As a general rule, if the tooth starts to discolor after some trauma, then chances are, the nerve will not recover and eventually, irreversible pulpitis
sets in and root canal
treatment will be needed. The only times that the bruised tooth might heal up on its own is if the tooth was still developing and the tip of the root hasn't completely formed. During this time, the nerve is still developing and has a much higher chance of recovering from any trauma. However, once the tip of the root is formed, the regenerative capabilities cease and as noted earlier, the nerve of the tooth almost always dies.
What makes diagnosis difficult is because the traumatized tooth may or may not exhibit pain
/soreness even when the nerve dies. So some patients end up with a discolored tooth, and no history of pain, and thinks the tooth is just fine. But over the years, as we take follow-up X-rays, we get to see that an infection
has formed and caused some bone loss around the tips of these teeth, signalling a definite need for root canal treatment.
If you want a second opinion on the matter, I would recommend seeing an Endodontist (root canal specialist) for further evaluation. They can do a number of vitality test to see if the nerve of that tooth is still functioning properly. If it is, then consider yourself lucky because you'd be one of the rare cases where trauma occurred with discoloration, but no nerve damage.
However, if the nerve is damaged and dying/dead, then it's better to get the root canal done now, so we can try to prevent the tooth from getting darker. These type of discoloration do not respond well to external bleaching/whitening techniques. And if the tooth gets too dark, we may either have to try internal bleaching (after the root canal is done), or may need to explore the option of porcelain veneer
to mask and match the tooth.
Jul***** *****, DDS