The self-insured employer appeals from the compensation judge’s decision that the employee’s 2009 work injury is a substantial contributing cause of the employee’s left knee condition and need for knee replacement surgery.  The employee appeals from the judge’s decision that the employee did not sustain a Gillette^[1] injury culminating in 2010.  We affirm.

The employee started working in the service department of Coca-Cola [the employer] in approximately October of 1997.  That job involved delivery, installation, and removal of beverage coolers and the installation of beverage dispensing machinery.  The employee used assistive devices, called wheelers, in moving machines.

On March 16, 2009, the employee slipped and fell on ice while making a delivery.  He was seen by physician assistant Kelsy Kuehn on March 17, 2009, relating that he had slipped and landed directly on his left knee.  At that time, he complained of pain underneath the kneecap and “the knee popping.”  Ms. Kuehn diagnosed left knee contusion, prescribed 800 mg of ibuprofen, and instructed the employee to ice his knee.  The employee was released to return to work without restrictions and did not miss any work at that time.

The employee next treated for left knee pain on December 7, 2009, at Superior Health Center, complaining of sharp left knee pain radiating to the left thigh, crepitus, and tenderness.  It was noted that his work involved a lot of bending and kneeling and that his knee pain was worsened by kneeling.  X-rays showed moderate patellofemoral compartment degenerative joint disease.  He was advised to rest, apply ice and heat, use a knee brace, and take over-the-counter medications as needed for pain.

On September 29, 2010, the employee was installing drink dispenser machinery in a restaurant when he missed two rungs coming down a ladder and fell.  The employee’s report of injury indicated that he “went down hard on left knee.”

The employee received treatment at Superior Health Center on October 13, 2010.  Medical records describe a work injury in 2009 when the employee twisted his left knee, and an additional injury occurring when the employee stepped off of a ladder at work and twisted the same knee.  According to the office notes from that day, the employee’s left knee problem was worsening.  The employee was complaining of aching and sharp pain in the left knee radiating to the left thigh/calf.  Again the employee indicated that the pain was aggravated by kneeling.  No work restrictions were recommended, but an MRI was ordered.  The October 22, 2010, MRI revealed a tear of the posterior horn of the medial meniscus at the posterior meniscal root with advanced chrondromalacia of the patellofemoral articulation.

The employee was seen by orthopedist Dr. R.W. Hendricks on November 12, 2010.  He diagnosed left knee arthritis and a medial meniscus tear.  On December 15, 2010, Dr. Hendricks performed left knee arthroscopic surgery to repair the torn meniscus.  The employer paid for this surgery based on an injury date of March 16, 2009.

On December 22, 2010, Dr. Hendricks opined that the employee had a stable knee post arthroscopic surgery and prescribed three weeks of physical therapy.  The employee was not released to return to work at that time.

The employee was seen by Dr. Hendricks again on January 12, 2011.  At that time, the doctor’s the diagnosis was left knee arthritis.  The employee was given a Synvisc-One injection and kept off work for another month.  According to Dr. Hendricks’ notes, the “next formal step” for treating the arthritis would be a total knee arthroplasty.

On February 9, 2011, the employee reported to Dr. Hendricks that the injection had not helped at all.  Dr. Hendricks diagnosed “left knee arthrosis, severe symptomatic.”  Total knee arthroplasty was discussed, and the employee expressed a desire to have the surgery scheduled.  The doctor released the employee to a sit down job only, pending surgery.  The employee later testified that he returned to work on that date and subsequently missed no work due to his left knee condition.

The employee was examined by independent medical examiner Dr. Mark J. Carlson, at the request of the employer, on February 21, 2011.  Dr. Carlson’s history reflects only a March 16, 2009, work injury, where the employee slipped and struck his left kneecap on the ground.  Dr. Carlson diagnosed severe degenerative arthritis of the patellofemoral joint and medial compartment of the left knee.  He opined that the employee’s fall on March 16, 2009, had resulted in a temporary knee contusion that aggravated the employee’s preexisting patellofemoral arthritis.  It was his opinion that none of the employee’s symptoms at the time of his exam were related to the 2009 injury and that the employee had reached maximum medical improvement from the March 16, 2009, injury by May 1, 2009.  He agreed that total knee replacement would be reasonable medical treatment for the employee’s degenerative arthritis.

The employee filed a claim petition on March 3, 2011, listing a date of injury of March 16, 2009, and seeking payment of various benefits, including expenses for a left knee replacement surgery.  That claim petition was later amended to include a claim for a September 29, 2010, left knee injury.

On June 8, 2011, Dr. Hendricks examined the employee again and noted that there had been no significant change in the employee’s left knee. He also noted that the employee was back to work at full duties without restrictions.

On January 13, 2012, Dr. Hendricks’ deposition was taken.  During the course of the deposition, Dr. Hendricks was given a hypothetical describing the employee’s work activities for the employer.  He testified that twelve years of lifting at work did play a role in the employee’s development of the osteoarthritis that was seen on the 2009 x-rays and that the osteoarthritis had resulted in the need for knee replacement surgery.  He also testified that the two work injuries exacerbated the employee’s arthritis and accelerated the need for his total knee replacement.

On February 2, 2012, Dr. Carlson issued another report following his review of additional medical records and the deposition of Dr. Hendricks.  Dr. Carlson found no evidence that the employee had sustained a Gillette-type injury.  He noted that daily activity of any type could contribute to the progression of osteoarthritis.

This matter proceeded to hearing on March 2, 2012, at which time the issues were whether the employee had sustained a left knee injury on either March 16, 2009, or September 29, 2010, or, in the alternative, whether he had sustained a left knee injury in the nature of a Gillette injury on September 29, 2010.  The employee sought approval for a total knee replacement, and the intervenors sought reimbursement of medical expenses paid.  In the findings and order filed on March 19, 2012, the compensation judge found, in part, that the March 16, 2009, work injury was a substantial contributing cause of the acceleration of the arthritis in the employee’s left knee and the need for knee replacement surgery, that the September 29, 2010, incident was not causative, and that the evidence failed to establish that the employee had sustained a work-related Gillette injury on September 29, 2010.  The employer appeals, and the employee cross-appeals.^[2]

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 (2012).  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 employer contends that substantial evidence does not support the judge’s finding that the employee’s 2009 work injury was a substantial contributing cause of the employee’s left knee condition, arguing that the finding was also clearly erroneous.  We are not persuaded.

First, the employer contends that the judge erroneously relied on the employee’s testimony at hearing when it was inconsistent with contemporaneous medical records and the employee’s deposition testimony. We have carefully reviewed the alleged inconsistencies.^[3]  Assessment of a witness’s credibility, however, 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).  We find nothing in the record to persuade us that the employee’s testimony lacks credibility.

Second, the employer argues that substantial medical evidence does not support the judge’s finding that the March 16, 2009, work injury is a substantial contributing cause of the employee’s current left knee condition.  Again we are not persuaded.

It is undisputed that the employee had degenerative changes in his left knee prior to the 2009 work injury.^[4]  While the employee had treated for right knee difficulties in 2005, there is no evidence that he received left knee treatment prior to 2009.  The employee testified that he always had left knee pain after the 2009 injury, which he would treat with ice and ibuprofen, that he gave up golf because of his left knee condition, and that pain would occasionally shoot up into the groin area.

The employer also argues that, because the employee worked without restrictions for 19 months following his 2009 work injury, he could not have sustained anything more significant than a knee contusion at that time.  We disagree.  Dr. Hendricks testified that the employee’s failure to treat regularly following the 2009 injury did not mean that injury was not significant.  It is also obvious that this employee is able to work with pain, as evidenced by the fact that he was working full duties without restrictions on June 8, 2011, despite having a severely symptomatic left knee for which knee replacement had been recommended.

Dr. Hendricks began treating the employee on November 12, 2010.  At his deposition he testified that the surgery he performed in December of 2010 improved the meniscus tear but did not address the arthritis of the employee’s patellofemoral joint.  He has diagnosed the employee with Grade III “end-stage” arthritis and opined that the employee’s two work injuries exacerbated his arthritis and accelerated the need for a total knee arthroplasty.  While he acknowledged that the employee would “eventually need the total knee arthroplasty anyway, given his age and the status of the arthritis,” it was his opinion that the work injury accelerated that need.  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., 27 W.C.D. 379, 218 N.W.2d 458 (Minn. 1974).

Dr. Carlson disagreed with Dr. Hendricks, but a compensation judge’s choice between expert witnesses 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, 37 W.C.D. 364 (Minn. 1985).  There is no indication that Dr. Hendricks relied on any relevant facts not supported by the record.  We therefore affirm the judge’s finding that the 2009 injury was a substantial cause of the employee’s knee condition and need for surgery.

The employee appeals from the judge’s finding that he did not establish that he sustained a Gillette injury to his left knee on September 29, 2010, contending that the judge used an incorrect standard in evaluating this claim.  In making this argument, the employee focuses on a portion of one sentence in the judge’s memorandum.^[5]  The employee is correct in noting that he did not need to establish that specific work activities caused specific symptoms that cumulatively lead to disability.  The judge’s choice of wording is troublesome, but, nevertheless, we are not persuaded that the judge’s denial of the employee’s Gillette claim is clearly erroneous.

The hypothetical given Dr. Hendricks contained a description of the employee’s work activities from October of 1997 up to 2009.  Dr. Hendricks was then asked, “Do you have an opinion as to whether the work in the 12 year period before the x-rays [taken in 2009] caused or accelerated the arthritic spurring you’re seeing in those x-rays?”  The doctor responded, “When you have a physical job like this, it certainly makes sense that that’s going to lead to an acceleration of the osteoarthritic process.”  He went on to testify, “I believe his two injuries exacerbated his arthritis and accelerated the need for a total knee replacement.”^[6]  However, Dr. Hendricks did not provide an opinion as to whether the employee’s work activities up to September 29, 2010, caused a Gillette injury on that date.  Neither the hypothetical provided to Dr. Hendricks nor the testimony of the employee were clear about the employee’s specific work activities or the specific weights he lifted between May of 2009 and September 29, 2010.  In fact, the employee testified that his work had become easier over the years.  Dr. Carlson concluded that the employee had not sustained a Gillette injury, and this opinion, combined with the employee’s testimony that he had ongoing symptoms in his left knee following the March 16, 2009, injury, which did not change appreciably after September 29, 2010, provide substantial evidence to support the judge’s decision denying the employee’s claim for a Gillette injury on September 29, 2010.