Bone cement is an acrylic cement used to hold implants in bone in joint replacement. Bone cement is not really a glue. It is a filler, or grout used to keep the implants in place.
Acrylic cement was originally used for dental work in 1951. Later in the 1960s, Sir John Charnley in Wrightington, England reported the first use of bone cement for joint replacement procedures. Bone cement is a compound called polymethyl-methacrylate (PMMA). PMMA is a polymer.
It is a big molecule made up of a long repeating chain of smaller substances called monomers. Monomers may readily bond to themselves in a process called polymerization. Polymers are large molecules and the molecular weight of PMMA is 200,000.
In the operating room, PMMA is prepared by mixing a liquid and a powder. The liquid contains methyl-methacrylate monomer and an activating substance. The powder has smaller particles of PMMA. When the liquid and the monomer are mixed, they form the cement, which is used to implant the components.
The liquid is supplied in a glass vial, the powder in a sealed packet. They are both poured in a mixing bowl to form the cement.
The chemical reaction that occurs is an exothermic reaction. That means heat is given off as the chemical reaction takes place. The cement is mixed anywhere from one to four minutes. At first it is liquid, then it becomes thick and doughy.
A vacuum pump is sometimes attached to the mixing bowl to help remove air bubbles from the cement, which makes the cement stronger.
Early on, cement was placed by finger packing and pressure. Modern techniques have evolved where surgeons now use a special cement gun to inject the cement in the femoral canal. Other techniques allow for pressurizing the cement to make sure that there is a good fill in bone.
If the cement is to be delivered by a cement gun, it is poured as a liquid in to the gun. If the surgeon is going to pack the cement by hand, he will mix it longer until it becomes thicker, like modeling clay.
After the cement is placed in the joint, the surgeon will then implant the femoral or acetabular component. He will remove any excess cement. Bone cement is self-curing. Once the reaction has taken place, the cement begins to set. Usually, it takes between ten and eighteen minutes. Colder temperatures slow the reaction down, while warmer temperatures make the cement set faster. The surgeon will maintain pressure on the implanted component until the cement is dry.
PMMA is biologically compatible. Allergic reactions or sensitivity to the cement are rare. Operating room nurses have sometimes complained of headaches from cement fumes given off after the liquid and powder are mixed. Despite this, there is no evidence that the fumes have any toxicity.
In rare instances, implantation of the cement has been known to cause a sudden drop in blood pressure. The reason is not well known. The anesthesiologist will take steps to make sure that the patient is well-hydrated and blood pressure maintained while the cement is being implanted.
Attempts have been made to improve the fixation of the cement to the surface of the femoral component. Femoral stems were coated with a layer of acrylic during the manufacturing process. It was felt that the cement would bond to a like substance on the surface of the metal better than it would bond to the metal itself. Long-term results however were mixed.
A radiopaque material such as barium sulfate is included with the powder used to make the cement. This is done so that the cement will be visible on x-ray and any changes in the cement mantle can be identified later on.
Antibiotics can be mixed with the cement in cases where there is an infection. The two most common antibiotics used in cement are Gentamycin and Tobramycin. The antibiotics will gradually extrude from cement into the soft tissues over time. Antibiotic laden cement is used as a spacer in infection cases.
If the joint replacement has become infected, it is sometimes necessary to remove all of the components and wait several weeks before they can be reimplanted. The antibiotic spacers are shaped and inserted in the joint to temporarily fill the empty space when the components are taken out.
When bone cement was first developed for use with orthopaedic implants, FDA approval was required for its use. Ironically, as noncemented components with porous surfaces were developed, FDA approval was required to implant components that did not require cement.
Bone cement has other uses in orthopaedic surgery besides joint replacement. It has sometimes been used to fill defects in bone in fracture cases. Recently, a new procedure, kyphoplasty, has been used to treat compression fractures in the spine. In this procedure, bone cement is injected directly in to the vertebral body to restore the normal height of the vertebra that has col-lapsed from the fracture.