Bone cement, technically known as Polymethyl Methacrylate (PMMA), is a specialized acrylic material widely used in orthopedic surgery, particularly in joint replacements like the hip and knee. Its primary function is not to act as a glue but as a grout, filling the gap between a prosthetic implant and the patient’s bone to achieve immediate mechanical fixation and stability. This ensures that forces generated by body weight are distributed from the prosthesis to the skeletal structure. Determining if this cement is permanent requires separating its chemical nature from its functional performance over time.
Understanding the Material Permanence of Bone Cement
The material itself, Polymethyl Methacrylate (PMMA), is chemically permanent within the body. PMMA is a highly stable synthetic polymer that does not degrade, dissolve, or get absorbed by the body’s biological processes. When the powdered polymer and liquid monomer are mixed, polymerization occurs, causing the mixture to harden into a solid matrix. This chemical stability means the material will not disappear once implanted.
However, material permanence does not guarantee infinite functional permanence in the joint. Bone cement achieves fixation primarily through mechanical interlocking, flowing into the microscopic irregularities and porous structure of the bone surface. It does not form an adhesive chemical bond directly with the bone or the metal implant. This mechanical bond, or “cement mantle,” is subject to continuous stress and fatigue from daily activities, which eventually leads to mechanical failure.
Over years, microscopic cracks form in the cement mantle, often propagating from existing pores or weaknesses within the material. The constant mechanical loading of the joint causes these cracks to grow, leading to a breakdown of the interface between the cement, the bone, and the implant. While PMMA is chemically stable and permanent, the functional bond it creates is not, making the system prone to mechanical loosening.
Factors That Determine Functional Lifespan
The functional lifespan of cemented implants—the duration before mechanical loosening necessitates a new operation—is influenced by several variables. Patient activity level is a major contributor, as higher impact activities place greater cyclical stress and fatigue on the cement mantle. A highly active patient will see the mechanical bond degrade faster than a patient with a lower activity profile.
Body weight or Body Mass Index (BMI) also affects longevity; greater mass results in higher forces being transmitted through the joint, accelerating wear on the cement-bone interface. Poor quality bone, such as that caused by osteoporosis, can compromise the initial stability and long-term durability.
Surgical technique also plays a significant role. Proper preparation of the bone surface and precise mixing and application of the cement maximize the strength and uniformity of the cement mantle. Inadequate cement thickness or the presence of voids creates weak points where fatigue cracks begin, reducing the time until failure.
Identifying and Managing Cement Failure
The eventual failure of the mechanical bond is termed aseptic loosening, meaning the implant has detached from the bone in the absence of a bacterial infection. This process is driven by the body’s reaction to microscopic debris particles generated from the implant materials. Macrophages, a type of immune cell, engulf these wear particles and release chemical signals that trigger osteolysis—the breakdown and resorption of the surrounding bone.
Symptoms often begin with chronic joint pain aggravated by weight-bearing or movement. Patients may also experience decreased mobility, instability, or stiffness in the affected joint. Radiographic imaging diagnoses the issue by looking for a progressive radiolucent line—a visible gap greater than two millimeters around the implant or cement—indicating the loss of mechanical fixation.
Once confirmed, the standard treatment is revision surgery, a significantly more complex procedure than the initial replacement. This operation involves removing the old prosthetic components and the failed cement mantle. The surgeon must then clean the bone bed and manage any bone loss before inserting new implant components with fresh cement or an alternative fixation method.
Comparing Cemented and Cementless Procedures
Cemented fixation is compared to the cementless, or press-fit, method, which uses a different principle for achieving long-term stability. Cementless implants have a porous surface designed to encourage the patient’s own bone tissue to grow directly onto and into the implant, a process known as osseointegration or biological fixation. This method requires time for bone ingrowth to occur, meaning patients may have a more restricted initial recovery phase compared to cemented procedures.
The choice between the two methods is based on patient-specific factors, including age, bone quality, and activity level. Cemented fixation provides immediate stability, making it a preferable option for older patients or those with poor bone density, such as from osteoporosis, who benefit from immediate weight-bearing. The longevity of cemented implants is well-established, with many lasting 15 to 20 years.
Cementless implants are recommended for younger, more active individuals with healthy bone stock, as successful osseointegration can offer greater long-term durability. While cemented fixation has a known functional lifespan limited by the fatigue of the cement mantle, successful cementless fixation forms a living bond with the bone that may offer a more permanent solution.
Ultimately, both techniques are highly effective, and the decision rests on balancing the benefits of immediate stability against the potential for greater long-term biological fixation.