Patella height is an important anatomical variation measured as the height of the patella compared to the knee joint line. A patella that sits too high is known as patella alta and a patella that sits too low is known as patella baja.
The patella plays an important role in human ambulation, knee range of motion and extensor power generation. It acts as a fulcrum for the knee extensor mechanism and transfers the tension from the broad quadriceps tendon to the narrower patella tendon 1,2.
This increases efficiency and power by:
transmitting forces over a greater distance 2
increases the lever arm of the quadriceps 2
changes the direction of pull of the quadriceps 1
Terminology
Patella alta is a high-lying patella with a tendon that is too long. Causes are often idiopathic. Although a lengthened tendon increases the lever arm of the patella tendon, it predisposes the patient to other issues 3. These include a higher risk of patella subluxation or dislocation and an increased risk of patella osteoarthritis due to abnormal contact areas 3,4.
Patella baja is a low-lying patella with a tendon that is too short. Causes can be idiopathic, iatrogenic or trauma-related. Due to the shortened tendon, the lever arm of the patella tendon is reduced resulting in reduced extension efficiency, extension lag, reduced range of motion and crepitus 5.
Measurements
Assessing for patella alta or baja is most reliably done radiographically by measuring the patella height 6. There are multiple published methods for assessing the patella height using radiographs, CTs and MRIs.
Insall-Salvati ratio
The Insall-Salvati ratio is the ratio of the patella tendon length against the greatest pole-to-pole length of the patella body, measured off a radiograph. It is considered one of the more reliable techniques of measuring patella height with a higher intra- and inter-observer reliability than all other published commonly used measurement techniques.
uses: radiograph, CT and MRI 7,8
Modified Insall-Salvati ratio
The Modified Insall-Salvati aims to account for variable shapes of the inferior patella pole. It represents a ratio between the patella tendon inserting into the patella articular surface and the length of the patella articular surface.
reported indications: unusual or long non articular inferior patella pole
uses: radiograph, CT or MRI 9
Caton-Deschamps index
The Caton-Deschamps index is a measure of the patella height, that aims to remove variability in cases of an abnormal patella length by using an alternate measure of height. It is a ratio between the length of the inferior patella articular surface to the anterior angle of the tibial plateau.
reported indications: unusual patella length with clear articular margin
uses: Radiograph, CT, MRI 9,10
De Carvalho index
The De Carvalho index is a measure of patella height, similar to Caton-Deschamps. It is a ratio between the shortest distance between the tibial plateau and the inferior aspect of the patella articular surface, to the patella articular surface.
reported indications: unusual patella length with clear articular margin
validated uses: radiograph, CT, MRI 9
Blackburne-Peel index
The Blackburne-Peel index aims to remove the need to measure the patellar height off the tibial tuberosity, to remove variations measurements due to traction apophysitis. It is a ratio between the length of a line that connects the lower end of the patellar surface and runs perpendicular to a line projected anterior to the tibial plateau, and the length of the patellar articular surface.
reported indications: Osgood Schlatters, Larsen-Johansson’s disease, unusual patella morphology or length
uses: Radiograph, CT, MRI 9
Patellotrochlear index
The Patellotrochlear index aims to address unusual patella morphology and not measure the patella tendon length. It is measured on a sagittal MRI slice as a ratio between the length of the trochlear cartilage overlap with the patellar cartilage and the length of the patellar cartilage.
reported indications: unusual patella length or morphology
uses: MRI 11
Patellophyseal index
The Patellophyseal index is a variation on the patellotrocheal index with an indirect measure of the trochlear articular surface that can be measured on a radiograph. It is a ratio between the distance from the anterior physeal line of the femur to the superior margin of the patellar articular surface, with the length of the patellar articular surface.
reported indications: unusual patella length or morphology
uses: radiograph or MRI 12
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