The employee appeals from the compensation judge’s findings that the employee suffered a separate, distinct, and intervening injury to her left knee in January of 2007 and that her 2006 work injury was not a substantial contributing cause of her need for a total knee replacement.  We affirm.

The employee was working as a bus driver for Metropolitan Transit Authority [the employer] on February 20, 2006, when she fell while boarding the bus and injured her left knee.^[1]  An MRI performed on March 10, 2006, was interpreted as showing a radial-type tear involving the inner third and apical free edge of the posterior horn of the lateral meniscus.

On July 18, 2006, the employee was examined by orthopedist Dr. Alan Markman, who diagnosed a lateral meniscus tear and chondral surface abnormality and recommended arthroscopy with a meniscectomy and chondroplasty.  Dr. Markman operated on August 31, 2006.  During surgery, Dr. Markman observed a large, parrot beak-type tear of the posterior horn of the lateral meniscus.  The flap was quite mobile and unstable.  The torn portion of the lateral meniscus was removed.

The employee returned to her regular job duties in December of 2006 without restrictions.  She had some continuing symptoms but did not lose time from work because of her knee condition through January 16, 2007.

On January 17, 2007, while at home, the employee fell and landed on her left knee.  An MRI was performed on February 7, 2007, and compared to the 2006 MRI.  The 2007 MRI findings were compatible with a tear of the periphery of the posterior third of the meniscus, extending to the inferior articular surface.  According to the radiologist, this appeared to be a new finding not shown on the previous MRI.

On March 29, 2007, Dr. Markman performed a left knee arthroscopy with partial arthroscopic lateral meniscectomy and chondroplasty of the chondromalacic areas and patellofemoral joint.  During surgery, Dr. Markman observed an intact articular surface on the femur, but the lateral meniscus had tearing of the posterior horn, with extension over toward the lateral third with a cleavage plane at that level, and the posterior horn was markedly unstable in front of the hiatus.  The unstable posterior horn of the lateral meniscus was excised, as was the lateral third tear, and a substantial portion of the lateral third was removed.

At the employer and insurer’s request, the employee was examined by Dr. Paul Wicklund on May 14, 2007.  In his report dated May 19, 2007, Dr. Wicklund opined that the employee had sustained a parrot beak-type tear of the lateral meniscus of the left knee at the time of her 2006 work injury and that the 2006 arthroscopic surgery was causally related to that injury.  He also opined that the employee’s fall on January 17, 2007, was a superseding, intervening injury, which was not related to her 2006 work injury, and that the employee had reached maximum medical improvement [MMI] from her work injury on December 18, 2006.

The employee continued to experience symptoms after the second surgery, and she treated with physical therapy and injections.  A third MRI, performed on January 2, 2008, disclosed a 1.2 x 1 cm. full-thickness chondral defect in the posterior aspect of the lateral femoral condyle, which was noted to be new.  It was also noted that the chondromalacia in the lateral compartment was increased overall.  Dr. Markman recommended that the employee “proceed conservatively” and increased the dosage of her anti-inflammatory medications.

When seen on August 6, 2008, by Dr. Markman, the employee continued to complain of significant pain with movement of her left knee, which had not improved after a series of injections.  Dr. Markman diagnosed post-traumatic arthrosis, greatest in the lateral compartment, and recommended a left total knee arthroplasty.

Dr. Wicklund reexamined the employee in September of 2008 and issued a report on October 7, 2008.  In that report, he indicated that, while the recommended left total knee arthroplasty was reasonable and necessary surgery, it was not related to the parrot beak-type tear of the lateral meniscus that had occurred in 2006.

On October 27, 2008, the employee filed a medical request, seeking approval for the recommended surgery.  The employer and insurer responded by denying that the 2006 work injury was a substantial contributing cause of the employee’s need for the surgery.^[2]

During a deposition taken on January 6, 2009, Dr. Wicklund testified that there was a “definitive progression of the arthritis of the lateral femoral condyle between the day of the surgery in March ’07 and the day of the MRI in January of ’08 and that’s solely due to the removal of the meniscus” in the March 2007 surgery.  It was further his testimony that the February 2006 work-related meniscus tear played no role in the changes seen on the January 2008 MRI and that the 2006 work injury was not a substantial contributing cause of the employee’s need for a left knee replacement.

Dr. Markman’s deposition was taken on January 14, 2009.  He testified that there was a progression of arthritis from August of 2006 to March of 2007 and that the 2006 work injury substantially contributed to the “more rapid wear-out phenomenon that [the employee] has experienced in her left knee,” which was his reason for recommending a left total knee replacement.

The matter proceeded to hearing, and in findings and order filed on March 2, 2009, the compensation judge found that the employee did not sustain a consequential injury to her left knee on January 17, 2007, but rather a separate, distinct, and intervening injury, and that the 2006 work injury was not a substantial contributing cause of the employee’s need for a left total knee replacement surgery.  The employee 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 (2008).  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 employee makes three arguments on appeal: 1) that the compensation judge erred by not allowing sufficient time for the employee to testify and by restricting her testimony during the hearing; 2) that the compensation judge erred in finding that the employee did not sustain a consequential injury to her left knee in January of 2007, causing her to need a total knee replacement; and 3) that the testimony of Mary Jo Carrier, in which she stated that the employee slipped on ice, was inaccurate.

The employee testified at hearing as to how her 2006 injury occurred.  At one point in the hearing, the employee’s attorney began to question her regarding notice, and the compensation judge intervened, indicating that it was an admitted injury for which the employer and insurer had paid benefits up through the time of the first surgery.  When the employee’s attorney argued that “how the injury happened . . . is important,” the compensation judge agreed but again pointed out that testimony regarding notice of the injury, and questioning regarding the ineffectiveness of medical treatment prior to the first surgery, served no useful purpose.  At that point, the employee’s attorney moved on to questions regarding the employee’s symptoms.

The transcript reflects that the employee thoroughly explained how her injury occurred and thoroughly described her symptoms both before and after her first surgery, including describing how her left knee gave out.  There is no suggestion in the record that the employee was prevented from testifying to any relevant facts.  The employee’s argument that she was not allowed sufficient time to testify or that she was restricted in her testimony is without merit.

At hearing the employee contended that her fall at home on January 17, 2007, was a consequential injury, in that it occurred because her left knee gave out.  On appeal, she contends that the testimony of Mary Jo Carrier and the memorandum from a disability conference, offered as evidence at hearing, do not credibly establish that the employee slipped and fell on ice at home.  We are not persuaded.

Several sources contradict the employee’s account of the January 2007 incident.  Ms. Carrier, whose duties include disability management at the employer, testified at hearing that the employee told her, within a day or two after January 17, 2007, that she had slipped on ice at home, hurt her left knee, and had to be carried into her home by her son.  Similarly, when the employee was seen at the Park Nicollet Clinic on January 22, 2007, Dr. Kevin O’Connell recorded that the January 17, 2007, fall had occurred “when she was attempting to walk on a slippery surface and she slipped and fell and struck her anterior aspect of her left knee.”  Then, at a February 16, 2007, disability management conference at the employer, attended by Ms. Carrier and documented in an internal memorandum of that date, it was again recorded that the employee gave a history of having slipped and fallen on ice outside her home in January of 2007.  Finally, Dr. Markman’s operative report of March 29, 2007, reported a history of the 2006 work injury and noted that when the employee returned to work she had “injured her knee twisting it, as she worked the pedals to turn the blinkers of the bus.”

At hearing, the employee testified that her knee had been giving out prior to her first surgery, that it continued to give out thereafter, and that, on January 17, 2007, her knee gave out, causing her to fall and hit her knee, as she was walking up the stairs at her home.  She denied that there was any ice on the surface of the steps and denied telling Dr. O’Connell or Ms. Carrier that she had slipped on an icy surface.

The compensation judge did not accept the employee’s testimony, “because this testimony was inconsistent with the employee’s January and February 2007 histories to her treating doctors and to supervisory personnel.”^[3]  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), citing Brennan v. Joseph G. Brennan, M.D., 425 N.W.2d 837, 839-840, 41 W.C.D. 79, 82 (Minn. 1988).  We see no basis to reverse the judge’s findings regarding credibility or her conclusion that the employee did not sustain a consequential injury, as claimed.

The compensation judge accepted the opinion of Dr. Paul Wicklund that the employee suffered a separate, distinct, and intervening injury to her left knee in January of 2007, in the form of a new tear of the lateral meniscus, that the surgery for that injury included removal of approximately two-thirds of the employee’s left lateral meniscus, which caused subsequent arthritic changes corresponding exactly to the location of the removed meniscus, and that the recommended total knee replacement was to treat those arthritic changes.  A judge’s choice between expert opinions is generally 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).  The employee points to no facts that are not supported by the evidence but rather argued, at oral argument, that Dr. Markman’s opinions were “more compelling and persuasive.”  The judge’s decision to accept Dr. Wicklund’s opinion, and her resulting finding that the 2006 work injury was not a substantial contributing cause of the employee’s need for left knee replacement surgery, are affirmed.