Last Updated on June 13, 2025
Patellar height refers to the vertical position of the patella (kneecap) relative to surrounding bony landmarks of the knee joint. Abnormal positioning of the patella is either patella alta (high-riding patella) or patella baja (low-lying patella). Both of these can affect knee function, stability, and biomechanics.
Accurate assessment of patellar height is essential in diagnosing
- Anterior knee pain
- Patellofemoral instability
- Post-traumatic or post-surgical alignment changes
- Congenital or developmental abnormalities
Several radiographic indices have been developed for patellar height measurement. These are Insall Salvati, modified Insall Salvati, Caton-Deschamps Indices and Blackburne-Peel.
Insall-Salvati Ratio
The Insall-Salvati ratio is one of the most commonly used methods to assess patellar height. It is measured on a lateral radiograph or sagittal MRI of the knee taken in 30 degrees of flexion.
How to Measure
- Insall-Salvai ratio can be measured on either
- Lateral view X-ray or sagittal MRI
- Knee flexed at 30 degrees
- The following distances are measured
- A – Patellar Tendon Length (TL): Measured along the posterior surface of the patellar tendon, from the lower pole of the patella to its insertion on the tibial tuberosity.
- B – Patellar Length (PL): The maximum distance between the upper and lower poles of the patella.
- Insall-Salvati Ratio – A / B (TL / PL)
Interpretation [1]
- <0.8 -Patella baja
- Normal range: 0.8 – 1.2
- >1.2 -Patella alta
On MRI, different thresholds are used due to differences in measurement technique, as the patellar tendon is directly visualized.
The accepted values are[2]
- Patella baja: <0.74
- Normal: 0.74-1.5
- patella alta: >1.5
Modified Insall-Salvati Ratio
The Modified Insall-Salvati Ratio was developed to improve the accuracy of patellar height evaluation by focusing on the articular surface of the patella, rather than its full anatomical length. It is particularly useful in cases where the patella is abnormally shaped or elongated.
How to Measure
- Lateral radiograph or sagittal MRI, knee flexed at 30 degrees
- A – Distance from the inferior margin of the patellar articular surface to the insertion point of the patellar tendon on the tibia.
- B – Length of the patellar articular surface.
- Modified Insall-Salvati Ratio- A / B
Interpretation
- Normal value: Around 1.25
- >2.0 is suggestive of Patella alta [3]
Caton-Deschamps Index
The Caton-Deschamps Index differs from the Insall-Salvati methods by using bony landmarks at the tibial plateau instead of tendon length. This makes it useful in postoperative knees or those with tendon injury.
How to Measure:
- Lateral knee X-ray or sagittal MRI/CT, knee flexed at approximately 30 degrees
- A – Distance from the inferior margin of the patellar articular surface to the anterior superior edge of the tibial plateau.
- B – Length of the patellar articular surface.
- Caton-Deschamps Index = A / B
Interpretation:
- <0.6 – Patella baja
- Normal range: 0.6 – 1.3
- >1.3 – Patella alta
Because this method measures the relationship of the joint surface to the tibia, it is can be valuable in postoperative assessment, such as after tibial tuberosity transfer or patellar tendon reconstruction.
The Caton-Deschamps ratio can also be applied to children, but age correction should be applied because patellar ossification begins at its proximal side[4].
Blackburne-Peel Ratio
The Blackburne-Peel Ratio is a patellar height measurement method that focuses on the vertical position of the patellar articular surface relative to the tibial plateau.
How to Measure:
- Lateral knee radiograph , knee flexed at 30 degrees
- A – Perpendicular distance from the inferior edge of the patellar articular surface to a horizontal line drawn at the tibial plateau.
- B- Length of the patellar articular surface.
- Blackburne-Peel Ratio = A / B
Interpretation[5]
- <0.5 – Patella baja
- Normal value: Approximately 0.8
- >1.0– Patella alta
This method is advantageous in skeletally immature patients or after tendon reconstruction, as it does not rely on tendon length. It can also be helpful when the lower pole of the patella is irregular or damaged.
Blumensaat Method
The Blumensaat Method evaluates patellar height using the Blumensaat line, which represents the roof of the intercondylar notch of the femur.
How to Measure
- Lateral knee radiograph, knee flexed at 30 degrees
- Identify the Blumensaat line (a radiodense line corresponding to the roof of the intercondylar notch).
- Assess the position of the inferior pole of the patella relative to this line.
Interpretation:
- In a normal knee, the lower pole of the patella aligns with or lies just above the Blumensaat line at 30° flexion.
- If the patellar pole lies significantly above the line, it suggests Patella alta.
- If the patellar pole is below the line, it indicates Patella baja.
This is a qualitative or semi-quantitative method and is often used as a quick screening tool in radiographic review.
Clinical Applications of Patellar Height Measurement
Accurate evaluation of patellar height is essential in several clinical scenarios.
Diagnosis of Patella Alta and Patella Baja
- Patella alta is commonly associated with patellofemoral instability, recurrent dislocations, and anterior knee pain.
- Patella baja may result from trauma, post-surgical scarring, or conditions like patellar tendon shortening after surgery.
Preoperative Planning
Precise measurement of patellar height helps in procedures such as tibial tubercle osteotomy, quadricepsplasty, or patellar tendon lengthening.
Postoperative Assessment
Identifies complications such as infrapatellar contracture syndrome or excessive lengthening.
Helps in evaluating the outcome of surgical interventions, especially after ACL reconstruction, total knee arthroplasty, or patellar tendon repair.
References
- W. Norman Scott. Insall & Scott Surgery of the Knee E-Book. (2011) ISBN: 9781455727896
- 1. Shabshin N, Schweitzer M, Morrison W, Parker L. MRI Criteria for Patella Alta and Baja. Skeletal Radiol. 2004;33(8):445-50. [PubMed]
- Grelsamer RP, Meadows S. The modified Insall-Salvati ratio for assessment of patellar height. Clin. Orthop. Relat. Res. 1992; (282): 170-6. Pubmed citation
- . Thévenin-Lemoine C, Ferrand M, Courvoisier A, Damsin J, Ducou le Pointe H, Vialle R. Is the Caton-Deschamps Index a Valuable Ratio to Investigate Patellar Height in Children? J Bone Joint Surg Am. 2011;93(8):e35. [Pubmed]
- Blackburne JS, Peel TE. A new method of measuring patellar height. J Bone Joint Surg Br. 1977;59 (2): 241-2. [PubMed]
