The employer and insurer SFM Mutual Insurance Company appeal from the compensation judge’s conclusion that the employee sustained a nonidiopathic permanent work injury to her right knee on June 14, 2010, and from the judge’s apportionment of liability for that injury equally between SFM Mutual Insurance Company and insurer RTW Group (the employer’s insurer on two earlier injuries to the left knee).  The employer and insurer, RTW Group, cross-appeal from the judge’s conclusion that the employee sustained a compensable work injury on June 14, 2010.  We affirm.

On October 17, 2006, Carole Baldwin sustained a work-related injury to her left knee while vacuuming a classroom in the course of her employment as a custodian with Independent School District #877, a job that she had held since August of 2004 and continued to hold on the date of the hearing below.  Ms. Baldwin [the employee] eventually sought treatment for her symptoms, and on January 15, 2007, on referral from Dr. Andrew Burgdorf, she saw orthopedist Dr. Brian O’Neill, who prescribed medication, assigned restrictions, and ultimately recommended a regimen of Cortisone shots based on a diagnosis of “left knee pain, mild degenerative disease.”  Dr. O’Neill eventually ordered an MRI scan, which was read to reveal a meniscal tear, and on April 23, 2007, he performed a left knee arthroscopy with partial medial meniscectomy.  The employee had had no record of any left knee symptoms prior to October 17, 2006, and Independent School District #877 [the employer] and its insurer at the time, RTW Group, acknowledged liability for the injury and commenced payment of benefits, including the expenses of the surgery and related wage replacement benefits.

On June 24, 2008, the employee sustained a second permanent injury to her left knee, as a result of her work activity shampooing carpet for the employer.  One week later, on July 1, 2008, the employer’s insurance coverage changed from RTW Group to SFM Mutual Insurance Company.  On October 16, 2008, the employee filed a claim petition, alleging against both RTW Group and SFM Mutual Insurance Company entitlement to rehabilitation benefits and undetermined medical benefits, while reserving a claim to temporary wage replacement benefits, all consequent to work injuries to her left knee on October 17, 2006, June 17 [sic], 2008, and also August 4, 2008.  Several months later, on December 17, 2008, the employee testified by deposition regarding her allegations of injury on those dates.  There was no reference in her testimony to any right knee symptoms, nor had there been any documentation of any right knee complaints during the year 2008.  The following day, December 18, 2008, the employee amended her claim petition to also allege entitlement to penalties against RTW Group and to reserve a claim for permanent partial disability benefits.

The matter eventually came on for hearing before a compensation judge on August 10, 2009.  In his findings and order issued October 2, 2009, the compensation judge concluded that the employee had sustained permanent injuries to her left knee on October 17, 2006, and June 24, 2008, but had not sustained a Gillette-type injury^[1] culminating in July or August of 2008.  Accordingly, the judge awarded benefits against the employer and RTW Group [collectively RTW], without apportionment of liability to the employer and SFM Mutual Insurance Company [collectively SFM].

Subsequent to her award, the employee continued under the care of Dr. O’Neill, who, on November 16, 2009, performed a total left knee arthroplasty, for which, along with the employee’s subsequent aftercare, RTW paid.  Following her recovery from surgery, the employee commenced physical therapy and returned on January 5, 2010, to full-time work with the employer, initially at lighter, essentially sedentary activities.  In February, the employee’s restrictions changed to permit walking for three hours per shift, during which time she worked at collecting trash, at which task she would support herself by leaning on her wheeled trash barrel.  During the month of March 2010, she worked on her feet for four hours each shift, alternating two hours on and two hours off.  On April 6, 2010, the employee returned to performing regular custodial duties eight hours a day on her feet, slowly and with occasional sitting, with the approval of her supervisor.  Effective that same date, she was restricted from squatting, crawling, kneeling, climbing ladders, or pulling bleachers, from pushing, pulling, lifting, or carrying over thirty pounds, and from frequent reaching above shoulder level or doing more than minimal stair climbing.

On May 4, 2010, the employee saw Dr. O’Neill in follow-up regarding her left knee surgery, complaining to him on that date also of right knee symptoms, for which he issued no treatment recommendations.  On June 14, 2010, at about 4:30 p.m., near the end of her shift, the employee felt her right knee buckle as she was walking on the mezzanine floor near the gymnasium.  She felt an increase in pain in the knee, but she was able to catch herself and did not seek medical attention until the following day, June 15, 2010, when she saw Dr. O’Neill regarding both of her knees.  The doctor noted on that date that she had “a stable left knee” with “satisfactory motion,” and he kept her restrictions the same and administered a Cortisone injection into the right knee.  The employee reported a right knee injury to the employer, and on June 16, 2010, the employer completed a First Report of an injury on June 14, 2010.  The employee subsequently returned to her work at the usual summer inside cleaning tasks, which were lighter than regular school-year work, entailing more sitting.  The employee had been sixty-four years old on June 14, 2010, and had been earning a weekly wage of at least $561.69.

On July 27, 2010, the employee returned to see Dr. O’Neill regarding a bilateral knee condition.  Dr. O’Neill diagnosed right knee arthrosis and status post left knee arthroplasty and continued the employee’s work restrictions, noting that the employee was “waiting to hear how they are going to sort out her work comp issues in the right knee.”  He saw the employee again on September 28, 2010, regarding continuing pain in both of her knees, on which date he diagnosed status post left knee arthroplasty and right knee degenerative disease, concluding, “I do believe she needs a knee replacement on the right at some point.”  The doctor noted also on that date that the employee “considers her right knee part of her work comp injury,” noting also in that regard, “It is difficult to say with certainty.”  Dr. O’Neill examined the employee again in follow-up regarding both knees on November 9, 2010, on which date he diagnosed “[s]tatus post left knee arthritis” and “[r]ight knee arthritis,” reporting that “[w]ork is a problem for her and they are trying to get it sorted out to see if her right knee arthritis is part of her work comp injury.”  He released the employee on that date to essentially sedentary activity, restricting her from doing any bending, stooping, crouching, squatting, crawling, or kneeling and advising her to avoid pushing or pulling on the bleachers, to avoid using stairs and ladders, and to sit as needed, noting also, “We are waiting for the work comp process to get the right knee replaced.”

On December 15, 2010, in a letter to the employee’s attorney, Dr. O’Neill reiterated his diagnosis of right knee arthrosis and opined that, although the employee’s work could not be said to have caused the arthritis, “because of the problems that she has had with the left knee over the years and subsequent knee replacement, she has put a lot of extra pressure on her right knee and I believe that has accelerated her arthritic condition in her right knee.”  In his summary paragraph, Dr. O’Neill expanded that opinion to state that the employee’s arthritis “appears to be aggravated by both her work activities and the problems she has had with her left knee.”  Dr. O’Neill saw the employee again regarding both of her knees on December 28, 2010, on which date he noted again that the employee “is going to need a knee replacement on [the right] side” and that “[s]he wants to continue working.  She is having a hard time getting by right now.  She is managing but just barely.”

On January 27, 2011, the employee filed a claim petition, alleging entitlement to further benefits consequent to her left knee work injuries in October of 2006 and June of 2008 and also to benefits consequent to a right knee injury on June 14, 2010.  The employee’s right knee continued painful, and on February 8, 2011, Dr. O’Neill administered another cortisone injection, noting that the employee’s left knee was also still bothersome.

On April 19, 2011, the employee was examined for SFM by orthopedic surgeon Dr. Jack Bert.  In his report on April 26, 2011, Dr. Bert indicated that the employee was having “aching, burning, stabbing, and sharp/dull” predominantly medially-based pain in her right knee.  He diagnosed seventeen months status post left total knee arthroplasty with persistence of swelling and pain, longstanding right medial joint osteoarthritis of the right knee, and severe exogenous obesity.^[2]  On that diagnosis, Dr. Bert concluded that the employee did need a total right knee replacement, consequent to her medial knee joint osteoarthritis.  It was Dr. Bert’s opinion that the employee’s work activities as a custodian since August 2008 were not a substantial contributing factor in her current right knee symptoms and need for knee replacement.  He attributed both the employee’s arthritis and her need for the surgery entirely to her obesity.  It was his opinion that the buckling event of June 14, 2010, was not a substantial contributing factor in the employee’s need for right knee replacement, noting that the employee continued to work after the incident and reportedly did not have worsening symptoms as a result of the event.

The employee had been examined for RTW on April 20, 2011, by orthopedic surgeon Dr. Alan Markman, who had previously examined her on January 7, 2009.  In his report on May 6, 2011, Dr. Markman noted that the employee had reported an increase in pain in the anterior medial and lateral aspect of her right knee, with increasing sharpness and a burning sensation, together with more swelling in that area since the buckling incident of June 14, 2010.  The doctor diagnosed “severe right knee arthrosis” and confirmed that the employee did need a total right knee arthroplasty as a result, concluding that a specific injury - - not a Gillette-type injury - - of June 14, 2010, was a significant aggravating factor with respect to her pain and the ongoing dysfunction of the knee.  He attributed the employee’s need for the surgery to “the previous injuries to her left knee, the non-work related idiopathic and/or degenerative conditions in the right knee, and the specific injury of June 14, 2010.”  Dr. Markman opined that the employee had reached maximum medical improvement with regard to her left knee injury, and he apportioned liability for the right knee condition 50% to the idiopathic and/or degenerative conditions, 25% to the left knee injuries of October 2006 and June 2008, and 25% to the buckling incident on June 14, 2010.

The matter came on for hearing again on August 17, 2011.  Issues at hearing, where no party contested the necessity of the employee’s proposed right knee surgery, were as follows: (1) whether the employee had sustained a work-related injury to her right lower extremity; (2) if so, (a) whether she had sustained an injury consequent to her June 24, 2008, left knee injury, (b) whether she had sustained a Gillette-type injury to her right lower extremity, (c) whether she had sustained an injury on June 14, 2010, and (d) whether, if she had sustained more than one right knee injury, apportionment of liability was appropriate; (3) whether either workers’ compensation insurer was liable for costs of the employee’s proposed right knee replacement; and (4) whether a named intervenor was entitled to reimbursement for payment of medical expenses.

Evidence admitted at hearing included the testimony of the employee, in part that her right knee “really was starting to bother me” when she first reported it to Dr. O’Neill on May 4, 2010, about a month and a half before the June 14, 2010, buckling incident, having been “something that had bothered me all along.”  In follow-up, the employee testified further that her right knee symptoms changed “[g]reatly” immediately after that buckling incident.  The employee testified that the symptoms in her right knee had been getting progressively worse during the time frame between her December 2008 deposition and the buckling incident on June 14, 2010, and that she also felt during that time that her work activities were contributing to the increase of her right knee pain.  She testified that the knee had actually buckled on occasion prior to that date, but not to the point of causing her almost to fall.  The employee acknowledged that, although she was walking at the time her knee buckled, she had not slipped and had recently even been sitting during as much of her work as possible.  She testified that her right knee was particularly painful when she was on her feet vacuuming or cleaning bathrooms or collecting trash, during which activities she apparently used her trash barrow as “pretty much my walker.”

By findings and order filed October 3, 2011, the compensation judge concluded in part that the employee did sustain “a permanent injury” to her right lower extremity on June 14, 2010, that was nonidiopathic and that was a substantial contributing factor in her need for medical treatment to her right knee beginning June 15, 2010, including her current need for total right knee replacement.  The judge found also that, by compelling the employee to compensate with overuse of her right knee, the employee’s left knee injuries of 2006 and 2008 were also substantial contributing factors in her need for that treatment, with liability for that treatment being apportioned 50% to the 2006 and 2008 injuries and RTW and 50% to the 2010 injury and SFM.  Both RTW and SFM appeal from the judge’s finding of a work injury on June 14, 2010.  In its responsive brief, RTW contends that, should this court affirm the judge’s conclusion that the employee’s right knee condition is work related, this court should also affirm the judge’s finding of a work injury on June 14, 2010, together with his finding of equal liability for both insurers.

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. Lyon 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.

The compensation judge found that the employee sustained a permanent work-related injury to her right knee on June 14, 2010, and that that injury has been a substantial contributing factor in her subsequent need for medical treatment, including total right knee replacement.  On appeal, SFM contends that this conclusion is unsupported by substantial evidence.  It contends primarily that any injury manifested on that date was an idiopathic injury, suggesting that the buckling of the right knee on that date might have been consequent to pre-existing problems in the employee’s left knee or to the employee’s obesity.  On cross-appeal, RTW, also, contends that the judge’s finding of a work injury to the right knee is unsupported by substantial evidence, arguing in the main that the employee’s right knee condition and need for total knee replacement are due entirely to pre-existing arthritis that is “not related to a consequential injury, a specific injury or to work activities.”  We are not persuaded.

In support of its contention that any injury on June 14, 2010, was idiopathic in nature, SFM cites case law for the premise that, for an injury to arise out of employment, there must be a causal connection between the employment and the injury, citing Lange v. Minneapolis-St. Paul Metro Airport Commission, 257 Minn. 54, 99 N.W.2d 915, 21 W.C.D. 61 (1959), and that such a connection requires that the employment pose an increased risk of the injury that is not present in everyday activities.  Citing Bohlin v. St. Louis County, 61 W.C.D. 69 (W.C.C.A. 2000); Spinks v. Ecowater Systems, 65 W.C.D. 32 (W.C.C.A. 2005).  Emphasizing that the judge made no finding of a specifically Gillette-type injury, although that had been at issue, SFM argues that the judge “has blurred the distinction between a specific injury and a Gillette injury.”  SFM argues that “[t]he June 14, 2010 incident was a specific injury (buckling episode), not a Gillette injury” and that, at the time of that incident - - while merely walking across the mezzanine floor - - the employee was not subject to the requisite increased risk.  In fact, SFM argues, most of the employee’s work on June 14, 2010, was performed while sitting down, not even while standing.  Even were we to construe his finding to be one of a specific injury however, the compensation judge cited case law precedent supporting the proposition that even mere standing at work may, in certain circumstances, reasonably constitute an increased risk of injury from falling.  See Caizzo v. McDonald’s, 65 W.C.D. 378 (W.C.C.A. 2005).  We acknowledge SFM’s argument that the employee was apparently spending at least some of her working hours sitting in June of 2010 and that there is no expert medical opinion to the effect that her right knee buckled on June 14, 2010, due to her standing or walking for any extended period of time on that date.  We conclude, however, that, given especially the employee’s obesity, the general condition of both of her knees, and evidence that her symptoms worsened substantially shortly after the buckling incident, it was not unreasonable for the judge to conclude that the employee’s knee buckled as a consequence of her work on that date, even in the simple act of walking.  On that basis we affirm that conclusion of the judge.  See Hengemuhle, 358 N.W.2d at 59, 37 W.C.D. at 239.

In support of its own contention that neither RTW nor SFM should be held responsible for the employee’s right knee condition, RTW argues, based on the opinion of SFM’s medical expert, Dr. Bert, that the employee’s right knee condition “is simply not of the type that could be caused by an acute injury, but rather is a degenerative condition that took place over a long period of time.”  Such, RTW argues, was the employee’s arthritic condition, “which, by definition, is a long-term degenerative disease marked by gradual deterioration and the narrowing of the joint spaces.”  However, just as certainly as Dr. Bert’s opinion may support such a position, the opinion of RTW’s own medical expert, Dr. Markham, supports the position that the employee’s current right knee condition is attributable to “the previous injuries to her left knee” and to a “specific injury of June 14, 2010,” as well as to certain “non-work related idiopathic and/or degenerative conditions in the right knee.”  Particularly in light of the employee’s own association of her right knee pain with her left knee problems even prior to the June 2010 buckling incident, and in light also of the opinions of Drs. O’Neill and Markman, it was not unreasonable for the compensation judge to conclude that the employee’s right knee condition was substantially related to her work activities.  Therefore we affirm that conclusion.  See Hengemuhle, 358 N.W.2d at 59, 37 W.C.D. at 239.

Having found a work-related injury on June 14, 2010, the compensation judge apportioned liability for related benefits - -including the cost of a total right knee replacement - - 50% to RTW and 50% to SFM.  The judge based the 50% attributable to RTW on the condition’s being half the result of her compensating for her left knee problems, and he based the 50% attributable to SFM on the condition’s being half the result of a specific new injury on June 14, 2010.  Based in large part on the employee’s earlier deposition testimony, SFM argues on appeal that the employee’s right knee problems long preceded the right knee buckling incident of June 2010, which, they argue, pursuant to the opinion of Dr. Bert, was due primarily to the employee’s obesity.  In its responsive brief, RTW contends, alternative to its position on its own cross appeal, that, should this court affirm a finding of a work injury on June 14, 2010, the compensation judge’s decision “should be affirmed in its entirety, which would preserve the finding of equal liability for both insurers.”  Unpersuaded by SFM’s argument, we agree with RTW’s position on the apportionment.

We would note initially that the compensation judge explained in the first sentence of his memorandum that “[t]he employee provided credible testimony throughout the hearing.”  The employee had testified in various contexts that her right knee pain had been continuous for some years and that she associated it with her left knee condition and the requirements of its eventual total replacement.  It was not unreasonable for the judge to credit this testimony.  See Brennan v. Joseph G. Brennan, M.D., 425 N.W.2d 837, 839-40, 41 W.C.D. 79, 82 (Minn. 1988) (assessment of a witness's credibility is the unique function of the trier of fact).  The judge noted that “[t]his is an employee with a significant condition in both knees which is work related” and that “[s]he had a precarious medical condition prior to [June 14, 2010] and was at risk for injury.”  These were reasonable and objective conclusions, and in this context it was not unreasonable for the judge to rely on the expert medical opinion of Dr. Markman, if only to the extent that responsibility for the employee’s ongoing right knee condition was the equal responsibility of “the two left knee injuries of 2006 and 2008” on the one hand and “the give-way incident of June 14, 2010” on the other.  See Johnson v. L. S. Black Constr. Co., slip op. (W.C.C.A. Aug. 18, 1994), quoting City of Minnetonka v. Carlson, 298 N.W.2d 763, 767 (Minn. 1980) (a fact finder generally “may accept all or only part of any witness’ testimony”).  Because we have already concluded that the employee’s right knee condition is work-related and not idiopathic or solely degenerative, and because the judge’s reliance in part on Dr. Markman’s expert medical opinion was not unreasonable, we affirm the compensation judge’s apportionment of liability for the employee’s right knee medical care, including the recommended total knee replacement.  See Hengemuhle, 358 N.W.2d at 59, 37 W.C.D. at 239.