Joe's doctor must be concerned that his bone growth will be hampered by damage to the epiphyseal plate.
We can arrive at this answer because:
In the case of Joa, who is still a teenager, a fractured forearm can damage his epiphyseal plate and affect his body's bone growth, so his doctor should be concerned about this.
It is important to note that during adult life, the epiphyseal plate is replaced by an epiphyseal line.
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