The employee appeals the finding that his current disability is not causally related to his June 30, 1998, right knee injury.  We affirm.

Troy Parker, the employee, sustained an injury to his right knee on July 30, 1998, while working for the University of Minnesota, the self-insured employer.  The employer admitted liability for the employee’s personal injury.

The employee received significant treatment for his right knee prior to his personal injury.  In 1993, the employee slipped and fell at his sister’s house injuring his right knee.  The employee saw Dr. Paul Crowe, who performed an arthroscopy and excision partial synovectomy and plica of the right knee.  The employee followed with Dr. Crowe with persistent right knee symptoms.  An MRI scan in December 1993, showed a complex tear involving the mid- and posterior thirds of the medial meniscus with an associated medial meniscal cyst.  Dr. Crowe referred the employee to Dr. Hauck for a second opinion.  Dr. Hauck ordered an MRI scan, reviewed the scan and recommended a partial medial meniscectomy.  The employee returned to see Dr. Hauck in July 1994, complaining of worsening right knee pain with popping and catching.  Dr. Hauck again recommended an arthroscopic debridement to repair the complex posterior horn meniscus tear.  The employee did not proceed with the recommended surgery.

The employee was seen at HealthWorks on July 30, 1998, complaining of right knee pain.  On examination, Dr. James Anderson noted pain at the medial aspect of the right knee but noted the knee was stable and the ligaments attached.  The doctor diagnosed a strain of the medial collateral ligament and restricted the employee’s work activities.

The employee returned to see Dr. Crowe on August 4, 1998.  The doctor recommended an MRI scan that showed the preexisting tear through the posterior horn of the medial meniscus and a new, very small meniscal tear of the middle one-third of the medial meniscus.  Dr. Crowe then recommended surgery which he performed on September 28, 1998, consisting of an arthroscopy with a patellar shaving.  In his surgical note, Dr. Crowe stated the posterior horn tear appeared to be healed and was solid enough that no additional surgery was performed, but the doctor removed a large chondral fragment from the patella.  In November 1998, Dr. Crowe released the employee to return to work with restrictions.  In January 1999, the doctor opined the employee had reached maximum medical improvement and rated a one percent whole body disability.

On March 9, 2000, the employee saw Dr. Benjamin Gulli, a partner of Dr. Crowe, complaining of a sudden onset of right knee pain.  Dr. Gulli diagnosed an acute inflammation of the right knee and performed an aspiration which relieved the employee’s pain.  The employee returned to see Dr. Gulli in December 2000, complaining of increased pain and swelling in his right knee over the prior week.  An MRI scan in May 2000, showed a signal abnormality in the lateral meniscus, chondromalacia patella and a complex grade III tear of the posterior horn of the lateral meniscus which the radiologist stated was new when compared to the September 4, 1998, MRI scan.  Dr. Gulli recommended surgery and performed an arthroscopic partial medial meniscectomy in March 2001.  The doctor noted an extensive complex tearing of the posterior one-third of the medial meniscus.

Dr. Mark Gregerson examined the employee in January 2001, at the request of the employer.  The doctor diagnosed chondromalacia with possible medial meniscal tearing and recommended an arthroscopy.  In May 2001, Dr. Gregerson reviewed the surgical report and notes of Dr. Gulli and concluded the employee’s diagnosis was medial meniscal tear and grade II lesion of the patella.  The doctor stated that, since the September 1998 arthroscopy showed normal findings of the medial meniscus, the employee’s current knee symptoms and the need for the 2001 arthroscopy were not related to the employee’s personal injury.

Dr. Gulli reexamined the employee in June 2001, and released the employee to return to work, without restrictions, although the doctor felt the employee might have difficulty with his usual job.  The employee had no further treatment for his right knee until July 9, 2004, when he returned to see Dr. Gulli complaining of a gradual onset of right knee pain over the past year.  On examination, Dr. Gulli noted a large effusion with significant varus of the right knee.  The doctor diagnosed post-traumatic degenerative joint disease with significant varus.  Dr. Gulli injected the employee’s knee with Cortisone and prescribed an unloader brace.

Dr. Edward Szapalski examined the employee in November 2004 at the request of the employer.  The doctor diagnosed osteoarthritis in the medial compartment of the right knee which he opined was unrelated to the employee’s personal injury.  The doctor based his opinion on the fact that Dr. Crowe noted no meniscal pathology at the time of the 1998 arthroscopic evaluation.  Accordingly, Dr. Szapalski concluded the employee’s meniscal tear developed sometime thereafter.  The doctor opined the July 30, 1998, personal injury was primarily a contusion of the right knee which caused no injury to the medial meniscus or the medial articular surface and was a temporary injury.  Finally, Dr. Szapalski opined the employee reached maximum medical improvement (MMI) from the July 1998 injury by December 1, 1998, and stated any treatment rendered thereafter would not have been related to that injury.

By report dated July 8, 2005, Dr. Gulli stated his diagnosis of the employee’s condition was post-traumatic degenerative joint disease as a result of a medial meniscal tear.  He recommended a surgical procedure, a high tibial osteotomy which, the doctor stated, would provide ten years of relief for the employee at which point knee replacement surgery would be necessary.  Dr. Gulli opined the July 30, 1998, personal injury was a substantial contributing cause for the arthroscopic procedure and the need for an osteotomy and knee replacement surgery.  Dr. Gulli based this opinion on the fact the employee had a clearly abnormal meniscus at the time of Dr. Crowe’s arthroscopy in 1998, that subsequently became more damaged leading to the need for the 2001 surgery.  Since the employee reported no subsequent injury, Dr. Gulli stated the 1998 injury caused partial tearing and attenuation of the meniscus which lead to a degenerative process, arthritis and further tearing of the meniscus.  The doctor concluded the employee was restricted to sedentary work.

The employee filed a claim petition and medical request seeking total disability benefits from July 2004 through August 2005, a rehabilitation consultation and payment of medical expenses.  Following a hearing, the compensation judge denied the employee’s claims finding the employee did not suffer a meniscal injury on July 30, 1998, and that injury was not a substantial contributing cause of the employee’s osteoarthritis or need for medical treatment.  The employee appeals.

The employee first argues the self-insured employer paid the employee’s workers’ compensation benefits following his personal injury through the 2001 surgery, including permanent partial disability benefits.  Accordingly, the employee apparently argues the employer is now estopped from denying liability for his current wage loss and medical expenses.  We disagree.

Prior to the current hearing, the employer voluntarily paid benefits to the employee.  But an admission of liability and payment of benefits does not obligate the employer to pay future benefits.  Rather, it remains the obligation of the employee to prove entitlement to benefits by a preponderance of the evidence.  Minn. Stat. § 176.021, subd. 1a.  Further, this court has on many occasions held that the doctrine of estoppel does not apply to bar an employer that voluntarily paid benefits from changing its position and asserting defenses to continuing liability.  Kingbird v. Anderson Fabrics, 63 W.C.D. 337 (W.C.C.A. 2002).  To hold otherwise would discourage voluntary payment of workers’ compensation benefits.  Enger v. General Sec. Servs., slip op. (W.C.C.A. July 28, 1998).

The employee next argues it is undisputed that his osteoarthritis is caused by the narrowing of the medial joint.  This condition, the employee contends, was the result of the 2001 surgery for which the employer accepted liability.  Accordingly, the employee argues the compensation judge erred in accepting the opinions of Dr. Szapalski and denying the employee’s claims for benefits.  We are not persuaded.

Dr. Szapalski maintained that the employee’s personal injury resulted in a contusion of the patella which showed up as bone marrow changes on the MRI scan and the fissures in the patella which Dr. Crowe repaired.  In the 1998 arthroscopic surgery, Dr. Crowe found a healed meniscal tear which, Dr. Szapalski stated, dated back to 1993 as demonstrated on the scan.  Dr. Szapalski testified the 1998 personal injury was not a substantial contributing cause of the need for the 2001 surgery.  Finally, Dr. Szapalski stated the employee’s osteoarthritis is in the medial compartment of the knee which was not injured in 1998.  Rather, the osteoarthritis resulted from the meniscal degeneration and removal, which process predated the employee’s personal injury. 

Certainly, there is evidence which, if accepted by the compensation judge, would warrant an award of benefits in this case.  The compensation judge, however, rejected the opinions of Dr. Gulli and accepted those of Dr. Szapalski.  It is the responsibility of the compensation judge, as trier of fact, to resolve conflicts in expert testimony.  See Nord v. City of Cook, 360 N.W.2d 337, 342, 37 W.C.D. 364, 371 (Minn. 1985).  The opinions of Dr. Szapalski are well founded and the compensation judge could properly rely upon them.  The opinions of Dr. Szapalski constitute substantial evidence in the case.  The decision of the compensation judge must, therefore, be affirmed.  Hengemuhle v. Long Prairie Jaycees, 358 N.W.2d 54, 59, 37 W.C.D. 235, 239 (Minn. 1984).