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The short answer
File against both employers and let them fight about apportionment. That fight is theirs.
What you must not do is let either one use the other as a reason to pay you nothing.
What is actually happening
Two claims, potentially two carriers, and a dispute over how much of your current disability belongs to each.
The old employer's carrier will say the new injury caused it. The new employer's carrier will say you were already damaged. Both arguments are made simultaneously, and if you only filed one claim, only one of them is in the room.
The rules they will be arguing about
§ 4663 — apportionment to causation. Each is liable only for the portion its injury caused, and any opinion assigning a percentage must explain how and why. Under Escobedo, a percentage without reasoning is not substantial evidence, and a great many are exactly that.
§ 4664(b) — if there was a prior award of permanent disability, that disability is conclusively presumed to still exist. Conclusively — it is not open to rebuttal. Which is why the first question is always whether there was an actual award, rather than a Compromise and Release that never produced one; § 4664(b) is asserted far more often than it applies. Note what (b) does not say: there is no "same region" limb in it.
§ 4664(c) is the separate rule people run together with it — the accumulation of all permanent disability awards for any one region of the body may not exceed 100 percent over your lifetime, with the regions defined in the statute. That is a lifetime cap by region; (b) is a presumption about a prior award.
§ 5500.5 — if any part of this is cumulative rather than a single event, liability is confined to a defined exposure window, and you may proceed against one liable carrier and let them seek contribution. How that works.
What to do
File both claims. Do not choose. Each has its own one-year clock under § 5405, and choosing wrong is how people end up with neither.
Disclose the prior injury early and completely. It is in the records and it will be found. Disclosed early it is a fact; disclosed late it is a credibility problem, and credibility decides disputed cases. Why that matters.
Get the prior file. The old medical reports, the old rating, and any award. The rating you previously received is the baseline everything gets measured against, and you cannot argue about a baseline you have not read. How to get your file.
And name every body part on both claims. A part left off one claim is a part that gets attributed to the other.
The thing worth knowing
Where a pre-existing disability combines with a new injury to leave you substantially more disabled than either alone, the Subsequent Injuries Benefits Trust Fund can pay the difference. Disability apportioned away from an employer is often precisely what SIBTF exists to address — so an aggressive apportionment defense sometimes points toward a claim nobody has mentioned. How that works.
Sources
Labor Code § 4663 (apportionment to causation) · § 4664(a)–(b) (employer liability limited to what it caused; prior award presumption) · § 5500.5 (cumulative injury liability) · § 5405 (limitations) · § 4751 (Subsequent Injuries Benefits Trust Fund) · Escobedo v. Marshalls (2005) 70 Cal.Comp.Cases 604 (en banc).
General information about California law, not legal advice about your case.
Impairment values described are from the AMA Guides, 5th Edition as applied under the California rating schedule; the Guides are a copyrighted medical text and figures here are summarized rather than reproduced. Your rating depends on your own examination findings.
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