The self-insured employer appeals from the compensation judge’s determination that the employee sustained a permanent, work-related injury to his low back on June 15, 2009, and that the work injury was a substantial contributing cause of the employee’s need for low back treatment, including surgery on January 17, 2012.  We affirm.

Rocky W. Johnson, the employee, began working as a dock worker loading and unloading trucks for the self-insured employer, USF Holland, Inc., in 1995.  His job duties included driving a forklift, operating pallet jacks, and occasionally lifting in excess of 100 pounds.  The employee began driving truck for the employer about three to five years later.  The employee testified he had his own pallet jack, and that most of the time he loaded and unloaded the trailer himself.  On June 15, 2009, the employee was assigned a trailer to unload.  He tried to pull the rolling door up, but it was stuck and wouldn’t move.  The employee then used a dock hook to pull up and back on the door.  While pulling, he felt a sharp pinch or twinge in his back.  He reported the injury and finished out the day.  The employee stated he didn’t think much of it at the time, but the next morning he was stiff and in terrible pain.

On June 16, 2009, the employee sought treatment at his primary care clinic, Multicare Associates, where he was examined by Rick Hathaway, PA-C.  A lumbar strain was diagnosed and Flexeril and Vicodin were prescribed.  The employee was taken off work and referred for physical therapy.  On June 22, 2009, PA Hathaway reported the physical therapist had not seen any real improvement and felt the employee was beginning to display some radicular symptoms.  PA Hathaway referred the employee to Dr. Domino and the employee’s Vicodin was refilled.

The employee was seen by Dr. Terry Domino on June 23, 2009, reporting low back pain and pain radiating down the back of the left leg to the knee.  On examination, there was tenderness and spasm in the lower lumbar area with limited range of motion, knee jerks were decreased symmetrically, and straight leg raising elicited pain radiating down the left buttock to the knee.  An MRI scan on June 24, 2009, showed a central to left paracentral disc herniation at L4-5, mild annular bulging at L3-4 and L5-S1, and bilateral facet degeneration at all three levels.  Dr. Domino diagnosed an acute left-sided lumbar strain with an L4-5 disc herniation, referred the employee for additional physical therapy, and continued the employee off work.

The employee returned to Dr. Domino in July 2009.  He continued to be symptomatic, but was slowly improving with physical therapy.  On July 20, 2009, Dr. Domino indicated the employee had returned to light duty work.  The employee continued to take Naprosyn and Vicodin^[1] for pain, but was reducing the amount he was taking.

On July 29, 2009, the employee was seen by Dr. Glenn Buttermann, an orthopedic surgeon.  The employee reported persistent pain in the low back and left leg.  On examination, the doctor noted tenderness along the left paraspinal muscles and limited lumbar flexion.  X-rays revealed retrolisthesis at L3-4.  The MRI scan was interpreted as showing disc degeneration from L3 to S1 including a left-sided disc herniation at L4-5.  Dr. Buttermann recommended further conservative treatment including physical therapy, anti-inflammatory medications, and injections.

The employee completed physical therapy on July 31, 2009, and was seen by Dr. Domino on August 3, 2009.  The employee reported he did not have much pain in the back and was not taking Vicodin.  The doctor assessed acute low back pain with L4-5 disc, improving.  Dr. Domino lifted the employee’s work restrictions and stated the employee could resume his truck driving duties.  The employee was last seen by Dr. Domino on August 10, 2009, stating his back condition felt normal.  He continued to take Naprosyn for pain and stiffness but had not taken Vicodin in over a week.

Three weeks later, on August 31, 2009, the employee was seen at Multicare Associates for bilateral hip pain.  X-rays showed advanced arthritic changes in both hips.  Naprosyn was discontinued and Mobic^[2] was prescribed.  The employee’s symptoms did not improve, and on November 13, 2009, the employee was seen by Dr. Erik Wetter at Orthopedic Partners.  Dr. Wetter noted anti-inflammatory medications were not helpful, and that the employee took narcotics from time to time which helped a bit but did not provide lasting relief.  The employee received cortisone injections in December 2009 which provided no relief.  In a chart noted dated December 17, 2009, a doctor at Multicare Associates noted the employee’s prescription for Vicodin had been refilled.

On February 25, 2010, the employee was evaluated by Dr. David Palmer at St. Croix Orthopedics.  The doctor diagnosed severe degenerative joint disease and recommended hip resurfacing surgery beginning with the right side.  The surgery was performed by Dr. Palmer on March 17, 2010.  The employee was seen in follow up on April 1, 2010, at which time he was off work and taking Percocet^[3] every 6 hours for pain.  The employee was given a new prescription for hydrocodone and continued off work.

On April 26, 2010, the employee was seen by Dr. Buttermann.  A chart note indicated the employee’s back pain was only noticeable when sitting or standing too long.  The employee indicated his back had popped out of place four times since the original injury and he wanted to follow up before returning to work.  Dr. Buttermann noted the employee continued to have low back pain with intermittently severe symptoms.

Hip resurfacing surgery for the left side was performed by Dr. Palmer on May 24, 2010.  At the two week post surgery follow up on June 9, 2010, Dr. Palmer noted the employee was taking Norco^[4] with good response.  The employee was attending physical therapy and was full weight bearing with cane assist.

On June 24, 2010, the employee was examined by Dr. Gary Wyard at the request of the self-insured employer.  The employee reported he was doing reasonably well in his recovery from the hip surgeries but was continuing to take hydrocodone for pain.  Dr. Wyard noted the employee had a history of low back problems, but it was not really discussed at the time as the examination focused on the hips.  Dr. Wyard’s impression included central disc at L4-5, neurologically intact.  The doctor observed the employee had degenerative disc disease of the lumbosacral spine, but opined the employee did not need restrictions or limitations as a result of his back condition at that point.

The employee was seen by Dr. Palmer on July 13, 2010, for his six weeks post surgery checkup.  He was doing well with a slight limp on the left.  He was discharged from care and was released to return to work as of July 26, 2010, without restrictions, although he was cautioned against repetitive impact loading and positions of dislocation.  The employee returned to work driving truck for the employer.

The employee returned to see Dr. Buttermann on November 1, 2010, for severe low back pain.  A CT scan showed bilateral sacroiliac ankylosis, moderate disc degeneration at L3-4, and a disc herniation at L4-5.  The doctor recommended epidural steroid injections which were performed on November 15, 2010.  The employee was seen at Multicare Associates that same day for renewal of his handicapped parking permit.  He stated he had a fairly physical job driving truck and forklift which required some loading and unloading and that after the work day his hips and back were much worse.

On April 11, 2011, the employee was seen by Dr. Peter Terry at Multicare Associates for back pain.  The employee reported onset of back pain three years previously and rated the pain as a 6 out of 10.  The employee stated the symptoms were aggravated by daily activities and relieved by pain medications.  The doctor assessed left L5 radiculopathy, acute, and referred the employee for physical therapy.  The employee’s symptoms improved somewhat with physical therapy, but he reported periodic exacerbations with low back and leg pain.

The employee returned to Dr. Buttermann on June 20, 2011.  The doctor noted the employee had undergone extensive non-operative treatment over the past two years and was now interested in proceeding with surgery as he was having a difficult time doing much of anything.  On July 11, 2011, the employee filed a claim petition seeking approval of the lumbar surgery recommended by Dr. Buttermann.  In a chart note dated July 20, 2011, Dr. Buttermann stated the employee currently had predominantly axial symptoms^[5] that had been present since his June 2009 injury.  X-rays showed narrowing and mild retrolisthesis at L3-4 and L4-5, and a CT scan showed bilateral lateral recess stenosis and diffuse disc herniations at L3-4 and L4-5.  The L4-5 disc herniation had decreased in size since the June 2009 MRI scan and, according to Dr. Buttermann, this finding was consistent with the employee’s predominantly axial symptoms.  The doctor noted the employee’s leg symptoms could be severe but were typically intermittent, consistent with retrolisthesis instability and lateral recess stenosis.

Dr. Wyard conducted a second examination of the employee at the request of the self-insured employer.  By report dated September 14, 2011, Dr. Wyard noted a normal low back examination with degenerative lumbar disc disease by MRI scan dated June 24, 2009.  Dr. Wyard maintained the employee had underlying degenerative disc disease consistent with his age, and his low back condition was not related to his June 2009 work injury.  The doctor explained the employee did not have any focal or neurological findings in the medical record or by his examination, was improving when he saw Dr. Domino on August 10, 2009, had continued to work, and currently had no objective findings that he could attribute to the work injury.

By report dated November 23, 2011, Dr. Buttermann opined the employee had a significant, permanent aggravation of underlying degenerative disc disease at L3-4 and L4-5 with a disc herniation at L4-5 as a result of his work injury on June 15, 2009.  The doctor observed that prior to the injury, the employee had no problems with his low back, but since the injury had had significant problems that required medical treatment including physical therapy, injections, radiographic tests, and medications, all of which, in his opinion, were reasonable and necessary and related to the work injury.  Dr. Buttermann opined the proposed surgery was necessary due to the employee’s persistent pain and significant degenerative disease at L3-4 and L4-5.  He further imposed work restrictions of no lifting over 20-25 pounds and no repetitive lifting, bending or twisting.

The self-insured employer denied payment for the proposed surgery, but the employee decided to proceed using his personal insurance.  Dr. Buttermann took the employee off work in early December in anticipation of the surgery.  The surgery, consisting of a spinal fusion and decompression at L4-5 and disc replacement at L3-4, was performed on January 17, 2012.  At a post surgery recheck on March 5, 2012, the doctor noted the employee had some incisional numbness and pain and some aching and stabbing in the low back and left thigh.  The employee was also noted to be in no apparent distress and was ambulating well.  The employee was taken off morphine and Percocet and switched to Norco.  He was to return for a recheck in six weeks and was continued off work.

A hearing was held on March 20, 2012, before a compensation judge at the Office of Administrative Hearings.  In a decision served and filed on May 23, 2012, the compensation judge found the employee had sustained a permanent injury to his low back on June 15, 2009, and that the June 2009 injury was a substantial contributing factor to the employee’s need for low back medical treatment, including the January 17, 2012, surgery.  The self-insured employer appeals.

The self-insured employer argues that substantial evidence does not support the compensation judge’s determination that the June 15, 2009, injury is a substantial contributing cause of the employee’s current low back condition and his need for medical treatment, including the low back surgery in January 2012.  We are not persuaded.

There is no dispute the employee sustained an injury to his low back on June 15, 2009.  Nor was there evidence of any low back problems prior to the injury.  After the injury, the employee testified that physical therapy did help and his symptoms improved, but there was never a point where he had no low back symptoms.  Shortly after the employee was released to return to his truck driving duties in August 2009, he began treatment for degenerative arthritis in his hips, ultimately undergoing surgery in both hips.  The employee was off work for at least five months during this period and was prescribed narcotic pain medications.  Slightly more than three months after returning to work as a truck driver in July 2010, the employee returned to see Dr. Buttermann for severe low back pain.  The employee received epidural injections and was again prescribed pain medications.  In June 2011, Dr. Buttermann recommended surgery, noting the employee continued to have difficulty with low back pain.  The employee was again off work due to the low back surgery from December 2011 through the date of hearing.  At the hearing, two months after the surgery, the employee testified he was better than he was before the surgery and no longer had radiating pain down his leg.

The employer points out various items in the evidence which it asserts impair the employee’s credibility, or support the conclusion that the work injury was not a substantial contributing cause of the employee’s ongoing low back problems.  We have carefully reviewed the record.  The evidence is amenable to more than one interpretation or explanation and we find nothing sufficiently contradictory to require a reversal of the compensation judge’s decision.

The employer asserts the compensation judge improperly relied exclusively on the employee’s testimony describing continuing, subjective pain, disregarding evidence to the contrary.^[6]  The employer cites Fitzgerald v. DOS Trucking, Inc., No. WC07-222 (W.C.C.A. May 29, 2008), for the proposition that a judge’s findings based solely upon the testimony of the employee are not supported by substantial evidence where the testimony is inconsistent with multiple, independent sources.  This court specifically stated in Fitzgerald, however, that we were “not setting forth a general rule that an employee’s testimony is not to be accepted when his or her testimony is at variance with medical records.”  There is substantial evidence other than the employee’s testimony in this case that supports the findings of the compensation judge.

The compensation judge accepted the employee’s testimony as credible.  Assessment of a witness’s credibility is the unique function of the trier of fact.  Even v. Kraft, Inc., 445 N.W.2d 831, 42 W.C.D. 220 (Minn. 1989).  The medical records are reasonably consistent with the employee’s testimony and we find nothing in the record to persuade us that the employee’s testimony lacked credibility.

The self-insured employer also argues that the compensation judge’s interpretation of the medical evidence was erroneous, and that his reliance on the opinions of Dr. Buttermann was inappropriate.  We disagree.

Dr. Buttermann saw the employee on multiple occasions between July 29, 2009, and March 5, 2012.  He took a history from the employee, reviewed the May 24, 2009, MRI scan, and ordered additional x-rays and CT scans of the low back.  In January 2012, Dr. Buttermann performed surgery on the employee’s low back.  Based on his experience with the employee and his expertise in the field, Dr. Buttermann opined the employee suffered a permanent, work-related injury to his low back on June 15, 2009, in the nature of an aggravation of his underlying degenerative disc disease and a left-sided disc herniation at L4-5.  If a work injury aggravates, accelerates or combines with a preexisting disease or condition to produce a disability, that disability is compensable.  Vanda v. Minnesota Mining & Mfg. Co., 218 N.W.2d 458, 27 W.C.D. 379 (Minn. 1974).  The compensation judge, on the record in this case, could reasonably rely on the opinions of Dr. Buttermann.

The self-insured employer argues the compensation judge improperly rejected the opinion of their expert, Dr. Wyard, who agreed the employee had degenerative disc disease with a disc herniation at L4-5, but opined the employee’s condition was age-related and was not causally related to his work injury in June 2009.  A compensation judge’s choice between expert witnesses is to be upheld unless the facts assumed by the expert in rendering his opinion are not supported by the evidence.  See Nord v. City of Cook, 360 N.W.2d 337, 37 W.C.D. 364 (Minn. 1985).  The compensation judge accepted the opinion of Dr. Buttermann.  There is adequate support in the record for the facts relied upon by Dr. Buttermann, and we, therefore, affirm the compensation judge’s determination that the employee sustained a permanent, work-related injury to his low back in June 2009 and that the work injury was a substantial contributing cause of the employee’s current low back condition and his need for medical treatment, including the January 17, 2012, low back surgery.