The student made a normal landing but as the aircraft slowed, the instructor noted the right wing was sinking and he assumed control of the aircraft. He held the wing up as long as he could and kept the aircraft straight without braking. The aircraft slowly veered to the right and came to rest off the sealed runway surface. The right hand landing gear wheel assembly had separated from the aircraft about thirteen metres after touchdown, with the result that the right hand flap, torque tube, aileron and stub of the landing gear cylinder were damaged by abrasion with the runway surface. The right hand landing gear strut had separated from the cylinder due to one of the two torque link securing lugs failing from fatigue, and the torque link retaining bolt failed due to overload. The main landing gear cylinder torque link lugs are subject to a visual inspection every 100 hours by aircraft maintenance engineers, but there is no existing requirement to inspect the strut torque link lugs.