What is the purpose of the posterior stabilizing box in the Attune (femoral knee implant)?

Purpose of the Posterior Stabilizing Box in Attune Femoral Knee Implant

The posterior stabilizing box in the Attune femoral knee implant primarily functions to provide anteroposterior stability by preventing posterior tibial translation, especially in the absence of the posterior cruciate ligament, while allowing for improved knee flexion and stability throughout the range of motion.

Mechanism and Function

The posterior stabilizing box works through a cam-and-post mechanism that includes:

• A central box cut in the femoral component that houses the cam mechanism
• A post on the tibial polyethylene insert that engages with the cam during knee flexion
• This engagement prevents excessive posterior translation of the tibia relative to the femur

Key Functions:

1. Anteroposterior Stability

   • Compensates for the absence of the posterior cruciate ligament (PCL)
   • Prevents posterior tibial subluxation during flexion
   • Maintains proper femoral rollback during knee flexion

2. Improved Range of Motion

   • Facilitates better deep flexion compared to cruciate-retaining designs
   • Helps achieve flexion angles of up to 127° as demonstrated in long-term studies 1

3. Enhanced Stability

   • Provides stability throughout the range of motion
   • Particularly important during activities requiring deep flexion

Design Improvements in Attune System

The Attune posterior stabilized design incorporates specific improvements over previous generations:

• Reduced Patellar Crepitus: The Attune design shows significantly lower rates of patellar crepitus (0.83%) compared to previous designs (9.4%) at two years post-operation 2
• Enhanced Primary Stability: Experimental studies show the Attune PS implant demonstrates lower overall micromotions and higher push-off forces, indicating better initial fixation 3
• Optimized Patellofemoral Geometry: Features a thinner and narrower anterior flange and reduced femoral intercondylar box ratio 2

Clinical Outcomes and Considerations

• Long-term Performance: Posterior-stabilized rotating-platform designs show 100% implant survival at 10-13 years with good flexion and stability 1
• Alignment Importance: Proper alignment of the posterior stabilizing mechanism is critical as malalignment is a major cause of early failure and revision 4
• Potential Complications:
  • Rare cases of polyethylene insert disengagement have been reported with posterior stabilized designs 5
  • Proper soft tissue balancing is essential even with perfect bone cuts 4

Radiographic Assessment

For proper evaluation of the posterior stabilizing mechanism:

• CT scans are most appropriate (rated 9/9) for measuring component rotation 4
• Standard protocol should include standing AP, lateral, and tangential axial views 6
• Full-length standing radiographs provide the most accurate information on mechanical axis alignment 6, 4

Surgical Considerations

Strong evidence supports no difference in outcomes or complications between posterior-stabilized and posterior cruciate-retaining arthroplasty designs 6, allowing surgeons to select the appropriate design based on specific patient needs.

In revision scenarios, posterior stabilized implants can provide satisfactory stability without requiring more constrained implants in many patients 7.

The posterior stabilizing box is a critical component of the Attune knee system that provides essential stability while allowing for improved range of motion and patient outcomes.