What are the problems associated with a lesser tibial slope in Total Knee Replacement (TKR)?

Problems Associated with Reduced Tibial Slope in Total Knee Replacement

Reduced posterior tibial slope in TKR leads to decreased knee flexion, increased instability (particularly in flexion), and altered contact mechanics that can compromise long-term outcomes.

Biomechanical Consequences

Reduced Range of Motion

• Decreased posterior tibial slope directly limits knee flexion, with cadaveric studies demonstrating an average increase of 2.3° of flexion gained per degree of posterior tibial slope increase when moving from 1° to 4° 1
• Small increases in posterior tibial slope significantly increase both knee flexion and posterior tibiofemoral translation 1
• However, one navigation-assisted study of 120 posterior-stabilized TKR cases found no statistically significant change in ROM with changes in posterior tibial slope, though this may be implant-specific 2

Instability Issues

• Eliminating posterior slope (0° tibial slope) improves varus-valgus stability in flexion but increases anterior-posterior laxity in extension 3
• Conversely, excessive posterior slope (7°) loosens the knee to both varus-valgus and anterior-posterior stress in flexion 3
• The optimal balance appears to be 3° posterior slope, which achieves the most normal stability profile in both flexion and extension 3

Altered Contact Mechanics

• Reduced posterior tibial slope results in smaller tibiofemoral contact areas and higher peak contact pressures 4
• Larger posterior tibial slopes (6-9°) produce larger contact areas and smaller mean and peak contact pressures, which may be protective against polyethylene wear 4
• After TKR, tibiofemoral contact area is already reduced to approximately 130 mm² compared to 586 mm² pre-operatively, with contact pressures increasing from 1.85 MPa to 7.56 MPa 4

Clinical Implications

Patellofemoral Complications

• Component malposition, including inadequate tibial slope, affects patellar alignment and tracking 5, 6
• Patellofemoral instability occurs in 1-12% of TKA patients, often related to component malrotation issues 5, 6
• Axial radiographs demonstrate the degree of patellar tilt or subluxation, which should be evaluated when tibial slope is suboptimal 7

Assessment and Detection

• Weight-bearing anteroposterior and lateral radiographs are the initial imaging modality for evaluating alignment and component positioning 7
• CT is the most commonly used modality for measuring axial malrotation and precise component positioning when alignment issues are suspected 7, 5
• Full-length standing radiographs of the lower extremity are recommended for comprehensive evaluation of malalignment 7

Critical Pitfalls to Avoid

Technology-Assisted Surgery Considerations

• Computer-navigated and robotic TKR systems using the transmalleolar axis underestimate posterior tibial slope by an average of 1.9° compared to traditional intramedullary axis measurements 8
• In 60% of cases, this results in 0-2° less posterior slope, and in 13% of cases, more than 4° less slope than intended 8
• Surgeons using navigation or robotics must account for this systematic difference to avoid inadvertently creating insufficient posterior slope 8

Long-term Consequences

• Instability represents the third most common cause of prosthesis failure overall, accounting for 7.5% of all revision cases 7
• The concepts of instability, malalignment, and loosening are closely interrelated—minor instability from inadequate slope can progress to significant malalignment and eventual loosening 7
• Annual weight-bearing radiographs are suggested for detecting subclinical wear and progressive malalignment 7