Dr. Johnson joined the College of Dentistry in 1991 after eight years at the University of Nebraska Medical Center College of Dentistry where he served as chair of the Department of Endodontics from 1989 to 1991. In 1991, he returned to the University of Iowa and directed the Advanced Education Program in Endodontics from 1991-1997. In 2003, he was appointed the Departmental Executive Office for Endodontics, serving until he initiated phased retirement in 2014. Dr. Johnson was appointed the first Richard E. Walton Professor of Endodontics in 2007.
He served as chair of the Endodontic section of the American Association of Dental Schools in 1992 and subsequently served as counselor to the Endodontic Section of the American Association of Dental Schools in 1992, and then served as counselor to the Endodontic Section of the American Dental Education Association. He was chair of the Educational Affairs Committee of the American Association of Endodontists (AAE) in 1994 and served on the Association's Ad Hoc Committee to Address Endodontic Nomenclature. Dr. Johnson also served on the American Association of Endodontists Task Force to Revise the Standards for Advanced Education. In 1994, he was elected by the American Association of Endodontists to serve six years as a director of the American Board of Endodontics. He served as secretary of the board for two terms in 1997 and 1998. He was also president of the board for two terms in 1999 and 2000. In 2001, Dr. Johnson was elected to a three-year term on the Board of Directors of the American Association of Endodontists representing 21 Midwestern and Western states. He served as board liaison to the Research and Scientific Affairs Committee and the Committee to Address Endodontic Nomenclature. In addition, Dr. Johnson served as the vice-program chair for the American Association of Endodontists Annual Session Committee in 2003, program chair in 2004, and general chair in 2005. Dr. Johnson served on numerous AAE committees, chairing the AAE Educational Affairs Committee as well as the AAE Continuing Education Committee. In 2010, Dr. Johnson was elected to serve as vice president of the American Association of Endodontics. He served the following year as president-elect and was president of the AAE in 2012. In 2017, for a second year, Dr. Johnson was appointed chair of the AAE Special Committee on Endodontic Education, which is charged with enhancing the quality of patient care and defining the national competency standards of endodontic care that is delivered by general dentists and specialists. Dr. Johnson served as an endodontic consultant to the ADA's Council on Materials, Instruments and Equipment from 1991 to 1995. He was an endodontic consultant to the Commission on Dental Accreditation of the ADA from 1994-2000. In 2004, he was appointed to serve a three-year term on the Endodontic Advisory Committee for the Commission. He is currently serving a second term, having been reappointed in 2016.
Dr. Johnson is editor of the endodontic textbook, Color Atlas of Endodontics. He was the senior author for the "Obturation" chapter for the 9th, 10th, and 11th editions of Pathways of the Pulp and is the senior author for the "Access" chapter (4th and 5th editions) and the "Instrumentation" chapters for Principles and Practice of Endodontics (4th edition).
After serving as a dental officer in the United States Army at Fort Sill from 1975 to 1977, Dr. Johnson was an active member of the 294th Medical Detachment of the Iowa Army National Guard from 1977 to 1995. In 1993, Dr. Johnson was awarded the "A" Proficiency Designator, the highest award for professional achievement given by the Office of the Surgeon General, United States Army. He has received the Army Commendation Medal with an oak leaf cluster, the Army Achievement Medal, an Overseas Service Medal, a National Defense Medal, and Army Service Medal with two oak leaf clusters, and a State Service Medal with two oak leaf clusters. In 1995, Dr. Johnson retired from the Iowa Army National Guard with the rank of Brigadier General.
Dr. Johnson directs the preclinical sophomore endodontics courses laboratory and in the junior endodontic clerkship, the senior Family Dentistry curriculum, and the Advanced Education Program in Endodontics.
He participates in the intramural dental practice in the Department of Endodontics.
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Dr. Johnson has served as chair and councilor of the Endodontic Section of the American Dental Education Association (ADEA). He completed four years of service to the American Association of Endodontists (AAE) as a representative on the Council of Hospitals and Advanced Education Programs of ADEA. Dr. Johnson served as a member of the Endodontic Review Commission of ADEA's Commission on Dental Accreditation and completed his term in 2008. Dr. Johnson has served as vice president and president-elect of the American Association of Endodontists and in April 2011, Dr. Johnson was elected president of the American Association of Endodontists.