This study found that back squats and front squats produced similar levels of quadriceps activation but participants used over 21% less weight in the front squat. This means that front squats can be a solid option to hit your quads if you want a lighter workout or want to work around some knee issues.
Reference: Gullett, J. C., Tillman, M. D., Gutierrez, G. M., & Chow, J. W. (2009). A biomechanical comparison of back and front squats in healthy trained individuals. The Journal of Strength & Conditioning Research, 23(1), 284-292.