Is there such thing a toe knuckle?

Yes, the toe knuckles are also called the interphalangeal joints. The toe has three joints: the metatarsophalangeal joint (MTP joint) at the base of the toe; the proximal interphalangeal joint (PIP joint) in the middle of the toe; and the distal interphalangeal joint (DIP joint) at the tip of the toe. The MTP joint connects the toe to the metatarsal bone in the foot. The PIP joint connects the proximal and middle phalanges, and the DIP joint connects the middle and distal phalanges.