I'm a huge fan of Bellamy's products so maybe you can try the Toothie Pegs from that brand? They are designed so that baby can bite down hard without cracking or splintering the rusk.
Apparently,they are a great alternative to teething rings because of the organic natural ingredients of milk and wheat, and contain best of all, there is no added sugar.
They cost about $5 for a box of 6 and you can either order them from the website or at an NTUC Finest.
http://www.bellamysorganic.com.sg/product/toothiepegs-milk-rusks/