an unloaded zone extended approximately
three times the height of the step-off, with an average width of 3.4
millimeters for the 1.0-millimeter step-offs and 1.6 millimeters for the
0.5-millimeter step-offs. After healing and remodeling, the unloaded zone
still averaged 2.5 millimeters in width for the 1.0-millimeter step-offs
but had decreased to only 0.35 millimeter in width for the 0.5-millimeter
step-offs. For seven of the nine 0.5-millimeter step-offs, the contact
pressure in the previously unloaded zone ranged from 0.5 to 1.5
megapascals, with a mean of 0.8 megapascal (40 per cent of the normal mean
contact pressure at this location). Under light microscopy, the cartilage
on the elevated side of the healed step-offs had decreased in thickness,
was displaced toward the defect and tapered toward the depressed side, and
ended in a hypocellular tissue flap. In contrast, the cartilage on the
depressed side had thickened as a result of hyperplasia of the chondrocytes
and hypertrophy of the cartilage and had failed to establish continuity
between the sides of the defect. There was a marked increase in the
subchondral vascular bed and re-establishment of the subchondral plate.
With the exception of the aforementioned hypocellular tissue flap, safranin
O stained the cartilage on both levels of the step-off uniformly, which
indicated the absence of glycosaminoglycan depletion.(ABSTRACT TRUNCATED AT
400 WORDS)
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