What are the factors influencing patellar tracking in total knee arthroplasty?

Factors Affecting Patellar Tracking in Total Knee Arthroplasty

Component malrotation—specifically excessive combined internal rotation of the femoral and tibial components—is the most critical modifiable factor affecting patellar tracking in TKA, with the degree of internal rotation directly proportional to the severity of patellofemoral complications. 1

Primary Determinants of Patellar Tracking

Component Rotational Alignment (Most Critical)

Internal malrotation of femoral and/or tibial components is the leading cause of patellofemoral instability after TKA, accounting for the majority of the 1-12% incidence of patellar tracking problems. 2

The evidence establishes clear rotational targets:

• Femoral component: Should be parallel to the transepicondylar axis 1
• Tibial component: Should be positioned in approximately 18 degrees of internal rotation relative to the tibial tubercle 2
• Combined excessive internal rotation of both components creates a dose-dependent relationship with patellofemoral complications 2

Patellar Component Positioning

Three patellar-specific factors critically influence tracking 3:

1. Patellar component medialization (most important patellar factor)
2. Patellar resection angle (must be parallel to avoid asymmetric thickness)
3. Patellar thickness changes

Recent evidence demonstrates that internal rotational patellar resection creates a thick medial remnant and thin lateral counterpart, generating high lateral patellofemoral pressure that induces maltracking 4. This is particularly problematic in bicruciate-substituting TKA designs, where the patellar resection angle and component position show strong correlations with postoperative patellar tilt (r = 0.46-0.63) 5.

Secondary Contributing Factors

Soft Tissue Imbalance

Patellofemoral instability occurs from extensor mechanism imbalance characterized by:

• Excessive lateral retinaculum tightness
• Valgus alignment of the extensor retinaculum 2

Pre-existing Patient Factors 6

• Severe preoperative valgus deformity
• Pre-existing patellofemoral dysplasia
• Native coronal alignment (varus knees show more central tracking; neutral-to-valgus knees demonstrate more lateral tracking) 7
• Patella alta (elevated Insall-Salvati ratio correlates with high lateral patellofemoral pressure) 4

Component Design and Sizing

• Femoral component design (anatomical components preferred)
• Size of femoral and tibial components
• Q angle modifications 6

Clinical Algorithm for Optimization

Intraoperative assessment sequence:

1. Verify component rotation using CT landmarks (when available):

   • Transepicondylar axis for femoral component
   • Whiteside line as alternative reference
   • 18-degree internal rotation for tibial component relative to tibial tubercle 1

2. Ensure parallel patellar resection:

   • Avoid any internal rotation of the resection plane
   • Maintain symmetric medial-lateral thickness 4, 5

3. Position patellar component centrally:

   • Medialization improves tracking but must be balanced
   • Avoid excessive medialization that could cause medial overload 3

4. Perform dynamic intraoperative assessment:

   • Observe tracking through full range of motion
   • Look for separation ≥2mm of patellar medial facet from medial femoral trochlea (indicates impaired tracking) 4

5. Consider lateral retinacular release only if:

   • Persistent lateral maltracking after component optimization
   • Demonstrated high lateral patellofemoral pressure in flexion 4

Critical Pitfalls to Avoid

• Do not adjust tibial component rotation or patellar thickness to correct tracking problems—these modifications are ineffective 3
• Do not rely solely on axial radiographs for rotational assessment—CT without IV contrast is the gold standard for measuring component malrotation 1
• Do not ignore the cumulative effect of small rotational errors—combined internal rotation has proportional impact on complications 2
• In bicruciate-substituting TKA designs, pay extra attention to patellar resurfacing technique, as these implants show greater susceptibility to lateral patellar tilt progression postoperatively 5

Assessment Modality

When patellar complications are suspected postoperatively:

• Initial assessment: Weight-bearing axial radiographs to demonstrate patellar tilt or subluxation 2
• Definitive rotational assessment: CT without IV contrast (coefficient of variation 0.87 for accuracy) 1
• Advanced imaging: MRI with metal artifact reduction techniques can detect complications when CT unavailable 2