The employer and insurer appeal the compensation judge’s finding that the employee’s work injury remained a substantial contributing cause of the employee’s cervical spine condition and need for surgery after a non-work-related motor vehicle accident.  We affirm.

On November 6, 2006, Alysa Keane, the employee, sustained a work-related cervical spine injury in a motor vehicle accident while working as an emergency medical technician and ambulance driver for Critical Care Services, the employer, which was insured for workers’ compensation liability by AIG/Chartis.  The employer and insurer admitted liability and paid various workers’ compensation benefits.  The employee was treated conservatively but experienced ongoing symptoms including neck pain, headaches, and numbness in her left arm.  In June 2010, the employee underwent an anterior cervical fusion at C6-7 and disc replacement at C5-6.^[1]

The employee’s headaches, neck pain, and a burning pain in her left shoulder, returned in October 2010.  Because of the progression and intensity of the employee’s symptoms, a CT scan of the cervical spine at C6-7 was ordered.  The November 16, 2010, CT scan revealed spot welding at C6-7, osteolysis at C7, but did not display an accurate interpretation of the C6-7 fusion.^[2]  The employee continued treatment for neck pain and was evaluated by a physician’s assistant, who recommended medial branch block injections.  On January 4, 2011, Dr. Louis Saeger performed medial branch blocks on the employee at the C-2 and C-3 levels.  The employee experienced some left-sided relief after that procedure.  The relief lasted less than one day.

Eight days later, on January 12, 2011, the employee was injured in a non-work-related motor vehicle accident.  The employee was transported to Regions Hospital by ambulance and was admitted for treatment from January 12 through January 17, 2011.  After being discharged, the employee was treated by Dr. Stefano Sinicropi.  A January 26, 2011, cervical spine CT scan showed a defect in the center of the fusion mass at C6-7.  On February 1, 2011, the employee reported significantly increased pain in her neck and severe left upper extremity pain to Dr. Sinicropi, who opined that the employee had sustained a failed cervical union at C6-7.  He also noted in his medical records that they had been working her up for a possible failed union at C6-7 before the January 2011 accident.  Dr. Sinicropi recommended posterior exploration and reconstruction of the fusion at C6-7.

On April 4, 2011, the employee was examined at the employer and insurer’s request by Dr. Mark Friedland.  In his report, Dr. Friedland stated that the employee’s prior injury of November 6, 2006, was a contributing cause of the employee’s need for medical care and treatment from and after the accident on January 12, 2011.  He also stated that “[h]ad [the employee] not undergone the surgical procedures by Dr. Sinicropi on [June 21, 2010,] the issue as to potential disruption of the C6-7 interbody fusion mass would not need to be considered.”^[3]  However, it was also Dr. Friedland’s opinion that without the non-work-related accident, it was more likely than not that the employee would have “gone onto a successful and complete interbody fusion at the C6-7 level.”^[4]  He therefore concluded that the employee’s subsequent motor vehicle accident was an intervening cause of the employee’s need for medical care after the accident.  At Dr. Friedland’s deposition, he indicated that no one knows with 100% certainty whether the employee’s fusion would have healed into a complete fusion or would have remained indeterminate, but nevertheless he opined that the indications were that it was progressing to a complete fusion.

The employee filed a medical request for approval of the recommended surgery on May 11, 2011.  The employer and insurer objected, claiming that the employee’s need for surgery was not causally related to the employee’s work injury.  On May 27, 2011, the employee underwent the surgery, performed by Dr. Sinicropi.  A hearing on the issues raised in the medical request was held on August 2, 2011.  On October 3, 2011, the compensation judge issued her decision, finding the employee’s November 6, 2006, work injury to be a substantial contributing factor to the employee’s need for surgery after the January 12, 2011, accident and ordering the employer and insurer to pay for the medical treatment associated with the May 27, 2011, surgery.  The employer and insurer appeal.

The Workers’ Compensation Court of Appeals must determine whether the findings of fact and order are clearly erroneous and unsupported by substantial evidence in view of the entire record as submitted.^[5]  Substantial evidence supports the findings if, in the context of the entire record, they are supported by evidence that a reasonable mind might accept as adequate.^[6]  Fact findings are clearly erroneous if the reviewing court, looking at the entire evidence, is left with a definite and firm conviction that a mistake has been committed.^[7]  Findings of fact should not be disturbed, even though the reviewing court might disagree with them, “unless they are clearly erroneous in the sense that they are manifestly contrary to the weight of the evidence or not reasonably supported by the evidence as a whole.”^[8]

The employer and insurer contend the compensation judge erred by applying an incorrect legal standard when she found the employee’s November 6, 2006, work injury to be a substantial contributing factor in the employee’s need for surgery on May 27, 2011.  The employer and insurer assert that the judge should have considered whether the employee’s need for surgery after a non-work related motor vehicle accident was a direct and natural consequence of the work injury.  If the judge had applied the direct and natural consequence rule, the employer and insurer maintain the only legal conclusion the judge could reach is that the employer and insurer are not responsible for the medical expenses from and after the May 27, 2011, surgery.  We are not persuaded.

The compensation judge framed the issue at the August 2, 2011, hearing as follows: “The dispute in this case is whether or not the employee’s work injury of November 6, 2006 is a substantial contributing factor in her need for surgery on May 27, 2011.”^[9]  As a general rule, an employer and its insurer are responsible for medical expenses caused by a work injury.^[10]  The work injury need not be the sole cause of the need for treatment but may be a substantial contributing factor of the employee’s condition.^[11]  Medical expenses incurred as a result of an aggravation of a work injury continue to be the responsibility of the employer and insurer if the work injury remains a substantial contributing factor in the need for treatment.^[12]  The causal relationship with a work injury may be broken by a superseding, intervening cause of the aggravation.

When the primary injury is shown to have arisen out of and in the course of employment, every natural consequence that flows from the injury likewise arises out of the employment, unless it is the result of an independent intervening cause attributable to claimant’s own negligence or misconduct.^[13]

“A superseding intervening cause is one which severs the causal link between the original personal injury and the resultant disability such that the original personal injury is no longer a substantial and contributing cause of the resultant disability.”^[14]  The determination of whether a subsequent incident or event is a superseding, intervening cause of disability or need for medical treatment is one of fact, and the employer and insurer have the burden of proof.^[15]  This case presents one critical issue for determination: whether the non-work-related automobile accident on January 12, 2011, severed the causal connection between the employee’s work injury and her need for surgery.

On November 6, 2006, the employee injured her cervical spine in a work-related accident.  Pain and progressive symptomatology led to a disc replacement at C5-6 and cervical fusion at C6-7 on June 21, 2010.  The recovery process from the disc replacement and fusion was measured and complicated.  Approximately four weeks post surgery the employee noticed the return of certain symptoms.  By October of 2010, she was actively seeking medical treatment for pain.  At the time of the second car accident, the employee’s fusion had not yet fully healed nor had she recovered from the effects of the June surgery.

The judge found the January 2011 non-work-related accident to be a substantial contributing factor in the need for surgery because the employee suffered a significant increase in symptoms after the incident.  The fact that there are multiple causal factors does not, as a matter of law, sever the ties between the work injury and the surgery.  Questions of medical causation fall within the province of the compensation judge.^[16]  The record contains conflicting medical opinions on the causation issue.  Dr. Friedland performed an independent medical evaluation on the employee and concluded that the employee’s work injury did not substantially cause or contribute to the employee’s need for surgery.  Dr. Sinicropi, the employee’s primary surgeon, performed both surgeries and concluded from his evaluations and treatment that the employee’s work injury was a substantial contributing factor in her need for surgery after the second car accident.

The employer and insurer argue that the employee was having neck pain from different levels and these symptoms did not indicate that the employee’s fusion was not healing properly at the time of the second accident.  Dr. Sinicropi testified in his deposition that the November 2010 CT scan suggested an indeterminate or equivocal fusion.  In other words, the scan could not identify the cervical fusion at C6-7 as a solid fusion.  Dr. Sinicropi also testified that the employee was actively seeking treatment for neck pain before the second accident.  Given the employee’s first surgical procedure, the susceptibility of injury to implants post surgery, as well as her post-surgical symptoms, diagnostic work up and treatment, Dr. Sinicropi concluded that the original work injury remained a substantial contributing factor in her need for the May 27, 2011 surgery.  Dr. Friedland also stated that the employee’s work injury was a contributing cause of her need for medical care after the accident, but unlike Dr. Sinicropi, Dr. Friedland opined that without the January accident occurring, it was more likely than not that the employee would have had a successful and complete fusion.

In essence, the employer and insurer’s argument draws different inferences from the medical evidence at hearing which are in line with the opinions and conclusions by Dr. Friedland.  However, where the evidence submitted reasonably permits different inferences, the choice of inference to be drawn rests with the compensation judge who is the finder of fact.^[17]  The compensation judge chose to adopt Dr. Sinicropi’s opinion.  It is the compensation judge’s responsibility, as trier of fact, to resolve conflicts in expert testimony.^[18]  Here, Dr. Sinicropi based his conclusions, in part, on his belief that the employee was in significant pain and that the cervical fusion was not solid at the time of the 2011 accident.  The compensation judge did not err by relying on Dr. Sinicropi’s opinion to support her finding that the employee’s work injury remained a substantial contributing factor of the employee’s need for surgery.

The employer and insurer further argue the dramatic onset of treatment and subsequent fusion surgery was directly attributable to the “abnormal” event of being involved in a severe car crash.  An aggravation of a permanently weakened physical condition caused by a personal injury that is the result of conduct by the employee which is unreasonable, negligent, dangerous or abnormal may constitute a superseding, intervening cause.^[19]  The employer and insurer contend that the employee’s activity of being in a motor vehicle accident was “abnormal” and should be found to be a superseding intervening cause on that basis, citing Morton v. Egan & Sons.^[20]  In Morton, this court, in a majority opinion, affirmed a compensation judge’s conclusion that a motor vehicle collision with a tree is an abnormal event and not part of a person’s customary and usual activities.  In doing so, the majority reversed the compensation judge’s finding that the effects of a collision was only a superseding intervening cause of the employee’s disability for thirty days and that an earlier work injury was a substantial contributing cause after the thirty day period.  A dissenting opinion stated that there was substantial evidence in the medical reports to support the compensation judge’s finding that the work injury was a substantial contributing cause after a thirty day period.  On appeal, the Minnesota Supreme Court reversed the majority decision of this court, adopting the analysis in the dissent and holding that there was substantial evidence to support the compensation judge’s finding that the work injury was a substantial contributing cause of the employee’s disability after thirty days.^[21]  In effect, the Morton case does not stand for the rule that a motor vehicle accident always breaks the causal connection between an employee’s work injury and disability for all subsequent medical treatment nor is Morton the controlling law in the present case.^[22]  In the case before us, the compensation judge found the January 2011 car accident was significant, but did not find that it rose to the level of a superseding, intervening event that would break the chain of causation to the work injury.

The employer and insurer also contend this court should follow Smith v. Timberland Lumber Co.,^[23] where this court affirmed a compensation judge’s finding that the employee’s work injury was not a substantial contributing factor to his need for surgery after a non-work-related motor vehicle accident.  In Smith, this court affirmed the compensation judge’s findings on substantial evidence grounds that an employee’s conduct of riding as a passenger in a vehicle that was involved in a motor vehicle accident did not constitute “unreasonable, negligent, dangerous or abnormal conduct.”^[24]  The court held that the “issue in intervening cause cases is not merely whether the intervening injury or condition is itself a substantial contributing cause of the employee’s subsequent disability but whether that intervening injury or condition has broken the causal connection between the employee’s work injury and that disability.”^[25]  This court analyzed the medical evidence in that case and determined there was substantial evidence in the record to support the judge’s finding that the employee’s work injury no longer remained a substantial contributing factor to his need for surgery after a non-work-related motor vehicle accident.^[26]

Like Smith, the record here contained conflicting medical opinions on whether the intervening event was itself a substantial contributing cause of the employee’s subsequent disability and whether that intervening event had broken the causal connection between the employee’s work injury and that disability.  In both cases, the compensation judge adopted one medical opinion over  the other opinion.  In this case, the compensation judge concluded that first, the non-work-related accident was a substantial contributing cause in the employee’s need for surgery; second, the work injury continued to be a substantial contributing cause in the employee’s need for surgery; and third, the January 2011 car accident did not break the chain of causation between the work injury and the second surgery.  Both cases involve a review of a compensation judge’s fact findings on a substantial evidence basis.  The decision in Smith does not compel a different result in this case.

The issue before the judge was whether the employee’s work injury remained a substantial contributing cause of the employee’s need for surgery after the second accident.  If the work injury continues to be a substantial contributing cause of the employee’s disability, the intervening injury or condition will not relieve the employer and insurer of liability for benefits.  Substantial evidence exists in the record to support the compensation judge’s decision that the January 2011 accident, the employee’s significant increase in symptoms, and her subsequent surgery did not sever the causal connection between the work injury and need for surgery.  We affirm.