The self-insured employer appeals the compensation judge’s denial of its motion for joinder and the judge’s findings that the employee’s 2004 and 2005 work injuries, incurred while he was in its employ, were substantial contributing factors to his current low back condition and need for treatment.  We affirm the denial of the employer’s motion for joinder, and affirm the findings that the 2004 and 2005 injuries substantially contributed to the employee’s current low back condition and need for treatment.

On November 4, 2004, and June 21, 2005, Joseph Johnson, the employee, sustained admitted neck and back injuries while working as a roofer for McDowall Companies, the employer, which was self-insured for workers’ compensation liability.  The employee’s 2004 injury occurred when a 40 to 60-pound roll of roofing materials thrown from a two-story roof hit the employee on the back of the head as he was bending forward slightly.  The employee was knocked down but did not lose consciousness.  He had immediate pain in the thoracic and cervical spine, was treated at the emergency room of the St. Cloud Hospital, and was diagnosed with a cervical strain.  He experienced neck pain for about a week after his injury and noticed low back pain within two weeks of the injury.  The employee, however, returned to work and did not undergo any further medical treatment for his injury at that time.

The employee’s 2005 work injury occurred when the employee twisted his back while stepping off a piece of heavy equipment he was operating at a job site, and experienced pain in his low thoracic and lumbar spine.  The employee was treated at Fairview Northland Hospital in Elk River, was assessed with thoracic strain and possible herniated disc in the lower thoracic region, and was assigned lifting and reaching restrictions for one week.  The employee also treated at Midwest Occupational Medicine and with a chiropractor through July 1, 2005, and reported much improvement in his symptoms.

The employee was not treated for low back injuries or symptoms from July 1, 2005, to August 12, 2009.  On March 17, 2006, and March 10, 2008, the employee underwent driver fitness physical evaluations.  He testified that he had ongoing low back problems at those times, but did not volunteer that information during the examinations because of his financial need to continue working.

The employee was laid off from his roofing position with the employer in November 2005.  He later worked as a truck driver for J.L. Jordan Trucking and for Watab Hauling; the employee testified that his back pain increased during this time.  In 2008, the employee began working full time for Matrix Manufacturing, as a machine operator.  The employee testified that his back pain continued to increase while he worked for that employer.

On August 12, 2009, the employee treated with Dr. Peter Jensen at Princeton Fairview Northland Clinic, reporting lumbar and thoracic pain; Dr. Jensen referred the employee for an orthopedic examination.  On August 24, 2009, the employee consulted Dr. David Edwards, orthopedist, reporting low back pain and right leg pain with numbness.  Dr. Edwards recommended an MRI.^[1]  The employee returned to the Princeton Clinic and treated with Dr. Jensen over the next several months.  Based on his review of the employee’s previous medical records, Dr. Jensen concluded that the 2004 and 2005 work injuries were substantial contributing factors in the employee’s current condition.

The employee again consulted Dr. Edwards in mid-January 2010, reporting continued low back pain and right leg pain.  Dr. Edwards again suggested an MRI scan and recommended a course of physical therapy.

On February 12, 2010, the employee filed a request for certification of dispute for approval of physical therapy and an MRI scan.  The Minnesota Department of Labor and Industry certified the dispute on February 12, 2010.  On March 4, 2010, the employee filed a medical request for payment of medical expenses as well as approval of physical therapy and the MRI scan.  In its medical response, the employer denied that the claimed medical treatment was causally related to the employee’s 2004 or 2005 work injuries.  The employee’s medical request was addressed at an administrative conference on April 29, 2010.  The employee later filed a request for formal hearing; the matter was scheduled for a hearing on August 4, 2010.

On June 24, 2010, the employee was evaluated by Dr. Randall Norgard at the employer’s request.  In a report also dated June 24, 2010, Dr. Norgard opined that the employee’s 2004 work injury was a cervical strain/sprain that had resolved within a few weeks after the injury, and that the employee’s 2005 work injury was a thoracolumbar sprain/strain that had resolved within four weeks.  Dr. Norgard also concluded that the employee’s condition and need for treatment were not causally related to his 2004 or 2005 work injuries, but instead were caused by his work for Matrix Manufacturing, where he had worked beginning in 2008.

On July 28, 2010, the employer filed a motion for joinder to add Matrix Manufacturing and its insurer to the proceedings, based on the causation opinion outlined by Dr. Norgard in his report.  In an August 3, 2010, order, a compensation judge denied the motion for joinder on grounds that it would delay the expedited hearing scheduled for August 4, 2010, and that joinder of the proposed parties was not necessary for resolution of the employee’s claims.

A hearing was held on August 4, 2010.  At the hearing, the employee testified that his back pain after his 2004 work injury had never resolved, but that he tolerated the pain.  He also testified his back and right leg pain never resolved after his 2005 injury, and his pain had continued to worsen over the years.  In her findings and order, served and filed August 19, 2010, the compensation judge found that the employee’s 2004 and 2005 work injuries were substantial contributing factors in his current low back condition and need for treatment.  The employer appeals these findings and the denial of its motion for joinder.

The employer argues the compensation judge erred by denying its motion to join Matrix Manufacturing (Matrix) and its insurer to the proceedings.  The employer bases its arguments on Dr. Norgard’s opinion, outlined in his June 24, 2010, report, that the employee’s condition and need for treatment were not causally related to the employee’s 2004 or 2005 work injuries, but were caused by his work for Matrix.  On July 28, 2010, the employer filed a motion for joinder to add Matrix and its insurer to the proceedings, arguing that they were necessary parties for full resolution of the matter; the employer attached Dr. Norgard’s report in support of its motion.  In an August 3, 2010, order, a compensation judge denied the motion for joinder on grounds that the proposed parties were not necessary for resolution of the employee’s claims, and that joinder of those parties would delay the hearing scheduled for August 4, 2010.  We agree.

The self-insured employer filed its motion for joinder on July 28, 2010.  The expedited hearing on the employee’s medical claims for admitted injuries was scheduled for August 4, 2010.  Granting the employer’s motion for joinder would have delayed the hearing.  Further, the employee sought to prove that the 2004 and 2006 work injuries were substantial contributing factors to his current condition and need for treatment, and was not obligated to pursue his claim against any other employer.  Birholz v. Wagner Spray Tech., No. WC05-301 (W.C.C.A. June 5, 2006).  The employer, in turn, was not prejudiced by the compensation judge’s denial of the motion for joinder since it may assert a contribution claim against Matrix and its insurer in a subsequent proceeding.  Id.; Romanick v. Crawford & Co., slip op. (W.C.C.A. Aug. 4, 1997) (denial of joinder not prejudicial to employer since action may be pursued through filing of petition for contribution and reimbursement).  The compensation judge did not err by denying the employer’s motion for joinder.  We therefore affirm the order denying that motion.

The employer appeals the compensation judge’s findings that the employee’s 2004 and 2005 work injuries were substantial contributing factors to his current low back condition and need for treatment.  Questions of medical causation fall within the province of the compensation judge.  Felton v. Anton Chevrolet, 513 N.W.2d 457, 50 W.C.D. 181 (Minn. 1994).  It is not necessary to show that an injury is the sole cause of disability, only that it is an appreciable or substantial contributing cause.  See Roman v. Minneapolis St. Ry. Co., 268 Minn. 367, 129 N.W.2d 550, 23 W.C.D. 573 (1964); see also Salmon v. Wheelbrator Frye, 409 N.W.2d 495, 497-98, 40 W.C.D. 117, 122 (Minn. 1987) (while an employee has the burden of proving that a work-related injury caused disability, it is not necessary that he or she show that the work-related injury was the sole cause, only that the injury was an appreciable or substantial contributing cause).

Dr. Jensen opined that the 2004 work injury was a substantial contributing factor in the employee’s current condition since the forced flexion caused by the impact to the back of his head, when a roll of rubber struck him from overhead, resulted in ligamentous and muscular damage that could cause chronic back stiffness, tightness, and pain.  Dr. Jensen also opined that the employee’s 2005 work injury caused further damage and exacerbation of his earlier ligamentous and soft tissue injuries.  In contrast, Dr. Norgard opined that the employee’s 2004 and 2005 work injuries had resolved within a few weeks after each injury and that the employee’s condition and need for treatment were not causally related to the employee’s 2004 or 2005 work injuries, but were caused by his work for Matrix Manufacturing.  It is the role of the compensation judge to consider the competing medical opinions and to determine which opinion is most persuasive.  A trier of fact's choice between experts is usually upheld 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, 342-43, 37 W.C.D. 364, 372-73 (Minn. 1985); Kelsey v. Lovegreen Indus. Servs, No. WC07-159 (W.C.C.A. Dec. 12, 2007).

In addition to medical opinions, a judge may base his or her conclusions on other reliable evidence in the record.  See Reimer v. Minnit Tool/M.I.T. Tool Corp., 520 N.W.2d 397, 51 W.C.D. 153 (Minn. 1994).  The judge’s decision was also based on the credibility of the employee’s testimony as to the continuity and gradual worsening of his symptoms from 2004 through 2009.  The employee is most familiar with the severity of his or her symptoms, and the judge did not err by relying on that testimony.  See Brening v. Roto-Press, Inc., 306 Minn. 562, 237 N.W.2d 383, 28 W.C.D. 225, (Minn. 1975).  Assessment of a witness's credibility is the unique function of the trier of fact.  See Brennan v. Joseph G. Brennan, M.D., 425 N.W.2d 837, 839‑40, 41 W.C.D. 79, 82 (Minn. 1988) (citing Spillman v. Morey Fish Co., 270 N.W.2d 781, 31 W.C.D. 187 (Minn. 1978)); see also Hatch v. Langhoff Enters., No. WC09-195 (W.C.C.A. Jan. 25, 2010).

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 (2010).  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).  In this case, the compensation judge’s factual conclusions concerning the employee’s 2004 and 2005 work injuries, and their role as contributing factors to the employee’s current condition, were not clearly erroneous as a matter of law given the evidence of record.  Substantial evidence, including expert medical opinion and the employee’s testimony, supports the compensation judge’s findings, and we therefore affirm.