These spikey balls are 10cm in diameter and are a softer plastic.  They promote circulation of the blood and a great massage for the bigger muscles, tendons and ligaments.  Not to be used directly on joints or bones.
 
Try lying on the floor with your knees bent, lift your bottom up and place them under the middle of your bottom, sway your hips side to side (like Elvis Presley) and the spikey balls roll under your glutes, it's a weird feeling, after a few minutes take them out, extend your legs long on the floor and you'll find you may feel like your bottom has disappeared into the floor, gone numb or you may even feel taller, or all of the above.  :)