CT Evaluation of Patellofemoral Pain Campbell, SC, Monu, JUV, Seo, GS University of Rochester, Rochester, NY Strong Memorial Hospital.

CT Evaluation of Patellofemoral Pain

Campbell, SC, Monu, JUV, Seo, GS

University of Rochester, Rochester, NY

Strong Memorial Hospital

Before we begin…

Before we can begin a discussion of the CT evaluation of

patellofemoral pain, the normal, basic, axial anatomy of the knee

must be reviewed.

Before we begin…Let’s review the necessary anatomy…

MedialPatellarFacet

LateralPatellarFacet

Femoral trochlea

Lateral FemoralCondyle

Medial FemoralCondyle

Now we can discuss pathology…

MedialPatellarFacet

LateralPatellarFacet

Femoral trochlea

Lateral FemoralCondyle

Medial FemoralCondyle

There are many causes of anterior knee pain…

• Chondromalacia patella• Plica syndrome• Meniscal Pathology• Patellar bursitis• Patellar tumors• Jumper’s Knee• Sindig-Johansson-

Larsen's disease • Bipartite patella• Patellar subluxation or

instability

Many of these pathologies are

best evaluated on plain film or MRI,

however…

• Chondromalacia patella• Plica syndrome• Meniscal Pathology• Patellar bursitis• Patellar tumors• Jumper’s Knee• Sindig-Johansson-

Larsen's disease • Bipartite patella• Patellar subluxation or

instability

CT is uniquely suited to evaluate

for patellar subluxation and

instability.

There are many causes of anterior knee pain…

Plain Film Views• Traditionally, plain film

views were used to evaluate the patello-femoral joint.

• They have the dis-advantage of only evaluating the knee at a fixed degree of flexion and not reliably displaying the knee joint at less than 30 degrees of flexion.

• Degrees of flexion in some traditional plain film views:

Merchant 45 Laurin 20 Sunrise 115

Merchant view of the knee

CT has the advantage of:

• Allowing a true axial view of the patellofemoral joint.

• Ease of patient positioning.

• Ability to reliably image the patellofemoral joint in multiple degrees of flexion.

So we’ll use CT to evaluate the patellofemoral joint.

Q. But what are we looking for?

A. Patellar Instability.

What is patellar instability?

• Also known as…MalalignmentSubluxationRecurrent dislocationPatellar tracking abnormality

In a nutshell, any abnormal biomechanical relationship between the patella and the

femoral trochlea.

But before we can define abnormal, we have to talk about what’s normal.

The Normal Patellofemoral Relationship

• The patella may be slightly lateral to the apex of the femoral trochlea at full knee extension.

• The patella “reduces” medially with increasing flexion of the knee joint.

• Contraction of the quadriceps applies lateralizing force to the patella but will not cause significant subluxation in a normal knee. This lateralizing force decreases with increasing knee flexion.

Q. Why does quadriceps contraction apply a lateralizing

force to the patella?A. The Q angle.

The Q Angle

• The patella is subjected to forces of the rectus femoris and the vastus medialis, lateralis and intermedius when these muscles contract.

• The net force of these muscles pulls along a line from the center of the patella to the anterior superior iliac spine.

The Q Angle

• The patella is attached to the tibial tubercle by the patellar tendon.

• The tibial tubercle lies laterally to the vector of the contracted quadriceps muscle force.

• With contraction, this pulls the patella laterally.

• With flexion of the knee, the tibia “derotates” bringing the tubercle more medial, and the Q angle decreases, bringing the force of the quadriceps in line with the tubercle and exerting a “reducing” force on the patella.

Q

So now that we know what to expect from the normal knee, how

do we use CT to evaluate the symptomatic knee?

CT Positioning

• Place the patient in a lateral decubitus position.

• Place a spacer between the knees and ankles to create an alignment similar to a relaxed stance.

CT Positioning

• Scan through the patellar apices at 0, 20, 30, 40 and 50 degrees of flexion.

• At each degree of flexion, have the patient contract his or her quadriceps muscles.

The Resulting ImagesQuadriceps Rest Quadriceps Flex

0 Degrees

40 degrees

How do we analyze the images?

• Select the image containing the patellar apex for each knee at each degree of flexion with and without quadriceps contraction.

• Assess four aspects of alignment:Depth of the femoral trochleaAngle of the femoral trochleaDegree of patellar displacementDegree of patellar angulation

Femoral Trochlear Depth

1. Draw a line parallel to the femoral condyles

2. Draw a line at the deepest point of the femoral trochlea, parallel to the first line.

3. Draw a third line at the highest point of the lateral trochlea and measure between the second and third lines. This is the femoral trochlear depth (FTD).

FTD

Femoral Trochlear Depth

1. Draw a line parallel to the femoral condyles.

2. Draw a second line parallel to the first at the deepest point of the femoral trochlea.

3. Draw a third line parallel to the highest points of the medial and lateral aspects of the trochlea.

4. Measure between the second and third lines at the trochlear apex. This is the femoral trochlear depth (FTD).

Alternatively…

Femoral Trochlear Angle

1. Draw two intersecting lines along the medial and lateral facets of the femoral trochlea.

2. This is the femoral trochlear angle (FTA, also known as the sulcus angle).

FTA

Patellar Displacement

1. Bisect the angle of the femoral trochlea.

2. Draw a line from the apex of the femoral trochlea through the apex of the patella.

3. The angle between these two lines is the congruence angle (CA).

4. If the patellar apex is medial to the trochlear apex the CA is negative. If it’s lateral, the CA is positive.

- +

CA

Patellar Displacement1. Draw a line parallel to the

femoral condyles.2. At 90 degrees to the first

line draw one line intersecting the patellar apex.

3. Then draw a line bisecting the trochlear apex.

4. The difference between the second and third lines is the measurement of patellar subluxation. If negative, the patella is medial; if positive, the patella is lateral to the femoral trochlea.

Alternatively…

Patellar Angulation1. Draw a line parallel

to the posterior femoral condyles.

2. Draw a second line parallel to the first at the patellar apex.

3. The angle between the lateral patellar facet and this second line is the patellar tilt angle (PTA).

+

-

Patellar Angulation

If the reference line is drawn along the anterior femoral condyles, the resulting angle is called the lateral patello-femoral angle (LPFA)

+

-

Alternatively…

Patellar Angulation

1. Draw a line parallel to the posterior femoral condyles.

2. Draw a second line along the axis of the patella.

3. The resulting angle is the patellar inclination.

Alternatively…

Normal Values

• Several studies have been performed to determine normal values in asymptomatic knees and results show a wide range with patellar displacement and patellar tilt being the most specific and sensitive indicators of patellar malalignment.

“There is no agreement on what constitutes normal anatomy of the patellofemoral joint.”

G.R. Scuderi

Evaluating the Measurements:Femoral Trochlear Depth

• Femoral Trochlear Depth > 5 mm

• Remember, the femoral trochlear depth will decrease with increasing flexion.

Evaluating the Measurements:Femoral Trochlear Angle

• Femoral Trochlear Angle <130 degrees

• The femoral trochlear angle or sulcus angle will normally decrease with increasing flexion.

• Some studies have indicated a normal trochlear angle as high as 156 degrees.

Evaluating the Measurements:Patellar Displacement

• Congruence Angle <0 degreesor

• Patellar Subluxation >0 mm• The patella may be slightly lateral at full

extension of the knee, but in the normal knee this will resolve by 20 degrees of flexion.

• Quadriceps contraction will exacerbate patellar lateralization, particularly at lesser degrees of knee flexion.

• The bottom line: The apex of the patella should align with or be medial to the apex of the femoral trochlea.

Evaluating the Measurements:Patellar Angulation

• Patellar Tilt Angle >8 degrees

• The normal patellar tilt angle should open laterally. If the lines drawing the tilt angle are parallel or open medially, the patella is tilted and abnormally aligned.

• Quadriceps contraction will exacerbate patellar tilt, particularly at lower degrees of knee flexion.

Evaluating the Measurements:

• The femoral trochlear angle and femoral trochlear depth will decrease with increasing flexion in the normal knee.

• The patellar displacement and patellar angulation will remain relatively constant throughout flexion in the normal knee but may increase with quadriceps contraction and decrease with progressive knee flexion.

• The type of patellar malalignment (i.e. displacement vs. angulation) may also change with quadriceps contraction.

Evaluating the Measurements:

• While a patient may only have only one abnormal measurement, the shape of the trochlea (including the angle and depth) and the location of the patella (including subluxation and tilt) are interelated and many patients will have more than one abnormality.

What if you can’t make a measurement?

• If there isn’t a femoral trochlea to measure at the level of the patellar apex, the rest of the measurements are meaningless.

• This may indicate patella alta and should be correlated with a lateral plain film.

Now for some examples…

• Shallow femoral trochlear angle

• Note: a shallow femoral trochlear angle does not necessarily mean an abnormal femoral trochlear depth.

131 deg.

Now for some examples…

• Shallow femoral trochlear depth

• While an abnormal femoral trochlear depth and angle often go together, they may be isolated findings.

4mm

Now for some examples…

• A subluxed patella.• The patella may be

subluxed in isolation without tilt.

+

Now for some examples…

• Abnormal patellar tilt.• The patella is not

lateralized.

6

Now for some examples…

• Abnormal patellar displacement and patellar tilt.

Significance of the Measurements

• There are no absolute radiographic measurements that indicate surgical correction.

• Each individual measurement should be considered as only one aspect of the patient’s potential problem. Many other factors, including ligamentous, cartilaginous and muscular components may be missed by this technique.

• Even meticulous radiographic evaluation is no substitute for a thorough history and careful physical exam.

Significance of the Measurements

• Each measurement has an associated cause or underlying anatomic abnormality.

• Shallow femoral trochlear depth and angle may indicate trochlear dysplasia or sequelae of previous dislocation as well as predisposition to recurrent patellar dislocation.

• An abnormal patellar tilt and angulation may indicate a tight lateral retinaculum, deficient medial ligamentous support, or malposition of the distal extensor mechanism (the tibial tubercle).

Significance of the Measurements

• Identification of the anatomic abnormality allows the orthopedist to select the appropriate treatment for the patient.

• Options for treatment of include rest, physical rehabilitation, NSAIDs or surgical intervention (lateral release, proximal or distal realignment, medial retinaculum repair, osteotomy or patellectomy).

Significance of the Measurements

This patient with abnormal patellar subluxation benefited from distal realignment (osteotomy and

tubercle transfer).

Conclusions

• Patellar instability is an important cause of anterior knee pain.

• Patellar instability can be easily and dynamically studied by CT.

• Measurements taken by CT of the patellofemoral joint can be used to identify the anatomic abnormalities contributing to patellar instability and help determine the patient’s therapeutic options.

• Beaconsfield T. Pintore E. Maffulli N. Petri GJ. Radiological measurements in patellofemoral disorders. A review. Clinical Orthopaedics & Related Research. (308):18-28, 1994 Nov.

• Biedert RM. Gruhl C. Axial computed tomography of the patellofemoral joint with and without quadriceps contraction. Archives of Orthopaedic & Trauma Surgery. 116(1-2):77-82, 1997.

• Dejour H. Walch G. Nove-Josserand L. Guier C. Factors of patellar instability: an anatomic radiographic study. Knee Surgery, Sports Traumatology, Arthroscopy. 2(1):19-26, 1994.

• Delgado-Martinez AD. Estrada C. Rodriguez-Merchan EC. Atienza M. Ordonez JM. CT scanning of the patellofemoral joint. The quadriceps relaxed or contracted? International Orthopaedics. 20(3):159-62, 1996.

• Fulkerson JP. Diagnosis and treatment of patients with patellofemoral pain. American Journal of Sports Medicine.  30(3):447-56, 2002 May-Jun.• Guzzanti V. Gigante A. Di Lazzaro A. Fabbriciani C. Patellofemoral malalignment in adolescents. Computerized tomographic assessment with or without quadriceps

contraction. American Journal of Sports Medicine. 22(1):55-60, 1994 Jan-Feb.• Imai N. Tomatsu T. Nakaseko J. Terada H. Clinical and roentgenological studies on malalignment disorders of the patello-femoral joint. Part II: Relationship between

predisposing factors and malalignment of the patello-femoral joint. Journal of the Japanese Orthopaedic Association. 61(11):1191-202, 1987 Nov.• Inoue M. Shino K. Hirose H. Horibe S. Ono K. Subluxation of the patella. Computed tomography analysis of patellofemoral congruence. Journal of Bone & Joint Surgery -

American Volume. 70(9):1331-7, 1988 Oct.• Laurin CA. Dussault R. Levesque HP. The tangential x-ray investigation of the patellofemoral joint: x-ray technique, diagnostic criteria and their interpretation. Clinical

Orthopaedics & Related Research. (144):16-26, 1979 Oct.• Martinez S. Korobkin M. Fondren FB. Hedlund LW. Goldner JL. Computed tomography of the normal patellofemoral joint. Investigative Radiology.  18(3):249-53, 1983 May-

Jun.• Masri BA. McCormack RG. The effect of knee flexion and quadriceps contraction on the axial view of the patella. Clinical Journal of Sport Medicine.  5(1):9-17, 1995.• Merchant AC. Mercer RL. Jacobsen RH. Cool CR. Roentgenographic analysis of patellofemoral congruence. Journal of Bone & Joint Surgery - American Volume.

56(7):1391-6, 1974 Oct. • Post WR. Teitge R. Amis A. Patellofemoral malalignment: looking beyond the viewbox. Clinics in Sports Medicine.  21(3):521-46, x, 2002 Jul.• Reikeras O. Hoiseth A. Patellofemoral relationships in normal subjects determined by computed tomography. Skeletal Radiology.  19(8):591-2, 1990.• Schutzer SF. Ramsby GR. Fulkerson JP. Computed tomographic classification of patellofemoral pain patients. Orthopedic Clinics of North America. 17(2):235-48, 1986 Apr.• Scuderi, GR, et al. The Patella, Springer-Verlag, New York, 1995• Vahasarja V. Lanning P. Lahde. Serlo W. Axial radiography or CT in the measurement of patellofemoral malalignment indices in children and adolescents? Clinical Radiology.

51(9):639-43, 1996 Sep.• Walker C. Cassar-Pullicino VN. Vaisha R. McCall IW. The patello-femoral joint--a critical appraisal of its geometric assessment utilizing conventional axial radiography and

computed arthro-tomography. British Journal of Radiology. 66(789):755-61, 1993 Sep.

Bibliography

THE END