Artificial Knee: Why Direct Injections Are Avoided and When They Are Used

Can you inject an artificial knee?

Generally, direct injections into the artificial knee joint itself are avoided due to significant risks, primarily infection and potential damage to the prosthetic components. While injections may be used in the surrounding tissues or as part of perioperative care, they are not typically administered into the sterile space of the replacement joint.

Understanding the Artificial Knee Joint

A total knee arthroplasty (TKA), commonly known as a knee replacement, is a surgical procedure that resurfaces a knee damaged by arthritis or injury. The diseased bone and cartilage are removed and replaced with prosthetic components made of metal alloys (such as cobalt-chromium or titanium) and high-grade plastic (polyethylene).

The primary components of an artificial knee include:

• Femoral Component: A metal cap that fits over the end of the thigh bone (femur).
• Tibial Component: A metal plate that covers the top of the shin bone (tibia), typically with a polyethylene insert (liner) that acts as the new cartilage surface.
• Patellar Component: A polyethylene button that may be placed on the back of the kneecap (patella).

Crucially, unlike a natural knee, an artificial joint does not have cartilage that can regenerate, nor does it produce synovial fluid that can be augmented. The space between the metal and plastic components is designed to be a highly sterile environment, essential for the longevity and function of the implant.

The General Principle: Why Direct Injections Are Avoided

The medical community strongly advises against injecting substances directly into the space of an artificial knee joint for several critical reasons:

• High Risk of Infection: This is the paramount concern. Introducing any foreign substance, even with strict sterile technique, carries a significant risk of bacterial contamination. A prosthetic joint infection is a devastating complication, often requiring extensive surgery to remove the infected implant, prolonged antibiotic therapy, and potentially a new replacement surgery. The lack of blood supply within the artificial components makes it difficult for the body's immune system or antibiotics to clear an infection once it takes hold.
• Damage to Prosthetic Components: Injections, particularly those involving needles, can potentially damage the smooth surfaces of the metal or, more critically, the polyethylene liner. Scratches, nicks, or punctures can accelerate wear and tear on the plastic component, leading to premature loosening or failure of the implant.
• Lack of Therapeutic Benefit: The rationale for injecting natural joints (e.g., corticosteroids for inflammation, hyaluronic acid for lubrication in osteoarthritis) does not apply to an artificial joint. There's no inflamed cartilage to calm, no natural synovial fluid to supplement, and no biological process to modulate within the prosthetic interface.

When Injections Are Used in the Context of a Knee Replacement

While direct intra-articular injections into the prosthetic joint are avoided, injections may be used in the broader context of knee health before or after a knee replacement, but with crucial distinctions:

• Prior to Surgery:
  • Corticosteroid or Hyaluronic Acid Injections: These may be administered into a natural knee joint suffering from osteoarthritis to temporarily relieve pain and inflammation, potentially delaying the need for surgery. These are stopped well in advance of the planned arthroplasty.
  • Nerve Blocks: Regional nerve blocks (e.g., femoral nerve block, adductor canal block) may be given around the knee joint (not into it) to manage pain during and immediately after surgery.
• Around the Time of Surgery (Perioperative):
  • Local Anesthetics: Anesthetic agents may be infiltrated into the tissues around the knee incision during surgery to provide immediate post-operative pain relief. This is not an injection into the joint space.
  • Antibiotics: Intravenous antibiotics are routinely given before and after surgery to prevent infection. In some cases, antibiotics are mixed into the bone cement used to secure the implant, but this is an intrinsic part of the implant fixation, not a post-surgical injection into the joint.
• Post-Surgery (But NOT into the Prosthetic Joint):
  • Injections for Surrounding Soft Tissue Issues: If a patient develops issues like bursitis (inflammation of a bursa) or tendinitis (inflammation of a tendon) adjacent to the knee replacement, an injection of corticosteroids or local anesthetic into the bursa or tendon sheath (not the joint) might be considered. This requires careful anatomical knowledge and sterile technique to avoid inadvertently entering the joint space.
  • Pain Management: Injections like trigger point injections (into muscle knots) or continued nerve blocks may be used for persistent pain outside the prosthetic joint.
  • Anticoagulants: Medications to prevent blood clots (e.g., low molecular weight heparin) are often given via subcutaneous injection (under the skin), not into the knee.

Risks and Considerations of Any Injection Near a Prosthetic Joint

Even when injections are administered into tissues around a prosthetic knee, significant caution is warranted:

• Proximity to Implant: Any injection in the vicinity of a joint replacement carries a higher theoretical risk of introducing bacteria that could migrate to the implant, even if the needle doesn't directly enter the joint.
• Sterile Technique: Meticulous sterile technique is paramount for any injection near a prosthetic joint.
• Communication with Surgeon: It is crucial for patients to inform their orthopedic surgeon about any planned injections near their knee replacement. The surgeon can provide guidance on the necessity, timing, and safest approach.

Alternatives to Direct Joint Injections for Post-Arthroplasty Issues

For pain or discomfort following a knee replacement that is not due to infection or implant failure, safer and more effective management strategies are typically employed:

• Physical Therapy and Rehabilitation: Targeted exercises to improve strength, flexibility, and mobility.
• Oral Medications: Non-steroidal anti-inflammatory drugs (NSAIDs), analgesics, or other pain relievers.
• Activity Modification: Adjusting activities to reduce stress on the knee.
• Weight Management: Reducing body weight can significantly alleviate stress on the knee joint.
• Rarely, Revision Surgery: In cases of severe pain, instability, or mechanical issues not amenable to conservative treatment, a revision arthroplasty may be considered to address the underlying problem with the implant.

Conclusion: Prioritizing Prosthetic Integrity and Patient Safety

In summary, injecting directly into an artificial knee joint is generally not recommended and carries substantial risks, particularly infection, which can jeopardize the success and longevity of the knee replacement. While injections may play a role in managing conditions around the knee or as part of perioperative care, these are distinct from injecting into the prosthetic joint itself.

Any decision regarding injections in a patient with a knee replacement must be made in close consultation with an orthopedic surgeon or a medical professional experienced in managing prosthetic joints, prioritizing the integrity of the implant and the long-term health of the patient.