What is involved in testing for patella tracking?

What Testing for Patella Tracking Means

Testing for patella tracking involves assessing the motion of the patella relative to the femur during knee flexion and extension, primarily through physical examination and imaging studies to detect abnormal lateral or medial displacement that may indicate patellofemoral instability. 1

Physical Examination Techniques

Manual assessment during active knee extension is the primary clinical method, where the examiner observes for excessive lateral patellar movement, often described as a "J-sign" when the patella exhibits abnormal lateral displacement. 2 However, visual assessment alone has significant limitations:

• Surgeons correctly identify patellar maltracking only approximately 68% of the time using visual assessment, with inadequate inter- and intraobserver reliability. 2
• The assessment involves determining whether the patella translates more than 2 quadrants laterally during active knee extension. 2

During surgery, specific intraoperative tests assess patellar tracking adequacy, including the "no thumb test," "towel clip test," and "vertical patella test" to evaluate lateral retinaculum tightness. 3 These techniques help surgeons determine if the patella moves appropriately through its range of motion and whether the lateral retinaculum requires release. 3

Imaging-Based Assessment

Weight-bearing axial radiographs are the recommended imaging modality for assessing patellofemoral kinematics and demonstrating the degree of patellar tilt or subluxation. 4, 5

Standard Radiographic Views

• Routine evaluation includes standing anteroposterior, lateral, and tangential axial views of the patellofemoral joint. 4
• Axial radiographs specifically demonstrate patellar tilt or subluxation patterns. 4, 5
• Weight-bearing views provide superior assessment compared to non-weight-bearing studies. 4

Advanced Imaging When Needed

CT is the most commonly used modality for measuring axial malrotation of knee components that may affect patellar alignment, particularly when component malposition is suspected. 4, 5 CT with metal artifact reduction techniques can detect patellar complications in patients with prostheses. 4, 6

Normal vs. Abnormal Tracking Patterns

The patella normally translates medially in early knee flexion and then laterally as flexion progresses. 1 Abnormal tracking (maltracking) manifests as:

• Excessive lateral displacement indicating potential instability. 5, 2
• Patellar tilt abnormalities visible on axial imaging. 4, 5
• Imbalance in the extensor mechanism from lateral retinaculum tightness, component malrotation, or valgus alignment. 5

Clinical Context and Pitfalls

The incidence of patellofemoral instability after total knee arthroplasty ranges from 1% to 12%, most often due to internal malrotation of femoral and/or tibial components. 4, 5 Common pitfalls include:

• Over-reliance on visual assessment alone, which has poor reliability even among experienced surgeons. 2
• Failure to obtain weight-bearing axial radiographs, which provide more accurate kinematic information than standard views. 4
• Not recognizing that surgeon experience does not correlate with ability to accurately identify maltracking visually. 2

Dynamic assessment during different activities matters, as patellar tracking patterns differ between sitting, squatting, and stepping motions. 7 The patella typically moves more laterally during squatting and stepping compared to sitting. 7