Best Demonstratives and Models to Describe Patella Baja
Weight-bearing axial radiographs are the best initial imaging method to assess patellofemoral kinematics and demonstrate patella baja, with CT being the gold standard for more detailed evaluation of component rotation and alignment. 1
Diagnostic Imaging Methods for Patella Baja
Radiographic Assessment
- Standard radiographs: Initial evaluation should include weight-bearing AP, lateral, and axial views of the knee 1
- Weight-bearing axial radiographs: Specifically recommended to better assess patellofemoral kinematics and demonstrate the degree of patellar tilt or subluxation 1
- Full-length standing radiographs: Important for evaluating overall lower limb alignment when patella baja is suspected 1
Advanced Imaging
- CT scans: Most commonly used for measuring axial malrotation of knee prosthesis and component rotation 1
- Provides better assessment of component positioning than standard radiographs
- Can be used with metal artifact reduction techniques to assess patellar complications
- Three-dimensional CT studies offer enhanced evaluation of component rotation 1
- MRI with metal artifact reduction: Can detect patellar complications when appropriate techniques are used 1
Measurement Methods for Patella Baja
Five validated measurement methods exist to quantify patella baja 2:
Blumensaat's line: Measures the relationship between the inferior pole of the patella and a line drawn through the intercondylar notch
Insall-Salvati ratio (ISR):
- Measures the ratio of patellar tendon length to patellar length
- Patella baja defined as ISR < 0.8 3
- Most widely used method for assessing patellar height
Modified Insall-Salvati ratio: Accounts for variations in patellar shape by measuring from the inferior articular surface of the patella rather than the inferior pole
Blackburne-Peel ratio (BPR):
- Measures the ratio of the perpendicular distance from the lower edge of the patellar articular surface to the tibial plateau
- Patella baja defined as BPR < 0.54 3
- Particularly useful in post-surgical settings
Caton-Deschamps ratio: Measures the ratio of the distance between the lower edge of the patellar articular surface and the anterior edge of the tibial plateau to the length of the patellar articular surface
MRI-Specific Criteria
- On MRI, patella baja is defined as a TL/PL (tendon length/patellar length) ratio of less than 0.74 4
- Gender differences exist: females have significantly higher TL/PL ratios than males (cutoffs of 0.79 for females and 0.74 for males) 4
Clinical Implications and Differentiation
True Patella Baja vs. Pseudo-Patella Baja
True patella baja: Distal positioning of the patella relative to the femoral trochlea or shortening of the patellar tendon 2, 5
- Characterized by ISR < 0.8 and BPR < 0.54 3
Pseudo-patella baja: Elevation of the joint line while the patella remains in normal position relative to the femoral trochlea 2
Causes of Patella Baja
- Congenital factors
- Post-surgical complications (TKA, ACL reconstruction, high tibial osteotomies)
- Trauma
- Infection (increases risk by 10.9 times after revision TKA) 3
- Joint instability (increases risk of pseudo-patella baja by 11.4 times) 3
- Scarring and shortening of the patellar tendon
- Scarring of the patellar tendon to the anterior tibia 2
Clinical Significance
- Patella baja can lead to decreased range of motion (average 113.3° with patella baja vs. 122.0° in normal knees) 3
- Other complications include decreased lever arm, extensor lag, impingement, anterior knee pain, increased energy expenditure, and potential tendon rupture 2
- Early identification is crucial for proper management and to prevent further complications
By using these measurement methods and imaging techniques, clinicians can accurately diagnose patella baja and distinguish between true patella baja and pseudo-patella baja, which is essential for appropriate treatment planning.