An interspinous spacer for placement between adjacent spinous processes includes a flexible, fillable container (e.g., a bag or balloon) for containing a material that is compressible during end use, for example, silicone after curing. The container is impermeable to the material it will be filled with. A structural mesh, for example, made of PET fabric and interwoven shape-memory alloy wire, provides structure for and containment of the container, as well as shape control. The material can be injected into the container through an optional conduit, for example, a one-way valve.