A Comprehensive Review of Injury Causation Analysis Methodology for the Assessment of Workers' Compensation and Motor Vehicle Collision Injuries

Abstract

Orthopaedic surgeons are frequently involved in treating patients with conditions resulting from occupational injuries or trauma from motor vehicle collisions. These circumstances may lead to disputes that are subject to litigation or medicolegal determinations by state agencies. As musculoskeletal experts, orthopaedic surgeons are frequently asked to opine on the causation and extent of injury in these patients. This comprehensive review details the methodology for injury causation analysis, which has been developed and validated in the peer-reviewed literature. These approaches evolved from the Hill criteria to encompass protocols used by the American Medical Association and taught at the American Academy of Orthopaedic Surgeons instructional courses. The various methodologies are defined in the context of their use for workers' compensation cases and motor vehicle collisions with the assistance of an illustrative case.

Musculoskeletal injuries commonly occur as a consequence of injuries both at work and outside work. Although nonfatal occupational injuries are generally regulated by the Workers' Compensation laws of that state, motor vehicle collisions (MVCs) occurring outside the scope of employment are a subset of personal injury cases, litigated through the court system.¹ Individuals may report pain or other symptoms attributable to an incident. However, many of these symptoms (eg, low back pain [LBP] or neck pain) are multifactorial and extremely prevalent in the population, and they may arise even in the absence of any inciting event.^2-4 Therefore, a detailed injury causation analysis (ICA) is necessary to compensate for medical treatments or to determine the cause of disability or impairment.

As experts in musculoskeletal injuries, orthopaedic surgeons play a critical role in the diagnosis and treatment of workplace injuries. Over the 2021 to 2022 time period, a total of 502,380 workplace musculoskeletal disorders that resulted in at least one day away from work were reported in the United States.⁵ Because of its large population, California is the state with the greatest number of injured workers, and therefore, it is the workers' compensation system that affects the greatest number of patients and physicians. Qualified Medical Evaluators are appointed by the State of California to conduct independent medical examinations (IMEs) as neutral parties. Orthopaedic surgeons spanning cases designated in general orthopaedics, spine surgery, and hand surgery are requested in 51.1% of contested workers' compensation cases in California as shown in Figure 1.⁶ Pain management and physiatry specialists are requested in an additional 9.5% of cases. Nonphysician providers (eg, chiropractors and podiatrists) are called in an additional 10.9% of cases, bringing the total to 71.5% of the cases requiring a neutral party involving musculoskeletal practitioners by specialty. Given the demand for workers' compensation injury causation opinions, it is beneficial for orthopaedic surgeons to be recognized as the musculoskeletal experts in medical-legal matters involving occupational injuries.

Figure 1.

A pie chart showing percentage of neutral expert witnesses required by California Division of Workers Compensation for contested cases, by specialty, 2022. Data from Workers' Compensation Information System, State of California Industrial Relations.

Not surprisingly, workers' compensation systems are largely dependent on orthopaedic specialists. In addition to treating these conditions, orthopaedic surgeons may also be responsible for providing information regarding the nature and extent of the injury, as well as providing recommendations about return-to-work capabilities. In many cases involving litigation, orthopaedic surgeons may be asked to decide whether a work injury or MVC may have caused permanent injuries involving the musculoskeletal system and/or aggravated preexisting conditions, such as degenerative joint disease. As part of this process, they will need to determine whether there is a causal relationship between a patient's clinical findings and the incident in question or if the two are simply coincidental in nature. Thus, determining the work-relatedness of an injury and, by extension, causality is a necessary skill for orthopaedic surgeons to master when caring for these individuals. It is imperative that causation analysis be based on facts and medical science, not merely on a temporal relationship and/or opinion alone.⁷

In this review, causation analysis methods are reviewed with regard to workers' compensation and MVC injuries. In particular, the approaches employed to determine work-relatedness, causation, and the extent of an injury are discussed to allow orthopaedic surgeons to opine on these matters with reasonable medical and scientific probability. ICA methodologies are detailed in sections specific to MVCs and work-relatedness for occupational injuries. An illustrative case example is then used to apply the methodologies discussed.

Review of Injury Causation Analysis Methodology

ICA is a scientific method used to analyze the mechanism and magnitude of injury in individuals who are involved in incidents, such as workplace trauma or MVC, which may or may not lead to impairment. ICA techniques are largely adapted from the classic scientific method.⁸ ICA methods have been subjected to extensive peer review over the past several decades, and these results have been published in the literature.^9,10 The steps for ICA techniques include⁸ stating the problem, acquiring and reviewing the existing literature on the problem, generating a hypothesis, testing the hypothesis, collecting and analyzing data from the test, and finally making a conclusion. In 1965, Austin Bradford Hill proposed a system to determine whether a causal inference is reasonable by detailing nine principles: strength of association, consistency, specificity, temporality, biological gradient, biological plausibility, coherence, experiment, and analogy.^11,12 The epidemiological principles of Bradford Hill have been adapted for modern physicians. Currently, the American Medical Association's Guides to the Evaluation of Permanent Impairment (AMA Guides), Sixth Edition,¹³ uses four criteria and states, “Causality requires determination that each of the following has occurred to a reasonable degree of medical certainty ….”. Another updated methodology to evaluate causation incorporating many of the Hill criteria was proposed at the Annual American Academy of Orthopaedic Surgeons Workers' Compensation and Musculoskeletal Injuries Course.¹⁴ Table 1 shows the different criteria for ICA including the Hill Criteria, the AMA Guides sixth Edition, Academy of Orthopaedic Surgeons Workers' Compensation and Musculoskeletal Injuries Course, and criteria used for causation of MVCs (also known as BioTab).

Table 1.

Major Criteria Used for Evaluating Causation

Hill criteria11

Strength

Consistency

Specificity

Temporality

Biological gradient

Plausibility

Coherence

Experiment

Analogy

AMA guides 6th edition13

A causal event took place

The patient experiencing the event has the condition

The event could cause the condition

The event caused or materially contributed to the condition within medical probability

Causation analysis from AAOS workers' compensation and musculoskeletal injuries course14

Is there temporality with exposure preceding the incident?

Did the incident result in expected symptoms in a timely manner, or was there a delay in the onset of symptoms?

Could the described incident cause the condition? (Association)

Is there a dose-response relationship?

Is there consistency? Do the symptoms correlate with objective findings?

Motor vehicle collision (MVC) causation criteria/BioTab15

State the claimed interactions between vehicles, or a vehicle and a stationary object, during the subject collision (effect analysis)

Review the data, including the occupant's motion (kinematics)

Analyze the occupant's interaction with surroundings (biomechanics)

Review the existing literature to determine injury potential during the collision

Test the hypothesis by comparing injuries noted in medical records and any relevant experimental analysis

Make a conclusion

AMA = American Medical Association

With any ICA protocol, the key concept is to recognize that the mechanism of injury could have caused the alleged injury. ICA requires a comparison of the mechanical forces involved in the incident with the injury tolerance of the body. For an injury to occur, the forces (eg, compression, tension, or shear) must be applied in a manner and with sufficient magnitude to exceed the tolerance or strength of the tissue in question.^10,15 For example, a fall from standing would exhibit the magnitude and type of force that could cause a pubic ramus fracture in an elderly female with osteoporosis.⁹

Evaluating Causation for Motor Vehicle Collisions

For MVCs, an effect analysis can be performed to delineate the change in velocity (delta-V) of the vehicle and the principal direction of force (PDOF). This information is valuable for determining the severity of a collision and the likelihood that an occupant may sustain an injury. The delta-V for MVCs is widely accepted as a reasonable representation of effect severity,^16,17 and multiple studies have shown that there is a dose-response relationship between the delta-V and the severity of injuries arising from an MVC.^10,17-19 A reconstruction of an MVC may be accomplished with photographs from the scene of the accident, photographs of the damaged vehicles, damage repair estimates, and/or data from the vehicle's event data recorder, which may all be used to calculate the delta-V and PDOF.^20-23 It is generally accepted that when the vehicles incur little to no damage, the delta-V is less than 10 mph, which is indicative of a minor or low-speed MVC.

Live human volunteers participate in low-speed (typically <10 mph delta-v) crash testing with analysis published in the peer-reviewed biomechanical literature.^9,10,16 Low-speed human subject crash testing is approved by institutional review boards and provides data useful for understanding occupant kinematics, injury countermeasures, and symptoms if any result from this level of effect. Data from epidemiological studies of real-world low-speed collisions compares well to the human volunteer literature with both showing no meaningful risk of injury beyond self-limiting muscle strains in collisions of this severity.^{9,16-19,24,25} The injuries include microtrauma to the soft tissues in the form of musculotendinous and muscle fascicle strains occurring at the time of effect, which may generate pain that frequently resolves on its own without any formal treatment in a relatively short period, even in a matter of hours to days.^16,26

Orthopaedic surgeons often encounter patients who report persistent pain or other symptoms that may be influenced either consciously or unconsciously by secondary gain. It is important to emphasize that these subjective symptoms do not represent objective evidence that a true injury has occurred as a consequence of an incident. In one study, 10 of 51 subjects reported having symptoms following a “placebo” low-speed rear-end MVC in which no actual effect was observed²⁷; participants who scored higher on a preimpact psychological assessment of psychosomatic disorders were much more likely to experience symptoms after the placebo MVC. In countries such as Lithuania and Greece where financial compensation for MVCs is relatively modest, it is unusual for individuals involved in low-speed collisions to have prolonged symptoms.^28,29

Acute onset or exacerbation of LBP is a common report following MVCs. For rear-end collisions, lumbar extension is restricted by the seat, and rebound flexion is limited by the shoulder belt (if in place at the time of impact), whereas the converse is true for frontal impacts. In the 1950s, researchers demonstrated that subjects easily tolerate up to 20 g (ie, 20 times the force of gravity) during front-to-back and back-to-front accelerations/decelerations (observed when a vehicle rapidly brakes and speeds up, respectively) without injury.³⁰ Given that accelerations in minor MVCs with less than 10 mph delta-Vs rarely exceed 10 g, these findings suggest that the LBP reported following such colinear collisions may not be related to a traumatic condition caused by the incident but simply reflect the high prevalence of intermittent axial back pain in the general population.

The human body experiences myriad forces with resultant accelerations and decelerations while performing normal activities of daily living, such as picking up objects and climbing stairs. For instance, the lumbar spine is subjected to relatively high forces with seemingly innocuous movements, which have been quantified by investigators. Using strain gauges implanted in lumbar vertebral bodies, Rohlmann et al³¹ observed forces that routinely exceeded 100 pounds even while performing largely mundane tasks. The forces applied to the spine during MVCs have also been simulated using anthropomorphic test devices (ie, crash-test dummies). According to this testing, rear-end collisions with delta-Vs up to 15 mph generate less than 100 pounds of lumbar axial compressive force,³² whereas frontal collisions with delta-Vs up to 15 mph may give rise to lumbar compressive loading of up to 200 pounds, both of which are well below the recommended safe occupational lifting limit of 764 pounds.²⁴ Compression forces present in the lumbar spine during rear-end collisions with delta-Vs of up to 45 km/hr (27.9 mph) are comparable to those encountered during everyday activities.³³ For both rear-end and frontal collisions with delta-Vs under 12 mph, the forces acting on the cervical or lumbar spine are well below the forces experienced during activities of daily living.^25,34,35

Evaluating Work-relatedness

To establish the relatedness of work injuries (which represents a subset of ICA), the National Institute for Occupational Safety and Health developed a set of criteria that are also set out in a stepwise fashion³⁶: identify evidence of disease, review and assess the available epidemiological evidence for a causal relationship, obtain and assess the evidence of exposure, consider other relevant factors, judge the validity of the testimony, and form conclusions about the work-relatedness of the disease in the person undergoing evaluation.

For cases of work-relatedness, the goal is to evaluate whether the findings of the injury are compatible with the effects of the traumatic insult to which the individual was exposed during the incident. Exposure must be assessed as to whether sufficient exposure was there to cause the injury in the first place. Exposure with its known biological and biomechanical effects must be consistent with current evidence-based epidemiology, although the weight of the evidence should support the injury as having an occupational rather than a nonoccupational etiology.

The determination of work-relatedness should use a reproducible method for decision making, including collecting, organizing, and appraising occupational exposure, and other available evidence. Accurate determination of work-relatedness is important to ensure that workers receive appropriate benefits (eg, appropriate workers' compensation benefits), to ensure that resources are appropriately used if there is a causal hazard that can be prevented, and to prevent exacerbations and recurrences of the condition when there is increased risk at the workplace. When there is elevated risk at the worksite, preventive efforts should be instituted to help other workers avoid similar injuries.^36,37

Work-relatedness is initially assessed during the patient's first clinical encounter. A work-relatedness determination is generally straightforward for acute traumatic injuries but more complex for subjective symptoms and occupational diseases. The initial assessment of causality is considered preliminary. More detailed analyses, including information from the patient, other providers, medical records, exposure records, epidemiological studies, other published literature, and an inspection of the worksite, are usually required for a definitive causal assessment of occupational diseases and disorders.³⁷

It is important to note that industrial causation is defined differently in each state. The Third, Fourth, Fifth, and Sixth editions of the AMA guidelines are used in different states for workers' compensation impairment rating determinations, with the Fifth and Sixth being the most commonly used editions, as detailed in Figure 2, which depicts the usage of the AMA guidelines for each state as of 2019.³⁸ Some states do not use the AMA guidelines or have exceptions for different injuries, and the legal code will override the AMA guidelines if causation is defined differently. For example, the Fifth edition of the AMA guidelines³⁹ defines causation as “an identifiable factor (eg, accident or exposure to hazards of a disease) that results in a medically identifiable condition.” This contrasts with the more detailed definition in the Sixth edition of the AMA guidelines, as detailed in Table 1.

Figure 2.

Map showing usage of AMA guides or state-specific sources for impairment rating determination by state or jurisdiction for Workers' Compensation as of 2019. AMA = American Medical Association.

Using California again as an example, the determination of causation is based on substantial medical evidence, requiring evidence to rise to the level of reasonable medical probability. This must be based on fact and not be speculative, according to California state case law in Escobedo v. Marshalls (2007) 70 CCC 604, and Garza v. WCAB (1970) 3 Cal.3d 312. A musculoskeletal claim in the California Workers' Compensation system must only be causative of 1% of the injury to qualify for causation, entitling the injured worker to 100% of the treatment required, according to California Labor Code §3600(a). It is worth noting that the permanent disability settlement is apportioned to other causes of disability, such as prior injury, whereas causation of the injury is not apportioned (as detailed in California Labor Code §4663). To reiterate this key concept, medical treatment (along with temporary disability) is determined by causation of the injury and is entirely covered by the employer, whereas permanent disability settlements are determined by causation of the disability, which can be apportioned to other causes. At times, the determination of causation is a legal decision and not a medical one. When an injury arises from employment, such as repetitive job duties in the normal course of employment, a physician may determine the causation of the injury. However, in injuries that occur during the course of employment but do not arise out of employment, such as slipping and falling during lunch off-site, causation is determined by the Trier of Fact (in essence determined by an administrative judge) and should be deferred by the physician.

The process of evaluating causal events in both MVCs and work injuries is illustrated in Figure 3. One of the main differences between the evaluations of MVCs and occupational injuries is the use of an IME. In some states, IMEs are called qualified medical examinations, with California as the prime example. In Texas, this role is performed by a designated doctor appointed by the state. An IME is an examination and review by a physician (in many cases, an orthopaedic surgeon) who is not involved in the treatment of the individual being examined. The requesting party, most commonly insurance companies and employers, pays the physician's fee for the IME.⁴⁰ A physician conducting an IME is in a role that does not involve assuming care of the patient and would not incur a patient-physician relationship. Instead, the role involves determining a diagnosis or extent of injury along with the severity of the injury/condition (including determining an impairment rating) and determining causality.⁴⁰ It is imperative that the physician performing the IME takes an exhaustive history, reviews all the medical records and relevant documents, and then conducts an examination in accordance with the state's laws to obtain an impairment rating, determine maximal medical improvement (the date when an injured worker has reached their full potential recovery and their condition is unlikely to improve with further treatment),⁴⁰ and extent of injury that goes to causality in order for financial compensation to be received by the injured worker. The causation opinions expressed in the IME report should be based on analytical reasoning and be backed up by peer-reviewed literature.³⁷

Figure 3.

Flowchart process of the injury causation analysis (ICA) from the initial causal event to final disposition. Delta-v = change in velocity at impact for the affected vehicle, IME = independent medical examination, PDOF = principal direction of force for the affected vehicle, QME = qualified medical examination.

Case Example

A 57-year-old man was involved in a MVC while working as a delivery driver. His truck was rear-ended while he was at a complete stop at a red light, causing mild damage to the rear bumper; a reconstruction of the collision revealed a delta-V of 12 mph and a PDOF of 6 o'clock based upon the amount of damage to the vehicles and the electronic data recorder of the vehicle, which initiated the contact. When the police compiled their report, it was noted that the patient reported of pain in his neck and lower back but initially declined to seek immediate medical attention. He informed his supervisor the day after the incident for which he was referred to an occupational medicine clinic for further evaluation. According to their documentation, he still had 6/10 neck pain and 7/10 lower back pain, with diffuse numbness radiating down both legs. On physical examination, he had a full range of motion of his cervical spine with pain at the extremes of motion and tenderness over his paraspinal muscles bilaterally at the C5 and C6 levels but no palpable spasms. Likewise, he also had full range of motion of his lumbar spine with pain at the extremes of motion and bilateral paraspinal tenderness at the L3 and L4 levels with spasms. As part of his neurological examination, he had 5/5 strength and intact sensation to both light touch and pinprick in his bilateral upper and lower extremities, normal and symmetrical deep tendon reflexes, and negative Spurling maneuvers and straight leg raising tests bilaterally. Plain radiographs of the cervical spine demonstrated mild degenerative changes at C5-C6 with disk space narrowing but no fractures or dislocations, and grade 1 anterolisthesis of L5 on S1 was evident on lumbar spine radiographs with no acute traumatic injuries. In addition to starting ibuprofen 800 mg three times a day and cyclobenzaprine 10 mg three times a day, the patient was prescribed 12 sessions of physical therapy. At the 6-week follow-up visit following the MVC, his neck pain had resolved; however, he still reported of 6/10 pain in his lower back and diffuse numbness traveling distally in his legs in a nondermatomal pattern. MRI of the lumbar spine was performed, which confirmed the presence of grade 1 L5-S1 spondylolisthesis as well as disk protrusions at multiple levels measuring 2 to 3 mm but no notable impingement of any nerve roots. At that point, a home exercise program was initiated, and he was cleared to return to his previous occupation as a delivery driver with limited work duties. At the 3-month follow-up visit, his LBP had improved to 3/10, and he only experienced intermittent numbness in his lower extremities; thus, he was permitted to resume working without any formal restrictions.

In this example, the extent of injury included the cervical and lumbar strains caused by the MVC. The forces of a low-speed MVC, such as that on a rear bumper would not have caused the other findings present in his imaging studies, such as cervical degenerative disk disease, lumbar disk herniations, or lumbosacral instability. Considering the mechanism of injury and the natural history of these relatively benign soft-tissue conditions, it is anticipated that symptoms related to cervical and lumbar strains would generally resolve within three months after the incident.

Review of the steps of the ICA:

• (1) Is there temporality with exposure preceding the incident?

  • Yes, the neck and back pain correspond temporally to the incident.

• (2) Did the incident result in expected symptoms in a timely manner or was there a delay in the onset of symptoms?

  • The incident gave rise to the expected symptoms of neck and lower back pain but not numbness, which appeared to be unrelated to the MVC.

• (3) Is there strength of association? Could the described incident cause the condition?

  • Symptoms of cervical and lumbar strains are commonly observed following MVCs; however, this particular incident would not have caused degenerative disk disease, disk protrusions, or spondylolisthesis.

• (4) Is there a dose-response relationship? Would more exposure lead to more injury and what if you remove the exposure? Would the condition still exist?

  • The dose-response curve is well established with delta-V in MVCs, such that more “exposure” leads to greater injury. Even if the exposure is eliminated, LBP can still occur given its prevalence, especially in individuals with preexisting conditions, such as lumbar disk herniations, spondylosis, and/or spondylolisthesis.

• (5) Is there consistency? Would you see the same results in different populations with the same or similar exposure and same injuries? Do the symptoms correlate with objective findings (eg, physical examination, testing, and imaging studies)?

  • This type of incident is consistent because the cervical and lumbar strain injuries are self-limited and resolve within several weeks to a few months. These symptoms correlated with physical examination findings and imaging studies, which were consistent with these soft-tissue conditions.

Summary

Whether for the preparation of workers' compensation reports or while serving as an expert witness in cases of litigation, it is important for orthopaedic surgeons to be knowledgeable about the methods used to delineate the extent and causation of an injury. A thorough analysis of the facts using validated ICA protocols will facilitate the establishment of conclusions based on reasonable scientific and medical probabilities.