The employer, Northwest Swiss Matic, and its insurer appeal from the compensation judge’s apportionment of liability between two admitted dates of injury.  We affirm.

The employee, Christopher J. Campa, began working for Alliant Tech Systems in 1984.  In 1994, he became a machinist’s helper and in 1996 he was promoted to machinist, working four ten-hour shifts per week.  This work involved lifting heavy bar stock and loading it into the machines, setting up and adjusting tools, weighing and inspecting parts, and doing clean up work.  Loading the bar stock required placing bars into spindles which held the stock while it was being machined, and which were closed by means of collets.  The employee testified that the collets often needed to be hammered shut.  He frequently ran more than one machine at a time, each of which held six to eight bars, so that he would be loading or unloading another machine while one was running.

On December 19, 1994, the employee was seen at the New Hope Medical Clinic, complaining of two or three days of right arm and shoulder pain, not associated with any known injury.  He was diagnosed with muscle spasm and tendinitis, given medications, and advised to return if his symptoms failed to improve.  He next sought treatment for right shoulder symptoms on June 1, 1998, at the New Hope Medical Clinic, relating his symptoms to his work as a machinist.  On examination, he had diminished range of motion in the right shoulder and pain over the AC joint with palpation.  He was diagnosed with right shoulder pain and AC joint inflammation and was treated with a corticosteroid injection.

The employee testified that his symptoms on both occasions resolved quickly, returning to a level of 0 on a scale of 1 to 10.  He had no ongoing problems with either shoulder until April 2000, when a gradual onset of right shoulder symptoms, accompanied with pain radiating down the right arm, led him to seek treatment again at the New Hope Medical Clinic on April 28, 2000.  The employee testified that his pain was then at a level of 8 out of 10.  On examination, the doctor noted reduced range of motion with crepitation.  The employee was diagnosed with rotator cuff tendonitis/impingement and a suspected rotator cuff tear.  He was given ibuprofen and Darvocet, placed under restrictions for the next four weeks, and referred for physical therapy.

Alliant and its insurer admitted a work injury, for which the parties have used a date of April 27, 2000.  The employee did not miss time from work, as the employer accommodated his restrictions.

The employee’s symptoms improved during a four-week program of physical therapy, but increased again in June 2000 while he was playing softball.  When he returned to the New Hope Medical Clinic on June 8, 2000, he continued to show a diminished range of motion on examination.  Physical therapy was continued and the employee was referred for an MRI scan of the right shoulder.  The MRI was performed on June 14, 2000, and was read as showing mild tendinosis or a partial-thickness tear of the posterior aspect of the supraspinatus muscle.  Mild degenerative change was noted at the acromioclavicular joint.  No full-thickness tears were identified. The employee returned to the New Hope Medical Clinic on October 10, 2000, and continued to complain that he had shoulder pain while at work.  He was referred to an orthopaedist.

The employee was seen by Dr. Joseph Flake at Orthopaedic Partners on October 16, 2000.  He told the doctor that his symptoms had improved with physical therapy and medication, but he was still having pain with range of motion at or above shoulder height.  Tenderness and slight swelling were present at the AC joint with some subacromial tenderness.  Dr. Flake reviewed the MRI report and found evidence for a rotator cuff tear.  He thought there might be a partial tear with mild   tendinosis, as well as degenerative changes at the AC joint.  Dr. Flake concluded the employee was doing very well and no different form of treatment was needed.  He did not believe surgery was appropriate.  Dr. Flake advised the employee to return for reevaluation if his symptoms increased or recurred.  The employee was released to work without restrictions.

On April 4, 2001, Dr. Flake signed a health care provider report which stated that the employee had reached maximum medical improvement with no permanent partial disability and no need for work restrictions.

The employee testified that he received no significant further treatment for about the next five years.  He was able to perform his usual job for the employer without formal accommodations to his injury, although he testified that he tried to do as much of the work as possible with his left hand, and he favored his right shoulder.  He further testified that he was able to continue to engage in bowling, softball, fishing, pool and golf, except that he began to throw the softball with a sidearm motion rather than overhand. The medical records show only a single treatment for right shoulder pain at the New Hope Medical Clinic on March 1, 2004.  The employee was given medication, advised to use ice and heat, and to return on an as-needed basis.  According to the employee’s testimony, he continued to have ongoing pain at the top of his shoulder which he rated at a 2 on the 1 to 10 scale.  He continued to work at Alliant with no lost time due to his shoulder through June 2004, when his position with that employer ended in a layoff because the employer relocated to another state.

The employee found another machinist job operating the same kind of machines for employer Northwest Swiss Matic, which he started around October 2004.  He testified that his shoulder pain was still at a pain level of 2 out of 10. The employee told Northwest about his prior right shoulder work injury and he underwent and passed a pre-employment physical.  The employee’s duties at Northwest were virtually identical to those he performed at Alliant, except that the machines used levers rather than collets, which required pulling back and pushing forward to secure the bar stock and which was sometimes difficult.  The employee testified that he at first experienced no difficulties in the work at Northwest.  However, after about six months he began to notice increased pain in his shoulder, particularly when carrying buckets full of heavy metal parts.  By June 2005, the pain had reached a level of 6 or 7 out of 10.

Despite the increased pain, the employee continued working.  On December 2, 2005, while pushing forcefully on a jammed lever on a machine, the employee experienced a sudden, severe onset of pain at the top of his shoulder, which he characterized as the worst pain he had ever experienced in his shoulder.  He rated it as a "10+" on a scale of 1 to 10. He stopped work, reported an injury to the employer, and left work.

The employee was seen at the New Hope Medical Clinic the same day.  He gave a history of right shoulder pain over several years associated with a workers' compensation related injury, and noted that he had previously been told he might have a partial rotator cuff tear.  He reported that the pain had recently increased.  His doctor diagnosed right shoulder bursitis and overuse syndrome. He advised the employee to take ibuprofen, avoid repetitive shoulder motion, and recommended physical therapy.  The employee was advised to restrict activities involving repetitive use of the right shoulder and to avoid pushing and jerking for the next three weeks.

The employee returned to work the next day and continued to perform his machinist job with Northwest Swiss Matic over the next several months.

The employee underwent a course of physical therapy from December 2, 2005, to January 9, 2006.  The records show no treatment between January and May 2006, and the employee testified that physical therapy had reduced his symptoms, although they did not fully resolve.  He did not treat further for his shoulder between mid-January and May 2006.  However, his symptoms again began to worsen in the spring of 2006, returning to what the employee characterized as a constant pain level of 5 or 6 on a 10-point scale.  His family physician referred him to an orthopaedist, Dr. Paul Crowe.

Dr. Crowe saw the employee on May 23, 2006.  On examination, Dr. Crowe found subacromial impingement particularly with flexion, but abduction was more comfortable for the employee and there was good shoulder stability and strength.  The doctor thought a rotator cuff tear unlikely, but recommended an MRI scan be performed to rule out that possibility.

The employee was seen at the Park Nicollet Clinic on June 8, 2006.  He reported a new right anterolateral shoulder pain that he had not experienced before, which was made worse with heavy lifting of objects.  On examination there was moderate tenderness in the bicipital tendon region, moderate tenderness in the subacromial region, and tenderness in the posterior shoulder region.  The diagnosis was of a right shoulder rotator cuff injury.

An MRI scan performed at Park Nicollet Clinic on June 16, 2006, was interpreted as showing infraspinatus tendinosis and possible intrasubstance partial thickness tear, a possible posterosuperior labral tear, and mild to moderate degenerative changes at the acromioclavicular joint.

The employee was seen on June 19, 2006, by Dr. Michael Walsh at the Tria Orthopaedic Center.  He described pain radiating from the right shoulder to the elbow which he blamed on overhead use of the arm at work.  Dr. Walsh diagnosed a partial-thickness rotator cuff tear of the right shoulder and told the employee that this was probably due to his job.  Dr. Walsh did not recommend surgery.  He gave the employee a subacromial injection and recommended further physical therapy.

The employee was seen in follow-up at the Park Nicollet Clinic on July 6, 2006.  He reported that the local injection had provided a few days of relief but that his pain had returned.  He was having difficulty at work with heavy lifting, setting up his machines, and doing repetitious rotation movements.

The employee returned to Dr. Crowe on July 10, 2006.  Dr. Crowe recommended a right shoulder arthroscopic acromioplasty.  Surgery was initially denied by Northwest Swiss Matic’s insurer pending a medical assessment scheduled with Dr. Michael D’Amato.

Dr. D’Amato saw the employee on September 22, 2006.  In his report, he offered the opinion that the injury of December 2, 2005, constituted only a temporary aggravation of the employee’s preexisting condition.  He concluded that the employee had reached maximum medical improvement from this temporary aggravation within six to 12 weeks from the initiation of treatment.  While he agreed that surgical treatment was appropriate for the employee’s current condition, he considered the need for surgery to relate solely to a natural progression of the employee’s condition following the April 27, 2000, work injury.

The employee was laid off by Northwest Swiss Matic for economic reasons in October, 2006.  He described his right shoulder pain and symptoms at that time as remaining at an 8 out of 10.

Alliant and its insurer agreed to payment of the proposed surgery, and of temporary total disability benefits thereafter, under a temporary order.  On April 5, 2007, the employee underwent right shoulder surgery conducted by Dr. Crowe.  A diagnostic arthroscopy of the glenohumeral joint showed no rotator cuff tear.  Dr. Crowe then went on to perform a right shoulder acromioplasty and distal clavicle excision to treat the employee’s right shoulder impingement syndrome.  The parties agree that the surgical procedure left the employee with a three percent whole-body permanent partial disability rating.

The employee continued to treat with Dr. Crowe in follow-up.  Permanent restrictions were imposed. He also underwent post-surgical physical therapy.  By the time of the hearing, the employee was no longer receiving ongoing medical treatment for his shoulder.  The employee testified that his right shoulder condition was improved following surgery, though it never diminished to the level it was at prior to the 2005 work injury.  He testified that he no longer can play softball or bowl, that he has had to modify how he plays golf, and that he finds certain shots at pool hurt his shoulder.  He began working for a new employer in an inspection job about January 2008.   At the hearing, he characterized his right shoulder pain level subsequent to recovery from surgery at a constant 4 to 6 out of 10.

The employee was examined by Dr. Loren Vorlicky on September 18, 2007, on behalf of employer Alliant and its insurer.  Dr. Vorlicky rated the employee with a three percent permanent partial disability based on the distal clavicle excision, pursuant to Minn.R. 5223.0450, subp. 2C.  He offered the opinion that the work injury of April 27, 2000, was merely a temporary exacerbation of a pre-existing condition which had largely resolved by October, 2000.  In his view, the December 2005 work injury constituted a permanent aggravation of the employee’s pre-existing condition and was the substantial contributing cause of the employee’s disability and subsequent need for surgery.

In a letter dated March 5, 2008, Dr. Vorlicky responded to a hypothetical situation posed by Alliant’s attorneys.  He was asked to provide an apportionment opinion predicated on the possibility that a compensation judge might find that both the 2000 and 2005 injuries had substantially contributed to the employee’s ongoing right shoulder condition and surgical treatment.  Dr. Vorlicky responded by reiterating his view that the employee’s 2000 work injury at Alliant was not a substantial contributing cause of his ongoing right shoulder condition and surgical treatment.  However, based on the hypothetical, he apportioned causation at 20 percent to the 2000 work injury and 80 percent to the 2005 work injury.

In a letter dated May 1, 2008, Dr. D’Amato similarly responded to the same hypothetical situation.  Dr. D’Amato also restated his view that the employee’s work activities with Northwest Swiss Matic were not a substantial contributing cause of his ongoing right shoulder condition and surgical treatment.  However, based on the hypothetical, he apportioned causation at 45 percent to the employee’s pre-existing condition predating the 2000 work injury, 45 percent to the 2000 work injury, and 10 percent to the 2005 work injury.

On June 25, 2008, Dr. Crowe, the employee’s treating surgeon, offered his opinion with respect to apportionment.  He considered the employee’s disability and surgery were best apportioned half to each of the 2000 and 2005 work injuries.

A hearing was held before a compensation judge on July 18, 2008, to determine whether the 2005 work injury was permanent or temporary in nature, and what apportionment of liability between the 2000 and 2005 injuries should be applied to the employee’s disability and cost of treatment.  Following the hearing, the judge found that the 2005 work injury was permanent in nature.  He apportioned the employee’s permanent partial disability entirely to the 2005 injury, and apportioned the remaining disability and expenses 10 percent to the work injury of 2000 and 90 percent to that of 2005.  Northwest Swiss Matic and its insurer appeal.

On appeal, the appellants contend that the injury of December 2005 was merely temporary in nature and argue that the compensation judge’s finding to the contrary is unsupported by substantial evidence.  Specifically, the appellants point out that the MRI scans performed before and after the 2005 work injury showed similar findings, and failed to show any acute injury or new process had taken place.  They further cite to the employee’s initial improvement following physical therapy in January 2006, at which time the employee had good range of motion and diminished pain. Finally, they note that the employee was able to continue working in his machinist job following the 2005 injury.  Accordingly, they argue, the employee was “back to normal” from the 2005 work injury by early January 2006.  The employee’s worsening of symptoms in spring of 2006, and subsequent surgery, was due only to a natural progression of his condition as it existed prior to the 2005 work injury, based on the opinion of Dr. D’Amato.

The compensation judge expressly adopted the contrary opinion of Dr. Crowe, the employee’s treating surgeon, who considered the 2005 work injury to have been a permanent aggravation of the employee’s pre-existing right shoulder condition and a causal factor leading to his need for surgery.  In so doing, the compensation judge noted that he had given significant weight to the employee’s testimony that the 2005 injury resulted in a dramatic increase in his pain, which never returned to its pre-December 2, 2005, levels.

It is the role of the compensation judge to resolve any differences or conflicts between the medical experts.  Generally, this court must uphold findings where primarily based on a compensation judge’s choice between conflicting expert opinion, unless the facts assumed by the expert on whom the judge relies are not supported by the evidence.  Nord v. City of Cook, 360 N.W.2d 337, 37 W.C.D. 364 (Minn. 1985).  The appellants have not shown any defect in the foundation for Dr. Crowe’s opinion.  We affirm the finding of a permanent injury.

The appellants also appeal the compensation judge’s determination of apportionment of liability.  Equitable apportionment is not a finding based on a precise formula but instead is based on all the facts and circumstances of a case.  Factors considered in determining apportionment include the nature and severity of the initial injury, the employee’s physical symptoms following the initial injury and before the second injury, the nature and severity of the second injury, and the period of time between the injuries.  Goetz v. Bulk Commodity Carriers, 303 Minn. 197, 226 N.W.2d 888, 27 W.C.D. 797, (1975); Harvala v. Noeske Lumber, 44 W.C.D. 118, 126 (W.C.C.A. 1990).

The compensation judge explained his reasoning on the apportionment issues in detail in his memorandum.  He gave great weight to the employee’s testimony regarding how his symptoms and pain level changed after each injury, and about what changes he made in his work and recreational activities.  The employee testified that the work injury in 2000 resulted at first in pain which he rated at an eight on a scale of one to ten, but that after a few months’ conservative treatment it had diminished to a level of two, remaining at that level until after the employee had been working for several months at Northwest Swiss Matic.  The employee characterized his pain level associated with the 2005 work injury as a 10-plus.  While it fluctuated during treatment, he testified that it never returned to the level it was before the 2005 injury; even after the employee’s surgery, his pain had diminished only to a constant four to six.  The compensation judge also considered it significant that the employee had been able to continue in his machinist job without formal restrictions or ongoing medical treatment for almost five years following the injury in 2000, while the 2005 work injury was followed by his surgery and the imposition of permanent restrictions.  On consideration of these factors, the judge determined on an equitable apportionment between the work injuries in the ratio of 10 percent to the 2000 work injury and 90 percent to the 2005 work injury.

In placing the entire liability for the employee’s three percent permanent partial disability on the 2005 work injury, the compensation judge relied on the same factors, and further concluded that the medical records and the employee’s testimony failed to show a significant functional loss prior to the 2005 injury.

In contesting the apportionment, the appellants rely initially on their argument that the 2005 work injury was merely a temporary aggravation of the employee’s pre-existing right shoulder condition and that it resolved by mid-January 2006, characterizing the employee’s subsequent symptoms and treatment from May 2006, through and after the surgery, as the result of the same “underlying pathological process” that was present in the employee’s right shoulder prior to the 2005 injury.  This argument is consistent with the opinions of their medical expert, Dr. D’Amato.  As we have already noted, however, the compensation judge adopted the contrary opinion of Dr. Crowe that the 2005 work injury was permanent in nature and that it was a substantial contributing factor in the employee’s subsequent disability and treatment, including the surgery.  We have already affirmed the judge’s choice between these medical opinions, and as a result, those of the appellants’ arguments against apportionment that are based on an assertion that the 2005 injury resolved in January 2006, require no further review.

The appellants next argue that, even if the 2005 work injury did contribute to the employee’s ongoing disability and treatment after May, 2005, the factors cited by the compensation judge do not support a 90/10 apportionment.  They contend that each of these factors was either similar following each of the two injuries, or showed less disabling effect after the 2005 injury than after the 2000 injury.  Specifically, they point out that the employee’s restrictions immediately after the 2005 work injury were of short duration, similar to the situation immediately after the 2000 work injury; that the employee did not miss a significant amount of work immediately following either injury, that the employee’s ongoing treatment after the 2005 work injury consisted only of a few weeks of physical therapy, less than the treatment following the 2000 work injury; and that the MRI scans after each injury showed essentially the same pathology.  This argument, however, considers each factor only during the initial weeks after the injuries, and presumes no causal link between the 2005 injury and the employee’s condition after May 2006.  This merely restates the “temporary injury” argument in a new guise.  As we have noted before, the compensation judge found that the resumed treatment in May 2006, leading to the surgery and permanent restrictions, was causally related to the 2005 injury.

The appellants next object to the extent of the compensation judge’s reliance on the employee’s testimony about his pain complaints, arguing that these should not have been given much weight as they were merely subjective.  We do not find this argument persuasive in the present context.  The employee’s testimony about his symptoms was a reasonable factor to consider on this issue.  The comparative weight to be given to it was a matter committed to the compensation judge.

In considering the apportionment opinions of Dr. D’Amato and Dr. Vorlicky, the compensation judge observed in his memorandum that neither believed any apportionment to be appropriate.  Dr. D’Amato considered the 2000 work injury and the employee’s prior shoulder condition entirely responsible for the employee’s disability and surgery, while Dr. Vorlicky placed all responsibility on the 2005 injury.  Both doctors did provide a hypothetical apportionment ratio based on the possibility that a compensation judge might find that both injuries were substantial contributing factors.  The compensation judge characterized his rejection of their opinions because they were not supported by an adequate foundation.  The appellants argue that this is technically incorrect, as the hypothetical situation on which the apportionment ratios were provided became consistent with the facts of the case when the judge found both injuries to be contributing factors.  While this is technically true, we think that the judge simply meant that he had not given the apportionment ratios offered by these physicians much weight, in light of their strongly expressed contrary medical opinions.

"Apportionment is a question of fact for the finder of fact, and the compensation judge is not bound by medical opinions with regard to apportionment."  Petersen v. Bethany Samaritan Heights, slip op. at 5 (W.C.C.A. July 16, 1992).  The compensation judge was not required to accept either of these opinions, or indeed any medical opinion on apportionment.  The use of the term “foundation” seems to us more a choice of terminology than an indication of an error of law.

The compensation judge's determination of equitable apportionment should be affirmed where it is supported by substantial evidence in view of the entire record as submitted.  Sundquist v. Kaiser Eng'rs, Inc., 456 N.W.2d 86, 88, 42 W.C.D. 1101, 1103 (Minn. 1990); DeNardo v. Divine Redeemer Mem. Hosp., 450 N.W.2d 290, 293, 42 W.C.D. 626, 629 (Minn. 1990).  While the evidence in this case could have supported a range of different apportionment ratios, depending on the weight given to the individual factors, we conclude that it provides substantial support to the determination made by the compensation judge, and affirm.