The self-insured employer appeals from the compensation judge's conclusion that the employee's work activities were a substantial contributing cause of her herniated disc.  We affirm.

Ann Boyd is subject to a scoliosis condition that was initially addressed surgically in 1982, when Ms. Boyd was in her late teens, with the installation of Harrington rod instrumentation from about T2 down to about L5 of her thoracic and lumbar spine.  About four years later, in about 1986, Ms. Boyd completed a degree as a registered nurse, in which capacity she worked thereafter for about two years in a nursing home.  In 1988, Ms. Boyd took a job as a floor nurse at Abbott Northwestern Hospital, a job that entailed substantial bending of her back, a certain amount of lifting, and being on her feet most of her working hours.  In that same year, 1988, she underwent a revision of her Harrington rod instrumentation, the original device evidently having broken.  She continued to work for Abbott Northwestern Hospital with her new instrumentation without any related difficulty for about six years, until 1994, when she found a job closer to home as a floor nurse at St. Francis Regional Medical Center.  Ms. Boyd worked at that job, performing all of its required duties for about nine years, until 2003, without any serious or continuing low back symptoms, evidently experiencing only some intermittent sciatic pain, sometimes in her right leg, sometimes in her left.  In 2003, Ms. Boyd temporarily left the labor market to concentrate on home schooling of her four daughters.  The latter endeavor involved sitting and standing at the kitchen table, sitting on the couch, and doing some occasional bending.  Ms. Boyd's daughters, between the ages of nineteen and thirteen at the time of the hearing below, did most of the housework, Ms. Boyd performing only the lighter duties.

In October of 2005, Ms. Boyd re-entered the labor market, taking a part-time job as a registered nurse at St. Gertrude's Health Center, where she evidently split her time between the more physically heavy duties of a floor nurse and the lighter and more administrative and supervisory duties of a charge nurse.  Her job description as a "Charge Nurse and Designated Lead RN" at that employer indicates that the job required her to be on her feet from seventy-five to one hundred percent of the day, stooping, bending, squatting, performing pushing/pulling/reaching movements, climbing, going up and down stairs, working on step ladders, perhaps walking on uneven surfaces, and lifting up to one hundred pounds, up to fifty pounds frequently.  On Wednesday March 5, 2008, after Ms. Boyd [the employee] had been working for St. Gertrude's Health Center [the employer] for about two and a half years, a supervisor at the employer completed a Supervisor Incident Analysis Report, documenting a complaint by the employee of pain in her right hip at work on February 26, 2008, that had radiated down her right leg throughout her work shift and had eventually rendered her, on that morning of March 5, 2008, unable to walk.  The employee had been forty-three years old on February 26, 2008, and had been earning a weekly wage of $492.77.  The employer, which was self-insured at the time, denied liability for any work-related injury.

The employee apparently remained off work on Thursday March 6, 2008, and the following day, March 7, 2008, she saw orthopedist Dr. Donald Asmussen, to whom she complained of low back and right leg radicular pain extending down the back of her thigh to her calf.  Upon examination of the employee, and noting her history of scoliosis surgery and of intermittent sciatic pain in years past, Dr. Asmussen suspected that the employee's pain on that date was due to an L5-S1 disc herniation.  On that provisional diagnosis, the doctor recommended a lumbar MRI scan, prescribed medication, and anticipated a need for epidural steroid injection upon corroboration of his diagnosis by the MRI scan.  A week later, on March 14, 2008, the employee evidently called in to Dr. Asmussen's office, outlining her work activities and emphasizing her feeling that they were an important cause of her current back symptoms.  In his office note on that date, Dr. Asmussen stated, "She has outlined all of these to me and I would agree that it is more than reasonable to conclude that her back pain is related to work activity."

On March 18, 2008, on referral from Dr. Asmussen, the employee underwent a physical therapy evaluation at Park Nicollet Clinic, following which the evaluator recommended a two- to four-week home program for her, with self-management of symptoms.  When she saw Dr. Asmussen again the following day, the employee complained of increased pain, and the doctor increased her medication dosage.  He noted on that date that the employee's biggest concern was that her workers' compensation claim had been denied on the ground that her pain was related to her scoliosis surgery.  Dr. Asmussen indicated that he disagreed with this conclusion, in that the distribution of the employee's radicular symptoms "would be the L5 and S1 nerve root distributions which [are] considerably inferior to her previous spinal surgery.  I do not believe that they are related."  On March 24, 2008, the employee filed a claim petition, alleging entitlement to various benefits consequent to a work injury on February 26, 2008.

Upon seeing her again on April 11, 2008, Dr. Asmussen found the employee clearly improved.  He continued to recommend an MRI study, which the employee had been putting off pending acceptance of liability by the employer.  Dr. Asmussen reiterated that the employee's

On April 24, 2008, the employee underwent an MRI scan of her lumbar spine, which was read to reveal "[m]ultilevel degenerative disc disease with postsurgical changes of spinal fusion instrumentation for scoliosis," together with a broad-based disc protrusion at L5-S1 that was mildly impinging on the S1 nerve root and some moderately severe bilateral facet arthropathy at that same level, but without any impingement on the L5 nerve roots.  The scan was also read to reveal "spinal fusion instrumentation . . . extending from the L5 level cranial to [at least] the T10 level."  On that same date, the employee also underwent an epidural steroid injection into the neural foramen on the right at S1 of her spine, which resulted in an eighty percent initial reduction of her pain.  On May 6, 2008, she saw Dr. Asmussen again, indicating to him that, while she was extremely pleased with her response to the injection with regard to her right leg symptoms, she was now experiencing radicular symptoms all the way down to her calf in her left leg.  Noting that the employee had told him that the employer had refused to take her back under any restrictions, Dr. Asmussen indicated that he thought it reasonable for the employee now to undergo also a left epidural steroid injection, in hopes of getting her back to unrestricted work.  The following day, the employee underwent the recommended left epidural injection, into the neural foramen on the left at S1, which resulted in ninety-five percent initial reduction of her left leg pain.

On May 14, 2008, Dr. Asmussen noted that he had had a long talk with the employee regarding her work and her condition and had found it reasonable for her to return to unrestricted work," although she knows very well to be extremely cautious with lifting, bending, patient transfers, etc.  Clearly, if a patient goes down, she will need extra help with them, and she is not to be doing heavy patient lifting on her own."  The doctor characterized the employee's status on that date as "[l]umbar disk herniation, now improved."  On May 23, 2008, the employee returned to working up to six hours a day as a charge nurse, a job that was normally not as physically stressful as the job that she had been doing on the date of her alleged work injury, February 26, 2008, and resulted in lower wages than she was paid at that date-of-injury job.

On June 30, 2008, the employee was examined for the employer by neurologist Dr. Daniel Randa.  Dr. Randa diagnosed right L5-S1 radiculitis, due to irritation but not significant compression of the right L5-S1 nerve root, together with multilevel degenerative lumbar spondylosis, including changes at L5-S1.  Dr. Randa concluded that these changes were "the anticipated consequence of her previous spinal fusion and Harrington rod placement" down to L5-S1 and were in turn the cause of the employee's right S1 nerve root irritation.  He explained that the employee's disc protrusion at L5-S1 was "secondary to the biomechanical distortions created  by the scoliosis of the thoracolumbar spine, and previous Harrington rod placement and spinal fusion from L5 rostrally up to approximately T2."  Dr. Randa concluded further that the employee had 0% permanent partial disability to the lumbar spine, based upon Minnesota Rules 5223.0390, subpart 3.A., and had never been disabled from working within restrictions since the date of her alleged work injury.  Further, he concluded that she was currently capable of working full time without restrictions, although he did recommend that she avoid "any type of heavy lifting as a result of her scoliosis, previous fusion, and degenerative lumbar spondylosis."

On July 2, 2008, the employee was examined at the request of her attorney by orthopedic surgeon Dr. Robert Wengler.  Upon examination, it was Dr. Wengler's opinion that the employee had sustained a right-side L5-S1 disc herniation as a consequence of her work activities at the employer on February 26, 2008.  He opined that her medical treatment to date had been reasonable and necessary, and he supposed that she would eventually require surgery.  He recommended that, consequent to her work injury, she restrict her lifting to ten pounds and not engage in activities requiring repetitive bending or stooping, pushing or pulling, or working in positions of postural stress.  Finally, Dr. Wengler rated the employee's related permanent partial disability at 12% of the whole body, pursuant to Minnesota Rules 5223.0390, subparts 4.D. and 4.D.(1), which rate disability for chronic radicular pain or paresthesia that persists despite treatment.

On July 9, 2008, the employee returned to Dr. Asmussen with complaints of renewed radicular pain, particularly on the right side, where it extended all the way down to her foot.  Dr. Asmussen recommended definitive surgery, but the employee opted instead for another epidural steroid injection, pending the employer's acceptance of primary liability in her workers' compensation claim.  Reiterating his opinion that "this clearly is a work-related issue an[d] not related to her previous Harrington rod surgeries," Dr. Asmussen recommended the steroid treatment, and on that same day the employee underwent "bilateral transforaminal SI diagnostic and therapeutic epidural injections."

On August 19, 2008, Dr. Randa testified by deposition, essentially reiterating his opinion that the employee's herniated disc at L5-S1 was a consequence solely of the extra stress that had been exerted on that level by the employee's Harrington rod instrumentation, which had fused the employee's back from T2 to L5 for so many years of her life, and that the employee's work activities were not a substantial contributing cause of her herniated disc.  Dr. Randa testified that he regularly sees such wear and tear changes at the extremities of spinal fusions even in people who don't work outside the house at all, that, given the employee's fusion down to L5, the disc at L5-S1 of her spine would have degenerated or protruded over time "regardless."  In a clinical progress note on August 29, 2008, having expressly acknowledged that the employee "does have a fusion extending from L5 superiorly to T3," Dr. Asmussen responded to Dr. Randa's opinion, indicating that,

On September 22, 2008, Dr. Wengler also testified by deposition, reiterating his conclusion that the employee was subject to a herniated disc at L5-S1, that that condition was causally related to her work for the employer on February 26, 2008, and that she was in need of surgery for her condition.  He explained that

The matter came on for hearing on September 29, 2008, on which date the parties stipulated as to the employee's weekly wage and to the fact that the medical treatment at issue was reasonable and necessary.  Issues at hearing included whether the employee had sustained a work-related injury to her low back on February 26, 2008, and, if so, what was its nature, was it temporary or permanent, whether she was subject to a 12% whole-body impairment as a result, and whether her subsequent medical treatment was causally related to that work injury.  Evidence introduced at hearing included the live testimony of the employee, in part that she informed Dr. Asmussen of her work injury at her first visit with him, though his office notes might not have reflected that.  Also submitted into evidence was the deposition testimony of Drs. Wengler and Randa.  By findings and order filed October 30, 2008, the compensation judge concluded in part that the employee had sustained on the date alleged a work-related permanent aggravation to her low back in the nature of a herniated disc at L5-S1.  On that finding he ordered the employer to pay all alleged temporary total and temporary partial disability benefits from March 6, 2008, through the date of hearing, as well as all medical bills at issue and compensation for a 9% whole-body impairment.  The self-insured employer appeals.

In reviewing cases 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.  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, "[f]actfindings are clearly erroneous only if the reviewing court on the entire evidence is left with a definite and firm conviction that a mistake has been committed."  Northern States Power Co. v. Lyon Food Prods., Inc., 304 Minn. 196, 201, 229 N.W.2d 521, 524 (1975).  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."  Id.

The compensation judge found that the employee's duties as a nurse at the employer were a substantially contributing cause of the herniated disc at L5-S1 of her back.  The employer contends that the herniated disc at issue was a consequence solely of the instrumentation that had been installed to fuse the employee's thoracic and lumbar spine in treatment of her preexisting scoliosis condition.  The employer argues that the medical experts relied on in support of a contrary conclusion were without proper foundation for their opinions, in that they were presuming that the employee's instrumentation did not end immediately above the L5-S1 level, thereby rendering unusual stress on the disc at that level.  The employer argues further that Dr. Asmussen's notes of his initial treatment of the employee on March 7, 2008, do not even mention a relationship between the employee's low back condition and her work activities on February 26, 2008, or any suggestion of one by the employee.  We are not persuaded.

With regard to the employee's own perception of a relationship between her low back condition and her work on February 26, 2008, the employee testified that she did first feel the onset of her symptoms on that date and that she did report that onset to Dr. Asmussen on March 7, 2008.  In his memorandum, the compensation judge indicated that he credited this testimony of the employee.  Assessment of a witness's credibility is the unique function of the trier of fact.  Brennan v. Joseph G. Brennan, M.D., 425 N.W.2d 837, 839-40, 41 W.C.D. 79, 82 (Minn. 1988).  Moreover, we note that already by March 5, 2008, two days before the employee's initial appointment with Dr. Asmussen and fully nine days before she evidently called in to Dr. Asmussen's office to underscore her perception that her condition was work related, the employer had already documented that perception in its Supervisor Incident Analysis Report.  We find reasonable and accept the compensation judge's conclusion in his memorandum that "[t]here is no question that the employee did not 'manufacture' the occurrence of the injury in issue between March 7 and 14, 2008."

With regard to the judge's conclusion that the employee's activities at work were a substantial contributing cause of her disc herniation at L5-S1, we rely initially on our very well established rule that "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."  Nord v. City of Cook, 360 N.W.2d 337, 342-43, 37 W.C.D. 364, 372-73 (Minn. 1985).  The employer contends that the facts assumed by Dr. Asmussen in rendering his opinion were not supported by the evidence, in that, at the time he first rendered that opinion, he understood the nerve root distributions at issue to be "considerably inferior to [the employee's] previous spinal surgery," whereas subsequent MRI evidence indicated that the fusion instrumentation extended all the way down to the L5 level.  We note, however, that Dr. Asmussen expressly reiterated his opinion on August 29, 2008, well after the April 24, 2008, MRI scan, expressly acknowledging the scan's evidence of a fusion all the way down to L5.  In his August 29, 2008, progress note, while acknowledging that "adjacent segment disease is clearly a contributing factor," Dr. Asmussen reasserted his opinion that the employee's work duties still "would be, at the very least, a contributing or augmentative phenomena," noting in support "that [the employee's] back and radicular leg symptoms did not begin until after she began working [for the employer]."  In further support of the compensation judge's conclusion is Dr. Wengler's opinion that neither the amount of weight that the employee had been lifting on the morning of February 26, 2008, nor the particular task that she was performing at the moment of the onset of her symptoms was of material importance as to the causal relationship between those symptoms and the employee's work for the employer.

Because it was not unreasonable for the compensation judge to credit the employee's testimony that the onset of her symptoms occurred while at work on February 26, 2008, as claimed, and because it was not unreasonable for the judge to conclude, in reliance on the expert medical opinions of Drs. Asmussen and Wengler, that the employee's disc herniation at L5-S1 was causally related to her work for the employer, we affirm the compensation judge's finding of a work-related injury and his consequent award of the benefits here at issue.  See Hengemuhle, 358 N.W.2d at 59, 37 W.C.D. at 239.