How to manage femoral cut in Total Knee Replacement (TKR) when there is significant femoral bowing?

Managing Femoral Cut in TKR with Significant Femoral Bowing

When significant femoral bowing is present (≥5 degrees), lateralize the femoral entry point by approximately 1 mm for every degree of bowing to achieve proper mechanical axis alignment and avoid malalignment complications. 1

Understanding the Problem

Femoral bowing creates a critical technical challenge during TKR because standard intramedullary (IM) guide placement through the intercondylar notch will follow the curved anatomy of the femur, resulting in:

• Excessive valgus cuts when using standard central entry points in bowed femora 2
• Malrotation of components leading to patellofemoral instability (occurring in 1-12% of TKA cases) 3
• Anterior femoral notching (prevalence increases to 16.7% with navigation systems in bowed femora) 4

Preoperative Assessment

Obtain full-length standing radiographs of the entire femur to:

• Measure the degree of femoral bowing (lateral and/or anterior) 1, 2
• Template the amount of distal femoral bone resection needed 2
• Identify patients at higher risk for alignment errors 2

Key measurements include:

• Mechanical axis deviation 1
• Degree of femoral bowing in both coronal and sagittal planes 4
• Canal diameter to identify capacious canals that allow guide rod divergence 2

Surgical Technique: Lateralized Entry Point

For lateral femoral bowing ≥5 degrees, lateralize the femoral entry point according to this formula: 1

• 3-5 mm lateral to intercondylar notch for moderate bowing (used in 48.8% of bowed femur cases) 1
• 6-10 mm lateral to intercondylar notch for significant bowing (used in 44.4% of cases) 1
• 10-15 mm lateral to intercondylar notch for severe bowing (used in 6.8% of cases) 1

This lateralization allows straighter passage of the long IM rod and achieves:

• Mechanical axis within 3 mm of knee center in 90.9% of cases 1
• Tibiofemoral angle of 6-10 degrees valgus in 96% of cases 1

Critical Technical Considerations

Target 5 degrees of valgus for the distal femoral cut to achieve cosmetically appealing alignment while avoiding excessive valgus and preventing thighs from rubbing together 5

Set femoral component rotation at approximately 3 degrees external rotation relative to the posterior condylar axis to ensure proper patellar tracking and gap balancing 5

Exception: In valgus knees with femoral bowing, this standard rotation could accidentally create internal rotation—adjust accordingly 5

When Anterior Femoral Bowing is Present

Modify the femoral cut when anterior bowing is observed because: 4

• Anterior femoral bowing was present in 61.5% of cases with anterior notching 4
• Combined anterior and lateral bowing significantly increases notching risk (p=0.021) 4
• Oversized femoral components compound the problem (occurred in 53.8% of notching cases) 4

Avoid oversizing the femoral component in patients with anterior bowing to reduce notching risk 4

Alternative Approaches for Severe Deformity

Consider extramedullary alignment guides when:

• Excessive femoral bowing is present on preoperative radiographs 2, 6
• Capacious femoral canal allows guide rod divergence 2
• Rotational positioning creates uncertainty (rotation can cause 2.5-degree variation between 20 degrees internal and external rotation) 6

Use both IM and extramedullary guides together when radiographs indicate unusual angles of femoral resection 6

Navigation System Considerations

Be aware that navigation systems may increase anterior notching risk in bowed femora (16.7% vs 5.7% with conventional technique, p=0.037) 4

If using navigation, actively modify the femoral cut when remarkable femoral bowing is observed rather than relying solely on the system's default settings 4

Common Pitfalls to Avoid

Do not assume standard central entry point will work in femora with ≥5 degrees of bowing—this leads to excessive valgus cuts in the majority of cases 1, 2

Do not ignore rotational positioning during guide placement, as femoral rotation affects the apparent alignment by up to 2.5 degrees 6

Avoid deep patellar cuts and ensure component placement tends medial and superior to prevent patellar complications that occur in 3.6% of TKA cases 7

Recognize that inadequate tibial slope affects patellar tracking and can lead to instability, which accounts for 7.5% of revision cases 8

Verification Steps

Confirm proper alignment intraoperatively by:

• Checking that the IM rod passes straight through the femoral canal without excessive force 1
• Verifying the resection angle on the sagittal plane 4
• Ensuring external rotation angle is appropriate for the specific anatomy 4

Obtain postoperative full-length standing radiographs to verify mechanical axis is within 3 mm of knee center 1