What do you recommend for building bone strength after having surgery on a sliding knee cap?

Balanced diet. Balanced diet and let your fearfully made body heal and rest.
Knee muscle strength. After surgery to remedy what i suspect is patellofemoral syndrome, the focus is not to improve your knee bone strength, but to improve the strength of the muscles that stabilize your kneecap. See physical therapy, which should include terminal knee extension exercises targeting the 'vmo' muscle. Maintaining bone mass depends on adequate calcium and vitamin d intake and weight bearing exercise.