The employee appeals from the compensation judge’s denial of payment for recommended surgery to his right knee.  We affirm.

In 1989, Robert Brustad underwent an open repair of the anterior cruciate ligament [ACL] in his right knee, which evidently involved the placement of a staple.  About eighteen years later, on October 9, 2007, he saw orthopedic surgeon Dr. Thomas Conner regarding renewed pain and intermittent swelling on the medial side of that knee.  Dr. Conner diagnosed internal derangement, status post ACL reconstruction, ordered an MRI scan of the knee, and discussed with Mr. Brustad the “potential of his arthritic changes versus a more acute and chronic meniscal tear accounting for his symptoms.”  The scan was performed on October 10, 2007, and was read to reveal a complex tear of the posterior horn and body of the medial meniscus, a complex tear of the posterior horn of the lateral meniscus, severe chondromalacia at the patella, some joint effusion, and osteophytic bone spurring around all three compartments of the knee.  In follow-up on October 12, 2007, Dr. Conner diagnosed right knee internal derangement with medial and lateral meniscal tears and recommended arthroscopic evaluation and debridement with partial meniscectomies.  Mr. Brustad underwent the surgery on October 15, 2007, at which time the doctor performed a partial medial meniscectomy only, finding the lateral meniscus intact.  The surgery was apparently successful, and on January 3, 2008, Dr. Conner released Mr. Brustad to “resume activities as tolerated.”

On February 27, 2008, Mr. Brustad [the employee] sustained a work-related injury to his right knee while preparing to draw blood from a patient in the lockdown behavioral unit at a nursing home in the course of his job as a phlebotomist with HealthEast/St. Joseph’s Hospital [the employer].  On the date of the injury, the employee was seen in the emergency room at St. Joseph’s Hospital by Dr. Timothy Thompson, to whom the employee reported being “attacked” by the nursing home patient.  Dr. Thompson noted,

On March 5, 2008, the employee saw his orthopedist, Dr. Conner, who noted that the nursing home patient who had attacked the employee, “in addition to throwing her walker at him, had taken a wheelchair and had run this back and forth into his knee as he was up against a wall of the room.”  Noting that the employee’s pre-injury MRI findings did show “apparent broken hardware at the knee and so this is not an obvious new injury,” Dr. Conner diagnosed right knee contusion and referred the employee for physical therapy, which the employee commenced on March 19, 2008.  The therapist on that date reported that the injury occurred “when an agitated patient hit [the employee] several times in the knee with a wheel chair” and that there was “also possible twisting involved.”  By May 5, 2008, after nine sessions, the employee had shown minimal improvement, and physical therapy was discontinued.

The employee continued to have persistent pain in his right knee during the summer of 2008, and on August 11, 2008, he underwent another MRI scan of the knee, which was read in comparison with his scan of October 10, 2007.  The 2008 scan was read to reveal tricompartmental degenerative disease, a complex degenerative tear of the posterior horn of the lateral meniscus, and evidence of the partial meniscectomy of the medial meniscus.  There appeared to be no significant change in that medial meniscus when considered in light of Dr. Conner’s operative report of October 15, 2007, about half a year before the employee’s February 2008 work injury.  In follow-up with the employee on August 27, 2008, Dr. Conner opined that the 2008 MRI showed primarily degenerative changes, stating in his notes on that date that the employee “does have some degenerative changes in the lateral meniscus, though he does not appear to have significant clinical symptoms in that area and, having had his recent surgery, I doubt that there is any significant new finding present.”  On that note, Dr. Conner administered a cortisone injection to the employee’s right knee and did not see the employee again for about four months.

Upon examination of the employee on December 17, 2008, Dr. Conner found the employee’s right knee unremarkable, with no significant effusion or swelling, satisfactory range of motion, stable ligaments across the knee, and intact motor function.  Examination about three weeks later, on January 7, 2009, revealed essentially the same findings.  Dr. Conner’s examination about two months later, on March 9, 2009, revealed tenderness along the lateral joint line, pain with McMurray’s maneuvers laterally, and some pain with patellar compression.  On these findings, Dr. Conner recommended that the employee undergo another right knee arthroscopy with debridement and partial meniscectomy.

On April 28, 2009, the employee was examined for the employer by orthopedic surgeon Dr. Tilok Ghose, who, in his report on May 21, 2009, reported the mechanism of the employee’s injury as follows:

The employee’s symptoms apparently continued, and on July 13, 2009, the employee filed a medical request, alleging entitlement to payment for the arthroscopic surgery recommended by Dr. Connor, consequent to the work injury on February 27, 2008.  On September 28, 2009, Dr. Connor issued a narrative report to the employee’s attorney, in which he indicated that the employee’s treatment to date had been both medically necessary and reasonable and directly consequent to the work injury of February 27, 2008, “that caused or exacerbated his condition.”

The matter came on for hearing on October 1, 2009.  At issue at the hearing were whether the right knee arthroscopic surgery recommended by Dr. Conner was reasonable and necessary to cure or relieve the effects of the employee’s February 27, 2008, work injury and whether that surgery was causally related to that work injury.  Evidence admitted at hearing included the testimony of the employee, in part that he had for years been taking Celebrex for pain in his knees.  By findings and order filed November 2, 2009, the compensation judge concluded, in reliance on the expert opinions of Dr. Ghose, that the surgery at issue was neither causally related to the employee’s work injury nor reasonable and necessary to cure or relieve its effects.  On those conclusions, the judge denied the employee’s claim for payment for that surgery.  The employee 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. Lyion Foods 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.

Dr. Ghose had opined that the employee’s current symptoms were due not to the employee’s contusion work injury, or even to any meniscal tear traumatically sustained, but rather to a degenerative meniscal tear consequent to the employee’s degenerative arthritic condition.  Moreover, it was Dr. Ghose’s opinion that that arthritic condition would only be exacerbated by the surgery recommended by Dr. Conner.  In reliance on Dr. Ghose’s opinion, the compensation judge found that the right knee arthroscopic surgery recommended by Dr. Conner was neither causally related to the employee’s work injury nor reasonable and necessary to cure or relieve its effects.

The employee contends that that conclusion of the compensation judge was both clearly erroneous and unsupported by substantial evidence.  He concedes, apparently, that he is subject to preexisting meniscal damage and a pre-existing arthritic condition, but he suggests that those conditions were exacerbated by his work injury, arguing that “[a] disability or condition from an accident which aggravates an existing work related [sic] infirmity is compensable even though the subsequent incident would have caused no injury to a normal person.”  Citing Nelsen v. American Lutheran Church, 420 N.W.2d 588 (Minn. 1988); Rohr v. Knutson, 305 N.W.2d 26 (Minn. 1975).  The employee notes that the compensation judge “spent literally a page and a half grappling with the notion of whether or not [the employee] sustained a direct blow to his knee,” arguing, without citing authority, that “this is not the standard; . . . the exact mechanism of injury does not have to be proved to certainty.”  He notes that “[t]he workers[’] compensation statute mandates that employers pay for the medical care (including surgery) necessary to cure and/or relieve the effects from a work related incident.”  He argues further that by January 3, 2008, the employee was under no physical restrictions from any pre-existing condition, suggesting that any symptoms after that date are attributable therefore to the work injury and not to any pre-existing condition.  Further, the employee argues that “it is patently unfair and prejudicial that the Court relied upon a one time IME report, where there is no specific history of care between the IME physician and [the employee].”  Finally, the employee argues that “the court’s ruling may have been based on bias,” in that “it was clear the judge was insulted and offended” by counsel’s mistaken suggestion that the hearing before the judge was not a formal one.  We are not persuaded.

We agree that a work-related aggravation of a pre-existing condition is compensable even if the aggravating incident would not have resulted in a work injury without the pre-existing condition.  And it is true, of course, that the statute mandates payment for any treatment related to an employee’s work injury.  In this case, however, the work-related injury was originally diagnosed first as a strain by Dr. Thompson and then as a contusion by Dr. Conner.  The exacerbation of arthritis that was eventually diagnosed even by Dr. Ghose was concluded by Dr. Ghose to have been only a temporary one, which had “certainly resolved by January 7, 2009.”  Given the employee’s original diagnoses of strain and contusion and his later diagnosis by Dr. Ghose of only temporary exacerbation of his arthritis, the mechanism of the employee’s injury and the severity of any direct blow to his knee that might have caused such a contusion was an evidentiary matter entirely germane to the causation issue before the judge.

Nor was it unreasonable for the compensation judge in this case, in reliance on the expert opinion of Dr. Ghose over that of Dr. Conner, to conclude that the employee’s work-related contusion did not permanently aggravate the employee’s pre-existing arthritic condition and so was not a substantial contributing factor in any need for the surgery that is here recommended in treatment of that condition.  See Nord v. City of Cook, 360 N.W.2d 337, 342-43, 37 W.C.D. 364, 372-73 (Minn. 1985) (a 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).  Nor is our conclusion altered by the fact that the employee was not subject to arthritis-related restrictions immediately prior to his work injury.  The employee’s post-injury MRI scan revealed little change in comparison to his pre-injury scan, and, to the extent that some minor change was evident, it would not have been unreasonable for the compensation judge to rely on Dr. Ghose’s opinion in concluding that the employee’s arthritis, as a degenerative condition, was deteriorating further at its own pace, separate from his work-related strain/contusion/temporary aggravation.  Indeed, it was Dr. Ghose’s credited opinion that the surgery here at issue was not only unrelated causally to the employee’s work injury but also unreasonable and potentially dangerous given the complicated state of the employee’s degenerative arthritis.

Finally, it is not at all “patently unfair and prejudicial” for a compensation judge to credit the opinion of an independent medical examiner over that of a treating doctor; indeed, the law is well established to the contrary.  See Caven v. Ag-Chem Equip. Co., Inc., slip op. (W.C.C.A. Sep. 14, 1993) (while a finder of fact may, in appropriate circumstances, choose to afford greater weight to the opinion of a treating physician, the judge is not required to do so).  Nor do we find any evidence that the judge was biased against the employee on grounds that she felt somehow personally “insulted and offended” by counsel’s mistake as to the formality of the hearing there scheduled before her.  The judge’s memorandum is copious and detailed, and it is clear that she considered the evidence carefully and thoroughly.

Because it was neither factually unreasonable nor erroneous as a matter of law, we affirm the compensation judge’s denial of payment for the surgery here at issue.  See Hengemuhle, 358 N.W.2d at 59, 37 W.C.D. at 239.