Bone cement, scientifically known as polymethyl methacrylate or PMMA, is a composite synthetic material widely used in orthopedic surgery. Its primary function is to secure prosthetic implants within the body or stabilize compromised bone structures. Rather than acting as a glue that chemically bonds two surfaces, PMMA functions more like a grout or filler, creating a tight mechanical interlock between the implant and the surrounding bone. This material provides immediate fixation and stability, which is beneficial for patient recovery and early weight-bearing. The durability and long-term performance of PMMA are central to the success of many surgical procedures.
What Bone Cement Is and How It’s Used
Bone cement is typically prepared in the operating room by mixing two main components: a powdered polymer and a liquid monomer. The powder primarily consists of polymethyl methacrylate (PMMA) beads, while the liquid is mainly methyl methacrylate (MMA) monomer. This mixture often includes additives such as a radiopacifier, like barium sulfate or zirconium, which allows the cement to be visible on X-rays. Antibiotics can also be incorporated into the powder to help reduce the risk of infection at the surgical site.
Upon mixing, a chemical reaction called polymerization occurs, causing the paste-like mixture to harden into a solid. This hardening process is exothermic, meaning it generates heat. The consistency of the cement changes during this process, allowing surgeons a working phase to apply it before it fully sets.
Bone cement finds widespread application in orthopedic surgery, particularly in joint replacement procedures such as total hip and knee arthroplasties. In these surgeries, it is used to anchor the artificial joint components securely to the existing bone, improving stability and distributing loads across the bone-implant interface. Beyond joint replacements, PMMA is also employed in spinal procedures like vertebroplasty and kyphoplasty, where it is injected into fractured vertebrae to stabilize them and alleviate pain, often due to osteoporosis. It can also be used in certain fracture fixations, especially in patients with reduced bone quality.
How Long Bone Cement Typically Lasts
While bone cement is engineered for extended use in the body, it is not truly permanent and can experience changes over time. Many modern joint implants, including those secured with bone cement, are designed to last between 15 to 20 years, though some may perform well beyond this period. The actual lifespan of bone cement in a patient can vary significantly due to a combination of factors.
A patient’s activity level plays a considerable role, as higher impact activities and increased physical stress on the joint can contribute to the wear and tear of the implant and the surrounding cement. The quality of the patient’s bone, the specific design of the implant, and the surgeon’s technique during the initial procedure also influence longevity.
Over time, bone cement can undergo degradation or loosening, leading to potential complications. The body’s physiological environment, including the absorption of bodily fluids, can impact the mechanical properties of the cement. Micromotion between the implant and the cement, or between the cement and the bone, can also contribute to wear particles and subsequent loosening. While a certain degree of bone remodeling around the cement is normal, progressive changes can indicate issues.
Signs That Bone Cement May Need Revision
When bone cement begins to fail or loosen, patients may experience a range of symptoms indicating that medical evaluation is necessary. The most common symptom is new or worsening pain at the surgical site, which can be localized or more generalized, and often becomes more pronounced with activity or weight-bearing. This pain may be accompanied by swelling around the affected joint.
Other signs can include stiffness, instability, or a decreased range of motion in the joint. Some individuals might report a popping or clicking sound, or a sensation that the joint is giving out or moving out of its proper position. These symptoms collectively suggest a potential issue with the implant’s fixation.
One primary reason for bone cement failure is aseptic loosening, a process where the bond between the implant and the bone weakens without the presence of a bacterial infection. This can involve the body’s response to microscopic particles generated from the wear of implant components, leading to bone loss, a process known as osteolysis. Radiographic imaging, such as X-rays, can reveal signs of loosening, such as widening gaps (radiolucent lines) at the interfaces between the bone, cement, or implant, or even migration of the implant component. If these issues arise, further medical intervention, often involving revision surgery to replace the failing components, may be required.