Bone Cement: Mechanical Failures, Biological Risks, and Surgical Challenges

What are the disadvantages of bone cement?

Bone cement, primarily polymethyl methacrylate (PMMA), is a crucial material in orthopedic surgery for securing prosthetic implants. However, despite its widespread use, it presents several significant disadvantages ranging from mechanical failures and adverse biological reactions to complex surgical and long-term revision challenges.

Mechanical Limitations and Failure

While bone cement provides immediate mechanical stability, its long-term performance is subject to inherent mechanical limitations that can lead to implant failure.

• Fatigue and Aseptic Loosening: Bone cement is a brittle material that can undergo fatigue failure under the repetitive cyclic loading experienced in daily activities. Over time, micro-cracks can propagate within the cement mantle, leading to its degradation and ultimately to aseptic loosening of the implant. Unlike biological fixation, which relies on bone ingrowth, cement relies solely on mechanical interlock, making it susceptible to this type of failure without infection.
• Fracture of the Cement Mantle: The cement mantle itself can fracture, especially in areas of high stress concentration or if the mantle is inadequately thick or contains voids. This can compromise the stability of the implant and necessitate revision surgery.
• Lack of Biological Integration: Unlike uncemented implants that allow for biological ingrowth of bone, bone cement creates a mechanical interface with the bone. This means there is no direct biological bond between the cement and the host bone, which can be a point of weakness and contribute to micromotion and subsequent loosening over time.

Biological and Physiological Concerns

The chemical composition and polymerization process of bone cement can elicit various biological and physiological responses in the patient.

• Exothermic Polymerization Reaction: The polymerization of PMMA is an exothermic reaction, meaning it releases heat. Temperatures can rise significantly (up to 100°C or more) at the bone-cement interface. This localized heat can cause thermal necrosis (death) of surrounding bone cells (osteocytes) and soft tissues, potentially leading to a weakened bone-cement interface and contributing to early loosening or pain.
• Monomer Toxicity and Systemic Effects: The unpolymerized monomer (methyl methacrylate) is a potent vasodilator and can have systemic effects if absorbed into the bloodstream. While rare, rapid absorption during cementation, particularly in total hip arthroplasty, can lead to "bone cement implantation syndrome" (BCIS), characterized by hypotension, hypoxia, arrhythmias, and even cardiac arrest in severe cases. These effects are typically transient but underscore the need for careful surgical technique.
• Infection Risk: Bone cement is a foreign body and provides a non-vascularized surface where bacteria can adhere and form biofilms. Once established, these infections are challenging to treat with antibiotics alone, as the cement acts as a barrier to the body's immune response and antibiotic penetration. This significantly increases the risk of periprosthetic joint infection (PJI), a devastating complication requiring complex revision surgeries.
• Allergic Reactions: Although uncommon, patients can develop allergic reactions to components of the bone cement, such as the monomer or stabilizers. This can manifest as skin reactions or more systemic responses.

Surgical and Revision Challenges

The properties of bone cement also introduce specific challenges during surgical procedures and subsequent revisions.

• Limited Working Time: Once mixed, bone cement undergoes a rapid polymerization process, offering a limited working time for the surgeon to accurately position the implant. This requires precision and efficiency, as improper placement due to premature hardening can lead to suboptimal implant alignment and function.
• Difficulty in Revision Surgery: Removing well-fixed bone cement during revision surgery is a complex and often destructive process. The cement adheres strongly to the bone, and its removal frequently necessitates significant bone resection, leading to bone loss and making subsequent revisions more challenging. This bone loss can compromise the stability of future implants and increase the risk of periprosthetic fractures.
• Cement Extravasation: There is a risk of cement extruding or leaking into surrounding soft tissues, blood vessels, or even nerve structures during implantation. While often benign, extravasated cement can cause localized pain, nerve compression, or even vascular compromise in rare instances.

Long-Term Considerations

The long-term presence and behavior of bone cement can influence future orthopedic interventions and patient outcomes.

• Degradation Over Time: While designed for durability, bone cement can degrade over many years due to mechanical stresses, biological processes, and the harsh environment of the body. This degradation contributes to the potential for late aseptic loosening and the need for eventual revision.
• Impact on Future Interventions: The presence of a cement mantle can complicate future diagnostic imaging (e.g., artifacts on MRI) and make any subsequent surgical procedures more intricate due to the difficulty of cement removal and the altered bone morphology.

Despite these disadvantages, bone cement remains a cornerstone in modern orthopedics due to its immediate stability and established clinical track record, particularly in older or less active patients. However, understanding its limitations is crucial for both practitioners and patients to make informed decisions regarding implant selection and surgical planning.