A pre-existing condition is a medical condition you had before your accident. The insurance companies would like you to believe that a pre-existing condition precludes you from claiming damages. However, this is not the case.
If you have a pre-existing condition, you can still claim damages. This is true whether your new injuries are to the same body part as your previous condition or entirely in a different area.
If your injuries are to a different body part, they are two separate conditions, and your pre-existing condition should be irrelevant. If you injure yourself in the same location as your previous injury, you can still argue that the accident aggravated your pre-existing condition. Regardless, talk to an experienced personal injury attorney so they can work with your care providers to prove your claim.