The employee appeals the compensation judge’s finding that the employee failed to meet her burden of proof that her work activity was a substantial contributing cause of her bilateral arm condition and also appeals the judge’s failure to directly address her equitable estoppel argument.  We affirm.

Lindsey (Kloos) Davis, the employee, began working for Bio Company, Inc., the employer, in March 2012.  The employer was insured for workers’ compensation liability by Minnesota Assigned Risk Plan, as administered by SFM Risk Solutions.  The employee worked as an injector, which involved preparing and preserving animal specimens for biological dissection in high schools and colleges by injecting the specimens with latex and formaldehyde.  This work required use of a scalpel, syringes, and repetitive grasping with awkward hand positions.  In late December 2012, the employee began experiencing bilateral arm and wrist symptoms, including numbness, tingling, electric jolt feelings into her fingers, and pain from her elbows down.  Her symptoms increased with gripping and picking up items.  The employee noted a significant increase in symptoms in January 2013, and claimed work-related bilateral arm Gillette injuries culminating on January 22, 2013.  The employer and insurer admitted liability and paid medical treatment expenses, rehabilitation services, temporary total disability benefits, and temporary partial disability benefits. 

The employee treated at Heartland Orthopedics with a physician’s assistant, who noted that the “symptoms appear to be secondary to generalized overuse” and recommended work restrictions of no lifting over 20 pounds and no injecting.  (Employee’s Ex. F.)  The employer did not have work within these restrictions and the employee received temporary total disability benefits.  The employee received occupational therapy which improved her symptoms.  On March 14, 2013, the employee returned to light duty part-time work with restrictions of no firm grasping or repetitive work.  The employee increased her hours in May 2013, and she experienced increased symptoms.

In August 2013, the employee underwent MRI scans of both wrists.  The right wrist MRI indicated mild tendonopathy and an 18-millimeter multilobulated cyst.  The left wrist MRI indicated a small effusion within the distal radioulnar joint and a full thickness tear of the triangular fibrocartilage.  On August 27, 2013, the employee was evaluated by Dr. Patrick Hurley at Heartland Orthopedics.  Dr. Hurley recommended aspiration of the right wrist ganglion cyst.  The employee’s left wrist was treated with bracing, occupational therapy and medication.  The right wrist cyst aspiration was performed on August 28, 2013, and resulted in a decrease of the employee’s symptoms, but they came back when she returned to work.  She was referred to a hand/wrist specialist for both conditions.

The employee was evaluated by Dr. Christopher Widstrom at St. Cloud Orthopedics on November 7, 2013.  Dr. Widstrom treated the employee’s left wrist with an injection and recommended surgery to remove the cyst on the right wrist.  He also stated: “Given the fact she has had long-term problems with both hands, I am not certain that long-term staying in a repetitive type job will be a successful strategy for her.”  (Employee’s Ex. J.)  The employer and insurer approved the recommended surgery, which Dr. Widstrom performed on December 12, 2013.  The employee reported pain relief after the surgery, but ongoing numbness around the scar.  Her left wrist symptoms increased.  On January 9, 2014, Dr. Widstrom recommended a repeat left wrist MRI scan and surgical repair of the triangular fibrocartilage tear on the left wrist and possible cyst removal.  The employee was released to return to work pending surgery with restrictions of no repetitive motion, lifting, pinching, or grasping with either arm.

The employer and insurer requested an independent medical evaluation with Dr. William Call on February 12, 2014.  Dr. Call concluded that there was no evidence of carpal tunnel syndrome in either arm or of a ganglion cyst in the left wrist.  He opined that the formation of ganglion cysts was not related to the repetitive nature of the employee’s job and that the job activities were not irritative, causal, exacerbatory, or contributory to the employee’s condition, and did not result in a temporary aggravation of an underlying condition.  He also stated that the triangular fibrocartilage tear in the employee’s left wrist was not related to her work activity, but was idiopathic and developmental, and that surgery on this condition was not reasonable and necessary.  (Employer and Insurer’s Ex. 1.)

On April 4, 2014, the employer and insurer filed a notice of intention to discontinue temporary total disability benefits, asserting a denial of primary liability based on Dr. Call’s opinion, and upon alternative bases that the employee had a temporary aggravation which had resolved or that she was 90 days post-maximum medical improvement.  On June 14, 2014, the employee filed a medical request for a repeat left wrist MRI scan and the recommended surgery.  The employer and insurer denied the request and filed a petition to discontinue benefits on July 10, 2014, and a rehabilitation request to discontinue rehabilitation benefits on July 15, 2014.  In an August 19, 2014, report, Dr. Widstrom again recommended a repeat MRI scan of the left wrist before surgery and also stated: “I do not believe the repetitive work caused the ganglion cyst, but certainly repetitive work could aggravate a pre-existing ganglion cyst.”  (Employee’s Ex. E.)

The compensation judge consolidated the pleadings in an August 1, 2014, order and a hearing was held on August 20, 2014.  At the hearing, the employee withdrew her request for approval of the recommended surgery, but continued to seek approval of the repeat MRI scan.  The employee relied on Dr. Widstrom’s opinion regarding causation.  The employee also argued that the employer and insurer should be equitably estopped from denying primary liability.  The judge stated:

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 employee first argues that the compensation judge erred by failing to address her argument that the employer and insurer should be equitably estopped from retroactively denying primary liability after approving the employee’s initial December 2013 surgery.  The doctrine of equitable estoppel may be invoked “to prevent a party from taking unconscionable advantage of [its] own wrong by asserting [its] strict legal rights.”  Northern Petrochemical Co. v. U.S. Fire Ins. Co., 277 N.W.2d 408, 410 (Minn. 1979).  Equitable estoppel applies in some circumstances where an employee has been prejudiced by an employer’s misrepresentation.  Lofgren v. Pieper Farms, 540 N.W.2d 834, 837, 53 W.C.D. 464, 467 (Minn. 1995); Neuberger v. Hennepin County Workhouse, 340 N.W.2d 330, 332, 36 W.C.D. 348, 360 (Minn. 1983); Sandnas v. Iron Range Lumber, Inc., 52 W.C.D. 392, 398 (W.C.C.A. 1994), summarily aff’d (Minn. Apr. 7, 1995).  Whether equitable estoppel should apply is generally a question for the fact finder.  See, e.g., O’Donnell v. Continental Casualty Co., 263 Minn. 326, 331, 116 N.W.2d 680, 684 (1962).

Estoppel does not apply to bar an employer and insurer which voluntarily paid benefits from changing their position and asserting defenses to continuing liability.  Kingbird v. Anderson Fabrics, 63 W.C.D. 237 (W.C.C.A. 2002), summarily aff’d (Minn. Mar. 27, 2003).  Generally, voluntary payment of benefits is not a waiver of an employer’s right to dispute entitlement to benefits at a later time.  Minn. Stat. § 176.221, subd. 1.  When an employer and insurer make voluntary payment of benefits, they may subsequently assert that payments were made under a mistake of law or fact.  Parker v. Univ. of Minn., 66 W.C.D. 373 (W.C.C.A. 2006), summarily aff’d (Minn. Oct. 2, 2006); Hoch v. Duluth Clinic, No. WC06-311 (W.C.C.A. Aug. 3, 2007).  “To hold otherwise would be to discourage voluntary payment of benefits.”  Enger v. General Sec. Servs., slip op. (W.C.C.A. July 28, 1998).

The employee argues that she attempted to return to work for the employer based on the employer’s approval of the December 2013 surgery, and claims that if the employer had denied liability before that surgery, she may have decided to stop working for the employer and her arm condition may have improved.  While equitable estoppel may apply in cases where primary liability was denied after surgery and the surgery resulted in increased disability or other prejudice, the employee’s claim of prejudice in this case is too speculative.  There is no medical evidence to support the employee’s assertion that if she had stopped working for the employer sooner, her arm condition would have improved.  Given the evidence in the record, the compensation judge did not err by failing to directly address the employee’s estoppel argument.  See Vukelich v. Potlatch Corp., 65 W.C.D. 309 (W.C.C.A. 2005) (compensation judge’s failure to directly address the employee’s estoppel argument for overpayment based on a mistake of law was not reversible error under the facts of the case).

The employee argues that substantial evidence does not support the compensation judge’s finding that the employee failed to meet her burden of proof that her work activity was a substantial contributing cause of her bilateral arm condition.  The employee has the obligation to prove entitlement to benefits by a preponderance of the evidence.  Minn. Stat. § 176.021, subds. 1 and 1a. 

The employee claims that the judge’s decision is based exclusively on Dr. Call’s opinion and that his opinion lacked foundation.  The employee’s foundation argument is based on her assertion that Dr. Call failed to personally review the employee’s MRI scans, his statement that the employee did not have left volar ulnar wrist pain until November 2013 even though the employee reported bilateral wrist pain in January 2013, and his allegedly inaccurate description of the employee’s work activities.  A purported lack of information, however, goes to the weight to be given to the medical opinion, not its foundation.  Schulenburg v. Corn Plus, 65 W.C.D. 237, 245 (W.C.C.A. 2005), summarily aff’d (Minn. May 25, 2005).   Dr. Call took the employee’s medical history, performed an evaluation, had x-rays taken of both wrists, and reviewed the employee’s medical records.  This level of knowledge is sufficient to afford foundation for a doctor to render an expert medical opinion.  See Caizzo v. McDonald’s, 65 W.C.D. 378, 382 (W.C.C.A. 2005).  This court will generally affirm a compensation judge’s findings of fact based on the choice between expert opinions, “so long as the accepted opinion has adequate foundation.”  Smith v. Quebecor Printing, Inc., 63 W.C.D. 566, 573 (W.C.C.A. 2003) (citing Nord v. City of Cook, 360 N.W.2d 337, 37 W.C.D. 364 (Minn. 1985)), summarily aff’d (Minn. Aug. 15, 2003).

In this case, the compensation judge specifically rejected Dr. Widstrom’s opinion.  The judge read Dr. Widstrom’s opinion as indicating that it was “only a possibility that repetitive work ‘could’ aggravate a pre-existing ganglion cyst” and concluded that this opinion did not meet the employee’s burden of proof.  (Memorandum at 6.)  The judge also noted that Dr. Widstrom did not give a causation opinion for the left wrist tear.  Substantial evidence supports the compensation judge’s finding that the employee failed to meet her burden of proof that her work activity was a substantial contributing cause of her bilateral arm condition.  Accordingly, we affirm.