On September 18, 2010, approximately 1145 central daylight time, a Cessna 320, N56VP, registered to and operated by the Association of Orthopedic Services PA, Grand Prairie, Texas, was substantially damaged when the nose gear folded on landing at Denton Municipal Airport (DTO), Denton, Texas. Visual meteorological conditions (VMC) prevailed at the time of the accident. The instructional flight was being conducted under the provisions of 14 Code of Federal Regulations (CFR) Part 91 without a flight plan. The instructor and pilot on board the airplane were not injured. The local flight originated at Grand Prairie Municipal Airport (GPM) approximately 0930. According to the flight instructor's accident report, the purpose of the flight was to administer a flight review and insurance endorsement to the pilot. After making 4 touch-and-go landings at Decatur (LUD), Texas, Municipal Airport, the flight proceeded to Decatur's Bishop Field (76T) to make a soft field landing on the grass runway. When the landing gear was extended, the GEAR DOWN AND LOCKED light did not illuminate. The annunciator lights were checked (press-to-test) and operated normally. Checking the convex mirrors on the engine nacelles confirmed the nose gear was only partially extended. The landing gear was cycled several times and manually extended at least 3 times. "G snaps" were made but to no avail. The instructor landed the airplane on DTO's runway 17 and the nose gear folded. Post-accident inspection revealed the push-pull rod that goes through the main spar and connects the gear transmission to the nose gear had separated, damaging the spar and several fuselage ribs. The Cessna 320 Maintenance Manual requires that the main and nose gear drive tubes and idler mechanism, in addition to rigging and clearance, be checked every 100 hours or 12 months, whichever comes first.