Large joint implants can either have a porous and textured surface to induce bone growth, or they can be smooth. The smooth varieties require bone cement to permanently adhere implants to bone. Rather than acting purely as an adhesive, bone cement accomplishes fixation by filling the space between the bone and implant. Its chief constituent is polymethylmethacrylate, which is the same material used to make Lucite® or Plexiglas®. The other key ingredient is a liquid monomer of methylmethacrylate. Each is stored separately until the time of the operation and they are mixed during the time of the operation. Doing so produces a polymerization reaction that solidifies the substances.
Microscopically, the structure of hardened bone cement is similar to honeycomb, which gives it the ability to absorb loads under compression. Therefore, although it appears strong and hard, bone cement plays a role in dampening shock that is transmitted from the implant through to the bone. Typically, one package of bone cement contains 40 grams of bone cement material; one unit refers to one package of bone cement.