The Journal of Bone and Joint Surgery, Vol 75, Issue 5 704-713, Copyright © 1993 by Journal of Bone and Joint Surgery, Inc
Scanning electron-microscopic and magnetic resonance-imaging studies of injuries to the patellofemoral joint after acute transarticular loading
RC Thompson, MJ Vener, HJ Griffiths, JL Lewis, TR Oegema and L Wallace
Department of Orthopaedic Surgery, School of Medicine, University of Minnesota, Minneapolis.
To examine the effects of transarticular loading on articular cartilage and
subchondral bone, we used a canine model that we had developed previously,
in which a standardized load of approximately 2000 newtons is delivered
across the patellofemoral joint. The purpose of the study was to define and
describe the initial changes, as seen on histopathological and magnetic
resonance-imaging studies, that occur in the early stages after injury to
the joint by transarticular loading. Scanning electron microscopy was used
to define the extent and characteristics of the fractures produced in the
subchondral bone of four patellae that were examined on the day of loading.
We found multiple, extensive fractures through the zone of calcified
cartilage and the subchondral bone, frequently with step-off displacement,
and with little or no change in the gross appearance of the articular
cartilage. Specimens from four patellae were examined histologically two
weeks after loading, and the observed changes were correlated with those
that had been demonstrated by scanning electron microscopy. Fractures
through the zone of calcified cartilage and the subchondral bone, with
step-off displacement, were prominent. Clefts were present in the surface
of the articular cartilage and, in some areas, there was a focal loss of
proteoglycan from the extracellular matrix, as indicated by the complete
absence of staining with safranin O. Six dogs were examined one year after
loading. There was healing of the subchondral fractures and restoration of
proteoglycan in the extracellular matrix. However, superficial clefts and
fissures were still present in the articular cartilage. Sequential magnetic
resonance-imaging studies were also carried out on these six dogs, at two,
eight, sixteen, thirty-six, and fifty-two weeks after loading. Two weeks
after loading, all knees had soft-tissue swelling, effusion, and a
decreased marrow signal in the medullary cavity of the patella. The
decreased marrow signal and effusion were still present eight weeks after
the impact, and then the findings gradually returned to normal. One year
after loading, it was found that the histopathological changes had not been
progressive; in fact, they had been ameliorated and, to some extent,
reversed by repair processes. The early, severe magnetic resonance-imaging
changes had also been reversed, so that this study demonstrated normal
findings by one year after loading.
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