The employers and insurers appeal and cross-appeal from the compensation judge’s decision that the 1987 work injury to the employee’s right knee and the 1990 work injury to the employee’s left knee were substantial contributing factors in the need for bilateral knee repair replacement surgery.  We affirm.

The employee’s treating orthopedist, Dr. Mark Engasser, has recommended bilateral knee replacement surgery and the parties have stipulated that the proposed treatment is reasonable and necessary for the employee’s condition.  The issue for the compensation judge at the hearing was whether the employee’s work injury to his right knee in 1987 and his work injury to his left knee in 1990 were substantial contributing factors in the need for replacement surgery.

The first injury at issue here occurred on September 21, 1987, when the employee was working for Walker Roofing, insured by Employer’s Insurance of Wausau.  The employee had worked as a roofer for various companies since 1965 and on the date of injury was lifting a very heavy roll of roofing material with some co-workers when he experienced right knee pain.  Subsequent medical records, however, focused on the employee’s usual physical activity associated with his roofing work rather than a specific incident as being the origin of the employee’s right knee complaints.  Walker and its insurer accepted liability for a right knee injury.

The employee injured his left knee on November 25, 1990, while he was working at Dalco Roofing, insured by Minnesota Assigned Risk Plan, administered by Berkley Risk Administrators.  The employee slipped off the third rung of a ladder and twisted his left knee.  The employee stated that he had swelling in his knee the next day and required medical care.  Dalco and its insurer accepted liability for the 1990 left knee injury.

The employee had extensive medical care for both knees.  His treatment for his right knee began in 1987, after the admitted injury.  By 1988, medical records referenced the employee having left knee symptoms as well.  The employee had a number of arthroscopic surgeries.  In 1997, the employee came under the care of Dr. Mark Engasser.  Dr. Engasser eventually recommended bilateral knee replacement surgery.  Dr. Engasser’s records indicate that he began discussing the possibility of knee replacement surgery with the employee in 2000.  This litigation followed Dr. Engasser’s surgery proposal.

Dalco denied any responsibility for the proposed surgery based on the opinion of Dr. Paul Dworak.  Dr. Dworak had evaluated the employee on behalf of Dalco on July 15, 2005.  He reviewed extensive medical records relating to the employee’s treatment for his knees and he conducted a physical examination of the employee.  It was Dr. Dworak’s opinion that the employee’s knee condition was not related to his employment at Dalco or Walker but was instead the result of inflammatory arthritis.  Dr. Dworak set out his opinion in his reports of July 15 and August 22, 2005, and in his deposition testimony on October 7, 2005, just before the hearing.

Dr. Dworak pointed out that the employee had a number of arthroscopic surgeries to his left knee, all of which showed significant synovitis.  Dr. Dworak further noted that Dr. Jonathan Asp, who treated the employee in 1990 and 1991, had diagnosed inflammatory synovitis or inflammatory arthritis.  A rheumatological consult in 1992 showed the possibility of underlying lupus.  It was Dr. Dworak’s opinion, based in part on the synovial fluid analysis done after the December 1993 surgery, that  “it is evident that we are dealing with a chronic synovitis most likely secondary to an immunological standpoint as opposed to the classic osteoarthritis.”  Dr. Dworak stated that a traumatic injury would not bring about inflammatory arthritis and he did not believe that the employee’s work injuries were significant factors in the employee’s need for knee replacement surgery.

Walker denied liability for the knee replacement surgery on the basis of the report of Dr. Larry Stern, who had evaluated the employee for the insurer on September 8, 2005.  Dr. Stern’s impression, after examining the employee and reviewing the medical records, was that the employee had “recurrent left knee effusions with possible diagnosis of systemic Lupus erythematosus” and that the employee had “progressive mild to moderate degenerative joint disease, left and right knee.”  Dr. Stern concluded that the 1987 injury and treatment involved the patellofemoral joint which was not a factor in the need for knee replacement.  Dr. Stern also stated that the employee’s present left knee condition was not the result of a traumatic incident in 1990 but was instead due to synovitis and, possibly, a rheumatoid condition, lupus.

The employee’s claim was supported by Dr. Mark Engasser.  In addition to having available the chart notes from Dr. Engasser’s treatment beginning in 1997, the compensation judge also reviewed the report of October 10, 2005, prepared by Dr. Engasser at the request of the employee’s attorney, and also reviewed Dr. Engasser’s deposition, taken after the hearing on October 25, 2005.  In his report, Dr. Engasser concluded that the employee’s “injuries in 1987 and 1990 were sufficient to contribute to an inflammatory process which was ongoing.  Indeed this patient may have had some type of systemic inflammatory process, but in my opinion the work injuries he had at Walker and Dalco Roofing substantially contributed to his need for treatment as well as his need for bilateral total knee arthroplasty.”  In his deposition, Dr. Engasser amplified his statement, stating again that the existence of a possible inflammatory process did not mean that the employee did not also have osteoarthritis with its origin in the employee’s work injuries.  Dr. Engasser continued to be of the opinion that the 1987 and 1990 work injuries were substantial contributing factors in the employee’s need for bilateral knee replacement.

In her decision of December 15, 2005, the compensation judge found “the opinion of Dr. Engasser is more persuasive and convincing than the opinions of Dr. Dworak and Dr. Stern.”  She determined that the 1987 and 1990 work injuries were both substantial contributing factors in the need for total knee replacement surgery.  Walker was ordered to pay for the expenses of the surgery for the right knee and Dalco was ordered to pay for the expenses of the left knee surgery.  Both employer’s and insurer’s appeal.

On appeal, both employers and insurers argue that the compensation judge’s decision was erroneous and not supported by substantial evidence.  Both briefs provide detailed recitations of the evidence which, according to the appellants, indicates that the employee’s knee condition was a result of an inflammatory process.  From that, it is argued, that the determination which should have been reached by the compensation judge is that the work injuries were not responsible for the employee’s need for surgery.  We are not persuaded.

Determination of medical causation is a factual determination to be made by a compensation judge upon consideration of the evidence.  Felton v. Anton Chevrolet, 513 N.W.2d 457, 50 W.C.D. 181 (Minn. 1994), Foster v. Metro Produce Distrib., 63 W.C.D. 218 (W.C.C.A. 2002).  We have held on numerous occasions that substantial evidence which supports that determination may consist of a properly founded medical opinion.  McNeal v. Minn. Oncology, slip op. (W.C.C.A. January 30, 2004).  Further it is not unusual to find more than one opinion on a question of causation.  A compensation judge, as factfinder, is expected to choose between those competing medical opinions and this court, as a reviewing court, will affirm the compensation judge unless the facts assumed by the expert in rendering the opinion are not supported by the evidence.  Nord v. City of Cook, 360 N.W.2d 337, 37 W.C.D. 64 (Minn. 1985); Smith v. Quebecor, 63 W.C.D. 566 (W.C.C.A. 2003).  Finally, it is especially relevant in this case to note that the question is not whether the work injury is the sole cause of the condition at issue but whether it is a substantial contributing factor.  Swanson v. Medtronics Inc., 443 N.W.2d 534, 42 W.C.D. 91 (Minn. 1989); Enger v. The Bureau, slip op. (W.C.C.A. July 8, 2002).

Dr. Engasser clearly had adequate foundation for his opinion.  The compensation judge’s reliance on that opinion provides substantial evidence to support her determination that the 1987 and 1990 work injuries were substantial contributing factors in the employee’s need for bilateral knee replacement.  While we can appreciate that the appellants believe the results should have been different, we do not find a lack of supporting evidence in this record which would require us to reverse the compensation judge. The decision of the compensation judge is affirmed.