The self-insured employer appeals from the judge’s finding that the employee’s work activities caused a permanent aggravation of a pre-existing condition.  We affirm.

The employee sustained a work-related injury on March 13, 1994, while working for McGlynn’s as a forklift driver.  The injury occurred when he jammed his left knee between a sink and the forklift.  He saw the company nurse that day and was seen at the Airport Medical Clinic on March 26, 1994.  Dr. Daniel Lussenhop noted that the employee had no discomfort and was performing his regular work but had come in because of persistent swelling in the medial aspect of his knee.  The doctor recommended that the employee apply moist heat to his knee and use an anti-inflammatory.  Dr. Lussenhop later referred the employee to an orthopedist, Dr. Jerry Reese.  When he examined the employee on April 12, 1994, Dr. Reese diagnosed a resolving avulsion/abrasion/hematoma injury and pre-existing degenerative arthritis of the left knee.  The employee continued to treat with Dr. Reese, and, on June 7, 1994, Dr.  Reese noted no significant swelling and opined that “function wise, however, the knee is as good as it was prior to the incident of 3/13/94 and the patient full[y] agrees.”

On May 10, 1995, the employee had x-rays of both knees.  Radiologist Dr. Frank Mork found that the employee had either an old healed fracture of the medial tibial plateau or early joint disease of the left knee.  On October 10, 1995, the employee saw his family doctor, Dr. Sheldon Burns, complaining of right mid-back pain and right leg pain.  After examining the employee and reviewing x-rays, Dr. Burns noted that the employee’s left knee showed degenerative joint disease.  No specific treatment was recommended.

On March 16, 1999, the employee was seen by Dr. Paul Lysne with complaints of persistent left knee pain.  Dr. Lysne found mild swelling, no joint line tenderness, and slight crepitus with passive flexion/extension.  He prescribed medication.

On May 10, 1999, bilateral x-rays of the knees showed extensive degenerative change on the left, with osteophytic spurring at the joint margins, particularly medially.  The employee was seen by Dr. Renner Johnston on that date and reported that “things are coming along reasonably well.”  Dr. Johnston discussed injections and medications with the employee, and the employee reported that new anti-inflammatories had been helpful.  Dr. Johnston also “talked to him about the potential of proximal tibial osteotomy and trying to avoid joint replacement.”

At Dr. Lysne’s request, the employee was examined by Dr. John Opsahl on June 9, 1999, for evaluation of blood and excess protein in his urine.  In his office notes of that date, Dr. Opsahl mentioned that the employee “really has no complaints other than intermittent pain in his left knee, secondary to arthritis.”  Dr. Opsahl’s recommendations included further testing to determine the cause “of his renal disease.”

At some point, McGlynn’s was apparently purchased by Pillsbury/General Mills [the employer].  From and after 1991, the employee worked full-time for the employer, driving a standing forklift.  His job required him to move the forklift back and forth throughout the day to pick up and move loads.  Each time he stopped the forklift, his weight would shift to his left side.  The employee experienced more knee pain over the years “with standing on the forklift and with going up and down ramps and bumps.”^[1]

On May 10, 2005, the employee experienced pain and swelling in his left knee when his weight shifted as he stopped the forklift.  He went to the company nurse, iced his knee for a while, and then completed his shift, taking breaks every two hours.  Two days later, on May 12, 2005, the employee was seen again by Dr. Burns.  Dr. Burns recorded that the employee had a 14-year history of stress on his knee at work and that it had become gradually more stiff and painful, with a specific exacerbation on May 10, 2005.  Dr. Burns obtained an x-ray of the employee’s knee and noted that it was one of the worst he had ever seen.  He diagnosed left knee osteoarthritis, stated that the employee needed to get the knee replaced, and referred the employee to orthopedic surgeon Dr. J. Patrick Smith.

The employee was examined by Dr. Smith on June 6, 2005.  Dr. Smith noted that x-rays from an outside source revealed severe end-stage tricompartment degenerative arthrosis.  In his office note, Dr. Smith opined, “[w]ith no history of previous injury it is my medical opinion based upon his history, physical examination, and my knowledge as an orthopedic surgeon caring for knee injuries, that his work activities as forklift drive[r] did in fact contribute to his current disability and his need for treatment.”  Dr. Smith went on to recommend a left total knee arthroplasty.

On August 12, 2005, the employee was examined by independent medical examiner Dr. Thomas Nelson.  According to the history taken by Dr. Nelson, the employee’s symptoms had begun on May 10, 2005, and the employee had previously experienced knee problems in 1999, with treatment by Dr. Lysne.  It was Dr. Nelson’s opinion that the employee was suffering from severe tricompartmental osteoarthritis of the left knee but that he had sustained only a temporary aggravation of this pre-existing osteoarthritis on May 10, 2005.  It was his opinion that the main cause of the employee’s knee arthritis was family history, combined with obesity.  He noted that the employee weighed over 290 pounds.

The employee filed a claim petition on December 2, 2005, seeking payment of outstanding medical expenses and approval for the recommended surgery.  In response, the employer denied that the employee had sustained an injury on or about May 10, 2005.

The employee underwent total knee replacement surgery, performed by Dr. Smith, on January 5, 2006.

Dr. Nelson reviewed some additional medical records and issued another report on March 27, 2006.  That report detailed the additional records that he had reviewed, including the records from 1994 for treatment of the employee’s left knee, and records from Fairview Southdale Hospital, where the employee had been treated following a motor vehicle accident in 1975, when he fractured the transverse processes of L1-L3.  In that report, Dr. Nelson reiterated that the May 10, 2005, work injury did not contribute to the employee’s osteoarthritis.

Dr. Smith eventually released the employee to return to work without limitations, and the employee returned to his pre-injury job on June 4, 2006.

When the employee’s claim petition came on for hearing on June 7, 2006, the employee was claiming that he had sustained a permanent aggravation of his pre-existing osteoarthritis on May 10, 2005, and he alleged entitlement to temporary total disability benefits from January 5, 2006, to June 4, 2006, and payment of outstanding medical bills and out-of-pocket expenses and mileage.  In findings and order filed on August 15, 2006, the compensation judge found that the employee had sustained a permanent aggravation of his left knee on May 10, 2005, and awarded the employee the claimed benefits.  The employer appeals.

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 (2006).  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 the 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 the compensation judge improperly relied on Dr. Smith’s opinion and the employee’s testimony in concluding that the employee’s work activities permanently aggravated his pre-existing osteoarthritis.

The opinions of Dr. Smith are contained in both his medical records and in his deposition, taken on May 18, 2006.  In that deposition, Dr. Smith testified that he had reviewed both of Dr. Nelson’s reports.  Dr. Smith opined that the employee’s work activities at the employer were a substantial contributing or aggravating factor in his condition as of May 10, 2005.

The employer contends that Dr. Smith’s opinions are lacking in foundation, because Dr. Smith did not review the records of Fairview Southdale Hospital from 1975, the records of Dr. Reese from 1994, the records of Dr. Mork from 1995, the records of Dr. Opsahl from 1999, or the records of Dr. Johnston from 1999.  We are not convinced.

Dr. Smith testified that he had reviewed the two reports of Dr. Nelson.  Dr. Nelson’s March 27, 2006, report outlined the complaints made by the employee and treatment he received at Fairview Southdale Hospital in 1975.^[2]  That same report referenced and detailed the treatment rendered by Dr. Reese in 1994, including the dates of treatment, findings on exam, diagnosis, and recommendations for treatment.  Dr. Nelson’s August 12, 2005, report detailed Dr. Mork’s formal reading of the October 12, 1995, x-ray.^[3]  Finally, while Dr. Smith testified on cross-examination that he had not reviewed the records of Dr. John Opsahl or Dr. Renner Johnston,^[4] he did think that the 1999 records of Dr. Paul Lysne were a part of his chart, and the records of Dr. Opsahl and Dr. Johnston submitted at hearing were apparently carbon copied to Dr. Lysne.^[5]  As such, it is reasonably inferable that Dr. Smith had direct or indirect knowledge of all relevant medical treatment received by the employee prior to the 2005 injury.  In addition, Dr. Smith was given a detailed hypothetical about the employee’s work activities at the employer.  Under the circumstances, we find no merit to the employer’s argument that Dr. Smith’s opinions lack foundation.

A judge’s choice between expert opinions 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).  In the present case, the compensation judge was entitled to rely on the opinion of Dr. Smith.

The employer contends that the compensation judge relied on inadequate or contradictory testimony by the employee and failed to mention the employee’s diagnosed personality disorder when finding his testimony credible.  Again, we are not persuaded.

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 employer points to medical records dating ten to thirty years prior to the work injury and questions the employee’s credibility because he could not initially recall those treatments.  However, we do not view this to be evidence of deception that would render the judge’s finding of credibility unreasonable or unfounded.

Finally, the employer submitted as an exhibit at hearing an MMPI evaluation performed by James N. Butcher, Ph.D., in August of 1981, which concluded that the employee was an immature and impulsive person.  It is true that the compensation judge did not mention this report in her findings and order, but a judge need not refer to every piece of evidence.  See, e.g., Rothwell v. Minnesota Dep’t of Natural Resources, slip op., (W.C.C.A. Dec. 6, 1993); Regan v. VOA Nat’l Housing, 61 W.C.D. 142 (W.C.C.A. 2000).  In addition, we question the usefulness of such an old evaluation, especially since there is no current evidence that any doctor thinks that those findings have any bearing on the employee’s current complaints or claims of disability.

Because substantial evidence, in the form of the employee’s testimony and the records and testimony of Dr. Smith, supports the judge’s decision, we affirm the findings and order in their entirety.