The fact that both parents have good patellas, do not guarantee that the offspring will not have patella problems when they get older, but it improves your chances. Patellas may also be injured from play, jumping and weight. If your shiba tends to stretch out one of their rear legs after running or you see them favoring the same rear leg, have your vet check their knees. It is a physical exam that does not require an x-ray.
Other problems that may cause leg issues - Both of these conditions can be determined with an x-ray.
Hip Dysplasia - dislocation of the hip - the hip joint has not formed normally.
Legg Perthes Disease - spontaneous degeneration of the head on the femur bone, located in the dog's hind leg. This results in disintegration of the hip joint (coxofemoral) and bone and joint inflammation. It typically happens early in life.