The self-insured employer appeals from the compensation judge’s determination that the employee’s 2004 surgery was causally related to the work injury of January 27, 1979, and from the compensation judge’s award of medical expenses.  We affirm.

Clifford Kelly was employed as a police officer by the City of St. Paul on January 27, 1979, and was on patrol with his partner in Mounds Park on that date.  There had been a heavy snow fall that day and they helped push a citizen’s car out of the snow.  The squad car then became stuck and the employee pushed it while his partner drove.  As he was pushing the car, the employee heard a pop in his left knee and noticed pain in the inner part of the left knee.  The employee’s initial care was with his chiropractor, Dr. Arnal, who began seeing him on February 2, 1979, and who, in a physician’s report dated February 7, diagnosed left medial collateral ligament sprain.

The employee had begun working as a police officer for St. Paul in September 1975.  He testified that he took and passed a pre-employment physical and also completed basic training with no difficulty.  It was also the employee’s testimony that he had not had any treatment for his left knee before 1979, and that he had no injuries to his left knee before that time.

The employee stated at hearing that Dr. Arnal’s treatment did not improve the condition of his left knee so he went to see his primary care physicians at Group Health.  The employee saw Dr. Lakhbir Kang on May 4, 1979.  The chart notes for that date state, “Pt gives a Hx of a recurrent type of problem with his L knee over many yrs. and has re-injured it on several occasions.  Once in Jan. of 1979 and now again a few days back.  Pt gives a Hx of evaluation of his knee problems some years back when surgery was recommended.” At the hearing, the employee testified that this history was incorrect.  He denied a recurrent problem with his left knee or multiple injuries to his left knee and denied ever having received a surgical recommendation.  No medical records of earlier treatment or of a surgical recommendation are in evidence.

The chart notes also indicate that the employee saw an orthopedist, Dr. Robert Fielden.  The employee testified that he had no recollection of seeing this doctor.  There is a chart note in the Group Health records from Dr. Fielden dated July 13, 1979, which states that the employee’s knee “clinically is fine.”  There is no record of treatment.  In October 1979, the employee saw Dr. Kang again and the history recorded was of soft tissue injury to his lower left leg about a week previously while playing hockey and a re-injury with a car door the day before.  Dr. Kang’s assessment, after x-rays of the tibia, was of a probable soft tissue injury.  Dr. Kang advised activity modification until the pain and swelling resolved.

There are no medical records of any treatment for the left leg from then until October 1981.  The employee testified that in the interim he continued to have problems with his left knee.  He noted worsening of his knee symptoms with more fluid in his knee and a “giving way” sensation in his knee when walking.  On October 30, 1981, the employee saw Dr. Robert Larsen at Group Health.  The chart notes provide a history that “this Pt comes in with a somewhat vague Hx of multiple injuries to the L leg.  The first one being 5/80 in a baseball game.”  Dr. Larsen recommended exercises which the employee did not find helpful and the employee was then referred to Dr. Paul Wicklund.

The employee first saw Dr. Wicklund on November 12, 1981, and has treated with Dr. Wicklund for his left knee problems since that time.  Dr. Wicklund’s initial history was that the employee had three injuries to his left knee, one when playing softball about two years previously and the initial injury in 1974, when he was skiing.  In his hearing testimony, the employee denied a 1974 knee injury, while skiing or otherwise.  Dr. Wicklund found medial joint line pain and recommended arthroscopic surgery.  The surgery, performed on November 24, 1981, revealed a defect in the medial femoral condyle which Dr. Wicklund indicated would heal with vitamin C and E.

Since that procedure the employee has had a number of surgeries performed on his left knee.  On July 22, 1985, Dr. Ivance did an arthroscopic procedure.  The operative note for that procedure was not available for the record.  On February 4, 1992, Dr. Wicklund removed loose bodies in the left knee.  On November 22, 1995, surgery removed loose bodies in the suprapatellar pouch.  The surgery also demonstrated Grade II to III changes of chrondromalacia at the medial femoral condyle.  Arthroscopy on March 12, 1999, showed chrondromalacia of the trochlea and patella, chondromalacia at the medial femoral condyle, spurring on the anterolateral tibial surface, and chondromalacia of the weight-bearing surface of the lateral femoral condyle.  On August 12, 2004, Dr. Wicklund performed surgery and found loss of articular cartilage from the trochlea.  Dr. Wicklund also noted chondromalacia of the medial femoral condyle and the lateral compartment.  The lateral compartment changes were smoothed off.

The city accepted liability for the employee’s January 27, 1979, work injury and paid for the surgeries before 2004.  The employer had the employee evaluated by Dr. Mark Friedland on February 25, 1998 and on May 26, 2005.  It was Dr. Friedland’s opinion that the treatment of the employee in 2003 and 2004 was not the result of the employee’s 1979 work injury.  According to Dr. Friedland, the 1979 work injury was a temporary exacerbation of a pre-existing condition.  Dr. Friedland stated that there were no indications of chondromalacia in 1981 and the changes which have occurred since that time would not be the result of the 1979 injury.  Based upon Dr. Friedland’s reports, the employer denied liability for the employee’s ongoing left knee problems and the employer alleged that payments for the previous surgeries were made under a mistake of fact.

The employee filed a medical request seeking payment of medical expenses related to the August 2004 surgery.  Following an administrative conference, the employer filed a Request for Formal Hearing.  The hearing was held before Compensation Judge Rolf Hagen on July 14, 2005.  The employee testified at the hearing and the parties introduced medical records of the employee.  Dr. Wicklund testified by deposition post-hearing.  Dr. Wicklund’s opinion was that the left knee condition and need for surgery in 2004 was causally related to the 1979 injury.

In his Findings and Order of September 8, 2005, the compensation judge found the employee to be a credible witness, accepted the opinions of Dr. Wicklund, and determined that the preponderance of the evidence supported a finding that the 1979 work injury was a substantial contributing factor in the need for the employee’s medical treatment, including the surgery on August 12, 1004.  The employer appeals.

The employer argues on appeal that the compensation judge erred in his causation determination by failing to consider medical records which refer to causes for the employee’s left knee problems other than the 1979 work injury.  According to the employer, the compensation judge’s failure to consider the medical records is demonstrated by his determination that the employee was a credible witness.  The employer contends that the employer could not be found to be credible when his testimony was at such variance with the medical records.  We disagree.

Given the differences between the history set out in some of the records and the history provided by the employee at hearing, the compensation judge had to choose which evidence to accept.  The employee was cross-examined at length on the differences between his testimony and the records and the compensation judge had opportunity to observe the employee’s demeanor and answers at hearing.  We are not aware of any authority which requires a compensation judge to accept medical records as more persuasive than an employee’s testimony in a case such as this.  Similarly there is no obligation for a compensation judge to accept the employee’s testimony as more persuasive than the medical records.  While the compensation judge could have decided otherwise, he concluded the employee was credible and accepted the employee’s history of the onset of his knee problems.  We have said on numerous occasions that determination of credibility is a unique function of the compensation judge.  Tewes v. George A. Hormel & Co., 430 N.W.2d 178, 41 W.C.D. 410 (Minn. 1988); Moreno v. Northwest Surfacing Co., slip op. (W.C.C.A. September 29, 2003).

The employer also argues that the compensation judge must not have considered the employee’s early medical records because the compensation judge did not refer to the records in his findings or memorandum.  A compensation judge is not required to relate or discuss every piece of evidence introduced at the hearing.  Braun v. St. John’s University, slip op. (W.C.C.A. July 20, 1992); Midtling v. Schwan’s Sales Enters., slip op. (W.C.C.A. September 22, 2003).  We note that the significance of the medical records was the primary issue raised by the employer in its trial memorandum and that, as noted earlier, the employee was cross-examined on this issue extensively.  We find no basis, therefore, for a conclusion that the compensation judge did not consider all of the evidence submitted to hin at the hearing.

In support of its position, the employer also quotes a statement by the compensation judge in his Findings and Order that there is no evidence of a pre-existing condition before 1979.  The employer points again to the medical records from Group Health which contain references to earlier injuries.  We believe this argument misconstrues the compensation judge’s comment.  The compensation judge is correct in noting that there are no medical records before 1979 which indicate any left knee injuries or treatment.  The evidence of a pre-existing condition are in medical records generated after January 1979.  The compensation judge found that the employee credibly denied the accuracy of that history.
  
The employer also argues on appeal that the compensation judge based his decision, not on the evidence, but on the fact that the employer had previously admitted liability for the employee’s medical care.  No support is provided for this argument.  Although the compensation judge noted in one of his findings that the employer had paid for the earlier surgeries, he specifically adopted the opinion of Dr. Wicklund on causation and stated that this was the reason for his conclusion.  There is no indication that the employer’s previous payments was a factor in the compensation judge’s causation determination.

The question for this court is not whether the evidence would support a contrary result but whether substantial evidence supports the decision of the compensation judge.  Hengemuhle v. Long Prairie Jaycees, 358 N.W.2d 54, 37 W.C.D. 235, (Minn. 1984).  We find substantial evidence in the employee’s credible testimony and the opinion of Dr. Wicklund for the compensation judge’s determination that the 1979 injury was a substantial contributing factor in the employee’s need for medical care in 2004.  We affirm.