Dr. Joseph L.T. Appleton, the retired dean of the University of Pennsylvania School of Dental Medicine and a person who had been associated with the school for 66 years, died at the Bryn Mawr Convalescent Home. He was 91.   Dr. Appleton remained active at the school through last fall, working at least three days a week, a colleague said, but his health had kept him away from the office for several months.   The dental school is planning a memorial service, but a date has not been set.  Dr. Appleton received his doctorate degree in dental science from Penn in 1914 and immediately joined the dental school’s teaching staff.  During World War I, he served as the first lieutenant in the Army Dental Corps.  In 1941, Dr. Appleton was made dean of the dental school. At the same time, he was also serving as co-chairman of the Philadelphia Public Health Survey, vice president of the executive committee of the Philadelphia Mouth Hygiene Association, consultant to the Philadelphia Department of Public Instruction (school board), and chairman of the editorial board of Dentistry, a professional digest.    During WWII, Dr. Appleton was a consultant to the bureau of medicine and surgery for the Navy and a member of the scientific advisory board of the Army Institute of Pathology.    In 1951, his last year as dean,  he was named chairman of the U.S. Public Health Service.  In 1952, he retired as dean and returned to lecturing in the school of dental medicine as a full professor.  In 1958 he wa given a life membership in the Pennsylvania State Dental Society and was named dean emeritus.  He is survived by a son, Joseph Appleton 3rd, a daughter, Mrs. William Blackmore, and five grandchildren.