Knee Spacer: Range of Motion, Purpose, and Living with One

What is the range of motion with a knee spacer?

The range of motion (ROM) with a temporary knee spacer, typically used in the context of a two-stage revision knee arthroplasty for infection, is significantly limited and primarily focused on preserving joint space rather than achieving functional movement. Patients can generally expect a restricted arc of motion, often less than 0-60 degrees, depending on pain, swelling, and the specific spacer design.

Understanding the "Knee Spacer"

In orthopedic surgery, the term "knee spacer" most commonly refers to a temporary, antibiotic-impregnated device used in the treatment of periprosthetic joint infection (PJI) following total knee arthroplasty (TKA). This is part of a "two-stage revision" approach:

1. Stage 1: The infected knee implant components are removed, the joint is thoroughly debrided, and a temporary knee spacer is implanted. This spacer is typically made of bone cement (polymethylmethacrylate, PMMA) mixed with high doses of antibiotics.
2. Stage 2: After a period (weeks to months) during which the infection is treated with the local antibiotic delivery from the spacer and often systemic antibiotics, the temporary spacer is removed, and a new, permanent knee replacement implant is inserted.

It is crucial to differentiate this temporary, infection-management device from a permanent knee replacement implant, which is designed for long-term functional mobility.

Purpose of a Temporary Knee Spacer

The primary objectives of implanting a temporary knee spacer are multifaceted:

• Local Antibiotic Delivery: To provide a high concentration of antibiotics directly to the infected joint space, effectively targeting and eradicating the bacterial infection.
• Maintenance of Joint Space: To prevent the joint from collapsing or scarring down (arthrofibrosis) during the period between the two surgical stages. This preserves the soft tissue envelope and makes the second-stage surgery technically less challenging.
• Preservation of Soft Tissue Tension: To maintain the length and tension of the collateral ligaments and surrounding soft tissues, which helps prevent contractures and facilitates better outcomes after the definitive second-stage procedure.
• Limited Mobility: While not designed for full functional motion, some limited movement is allowed to prevent severe stiffness and maintain a baseline level of joint articulation.

Expected Range of Motion with a Temporary Spacer

Given its temporary nature and primary purpose of infection management, the range of motion achievable with a knee spacer is deliberately limited and significantly less than that of a healthy knee or a well-functioning permanent knee replacement.

• Restricted Arc: Patients can typically expect an arc of motion ranging from 0 degrees (full extension) to approximately 60-90 degrees of flexion, or even less. The actual range will vary widely based on individual factors such as pain, swelling, the specific design of the spacer (e.g., articulating vs. static), and the surgeon's protocol.
• Pain and Swelling: Significant pain and swelling are common after the first stage of surgery, which inherently restricts movement.
• Non-Weight Bearing or Limited Weight Bearing: Many patients are advised to be non-weight bearing or to place only partial weight on the affected leg to protect the spacer and the healing tissues, further limiting functional use.
• Focus on Preservation, Not Function: The goal during this phase is not to achieve maximal functional ROM but rather to maintain enough motion to prevent severe stiffness and facilitate the subsequent definitive surgery.

Factors Influencing ROM with a Temporary Spacer

Several factors can influence the actual range of motion experienced with a temporary knee spacer:

• Type of Spacer:
  • Articulating Spacers: These are designed to allow for more motion, often resembling a simple knee replacement component. They are typically preferred as they can lead to better ROM and patient comfort.
  • Static Spacers: These are solid blocks of cement that fill the joint space and allow for very little, if any, motion. They are less common now but may be used in specific cases where motion is deemed detrimental or impossible.
• Presence and Severity of Infection: Ongoing inflammation and pain from an active infection will significantly limit ROM.
• Pain Tolerance and Management: Effective pain control is crucial to allow for even limited movement.
• Swelling and Effusion: Accumulation of fluid within the joint can physically restrict motion and increase pain.
• Soft Tissue Status: The condition of the surrounding muscles, ligaments, and capsule after the initial infection and surgery can impact flexibility.
• Patient Compliance with Rehabilitation: Adherence to physical therapy guidelines, which balance gentle motion with protection, is key.
• Surgeon's Protocol: Individual surgeons may have specific post-operative instructions regarding activity levels and ROM goals.

Rehabilitation Goals with a Temporary Spacer

Physical therapy during the period with a temporary knee spacer focuses on several key areas, distinct from post-permanent TKA rehabilitation:

• Pain and Swelling Management: Utilizing ice, elevation, and medication to control discomfort and inflammation.
• Gentle Range of Motion: Performing passive or active-assisted exercises within the pain-free range to prevent severe stiffness and maintain some joint mobility.
• Muscle Activation and Strengthening: Light isometric exercises for the quadriceps, hamstrings, and glutes to minimize muscle atrophy, which is common during periods of inactivity.
• Circulation and Edema Control: Encouraging ankle pumps and other exercises to prevent deep vein thrombosis (DVT).
• Patient Education: Instructing the patient on precautions, weight-bearing status, and signs of potential complications.

The emphasis is on maintaining the joint and preparing for the second stage, not on aggressive ROM or strength gains.

Distinguishing from Permanent Knee Implants (Total Knee Arthroplasty)

It's vital for patients and fitness professionals to understand the difference in ROM expectations between a temporary knee spacer and a permanent total knee replacement:

• Permanent Total Knee Arthroplasty (TKA): The goal of a successful permanent TKA is to restore significant functional range of motion, typically aiming for 0-120 degrees or more of flexion, allowing for activities like walking, climbing stairs, and getting out of a chair. The implant components are designed for smooth articulation and long-term durability.
• Temporary Knee Spacer: As discussed, the ROM is much more limited, serving as a bridge to infection eradication and subsequent definitive surgery.

The Role of Physical Therapy

Physical therapy is indispensable during the period a patient has a temporary knee spacer. A qualified physical therapist will:

• Assess and Monitor: Continuously evaluate the patient's pain, swelling, and current range of motion.
• Guide Safe Movement: Prescribe appropriate exercises that promote gentle motion without compromising the spacer or the healing tissues.
• Educate on Precautions: Reinforce weight-bearing restrictions and activities to avoid.
• Prepare for Next Stage: Help the patient maintain strength and flexibility to optimize outcomes for the second-stage revision surgery.

Living with a Knee Spacer: Important Considerations

Patients with a temporary knee spacer must adhere strictly to their surgeon's and physical therapist's instructions.

• Activity Restrictions: High-impact activities, heavy lifting, and any movements that cause significant pain or stress on the knee are typically prohibited.
• Weight-Bearing Status: Strict adherence to non-weight bearing or partial weight-bearing protocols is crucial to prevent damage to the spacer or surrounding tissues.
• Monitoring: Patients should be vigilant for any signs of worsening infection (e.g., increased pain, redness, swelling, fever, drainage) and report them immediately to their medical team.
• Patience: This phase requires significant patience, as it is a temporary, transitional period focused on healing and preparing for the definitive solution.

Conclusion

The range of motion with a knee spacer, in its common medical context as a temporary device for infection management, is inherently restricted. It is not designed to provide functional mobility comparable to a healthy knee or a permanent knee replacement. The primary focus during this critical phase is on eradicating infection, preserving joint space, and maintaining some foundational mobility to facilitate a successful second-stage revision surgery. Understanding these limitations and adhering to medical and rehabilitation guidance is paramount for optimal outcomes.