Abstract
Background:
Patellofemoral anatomic abnormalities can result in a variety of issues and require thorough clinical evaluation as well as imaging analysis to determine the cause and help guide treatment. Multiple measurements across multiple imaging modalities are used to aid in proper diagnosis.
Indications:
Diagnostic findings on imaging evaluation require assessment of the imaging modality, as well as the use of appropriate measurements to ensure results are reliable and accurate.
Technique Description:
Rotational malalignment of the knee can be evaluated with plain-film radiographs of the knee to assess for abnormal asymmetry. Computed tomography (CT) or magnetic resonance imaging (MRI) axial cuts of the knee can measure the knee rotation angle, and a full-length lower extremity CT can evaluate for rotational deformities of the femur and tibia. The quadriceps vector plays a crucial role in the patellofemoral joint. While it can be assessed on radiographs, it is best examined on CT or MRI. Important measurements here are the tibial tubercle–trochlear groove (TT-TG) distance and TT-TG angle, as well as the TT–posterior cruciate ligament distance. Patellar height is often assessed on lateral radiographs of the knee, with the Insall-Salvati and Caton-Deschamps ratios. It can also be evaluated with the patellotrochlear index and patellar tendon length, seen on sagittal MRI cuts. Trochlear dysplasia plays an important role in patellofemoral pathology. Among multiple measurements, the Dejour classification is evaluated on lateral radiographs of the knee, but it has been updated recently to include MRI criteria. Finally, patellar tilt should be evaluated using either the lateral patellofemoral angle or the patellar tilt.
Results:
Treatment of patellofemoral pathology relies on a detailed understanding of a patient’s patellofemoral anatomy. This relies on evaluating the quality of the imaging modality used, proper selection of relevant imaging slides, and appropriate references for each measurement to have reproducible and accurate measurements.
Discussion/Conclusion:
Evaluation of patellofemoral instability with imaging is crucial to determine the underlying pathology. Consistent, reproducible, and accurate measurement techniques are important to obtain the correct diagnosis and determine the appropriate treatment.
Patient Consent Disclosure Statement:
The author(s) attests that consent has been obtained from any patient(s) appearing in this publication. If the individual may be identifiable, the author(s) has included a statement of release or other written form of approval from the patient(s) with this submission for publication.
This is a visual representation of the abstract.
Keywords
Video Transcript
Introduction
Patellofemoral anatomic abnormalities can result in a variety of issues, including pain, cartilage lesions, and arthritis. Diagnosis requires a thorough clinical evaluation, as well as imaging analysis of the patient’s anatomy to determine the underlying cause of instability and guide treatment. The aim of this video is to highlight various measurements for the evaluation of the patellofemoral joint, with emphasis on key points for reliable and accurate measurements. Our disclosures are listed here and available online.
A variety of imaging modalities are used to assess the patellofemoral joint. We will include a discussion on plain-film radiographs that can be obtained in the office setting, as well as advanced imaging techniques such as computed tomography (CT) and magnetic resonance imaging (MRI). Three-dimensional reconstructions can also be used to aid in the evaluation of trochlear dysplasia. The image modality is important to consider, as measurements are often not interchangeable due to variations in radiographs being taken at 20° to 30° of flexion, whereas MRIs are often done at 10° of flexion and CTs in full extension. When performing all measurements, it is first vital to critique the quality of the image being used. Here is an overview of the topics we will be discussing, beginning with rotational malalignment.
Rotational Malalignment
We will begin with the subjective evaluation of radiographs. The profile of the tibial plateau and femoral condyles should be symmetric in both extension and flexion views, and an example of asymmetry is shown here. It is important to evaluate the quality of the radiographs. For a true anteroposterior (AP) view of the knee, the lateral aspect of the tibia should cross through the middle of the fibular head. For a true lateral view of the knee, the medial and lateral femoral condyles should be superimposed on each other. Additionally, the width of the femoral condyles should be symmetric. A rotational malalignment should be suspected with any asymmetry.
Another sign of rotational malalignment is the wink sign, which is a superimposition of the lateral tibial eminence on the femoral condyle on a standing AP radiograph. 2 Overlap greater than 2 to 4 mm is considered abnormal.
Rotation can also be evaluated on advanced imaging. The knee rotation angle can be evaluated on axial cuts of CT or MRI. The first line is drawn across the bony aspect of the posterior femoral condyles at the most posterior portion on axial cuts. A second line is drawn across the bony tibial condyles at an axial cut sitting below the joint surface and above the fibular head. The rotational angle formed between these 2 lines should measure between 0° and 8°. Full lower extremity CT is used to evaluate for increased femoral anteversion and tibial torsion. Anteversion greater than 30° and torsion greater than 40° are considered abnormal. 5
Quadriceps Vector
Next, we will discuss the evaluation of the quadriceps vector, which can be evaluated with long alignment radiographs. The angle between the line connecting the center of the femoral head and the center of the knee, as well as the line from the center of the ankle to the center of the knee, is measured. A >5° valgus alignment can be implicated in patellofemoral pathology.
The tibial tubercle–trochlear groove (TT-TG) is most often obtained on MRI axial cuts but can also be performed on CT axial cuts. A reference line is first created across the posterior condyles at the deepest point of the TG. A perpendicular line is drawn through the lowest point of the TG. Next, the axial cut containing the insertion of the patellar tendon onto the TT is selected. Another perpendicular line is drawn to the reference that intersects the midline of the patellar tendon. The distance between these lines is the TT-TG. On MRI, 10 mm is considered normal, and anything >15 mm is abnormal. On CT, a normal TT-TG is 13 mm, and anything >20 mm is considered abnormal. Ensure an appropriate reference line is drawn and the cuts containing the deepest portion of the TG and the insertion of the patellar tendon onto the TT are used for accurate measurements. 4
Another measurement that can be obtained here is the TT-TG angle. Same as with the TT-TG distance, start with the axial cut that contains the deepest point of the TG. Draw a reference line across the posterior femoral condyles and a perpendicular line through the deepest point of the TG. Another reference line is then drawn through the medial and lateral epicondyles. The intersection of these 2 reference lines marks the center of the knee for this measurement. Again, select the axial cut containing the patellar tendon insertion onto the TT, and draw a line from the center of the knee to the center of the patellar tendon insertion. This angle should measure less than 27°. 3
The next measurement is TT–posterior cruciate ligament (PCL), or TT-PCL, distance. Obtained from axial cuts on MRI in order to visualize the PCL, a reference line is drawn across the bony aspects of the posterior lateral and medial condyles of the proximal tibia, choosing an axial cut that sits below the joint surface and above the fibular head. Superimpose the most inferior slice where the PCL is clearly identified, and draw a line perpendicular to the reference line that touches the medial border of the PCL insertion on the tibia. A second line is then drawn through the midline of the patellar tendon insertion onto the TT perpendicular to the reference line. The distance between the 2 lines gives the TT-PCL, and a measurement >24 mm is considered abnormal. 6
Patellar Height
Evaluation of patellar height is commonly performed on radiographs. The Insall-Salvati ratio is the ratio between the length of the patellar tendon and the length of the patella, demonstrated here as A and B, respectively. 7 It is important to maximize the length of B in the patella. Measurement A is measured along the posterior aspect of the patellar tendon. A ratio >1.2 indicates patella alta, and a ratio <0.8 indicates patella baja.
Another measure of patellar height is the Caton-Deschamps ratio. 7 This is the measurement between the articular surface of the patella and the distance to the anterosuperior tibial plateau. This ratio is indicative of patella alta if it is >1.2, and patella baja if it is <0.6.
The patellotrochlear index is another measure of patellar height obtained on sagittal MRI. It is important to ensure the MRI is a true sagittal view of the knee, not an oblique sagittal view, as is often done for evaluation of the anterior cruciate ligament. Choose the sagittal cut that contains the greatest patella length. The first line, D, is drawn along the articular cartilage of the patella. A reference line is created perpendicular to this line. The second line, E, is drawn perpendicular to the reference line and through the superior-most portion of the femoral articular cartilage; the ratio between E and D is then taken. A ratio <12.5% is diagnostic of patella alta, and a ratio >50% is diagnostic of patella baja.
Patellar tendon length can also be measured on sagittal MRI. Choose the sagittal cut that sits midline of the patella and measure along the posterior aspect. A measurement greater than 51 mm is considered abnormal.
Trochlear Dysplasia
Our evaluation of trochlear dysplasia begins with an evaluation of the ventral trochlear prominence. This is measured on sagittal MRI at the cut containing the bottom of the TG. The first line is drawn along the anterior femoral cortical surface in the supratrochlear region. A second line is drawn parallel to the cartilaginous surface of the TG. The distance between these lines should be <5 mm. A measurement between 5 and 8 mm or greater indicates trochlear dysplasia.
The sunrise view of the knee can be used to obtain the sulcus angle. This is formed between 2 lines originating at the deepest point of the TG and extending to the peak of the medial and lateral condyles, respectively. This angle should measure around 135°. Patients with trochlear dysplasia will have a sulcus angle >145º. Measurement done on MRI should be performed with the leg extended, so an abnormal angle is increased to one >160°.
Lateral trochlear inclination (LTI) can be measured on axial MRI cuts. The most proximal cut of the TG containing full cartilage coverage on the medial and lateral facets is used. The first line is drawn across the posterior condyles. The second line is drawn along the lateral trochlear. The LTI is the angle formed between these 2 lines and should be >11°.
Trochlear dysplasia can also be evaluated on lateral radiographs of the knee using the Dejour classification. Evaluation of the femoral condyle outline and trochlear sulcus is used. Grade A is noted by the “crossing sign” between these 2 lines. Grade B demonstrates a supratrochlear spur. Grade C again has the crossing sign, but the double contour is also seen both above and below the crossing sign, and grade D has both the double contour and a supratrochlear spur.
The Dejour classification was recently updated to include criteria from MRI measurements. 1 Type 0, also known as no dysplasia, has a sulcus angle <157° and an LTI >14°. Low-grade trochlear dysplasia is defined as either an abnormal sulcus angle or an abnormal LTI with a ventral trochlear prominence <5 mm. Moderate-grade trochlear dysplasia occurs with an unmeasurable sulcus angle or unmeasurable LTI, and high-grade trochlear dysplasia occurs with an abnormal or unmeasurable sulcus angle and an unmeasurable LTI and a trochlear prominence 5 mm or greater.
Patellar Tilt
Finally, we will discuss evaluation of patellar tilt. The lateral patellofemoral angle begins with a reference line drawn across the peaks of the medial and lateral femoral condyles. A line is then drawn across the lateral patellar facet. The lateral patellofemoral angle is the angle between these 2 lines. It should form an acute angle that opens laterally. It is considered abnormal if these lines are parallel or if the angle opens medially. Axial CT cuts can also be used for the evaluation of patellar tilt. Select the cut that maximizes the width of the patella. A reference line is drawn across the posterior condyles, and a second line is drawn through the patella at the points of maximal width. The angle between these two should open medially and measure <20°. 3
Conclusion
In conclusion, evaluation of patellofemoral instability with imaging is crucial to determine underlying pathology. Consistent, reproducible, and accurate measurement techniques are important to obtain the correct diagnosis and determine the appropriate treatment.
Footnotes
Submitted February 25, 2025; accepted August 17, 2025.
One or more of the authors has declared the following potential conflict of interest or source of funding: R.S.D. has received hospitality payments from Medical Device Business Services. L.C. has received hospitality payments from Zimmer Biomet and Stryker. B.B.H. has received education service payments from Pinnacle, Inc. AOSSM checks author disclosures against the Open Payments Database (OPD). AOSSM has not conducted an independent investigation on the OPD and disclaims any liability or responsibility relating thereto.
