Holcomb v. Itasca Medical Ctr.

CAROLYN HOLCOMB, Employee, v. ITASCA MEDICAL CTR. and ACE USA, Employer-Insurer/Appellants, and LAKEWALK SURGERY CTR., Intervenor.

WORKERS= COMPENSATION COURT OF APPEALS

OCTOBER 16, 2002

HEADNOTES

EVIDENCE - EXPERT MEDICAL OPINION. Where the employee=s doctor=s failure to note the absence of evidence as to causation of the employee=s pre-injury non-ACL-related knee problems was immaterial as to causation of the employee=s post-injury ACL tear, where most of the Adiscrepancies@ in the doctor=s opinions were resolved in the medical record, where the doctor=s mistake as to the date of one of the employee=s MRI scans did not demonstrate unfamiliarity with the scan or with the condition of the employee=s knee, and where the doctor=s opinion was apparently, though not expressly, rendered within a reasonable degree of medical certainty, the employee=s doctor=s opinion was not of insufficient foundation to be relied upon.

CAUSATION - SUBSTANTIAL EVIDENCE. Where the expert medical opinions upon which the judge relied were not purely speculative, where the judge did not apply an inappropriate legal standard, and where there was sufficient evidence that the ACL tear at issue was present from and after the date of the work injury, the compensation judge=s conclusion that the employee=s work injury was a substantial contributing cause of the employee=s anterior cruciate ligament tear and the employee=s need for its repair was not clearly erroneous and unsupported by substantial evidence.

Affirmed.

Determined by Stofferahn, J., Pederson, J., and Johnson, C. J.

Compensation Judge: Gregory A. Bonovetz

OPINION

WILLIAM R. PEDERSON, Judge

The employer and insurer appeal from the compensation judge's conclusion that the employee=s work injury was a substantial contributing cause of the employee=s anterior cruciate ligament tear and her need for its repair. We affirm.

BACKGROUND

On January 5, 1998, after a history of left knee problems dating back to at least June of 1993, Carolyn Holcomb underwent a partial meniscectomy to repair a tear in the medial meniscus in her left knee. Ms. Holcomb was off work for about six weeks after the surgery, and subsequently she returned fully recovered and without restrictions to work full time at her pre-surgery job as a housekeeper at Itasca Medical Center. By April of 1998 Ms. Holcomb was no longer experiencing any swelling or other problems with her left knee, was fully able to walk up and down stairs, and was in no need of further related medical care.

On May 2, 1998, at about 8:00 a.m., Ms. Holcomb [the employee] sustained a reinjury of her left knee, when she twisted the knee in the process of mopping a floor in the course of her housekeeping work at Itasca Medical Center [the employer]. At the time of the injury, the employee heard a Apop@ and experienced a cracking and crunching sensation on the outside of the knee. She continued to work, but by the end of her work shift she had developed pain and discomfort in the knee and had advised her employer of the incident. That night the employee was awakened from her sleep by continuing knee pain, and by the following day the knee had begun to swell. The employee consulted her family physician, Dr. Dennis Scherer, who diagnosed a strained knee and advised her to remain off work for a brief period of time and to apply ice to the knee. The employee was forty-five years old on the date of her injury and was earning a weekly wage of $394.33.

After missing a few days from work, the employee returned to full-time duty at her job as a housekeeper with the employer. The employee=s symptoms apparently continued, however, and by late autumn 1998 her swelling, pain, and difficulty kneeling, squatting, and walking up and down stairs had increased to the point where surgery was contemplated. On November 16, 1998, she saw orthopedic surgeon Dr. Leo Hise, who diagnosed A[m]edial meniscus tear and probably new injury.@ On December 29, 1998, Dr. Hise performed an arthroscopic examination of the knee and a debridement of a lateral meniscus tear and of the posterior horn of the medial meniscus in the knee, together with removal of several small loose bodies. The post-operation report indicates that, subsequent to the procedure, the knee was arthroscopically examined to inspect for additional pathology, and A[n]one was seen.@ The employer and its insurer accepted liability for the May 2, 1998, injury, paid temporary total disability benefits from December 29, 1998, to January 16, 1999, and paid temporary partial disability benefits from January 18 to April 12, 1999.

By April 1999, the employee had returned to full-time work, not in housekeeping but as a nursing assistant. In time the employee began again to experience swelling and pain and in her left knee and a sensation that the knee might Agive out.@ She was referred for physical therapy, and on June 23, 1999, she commenced treatment with orthopedist Dr. Mark Carlson, who diagnosed A[e]arly degenerative changes by history, possibly traumatic chondromalacia@ and ordered an MRI scan. The MRI, conducted on that same date, was read to reveal in part Asignificant foreshortening and blunting of the lateral meniscus consistent with a tear at that location,@ and on July 21, 1999, Dr. Carlson recommended a lateral meniscectomy. On July 26, 1999, Dr. Carlson performed this third surgery on the employee=s left knee, a lateral meniscectomy together with an extensive chondroplasty. On August 24, 1999, Dr. Carlson noted that the employee was recovering very well and that she had full motion and excellent stability in the knee. The employer and insurer related the need for the surgery to the admitted injury of May 2, 1998, and paid temporary total disability benefits through September 1, 1999, when the employee returned to work half time as a ward secretary.

The ward secretary position to which the employee returned might have been expected to put less stress on the employee=s repaired knee, but the employee continued to have difficulty ascending and descending stairs, and by September 21, 1999, she was complaining again to Dr. Carlson, now of Apain and discomfort in the retropatellar area.@ Dr. Carlson noted, however, that she had excellent stability in the knee on exam, with no evidence of pivot shift, and a ALachman=s@ test was negative. By October 28, 1999, the employee was reportedly tolerating her part time work satisfactorily and still had full range of motion in the knee on examination. On November 16, 1999, the employee was discharged from a two-month physical therapy regimen with decreased but continuing pain, particularly with ascending and descending stairs. The employer and insurer paid temporary partial disability benefits from September 2, 1999, through November 18, 1999, together with compensation for a permanent partial disability of 2% of the whole body.

By the summer of 2000 the employer was no longer able to accommodate the employee=s left knee limitations, and on about July 10, 2000, the employee=s work for the employer was terminated. The employee experienced no identified new trauma to her left knee in the months subsequent to her surgery, but the condition of the knee evidently continued to deteriorate. Eventually, on September 5, 2000, the employee began to complain also of additional pain in the posterolateral aspect of her knee, although medial lateral stability was still found to be satisfactory. On September 26, 2000, Dr. Carlson rated the employee=s permanent partial disability at 3% of the whole body, pursuant to Minnesota Rules 5223.0510, subp. 3.B.2.

On December 16, 2000, the employee was examined for the employer and insurer by orthopedic surgeon Dr. Jonathan Biebl, who diagnosed Astatus post-medial lateral meniscectomy of the left knee, and degenerative arthrosis of the left knee, with significant media/lateral and anterior/posterior instability of the left knee.@ Dr. Biebl indicated that, in his opinion, the work injury of May 2, 1998, was a small tear of the already degenerated medial meniscus, from which injury the employee reached maximum medical improvement [MMI] by March 1999, three months after the December 1998 surgery. He indicated that, while the employee=s second surgery, in October 1998, was contributed to by the May 1998 work injury, the employee=s third surgery, in July 1999, was due to naturally occurring, progressive degenerative changes in the knee. Characterizing the employee=s left knee as Amildly unstable,@ Dr. Biebl indicated also that it was his Afeeling that this has been a progressively deteriorating degenerative knee on the basis of wear and tear, and osteoarthritis@ and that he did Anot see any evidence that would constitute a Gillette-type injury.@

On December 23, 2000, the employee underwent another independent medical examination, this with orthopedic surgeon Dr. Stephen Olmsted. In his report on January 9, 2001, Dr. Olmsted indicated that his examination had revealed a positive Lachman=s test and a subtle but not painful pivot shift in the employee=s left knee. His review of the employee=s June 23, 1999, MRI, however, revealed a normal ACL, and Dr. Olmsted indicated that the employee=s lack of stability in the knee was most likely due to a lack of meniscal tissue, the ACL having demonstrated on examination Aa good firm, strong endpoint.@

With the employee=s pain continuing, Dr. Carlson ordered another MRI scan, which was conducted on March 5, 2001. The scan was read to reveal degenerative articular cartilage changes with Grade 2 and Grade 3 chondromalacia and focal bone spur formations, together with previous partial meniscectomy changes and Aevidence of an old anterior cruciate ligament [ACL] tear.@ On May 1, 2001, Dr. Carlson stated, ABased on the last MRI, [the employee=s] progressive symptoms of instability and pain, and physical exam today, I would conclude that the [employee] does have an anterolateral instability based on an injury. I would recommend that the [employee] consider arthroscopy with an endoscopic anterior cruciate reconstruction.@ On May 31, 2001, Dr. Carlson reported also that, A[b]ased upon the [employee=s] history, the condition that she would be presently treated for is directly and causally related to her twisting injury in 1998.@

On July 20, 2001, the employee filed a medical request, alleging entitlement to payment of certain outstanding medical expenses and to payment for the reconstructive left knee surgery prescribed by Dr. Carlson, all consequent to her work injury on May 2, 1998. On August 3, 2001, the employer and insurer filed a medical response, refusing to pay for the treatment at issue, based on the December 16, 2000, report of Dr. Biebl. On October 24, 2001, the employee underwent the recommended ACL reconstruction surgery. The operative report indicated that the ACL Adid show indeed a femoral detachment.@

On December 7, 2001, Dr. Biebl testified by deposition, in part that, on the employee=s June 23, 1999, MRI scan, the employee=s left ACL appeared normal, Aa nice straight border to it without sag.@ Dr. Biebl testified that Dr. Carlson=s examination findings in June of 1999 suggested no problem with the employee=s ACL, A[n]o positivity to the Lachman test, . . . , which would be a fairly sensitive test for anterior cruciate ligament tear,@ and Aa negative pivot shift.@ Dr. Biebl testified that in his opinion the employee=s October 2001 ACL reconstruction surgery was warranted based on the employee=s knee=s instability, but he testified that it was also his opinion that the employee=s May 2, 1998, work injury did not contribute to the need for that surgery. Dr. Biebl did acknowledge, however, that the ACL tear at issue appeared on the March 5, 2001, MRI scan to be at least eight to twelve weeks old, that if it were not that old the MRI would have picked up more inflammation and some swelling. He also acknowledged that, although an ACL may on occasion become Afunctionally incompetent@ by Aa very slow and gradual process@ as a consequence of being stretched over and abraded by bone spurs in the notch through which it passes, normally a torn ACL is the consequence of some sort of accident or trauma. He also acknowledged that, although Dr. Carlson=s operative findings indicate that the referenced notch was widened in the course of the employee=s reconstructive surgery, no mention was made in the operative findings of any spurring. Finally, Dr. Biebl acknowledged further that it was possible, though not highly likely, that an ACL could be detached from the femur and yet not be visible to a radiologist or other physician reading MRI scans of a knee. Dr. Biebl testified also that it would be more likely for a torn ACL to result in a meniscus tear than for a meniscus tear to result in a torn ACL.

On December 10, 2001, Dr. Carlson reiterated his causation opinion, explaining the ACL discrepancy between the March 2001 MRI and what he twice identified as the AJune of 2000" MRI by stating that the earlier MRI Awas done in such a way that the cuts taken by the MRI did not profile the ligaments to a satisfactory enough point to clearly define the tear.@ He went on to conclude also that the employee=s twisting motion at the time of her 1998 work injury was the mechanism of her torn ACL and that A[i]n all likelihood the ligament went on to heal but in a nonfunctional position, that is it healed in a lengthened position.@

The matter came on for hearing December 14, 2001. The sole issue at hearing was whether the admitted left knee injury of May 2, 1998, was a substantial contributing cause of the employee=s need for her ACL surgery of October 24, 2001. At the hearing, the employee testified in part that the feeling that her knee might give out, that she experienced a couple of months after her second surgery in December 1998, was a feeling that she had not experienced prior to her work injury of May 1998. By findings and order filed February 4, 2002, and finding the employee to be Aa most credible witness,@ the compensation judge concluded that the injury of May 2, 1998, was a substantial contributing cause of the ACL tear at issue and of and the employee=s consequent need for surgery. The judge based his decision primarily on the opinion of Dr. Carlson, indicating that he concluded as he did A[s]ince the overwhelming weight of the evidence establishes that in a case such as this trauma is necessary for the tearing of an ACL, since the only trauma suffered to the left knee occurred on May 2, 1998 and since the documenting MRI reflects the existence of a[n] Aold@ tear. The employer and insurer appeal.

STANDARD OF REVIEW

In reviewing cases on appeal, the Workers= Compensation Court of Appeals must determine whether Athe 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 (1992). Substantial evidence supports the findings if, in the context of the entire record, Athey 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, A[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 Food 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, Aunless 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.

DECISION

Apparently in reliance on the opinions of Dr. Carlson, the compensation judge found that the employee=s ACL tear and reconstructive surgery in October 2001 were causally related to the employee=s work injury of May 2, 1998. The employer and insurer contend that the judge erred as a matter of law in any reliance on the opinions of Dr. Carlson, which they contend were lacking in sufficient foundation, and they contend also that the judge=s conclusion that the surgery at issue was causally related to the employee=s 1998 work injury is otherwise unsupported by substantial evidence.

1. Foundation for Dr. Carlson=s Opinions

The employer and insurer contend that the sufficiency of the foundation for Dr. Carlson=s causation opinion is A[t]he primary issue@ in this case. They contend that Dr. Carlson=s opinion is unfounded (1) because there is no evidence that Dr. Carlson was aware of the employee=s long history of left knee problems unrelated to any specific event, (2) because Dr. Carlson=s causation opinions contained numerous discrepancies, (3) because Dr. Carlson was mistaken even as to the date of a material MRI scan, and (4) because the doctor=s opinion was not rendered expressly Awithin a reasonable degree of medical certainty.@ We are not persuaded.

The employer and insurer argue first that, while he appeared to place weight on the fact that the employee denied any left knee injury subsequent to the one that occurred in 1998, Dr. Carlson makes no mention of the employee=s at least five-year history of left knee problems without specific injury prior to that date. They argue that Athere is no evidence that Dr. Carlson considered the significance of this past medical history while formulating his opinions.@ The employer and insurer themselves do not, however, specify what Asignificance@ should be attached to this history. Before the admitted May 1998 work injury, there was no radiological evidence of an ACL tear, and no medical diagnosis of such a tear existed. Since that May 1998 work injury, however, there does exist radiological evidence of such a tear, and the medical diagnosis to that effect is uncontested. Moreover, there is also testimony that ACL tears are normally trauma-caused; there is no testimony that any of the employee=s prior conditionsBapparently mainly meniscal problems--are normally trauma-caused. Under these facts, we see no dispositive significance in the fact that that the employee had a pre-May 1998 history of non-ACL knee problems and that those problems were unrelated to specific incidents. We do not find Dr. Carlson=s opinion insufficiently founded merely on grounds that the doctor did not reveal that he was aware of the employee=s previous history of left knee problems that were unrelated to a specific event. While adequate foundation is necessary for a medical opinion to be afforded evidentiary value, the expert need not express or even be made aware of every relevant fact for his opinion to have validity. See Bossey v. Parker Hannifin, slip op. (W.C.C.A. Mar. 14, 1994).

The employer and insurer also contend that Dr. Carlson=s opinions are unreliable because of numerous discrepancies between the doctor=s examination findings and his causation conclusion. They argue that Dr. Carlson failed to consider the implications of his own negative findings on application of the Lachman=s test and the pivot shift test in the course of his examination of the employee on June 26 and September 21, 1999, of his own findings of no instability on June 26, 1999, and excellent stability on August 24, 1999, and of his own findings of full range of motion on July 21, August 24, and October 28, 1999, and September 5 and October 10, 2000. All of these findings, they argue, support a conclusion that the employee had no ACL damage prior to at least October 2000 or otherwise consequent to her May 1998 work injury. Most of these alleged Adiscrepancies,@ however, are implicitly if not explicitly resolved in the medical record.

As of April of 1998, prior to her May 1998 work injury, the employee was, according to uncontested Finding 4 of the compensation judge, free of any symptoms in her left knee and was fully able to walk up and down stairs. At the time of her injury on May 2, 1998, the employee heard a Apop@ in the knee and thereafter was evidently never without symptoms. By late autumn 1998, the employee=s swelling, pain, and difficulty kneeling, squatting, and walking up and down stairs had increased to the point where surgery was contemplated, her surgeon diagnosingA[m]edial meniscus tear and probably new injury@ (emphasis added). Eventually, based in part on early 2001 radiological evidence of an Aold@ ACL tear, Dr. Carlson diagnosed one element of the injury that appeared Anew@ to Dr. Hise in late 1998 as a tear of the employee=s ACL that had occurred on May 2, 1998, reasonably opining on December 10, 2001, that A[i]n all likelihood the ligament went on to heal but in a nonfunctional position, that is it healed in a lengthened position.@ Although Dr. Carlson did have findings of a negative Lachman=s test in September 1999 and did at times in his records attest to the stability of the employee=s knee, the only testimony as to the diagnostic significance of the Lachman=s test is that of Dr. Biebl for the employer, and there is ample evidence, both in the medical records and in the employee=s testimony at hearing, that the employee frequently felt at risk of her knee Agoing out@ on her. Moreover, we find no testimony as to the diagnostic significance of range of motion tests with regard to ACL tears.

The employer and insurer argue also that Dr. Carlson=s repeated reference to an MRI scan completed in June 2000 indicates less than sufficient familiarity with the employee=s radiological studies, since the material MRI scans were conducted on June 23, 1999, and March 5, 2001, and no MRI scan was even conducted in June 2000. We conclude, however, notwithstanding the repetition of the error in Dr. Carlson=s report, that Dr. Carlson=s reference to the earlier scan as a June 2000 scan instead of a June 1999 scan is an essentially meaningless, certainly less than dispositive, mistake. Dr. Carlson, after all, ordered the scan himself and reported on it at the time, and his use of the accurate month demonstrates sufficient familiarity with it, as well as sufficiently distinguishes it from any other MRI, so as to preserve his opinion=s validity.

In admitting the doctor=s records into evidence, the compensation judge acknowledged that Dr. Carlson had not expressly indicated that his opinions were rendered within a reasonable degree of medical certainty. The judge indicated, however, that he was Agoing to take judicial, administrative note that Dr. Carlson when he indicatesBissues reports it=s done to a reasonable degree of medical certainty.@ Citing case law, the employer and insurer contend, finally, that A[j]udicial notice is limited to adjudicative facts >which may be regarded as common knowledge of every person of ordinary intelligence=@ and that A[i]t cannot be argued that [the] question of whether Dr. Carlson=s opinions are generally issued to a reasonable degree of medical certainty is answered by an average person=s common knowledge.@ We conclude, however, that the compensation judge=s taking Aadministrative note@ of the reasonable certainty of Dr. Carlson=s opinion is something quite different from Ajudicial notice@ as that concept is addressed in case law. As the supreme court has indicated, in workers= compensation proceedings, "[r]easonable probability [with regard to causation] is determinable by consideration of the substance of the [expert's opinion] and does not turn on semantics or on the use by the witness of any particular term or phrase." Boldt v. Jostens, Inc., 261 N.W.2d 92, 94, 30 W.C.D. 178, 181 (Minn. 1977), quoting Insurance Co. of North America v. Myers, 411 S.W.2d 710, 713 (Tex. 1966). The context of Dr. Carlson=s opinions in the substantial history of his treatment of the employee establish an adequate degree of certainty for his opinions to have been credited by the compensation judge.

We find no basis in either the law or the facts for reversal of the judge=s decision on grounds that the opinions of Dr. Carlson upon which he relied were of inadequate foundation.

2. Substantial Evidence of Causation

The employer and insurer contend that, even aside from foundational weaknesses in Dr. Carlson=s causation opinions, the compensation judge=s conclusion that the employee=s October 2001 ACL reconstruction was causally related to the employee=s May 1998 work injury is unsupported by substantial evidence. Initially they cite Dille v. Knox Lumber, 452 N.W.2d 679 (Minn. 1990), for the premise that A[w]here two opposing inferences can be drawn with equal justification from the same circumstantial evidence, . . . the party having the burden of proof [here the employee] must lose,@ suggesting that the employee cannot, therefore, prevail in this case. They go on then to argue that the judge=s conclusion fails (1) because the opinions of Dr. Carlson upon which that conclusion is based are purely speculative, (2) because that conclusion does not comport with appropriate legal standards, and (3) because there is insufficient evidence that the ACL tear at issue was present from and after the May 1998 date of the work injury. We are not persuaded.

With regard to their contention that Dr. Carlson=s opinions are speculative, the employer and insurer argue that Athere was no evidence that [Dr. Carlson] even reviewed the MRI scans,@ since the doctor=s explanation of the absence of evidence of the ACL tear on the earlier scan was expressly based on a scan dated June 2000, the employee having undergone no scan in that year. They contrast this weakness in Dr. Carlson=s opinion with the detailed opinion of Dr. Bieble in this regard, which was expressly based on first-hand review of the scans at issue. As we have indicated earlier, however, Dr. Carlson=s opinion need not be discredited on grounds of his mistake as to the year of the June MRI. Whether or not Dr. Carlson personally reviewed the June 1999 scan, and although we acknowledge that it is effective evidence, we cannot conclude that Dr. Biebl=s attestation that the ACL was visible and apparently normal on the scan dispositively rebuts Dr. Carlson=s suggestion the ACL may well have torn in May 1998 and then gone on to heal in a less than fully functional position. In this case, the compensation judge credited the opinion of Dr. Carlson over that of Dr. Biebl, and we will not disturb the judge=s decision on that basis. 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).

The employer and insurer contend also that Athe Compensation Judge applied an incorrect legal standard, finding that the surgery was compensable because the employer and insurer failed to present any alternative legal theory,@ which Aeffectively and incorrectly shifted the burden of proof to the employer and insurer.@ This contention is apparently based on the following statement by the judge at the end of his memorandum:

Since the overwhelming weight of the evidence establishes that in a case such as this trauma is necessary for the tearing of an ACL, since the only trauma suffered to the left knee occurred on May 2, 1998 and since the documenting MRI reflects the existence of a[n] Aold@ tear, the Court has concluded that a preponderance of the evidence does establish that the injury of May 2, 1998 was a substantial contributing cause for the need for the surgery of October 24, 2001.

The employer and insurer contend that the burden was on the employee to establish a causative traumatic event and that Ano specific traumatic event was identified as the cause of these concededly preexisting problems.@ We conclude that the employee did allege a causative traumatic eventBthe leg twist while mopping on May 2, 1998, which was accompanied by a popping sound and a cracking/crunching sensation on the outside of the knee. That the compensation judge chose to note that he could not identify any competing trauma does not, in our view, constitute a shift of the burden of proof to the employer and insurer. The employer and insurer also reiterate arguments that they posed challenging the foundation of Dr. Carlson=s opinions based on inconsistency between the doctor=s early examination findings and his ultimate causation opinion; we have already addressed these arguments earlier, having found them unpersuasive. Nor do we find any significance in the fact that no specific traumatic event was identified for the knee problems preexisting May 2, 1998, causation of those problems being not here at issue.

Finally, the employer and insurer argue that little evidence of ACL instability or damage existed until about nine months after the employee ceased working for the employer and that, indeed, some examination evidence even affirmatively implied ACL stability prior to that time. As indicated above, however, we do not find unreasonable the compensation judge=s acceptance of Dr. Carlson=s apparent crediting of the employee=s earlier symptomology as not contrary to an ACL tear, nor do we find unreasonable his explanation that the earlier radiological evidence was probably inadequate in its representations for technical reasons. The employer and insurer argue also that, because all Lachman=s and pivot shift tests were normal until clinical testing conducted by Dr. Olmstead in December 2000, the employee=s ACL tear could not have occurred until some time between September 1999, the date of Dr. Carlson=s negative Lachman=s and pivot tests, and December 2000. Thus, they suggest, the employee=s ACL tear, though described as Aold@ by the radiologist reading the March 5, 2001, MRI, was not so old as to have occurred at the time of the employee=s May 1998 work injury, Dr. Bieble having testified that an ACL tear described as Aold@ need not be any older that about eight weeks old. Dr. Biebl also conceded in testimony, however, that ACL tears are normally the result of trauma, and the employee testified that she recalled no trauma to the knee subsequent to the twisting motion that resulted in her May 1998 work injury. We conclude that it was the judge=s prerogative to credit the employee=s testimony to this effect, 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), citing Spillman v. Morey Fish Co., 270 N.W.2d 781, 31 W.C.D. 187 (Minn. 1978), and that it was not unreasonable for the judge then to weigh that testimony in light of what Dr. Biebl conceded to be normal mechanisms of injury.

Concluding that it was neither erroneous as a matter of law nor factually unreasonable and so unsupported by substantial evidence, we affirm the compensation judge=s award of benefits in this case. See Hengemuhle, 358 N.W.2d at 59, 37 W.C.D. at 239.