A breakwater-shaped bone graft worked in animals. A near-twin didn't.

The four-legged Tetrapod and the knuckled H-shaped Dolos, concrete blocks most readers have seen armoring coastlines, are the basis of a 2026 Biomaterials study on bone void fillers. Researchers used light-based 3D printing to build six interlocking medical-grade ceramic shapes, then implanted them into unhealed femoral defects in animals to see which would hold.

Three of the six shapes, Tetrapod, Dolos, and a six-legged variant called Hexaleg, packed themselves into stable arrangements with enough open space for bone to grow through. Two of those three, Tetrapod and Dolos, filled the defects completely, supported blood-vessel ingrowth, and produced well-formed new bone.

Hexaleg, the near-twin, did not. It had looked promising in cell culture, where the ceramic's large pores encouraged bone-forming cells. Inside living bone, the same granules produced loose defect filling and impaired remodeling. The authors frame that mismatch as the most informative result in the study.

The paper's load-bearing claim is architectural, not clinical: how the granules interlock when they pack together dominates whether bone regenerates, outweighing the design of any individual granule. The work is preclinical, in a single animal model, with no human data, no third-party replication, and a long way from a regulatory review.