The employer and insurer appeal from the compensation judge’s finding that the employee sustained a permanent injury to his low back on August 20, 2008, and from her finding that the employee sustained a Gillette-type injury^[1] culminating on March 31, 2009.  We affirm.

Brian T. Peterson [the employee] sustained an admitted injury to his low back on August 20, 2008, while working as a paramedic for North Memorial Health Care [the employer].  On that date, the employee, who had been working for the employer since January 2007, was thirty-six years old and was earning a weekly wage of $1,022.04.  At the time he was hired, the employee had taken and passed a pre-employment physical and had no restrictions on his work ability.

Prior to his work injury, the employee’s work schedule called for working thirty-six hours of primary duty and an additional thirty-six hours of back-up (on-call) duty each week.  When the employee was on call, he would not need to respond to a call unless the primary crew was out on a run.  The employee participated in three different types of calls or runs: a response to a patient transfer, a back-up call, or a 911 call.  The most physically strenuous call would likely be the 911 call where the employee was subject to performing heavy lifting in awkward positions.  There was no way to determine what types of runs the employee might have on any given shift.  The duties of the job varied considerably in terms of frequency and physicality.  In addition to lifting patients, the employee regularly handled oxygen tanks, cardiac monitors, backboards, and stretchers.  There was also a lot of down time spent at the garage.  Most of this time was spent performing light duty general housekeeping tasks, such as washing of the vehicles, sweeping, and other cleaning.

On August 20, 2008, the employee was transporting a 400-pound patient out from the lower level of a split-level home.  Two police officers were assisting the employee and handling the lower part of a stair chair used in the transfer.  The employee slipped on the stairs and twisted awkwardly as the police officers continued to push the stair chair toward him.  The employee felt a pop in his back and some pain.  Later that same shift, the employee encountered another awkward transfer and noticed worsening of his back pain.

The employee sought medical treatment the following day at the Douglas County Hospital, where he was diagnosed with a work-related low back soft tissue strain.  He was taken off work and referred for follow up with Dr. George Erhard on August 26, 2008.  Dr. Erhard kept the employee off work and arranged for physical therapy.  The employee attended three physical therapy sessions through September 9, 2008.  At the employee’s final session, the therapist noted that the employee exhibited a full active range of motion of the low back without pain.  The employee reported that he felt “as though he’s back to regular feeling.”  The therapist advised the employee to avoid activities that would exacerbate his pain and to engage in a therapeutic exercise program.  The employee was discharged from therapy with all goals having been met.  The employee also saw Dr. Erhard on that same date, September 9, 2008.  The employee reported to Dr. Erhard that his condition was ninety percent improved, that he had no back pain, and that he was not taking any medications, and Dr. Erhard released him to return to work without restrictions.  In a Health Care Provider Report issued that same day, Dr. Erhard indicated that the employee had reached maximum medical improvement [MMI] and had not sustained any permanent partial disability.

The employee returned to work as a paramedic on September 9, 2008, and performed his regular job without work restrictions through March 31, 2009.  During this time frame, the employee did not report ongoing back problems to his employer or seek any treatment for his low back.  During this same period, however, the employee did treat on several occasions for cluster migraine headaches, but he did not report any back problems to any of his medical providers.  On about March 30 or 31, 2009, while riding in an ambulance after a patient transfer to the Twin Cities, the employee experienced a sudden onset of radicular pain into his right leg.  He has not returned to work for the employer since March 31, 2009.

On April 1, 2009, the employee returned to the Alexandria Clinic with complaints of recurring back pain and now right leg pain.  At that visit, Dr. Michael Bristow recommended work restrictions, prescribed medication, and, “[g]iven the length of the symptoms and [their] recurrent/progressive nature,” referred the employee for a lumbar MRI to rule out a possible disc herniation.

On April 8, 2009, the employee returned to Dr. Erhard, who had not seen the employee with regard to his low back since September 9, 2008.  The employee now reported that he had “had some minimal back discomfort ever since that time, which became worse, perhaps 2-3 weeks ago without precipitating event.”  The MRI ordered by Dr. Bristow had revealed “a herniated disc at L5-S1 with a spur touching but not compressing the ganglion exiting at the L5-S1 foramen,” together with facet arthrosis at L4-S1, and Dr. Erhard recommended an epidural steroid injection and physical therapy.

On May 19, 2009, the employee was seen at Central Minnesota Spine Center on referral from Dr. Erhard.  The employee provided a history there of low back pain since lifting a patient on August 20, 2008, and of a sudden onset of right leg pain while sitting in an ambulance on March 28, 2009.  The physician’s assistant who examined the employee diagnosed low back pain, a mild L5-S1 posterior disc herniation, and facet joint arthrosis, with narrowing of the right foramina.  The employee was eventually seen by neurosurgeon Dr. Sunny Kim, who, after a CT discogram revealed a concurrent pain reproduction at L4-5 and L5-S1, recommended an L4-S1 spinal fusion.

On August 23, 2009, in response to a letter from the employee’s attorney regarding issues of causation, Dr. Erhard explained that, following the August 20, 2008, injury, the employee “experienced progressive development of pain in his low back with a subsequent radicular component going into his right lower extremity.”  He noted further that the injury of August 20, 2008, had also not entirely resolved before the employee returned to work.  Regarding the progression of the employee’s low back condition, Dr. Erhard stated:

On September 14, 2009, Dr. Kim performed the L4-S1 fusion surgery that he had recommended.  On November 15, 2009, also in response to inquiries from the employee’s attorney, Dr. Kim also opined that the employee’s lifting incident on August 20, 2008, was a substantial contributing factor in the employee’s ongoing low back condition and his need for surgery.  Dr. Kim stated that had the employee not had this accident he would not have required spinal fusion surgery.  It also was Dr. Kim’s opinion that the employee’s “subsequent activities of physically demanding work while working as an ambulance attendant between September 9, 2008, and March of 2009 was a further aggravating factor of his low back condition eventuating in the spinal fusion surgery.”

The employee filed a claim petition for workers’ compensation benefits on November 30, 2009.  He claimed entitlement to temporary total disability benefits continuing from April 1, 2009, as a result of the of the admitted August 20, 2008, injury, as well as a Gillette-type injury culminating on or about March 30, 2009.  In their answer to the employee’s claim petition, filed December 14, 2009, the employer and insurer admitted only that the employee had sustained a temporary injury to his low back on August 20, 2008, and they denied that the employee had sustained a Gillette-type injury in March 2009.

The employer and insurer arranged an independent medical examination of the employee by orthopedist Dr. Mark Friedland on February 24, 2010.  After obtaining a history, reviewing the employee’s medical records, and performing a physical examination, Dr. Friedland opined that the employee had sustained a temporary lumbosacral strain/sprain as a result of the work incident of August 20, 2008.  It was his opinion that that injury had fully resolved by September 9, 2008, when the physical therapist noted an objectively normal examination and when Dr. Erhard released the employee to unrestricted work activities.  Dr. Friedland opined also that the employee did not sustain a Gillette-type injury to his low back culminating in March 2009.  He based his conclusion in part “upon the fact that [the employee] could not confidently tell me today that his work activities were exacerbating his low back symptomatology.”  He also found it significant that the employee did not seek any care or treatment for low back symptomatology for seven months after being released from care by Dr. Erhard on September 9, 2008.  Dr. Friedland concluded also that, based upon the employee’s multiple radiographic studies, the employee’s symptomatology “was merely due to chronic persistent and longstanding L4-5 and L5-S1 degenerative disc disease.”

The employee’s claim for benefits came on for hearing before a compensation judge on August 5, 2010.  Evidence presented at trial included the employee’s testimony, his medical records, and records from the employer reflecting the employee’s work schedule and ambulance runs between September-October 2008 and March 31, 2009.  The employee testified on direct and cross-examination about certain previous injuries and low back strains, noting that he had no recollection of several of the incidents.  Medical records detailing these incidents were introduced into evidence and reflected treatment of short duration that did not entail physical therapy or any diagnostic scans.  The employee testified that he had recovered fully from these incidents, that he had no lower back problems between September of 2005 and his injury on August 20, 2008, but that he never fully recovered from the latter injury.  He testified that when he returned to work in September of 2008 he still had lingering pain in the low center of his back, that the nature of his pain didn’t ever really change except to become progressively worse over the next several months, the level of his symptoms varying depending on the level of his activity.

In a findings and order issued October 4, 2010, the compensation judge found that the employee’s August 20, 2008, injury was a permanent injury and a substantial contributing factor in the employee’s disability and need for medical treatment after April 1, 2009.  She found also that the employee’s continuing work after the August 2008 injury resulted in a Gillette aggravation culminating on March 31, 2009.  The judge apportioned liability between the injuries, seventy-five percent to the August 20, 2008, injury and twenty-five percent to the aggravation caused by the employee’s continued work.  The employer and insurer appeal.

On 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.”  Minn. Stat. § 176.421, subd. 1 (2010).  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.”  Hengemuhle v. Long Prairie Jaycees, 358 N.W.2d 54, 59, 37 W.C.D. 235, 239 (Minn. 1984).  Where evidence conflicts or more than one inference may reasonably be drawn from the evidence, the findings are to be affirmed.  Id. at 60, 37 W.C.D. at 240.  Similarly, 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 evidence or not reasonably supported by the evidence as a whole.”  Northern States Power Co. v. Lyon Food Prods., Inc., 304 Minn. 196, 201, 229 N.W.2d 521, 524 (1975).

The compensation judge found that the employee’s August 20, 2008, injury was a permanent injury rather than a temporary injury as alleged by the employer.  The judge accepted as credible the employee’s testimony that, despite the fact that he worked with no restrictions from September 9, 2008, through March 31, 2009, the employee was experiencing increasing low back pain during that time and his low back never did return to its pre-injury status.  The judge accepted also Dr. Erhard’s opinions that the August 20, 2008, injury was not entirely resolved when the employee went back to work.  The judge noted that the employee had some prior low back problems before he began work for the employer, but she accepted the employee’s testimony that these episodes of low back pain were minor and that the employee did not have any ongoing low back symptoms at the time he began working for this employer.  The employer and insurer argue that the judge erred as a matter of law in reaching this conclusion, by failing to consider the several factors relevant in determining whether an injury is temporary or permanent that are articulated in this court’s holding in Wold v. Olinger Trucking, slip op. (W.C.C.A. Aug. 29, 1994).^[2]  Furthermore, they argue, an analysis of the six factors outlined in Wold reveals that the judge’s determination that the August 20, 2008, injury was permanent in nature is not reasonably supported by the evidence.  We are not persuaded.

First of all, contrary to the employer and insurer’s argument, the compensation judge was not required by Wold to address the factors discussed in that case.  Heckel v. Crown, Cork & Seal, slip op. (W.C.C.A. Dec. 21, 1999).  The judge’s findings and memorandum adequately set forth her rationale for finding a permanent injury, and we find no basis for reversal as a matter of law on grounds of the Wold case.  Furthermore, we conclude that the judge’s determination is supported by substantial evidence in view of the record as submitted.

We acknowledge that neither Dr. Erhard nor Dr. Kim referenced the employee’s history of back strains, but we find nothing in the record to suggest that those histories would be material to their opinions.  Neither did Dr. Friedland comment on the significance of the employee’s prior history.  The employer and insurer’s argument that the employee’s 2008 injury was essentially consistent with his previous history of temporary injuries was a factual issue for the judge’s consideration.  Dr. Friedland attributed the employee’s condition to degenerative disc disease, but it is only reasonable that Dr. Erhard and Dr. Kim were also aware of that condition.  What Dr. Erhard evidently found significant was the employee’s history that his 2008 injury had not resolved.  It is well settled that injuries are compensable not only if the employment substantially contributes directly to a new condition but also if it contributes to the aggravation or acceleration of a pre-existing condition.  Wallace v. Hanson Silo Co., 305 Minn. 395, 235 N.W.2d 363, 28 W.C.D. 79 (1975).  An employee need not prove that the employment was the sole cause, only that it was a substantial cause, of the disability for which benefits are sought.  Roman v. Minneapolis St. Ry. Co., 268 Minn. 367, 380, 129 N.W.2d 550, 558, 23 W.C.D. 573, 592 (1964).

In her memorandum, the compensation judge stated that she accepted the employee’s position that his previous low back episodes were minor and that he had no low back symptomatology or medical treatment between September 2005 and August 20, 2008.  The judge noted also that the employee had passed a pre-employment physical before starting work for the employer and that he performed physically demanding work for the employer with no problems until August 20, 2008.  Most importantly, the judge found the employee to be a credible witness, accepting his testimony that, despite having no formal work restrictions or medical treatment between September 9, 2008, and March 31, 2009, he continued to experience low back pain and did not return to his pre-injury status.  Lastly, the judge accepted Dr. Erhard’s opinion that the August 20, 2008, injury was not entirely resolved when the employee went back to work.  We conclude that these findings by the judge adequately support her conclusion that the employee’s August 20, 2008, injury was permanent in nature.  See Brennan v. Joseph G. Brennan, M.D., 425 N.W.2d 837, 839-40, 41 W.C.D. 79, 82 (Minn. 1988) (assessment of a witness's credibility is the unique function of the trier of fact); Nord v. City of Cook, 360 N.W.2d 337, 342-43, 37 W.C.D. 364, 372-73 (Minn. 1985) (“the trier of fact's choice between experts whose testimony conflicts is usually upheld [unless] the facts assumed by the expert in rendering his opinion are not supported by the evidence”).

The compensation judge concluded that the employee sustained a Gillette-type injury culminating on or about March 31, 2009.  She based her determination on the employee’s hearing testimony and its consistency with the medical records and on the opinions of Drs. Erhard and Kim.  The employer and insurer argue that the treating doctors based their opinions on the employee’s contention that his work activities required heavy and awkward work, whereas, in fact, the evidence as a whole reveals that the employee’s physical activities were extremely varied and often very light.  Neither Dr. Kim nor Dr. Erhard, they contend, was aware of the employee’s actual work duties.  By contrast, they argue, Dr. Friedland did have proper foundation for his opinion and correctly stated in his report that even the employee could not definitively establish any type of pattern of association between his work activities and the development of a Gillette injury.  Because Dr. Friedland’s opinion was supported by the evidence of record whereas the opinions of the treating physicians were not, they contend, the judge erred in finding that the employee sustained a Gillette-type injury on March 31, 2009.  We are not persuaded.

To establish a Gillette injury, an employee must “prove a causal connection between his ordinary work and ensuing disability.”  Steffen v. Target Stores, 517 N.W.2d 579, 581, 50 W.C.D. 464, 467 (Minn. 1994).  The determination of a Gillette injury “primarily depends on medical evidence.”  Id., citing Marose v. Maislin Transp., 413 N.W.2d 507, 512 (Minn. 1987).  This case turns largely upon the compensation judge’s resolution of conflicting expert medical opinion.

The compensation judge specifically accepted the medical opinions of Dr. Kim and Dr. Erhard, the employee’s treating doctors, over the medical opinion of Dr. Friedland.  In her memorandum, the judge explained as follows:

Dr. Erhard and Dr. Kim treated the employee over an extended period of time, took multiple histories from the employee, and discussed the employee’s work activities with him.  Also, both doctors were provided with a history by the employee’s attorney when she solicited their causation opinions by letters dated August 12, 2009, which were in evidence before the judge.  As implied in the judge’s memorandum, there may be some inconsistencies and inaccuracies with respect to the facts relied upon by the medical experts, and they may have lacked complete knowledge about every aspect of the employee’s work activities.  These concerns, however, go to the evidentiary weight of the medical opinions offered, not to their foundation.  See, e.g., Bossey v. Parker Hannifin, slip op. (W.C.C.A. Mar. 14, 1994) (while adequate foundation is necessary for a medical opinion to be afforded evidentiary value, the expert need not be made aware of every relevant fact).  Given the entire record as submitted, the judge’s opportunity to observe the employee’s testimony, and the explanation provided by the judge in her memorandum, we find no basis for reversal on foundational grounds.

Whether an employee has proved a Gillette injury is a question of fact for the compensation judge.  See, e.g., Carlson v. Minneapolis Pub. Hous. Auth., slip op. (W.C.C.A. June 19, 1977).  As the trier of fact, it is the compensation judge’s responsibility to resolve conflicts in expert medical testimony, and, where there is adequate foundation for the opinions adopted by the judge, this court normally upholds the compensation judge’s choice among medical experts.  See Nord, 360 N.W.2d at 342-43, 37 W.C.D. at 372-73.  On the facts of this case, we cannot say that the compensation judge erred in accepting the causation opinions of Dr. Erhard and Dr. Kim.  We conclude that, based on the medical evidence presented here, it was reasonable for the compensation judge to rely on those opinions in concluding that the employee sustained a Gillette injury to his lower back on or about March 31, 2009, as a result of his work activities with the employer.  Therefore we affirm that conclusion.  See Hengemuhle, 358 N.W.2d at 59, 37 W.C.D. at 239.^[3]