Short answer. The income benefit for the new disability. Article 197 makes the State Insurance Fund liable where an employee under permanent partial disability suffers another injury resulting in a greater compensable disability — but where the new disability is related to the old one, only the difference in income benefits is paid.
What the law says
If any employee under permanent partial disability suffers another injury which results in a compensable disability greater than the previous injury, the State Insurance Fund shall be liable for the income benefit of the new disability
Labor Code, Article 197 — Second Injuries. Read the full provision →
Two conditions before the article does anything
The provision is narrower than it first reads. It applies where you were already under permanent partial disability and where the fresh injury results in a compensable disability greater than the previous injury. Both have to hold. A second injury that leaves you no worse off than before does not engage this article, and neither does one that is not compensable in its own right. So the starting point in any such claim is the comparison: what your rated condition was before, and what it is now. Everything else follows from that.
Related or unrelated decides the arithmetic
If the two disabilities are unrelated, the Fund answers for the income benefit of the new disability. If the new disability is related to the previous disability, the System is liable only for the difference in income benefits. The logic is that you should not be paid twice over for the same lost function. Where the new injury simply worsens an impairment already compensated, the law tops you up to the higher level rather than paying the higher level afresh — which is why two claimants with identical new injuries can be paid very differently.
Why 'related' is the word that gets fought over
The statute does not define it, and the difference in money is substantial, so this is where these claims are usually contested. An injury to the same limb, the same joint or the same body system as the earlier one invites the finding; a distinct injury to an unaffected part of the body does not. The harder cases sit in between — a back injury following an earlier leg impairment that changed how you walk, or a condition that the first disability made more likely. That is a medical question decided on medical evidence, not an administrative label.
The records that decide it
Everything turns on documents generated at two separate times, and claimants usually have only the recent set. Retrieve the earlier one: the original assessment, the rating assigned for the permanent partial disability, and the treating records from that injury. Then keep the new medical assessment, the incident report, and any imaging or specialist opinion addressing whether the new impairment overlaps the old. Ask your physician to say expressly whether the conditions are related and why. Bring both files together when you take advice, because neither alone answers the question.