Crystals in the frame.
The Dilithium articulation frame holds the Dilithium in alignment with the colliding matter/antimatter stream.
The frame suspends the dilithum in the reaction chamber using a field effect to keep the crystal faces in alignment with the matter/antimatter streams. The correct alignment of the crystal faces and planes maximizes the efficiency of the plasma conversion that takes place. By the 2360s, the theta-matrix compositor was integrated with the articulation frame, allowing recrystallization to take place in the warp core with the core online. (TNG:"Relics")