The employer and insurer appeal the compensation judge’s determination that medical treatment provided to the employee in 2006 and 2007 was causally related to the employee’s May 19, 1987, personal injury to the low back.  We affirm.

Edwin I. Skari, the employee, sustained an admitted injury to the low back on May 19, 1987, while working as a machinist for Fluidyne Engineering, the employer, then insured by TIG Insurance Company/REM.

The employee initially sought treatment from Dr. Richard Schoewe reporting right lower back pain radiating down the right leg.  In a follow-up visit, the doctor noted complaints of left low back and left leg pain.  The doctor prescribed medications and physical therapy.  In July 1987, Dr. Schoewe referred the employee to Dr. Richard Foreman, a neurologist.  Dr. Foreman reported the employee was totally deaf and communicated by sign language.  The doctor recorded a history of prior low back pain related to sustained postural positions.  Dr. Foreman ordered a CT scan that he stated suggested a disc abnormality at L4-5 extending to the right.  He recommended the employee avoid heavy work for the next month and provided work restrictions.  Thereafter, the employee continued to treat with Dr. Schoewe who diagnosed an L4-5 disc herniation.  In May 1988, the employee returned to Dr. Schoewe for follow-up.  The doctor referenced a herniated lumbar disc found on CT scan at L4-5 on the left, and reported the employee had pain on the left radiating into his left leg.  The doctor noted the employee had returned to his regular job as a machinist but was careful with lifting.  In 1989, the employee complained to Dr. Schoewe of right leg pain which the doctor related to the 1987 personal injury.

The employee returned to Dr. Foreman in 1995, who ordered a repeat MRI scan.  The May 1995 study showed disc degeneration at L2-3, a small anterior disc herniation at L3-4, and prominent anterior spur formation and a small left lateral disc herniation at L4-5.  In 1996, the employee was seen at the University of Minnesota orthopedic surgery clinic.  A history was obtained of chronic back pain since a work injury in the 1980s.  A diagnosis of degenerative lumbar spondylosis with spinal stenosis from L3 to S1 was made, and anti-inflammatory medication was prescribed.

The employee was seen by Dr. Paul Chlebeck in March 1998 complaining of left leg pain.  A CT scan in April 1998 showed moderate disc bulging at L3-4 and L4-5 without evidence of central or neural foraminal stenosis, and mild facet arthropathy at L4-5.  In August 2001, the employee returned to Dr. Foreman reporting a history of right leg problems for a number of years.  The doctor also noted a known diminished left knee reflex.  An MRI scan in September 2001 showed a right herniated disc and disc bulging with mild left neural foraminal stenosis at L3-4, and mild disc bulging with mild bilateral foraminal stenosis at L4-5.

The employee was referred by Dr. Chlebeck to the Twin Cities Spine Center where he was seen by Dr. Francis Denis in November 2001.  The employee’s chief complaints were back and right leg pain with occasional left-sided symptoms.  The employee noted a fall at work in 1987 with similar symptoms resulting in a permanent partial disability rating of 14%.  Following his examination, the doctor diagnosed lumbar degenerative disc disease with significant right L3-4 foraminal stenosis.  A selective nerve root block of the right L3 nerve root provided good, but temporary, relief of the employee’s symptoms.  Dr. Denis concluded the result confirmed the MRI findings, and in January 2002, performed a right L3-4 foraminotomy and discectomy.  By February 2002, the employee noted significant improvement in his pain symptoms and he was released to care as needed.

By a Temporary Order, served and filed June 19, 2002, and an Amended Temporary Order, served and filed July 26, 2002, the employee, the employer and insurer, and two additional insurers^[1] stipulated the employee became permanently and totally disabled on March 5, 2000.  In April 2005, the parties entered into a Stipulation for Settlement in which the employer and insurer acknowledged they had paid the employee for a 14% whole body disability secondary to an admitted personal injury on May 19, 1987.  The insurers further settled reimbursement claims among them for wage loss and medical benefits.

In March 2006, the employee was seen at St. John’s Hospital emergency room complaining of low back pain over the past several days exacerbated by bending and twisting.  The diagnosis was left lumbar strain.  In June 2006, the employee was seen by Dr. William Park at Summit Orthopedics at the request of Dr. Chlebeck.  The employee reported chronic, recurrent low back pain since his 1987 injury, increasing over the past month with left leg pain.  An MRI scan in September 2006 showed multilevel degenerative disc disease from L2-3 to L4-5, a left-sided disc protrusion at L4-5 impinging the left L4 nerve root and moderate facet arthropathy at L4-5 with mild to moderate narrowing of the neural foramina at L4-5.  The L4-5 disc protrusion was a new finding compared to the prior study of September 12, 2001.  Dr. Park referred the employee to his partner, Dr. John Dowdle.  The employee described pain in the low back radiating into the anterior thighs in both legs.  The doctor diagnosed mechanical low back pain and degenerative disc disease at L3-4 and L4-5.  Dr. Dowdle recommended an epidural injection at L3-4 and later recommended a medial branch block on the left from L3-4 through L5-S1.  The injections were administered by Dr. Lon Lutz at the Midway Pain Center in November and December 2006.

The employee returned to Dr. Denis in February 2007 complaining of left-sided symptoms.  The doctor noted this was a new symptom since 2002, but similar to what the employee previously had on the right side.  Dr. Denis diagnosed left L3-4 and L4-5 foraminal stenosis with herniation and recommended decompression surgery.

At the request of the employer and insurer, Dr. Paul Wicklund examined the employee in April 2002 and April 2003 and the doctor’s deposition was taken in May 2003.  Dr. Wicklund testified the employee’s symptoms in 1987 were due to an L4-5 disc disorder.  The doctor stated the April 1998 CT scan showed only a bulge at L3-4.  It was not until the September 2001 MRI scan, Dr. Wicklund testified, that an L3-4 disc herniation on the right was confirmed.  Accordingly, the doctor opined there was no correlation between the L4-5 disc problems and the treatment by Dr. Denis in 2001 and 2002 at the L3-4 level.  Subsequently, Dr. Wicklund was provided with additional medical records and the doctor prepared a report dated March 20, 2007.  Dr. Wicklund opined the additional medical treatment the employee received for his low back and left leg was not in any way related to the 1987 low back injury.  The doctor noted the employee’s complaints in June 1987 indicated right lower back pain radiating down the right leg with no evidence of any left-sided disc problems.  Accordingly, Dr. Wicklund concluded the employee’s current treatment was likely unrelated to the 1987 injury.

By report dated May 4, 2007, Dr. Denis stated it was his opinion that the employee’s present nerve root compression and foraminal stenosis at L3-4 and L4-5 were directly related to the 1987 personal injury.  The doctor opined the 1987 injury resulted in a permanent injury at L3-4 and L4-5 which was aggravated in 2002 resulting in a right L3-4 foraminotomy and was recently aggravated resulting in left-sided symptoms originating at L3-4 and L4-5 on the left.  Dr. Denis stated his review of Dr. Wicklund’s report did not change his opinion, and he maintained the 1987 injury was a substantial contributing factor in the employee’s present symptoms and his need for surgery.

The employee filed a claim petition seeking payment of various medical expenses.  Following a hearing, the compensation judge found the May 19, 1987, injury was a substantial contributing factor in the employee’s current symptoms and need for medical treatment.  The employer and insurer appeal.

The appellants argue the objective medical evidence does not support a conclusion that the employee injured the L3-4 level of his spine in 1987.  Rather, the appellants assert, the 1987 injury caused low back and left leg pain emanating from a left-sided L4-5 disc herniation that resolved by 1988, as opined by Dr. Wicklund and as documented by the April 3, 1998, CT scan which demonstrated only a diffuse bulging disc at L4-5.  Beginning in August 2001, the employee’s symptoms were low back and right leg pain.  These symptoms, the appellants contend, were attributable to a disc herniation at L3-4 as demonstrated on the September 2001 MRI scan that showed a right-sided herniated disc at L3-4.  Similarly, the appellants argue the employee’s current symptoms are also caused by an L3-4 disc herniation.  The employer and insurer assert the L3-4 herniation was a separate and distinct condition not caused by the 1987 injury.  The appellants contend, accordingly, that both the surgery in 2002 and the employee’s current need for medical treatment are unrelated to the work injury, and that the evidence does not support the compensation judge’s finding that the 1987 injury was a substantial contributing cause of the employee’s current need for medical treatment.  We are not persuaded.

This case involves, at its core, a disagreement between medical experts with respect to whether treatment provided to the employee in 2006 and 2007 was causally related to the employee’s work injury to the low back in May 1987.  As the trier of fact, it is the compensation judge’s responsibility, not this court’s, to weigh the evidence and chose between or among conflicting medical expert opinions.  See Nord v. City of Cook, 360 N.W.2d 337, 37 W.C.D. 364 (Minn. 1985).  In this case, the compensation judge specifically accepted the opinion of Dr. Denis over that of Dr. Wicklund.  Dr. Denis unequivocally related the employee’s current need for medical treatment and surgery to the 1987 injury.

The appellants, however, argue Dr. Denis apparently assumed that the insurer’s payment of benefits for the 2002 surgery at the L3-4 level was an admission of liability for that condition.  Further, the appellants assert Dr. Denis failed to explain how the 1987 injury caused an injury at L3-4.  The competence of a witness to render an expert opinion depends upon the witness’s scientific knowledge and the extent of the witness’s practical experience with the matter at issue.  Reinhardt v. Colton, 337 N.W.2d 88, 93 (Minn. 1983).  Dr. Denis was qualified, as an orthopedic surgeon, to provide a causation opinion.  The doctor had access to various medical records, including Dr. Wicklund’s report, and was familiar with the employee’s condition, having examined and provided treatment to the employee on multiple occasions.  An expert medical opinion does not lack foundation because the doctor did not explain the mechanism of the injury or the underlying reasons for his or her opinion.  All that is required, considering the facts of the case as a whole, is that a competent medical witness opined the injury causally contributed to the need for treatment.  Caizzo v. McDonald’s, 65 W.C.D. 378 (W.C.C.A. 2005).  Here, Dr. Denis clearly stated, in his opinion, the 1987 injury was a substantial contributing cause to the employee’s need for the treatment in question.  Nor do we believe Dr. Denis’s report requires the interpretation argued by the appellants.

The appellants further contend the compensation judge “did not address the various CT and MRI scans that were performed throughout the years showing substantial changes in the Employee’s condition beginning with the herniation at L4-5 (which resolved) and the development of a herniation at L3-4, for which surgery was performed in 2002.”  (Er Br. at 12.)       A compensation judge is not required to relate or discuss every piece of evidence introduced at the hearing.  Midtling v. Schwan’s Sales Enters., slip op. (W.C.C.A. Sept. 22, 2003); Braun v. St. John’s University, slip op. (W.C.C.A. July 20, 1992).  While the judge did not specifically discuss each CT and MRI scan, she was clearly aware of the studies.  We see nothing in the compensation judge’s decision requiring a reversal on this basis.

The critical issue before the compensation judge was medical causation, an issue almost always resolved by a judge’s choice between conflicting expert opinions.  See, e.g., Leschefske v. Lakeview Methodist HCC, No. WC05-289 (W.C.C.A. Apr. 28, 2006) citing Ruether v. State, Mankato State Univ., 455 N.W.2d 475, 478-79, 42 W.C.D. 1118 (Minn. 1990).  We conclude the compensation judge’s finding that the employee’s May 19, 1987, personal injury was a substantial contributing cause of his need for medical treatment to the low back in 2006 and 2007, is adequately supported by the record as a whole, and must, therefore, be affirmed.