At 1426 eastern time, a Cessna 210E was damaged on landing at Allegheny County Airport when one main landing gear leg would not fully extend. The pilot and passenger were not injured. The pilot said the left main landing gear would not extend into the down and locked position and he decided to land with the right main gear and nose gear down and the left main gear hanging. After touchdown, the airplane slid off the left side of the runway. Examination showed a crack in the left main landing gear saddle.