Introduction
Workers' compensation practitioners frequently encounter complex scenarios involving multiple injuries to the same body part. When these injuries occur years apart, with one claim still within the five-year reopening period and another more recent, questions of apportionment and credit become particularly challenging. This blog explores a fascinating scenario involving sequential knee injuries and a future knee replacement surgery, highlighting the critical legal principles that determine how permanent disability benefits are calculated and credited.
The Scenario: A Tale of Two Knee Injuries
Consider this increasingly common scenario in California workers' compensation:
An injured worker has two workers' compensation claims involving the same left knee:
- First Claim (2018) :
- Injury date: June 5, 2018
- Stipulated settlement in 2020: 14% permanent disability ($13,412.50 paid)
- Timely Petition to Reopen filed June 2023 (within 5-year window)
- AME found same 10% WPI rating as original settlement (no additional PD yet)
- Second Claim (2023) :
- January 29, 2023 cumulative trauma/aggravation to same left knee
- QME rated 1% WPI with 70% apportionment to "the industrial injury in question" (without distinguishing which industrial injury)
- Current PD value: $875
- Future Medical Event :
- Both doctors agree applicant will likely need total knee replacement surgery
- Expected rating: 20-30% WPI (17.5-26.25% PD after apportionment)
- This surgery would create substantial new disability attributable to industrial injury
This scenario raises several critical legal questions that can dramatically affect the applicant's recovery—potentially creating a $5,000-$19,000 difference in benefits depending on how these questions are resolved.
The Apportionment Ambiguity Problem
When Medical Evaluators Don't Distinguish Between Injuries
One of the most common issues in multiple injury cases is the failure of medical evaluators to distinguish between separate industrial injuries when providing apportionment opinions. In our scenario, the QME apportioned 70% to "the industrial injury in question" without specifying whether this referred to the 2018 injury, the 2023 injury, or some combination of both.
This ambiguity creates a significant legal problem. Under [Labor Code § 4663] and [Labor Code § 4664] , apportionment between multiple industrial injuries is required following the Benson decision. However, the burden of proving apportionment falls squarely on the defendant.
When a medical evaluator fails to distinguish between multiple industrial injuries in their apportionment opinion, the defendant's burden is not met. The evaluator's reference to "the industrial injury" (singular) does not create a presumption that all disability is apportioned to one specific injury. Rather, it represents a failure to provide substantial medical evidence of apportionment between the injuries.
The "Inextricably Intertwined" Doctrine
The Benson decision, while generally requiring apportionment between separate injuries, specifically recognized that "there may be limited circumstances ... when the evaluating physician cannot parcel out, with reasonable medical probability, the approximate percentages to which each distinct industrial injury causally contributed to the employee's overall permanent disability." In such limited circumstances, "the employer has failed to meet its burden of proof and a combined award of permanent disability still might be justified."
This has given rise to the "inextricably intertwined" doctrine, where disabilities from multiple industrial injuries are so interconnected that they cannot be reasonably separated. In such cases, the WCAB has held that an applicant is entitled to a single PD award without apportionment between the industrial injuries.
For example, in one case involving CIGA, "the AME reported that the exposure from the two injuries was so inextricably interwoven that it would be speculative to come up with any division of disability." The panel concluded that "because the AME was unable to parcel out the causal contribution of the two injuries, they were both liable for an indivisible award."
The Credit Conundrum: Percentage vs. Monetary Value
Two Different Credit Systems
Perhaps the most complex aspect of our scenario involves how credit for the prior permanent disability award should be applied. California workers' compensation law has two distinct methods for applying credit:
- Percentage Subtraction (Brodie Rule) : For subsequent injuries, the California Supreme Court in Brodie v. WCAB held that "the Fuentes rule remained the proper method for calculating apportionment following the changes by SB 899." Under this approach, the percentage of the prior award is subtracted from the current overall disability percentage.
- Monetary Credit Rule : For petitions to reopen, a different rule applies. As Sullivan on Comp explains: "It is important to note that it has been held that the Fuentes rule does not apply to awards of new and further permanent disability following a successful petition to reopen. Instead, the employee is entitled to the difference between the monetary value of the awards rather than the percentages of permanent disability."
The Dramatic Impact on Benefits
The difference between these two approaches can dramatically affect the applicant's recovery. In our knee replacement scenario:
- If attributed to 2018 Petition to Reopen (Monetary Credit): The defendant would subtract the $13,412.50 already paid from the new award value of approximately $23,000, resulting in a net recovery of approximately $9,587.50.
- If attributed to 2023 Subsequent Injury (Percentage Subtraction) : The defendant would subtract the 14% prior PD from the 17.5% new PD, resulting in a 3.5% net PD award of approximately $4,375.
This creates a $5,212.50 difference in recovery based solely on which credit method applies!
The "Double-Dipping" Dilemma
Can Defendants Have Their Cake and Eat It Too?
Another critical issue is whether a defendant can obtain both apportionment to a prior industrial injury AND a credit for the prior PD payment. This potential "double-dipping" raises serious equity concerns.
The WCAB has addressed this issue, finding that "it would be inequitable for defendant to obtain the benefit of separation of injuries for calculating PD while allowing defendant to essentially merge the cases for PD advances."
In our scenario, if the defendant attempts to apportion most of the disability to the 2018 claim (where credit would eliminate any new payment) while minimizing the 2023 claim value, the WCAB would likely find this inequitable and either deny the apportionment or limit the credit.
The WCAB has denied credit in cases where:
- The applicant's PD was already apportioned among multiple dates of injury, resulting in a smaller monetary award
- Allowing credit would result in the applicant not receiving any "new money" for a subsequent injury
The Apportionment-Then-Credit Sequencing Question
The Proper Calculation Method
When disability is apportioned between two industrial injuries, the proper calculation method is to first apportion the disability between the two cases and then apply credit only to the portion attributed to the earlier case.
In our scenario, if the 17.5% PD is apportioned equally between the two cases (8.75% to each):
- For the 2018 case: The monetary value of 8.75% PD ($11,500) would be subject to credit for the prior payment of $13,412.50, resulting in $0 additional recovery
- For the 2023 case: The 8.75% PD ($11,500) would not be subject to credit since there was no prior payment on this claim
This approach is consistent with the principle that each injury should be evaluated separately for purposes of apportionment and credit.
Medical Treatment: The Non-Apportionment Rule
100% Coverage for Knee Replacement Surgery
Unlike permanent disability, medical treatment cannot be apportioned between industrial injuries or to non-industrial factors. This principle is particularly important in our knee replacement scenario.
Sullivan on Comp is explicit on this point: "Unlike permanent disability, medical treatment cannot be apportioned between industrial injuries." Furthermore, "If an applicant's treatment is required due to multiple industrial conditions, the employer is fully liable to provide the treatment."
This means that regardless of how permanent disability is apportioned, both claims would remain jointly and severally liable for 100% of the knee replacement surgery costs (estimated at $43,000-$55,000) and related medical treatment.
Special Considerations for Knee Replacement Surgery
The Hikida vs. Justice Distinction
Knee replacement surgery presents some unique considerations in workers' compensation law. There is an important distinction between cases where:
- The knee replacement surgery itself causes a new condition or failed outcome (Hikida principle - no apportionment)
- The knee replacement is necessitated by both industrial and non-industrial factors with a good outcome (Justice principle - apportionment may apply)
This distinction can significantly impact how permanent disability following knee replacement surgery is evaluated and apportioned.
Strategic Considerations for Practitioners
For Applicant Attorneys
Based on these legal principles, applicant attorneys should consider the following strategies:
- Challenge Ambiguous Apportionment : When a QME or AME fails to distinguish between multiple industrial injuries in their apportionment opinion, argue that this fails to meet the substantial medical evidence standard for apportionment.
- Argue for Attribution to the Most Favorable Injury : If the disability cannot be parceled out between multiple injuries, argue that it should be attributed to the injury that provides the most favorable credit calculation for the applicant.
- Invoke the "Inextricably Intertwined" Doctrine : If the medical evidence supports it, argue that the disabilities from multiple injuries are "inextricably intertwined," potentially leading to a joint and several award without apportionment.
- Challenge "Double-Dipping" : If the defendant attempts to both apportion most disability to an earlier claim AND claim credit for the prior payment, argue that this constitutes inequitable "double-dipping" that should not be permitted.
- Ensure Full Medical Coverage : Regardless of how PD is apportioned, ensure that all industrial claims remain jointly and severally liable for 100% of medical treatment.
For Defense Attorneys
Defense attorneys should consider these strategies:
- Obtain Clear Medical Evidence on Inter-Injury Apportionment : Ensure that medical evaluators specifically address apportionment between multiple industrial injuries, not just between industrial and non-industrial causes.
- Document Overlap Between Prior and Current Disability : To obtain credit for a prior award, be prepared to prove both the existence of the prior award and the extent of overlap between the prior disability and the current disability.
- Consider the Equity of Credit Requests : Be mindful that the WCAB has discretion in allowing credit and considers equity factors. Requesting both substantial apportionment to a prior injury AND full credit may be viewed as inequitable "double-dipping."
- Address Medical Treatment Liability Early : Since medical treatment cannot be apportioned between industrial injuries, address contribution issues between carriers/employers early in the case.
Conclusion: The Importance of Careful Analysis
The intersection of apportionment and credit in workers' compensation cases involving multiple injuries to the same body part presents some of the most complex legal questions in the system. The difference between the various approaches can dramatically affect the benefits received by injured workers and the liability of employers and insurers.
As our knee replacement scenario demonstrates, the potential difference in recovery can range from $4,375 to $23,000 depending on how these legal questions are resolved. This underscores the importance of careful analysis and strategic advocacy in these cases.
By understanding the nuances of apportionment between multiple industrial injuries, the different credit methodologies for subsequent injuries versus petitions to reopen, and the prohibition against "double-dipping," practitioners can better navigate these complex waters and achieve more favorable outcomes for their clients.
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This blog is intended for educational purposes only and does not constitute legal advice. The analysis is based on California workers' compensation law as of June 2025. Specific cases may have different outcomes based on their unique facts and circumstances.