Where to Draw the Line:
Anatomical Measurements Used to Evaluate
Patellofemoral Instabilityy
Murray Grissom, MD1
Bao Do, MD2
Kathryn Stevens, MD2
1Santa Clara Valley Medical Center, San Jose, CA
2Stanford Hospital and Clinics, Stanford, CA

Objectives
1. Clinical Considerations
2. Anatomical Factors• Patellar height• Patellar tilt and displacement• Patellar congruence angle• Trochlear dysplasia
3. Translational forces• Q angle• Tibial tubercle-trochlear groove distance
4 Checklist for patellofemoral instability
4.

Patellar Instability: Clinical Considerations• Presentation
 Isolated anterior knee pain Overt dislocation• Progression

 Articular cartilage injuries
 Osteochondral fractures
 Patellofemoral osteoarthritis• Etiology

 Acute: traumatic dislocation
 Chronic: recurrent dislocation• Surgical Treatment: Trochleoplasty Elevation of lateral trochlear facet
pg trochleoplastypy Deepening Recreation of trochlear sulcus
 Recession wedge trochleoplasty
 Prominent trochlear groove recessed
to level of anterior femoral cortex

ClinicalAnatomic Factors Translational Forces

Checklist

Imaging Evaluation of Patellofemoral Instability
Anatomical Factors

Translational Forces

Measurement

NormalMeasurement

NormalPatellar Height

Insall Salvati & CatonInsall-Salvati
CatonDeschamps ratios = 1.0 ± 0.2

Quadriceps Angle

Males: < 1515°
Females: > 20°

Patellar Tilt - radiographIndex ∠ open laterally

Tibial Tubercle-Trochlear
Groove Distance< 2 cmPatellar Tilt - CT

Index ∠ < 2020°

Patellar Displacement

Patella intersects reference line

Patellar Congruence-28 ° < index ∠ < +16°

Crossing sign

Trochlear floor outline nevercrosses lateral femoral condyleoutlineTrochlear Inclination

Index ∠ > 11°

Trochlear Facet
Asymmetry

medial to lateral facet ratio >40%

Trochlear Depth> 3 mmClinicalAnatomic Factors Translational Forces

Checklist

Patellar Height: Insall-Salvati Ratio
Normal▪ Applicable to lateral film, ideallywith knee in 30°
30 flexion▪ Measure the greatest diagonallength of the patella (B, yellowline)▪ Measure the length of patellar
tendon (A, red line) from the lower
pole of the patella to the insertion
into the tibial tubercle▪ Normal ratio of A:B = 1.0 ± 0.2

PatellarPt lllength (B)Patellar tendon
length (A)Normal:A:B = 1.0 ± 0.2

ClinicalAnatomic Factors Translational Forces

Checklist

Patellar Height: Insall-Salvati Ratio

Patellarlength (B)Patellarlength (B)Patellar tendonllengthth (A)Patella alta: A:B > 1.2

ClinicalPatellar tendon
length (A)Patella baja: A:B < 0.8

Anatomic Factors Translational Forces

Checklist

Patellar Height: Caton-Deschamps Index
Normal▪ Applicable to lateral film, ideallywith knee in 30°
30 flexion▪ Measure the articular facet lengthof the patella (B, blue line)▪ Measure the distance between
the inferior edge of the patellar
Patellar cartilage
articular surface and the anterolength (B)superior angle of the tibia (A(A,Inferior patellar edgegreen line)
to anterosuperior▪ Normal ratio A:B = 1.0 ± 0.2

tibial angle length (A)Normal:
A:B
A B = 1.0
1 0 ± 0.2
02

ClinicalAnatomic Factors Translational Forces

Checklist

Patellar Height: Caton-Deschamps Index

Patellarcartilage
length (C)PatellarPt llcartilage
length (C)Inferior edge ofthe patellar
articular surfaceand anterosuperior angle ofthe tibia (D)Patella alta: A:B > 1.2

ClinicalInferior edge ofthe patellar
articular surface
and anterosuperior angle ofthe tibia (D)Patella baja: A:B < 0.8

Anatomic Factors Translational Forces

Checklist

Patellar Tilt And Displacement: Laurin Method

RLine C
mediallateralAxial radiograph technique▪ Patellar tilt
✓ Draw Line A: line tangent to thesummits of the femoral condyles✓ Draw Line B: line tangent to thelateral patellar facet✓ Normal: Angle (∠AB) between
Line A and Line B is open
laterallyNormal

Line B∠ABLine A▪ Lateral displacement✓ Draw Line C: line perpendicularto Line A and 1 mm lateral tosummit of medial femoral
condyle✓ Normal: Line C intersects patella

ClinicalNormal:∠AB
AB = open laterallyl t ll
Normal:Line C intersects patella

Anatomic Factors Translational ForcesChecklist

Patellar Tilt And Displacement: Laurin Method
Lateral Tilt & Displacement
RmedialmlaateralLine
Li CLine B∠ABLine A

Laterall patellar
Lll tilt:
il∠AB = open medially
Lateral patellar displacement:
Patella lies lateral to Line C
ClinicalAnatomic Factors Translational Forces

Checklist

Patellar Tilt: Dejour Method
NormalCT technique▪ Patellar tilt

Line BRmediaallateraal✓ Axial image at level of midpole of the patella✓ Draw Line A: line tangent toposterior femoral condyles✓ Draw Line B: line throughtransverse axis of patella✓ Normal: ∠A
AB < 20°✓ Lateral tilt: ∠AB > 20°∠ABLine AN
Normall patellar
ll tilt:il ∠AB < 20°
20°
ClinicalAnatomic Factors Translational Forces

Checklist

Patellar Congruence Angle: Merchant Method
NormalClinicalCR∠ABC = -11-11°

media
al (−°)−°lateral (+°)

Axial radiograph technique✓ Draw Line DB: line from apex oflateral femoral condyle (D) totrochlear groove (B)✓ Draw Line BE: line from trochlear
groove (B) to apex of medial femoral
condyle (E)✓ Draw Line BC: line bisecting Line DB& Line BE✓ Draw Line AB: line fromftrochlear
groove (B) to lowest point on thearticular surface of the patella (A)✓ Convention:
 ∠ABC: (+) if A is lateral to line BC ∠ABC: (-) if A is medial to line BCADEBNormal Congruence angle:-28 ° < ∠ABC < +16°

Anatomic Factors Translational Forces

Checklist

Patellar Congruence Angle: Merchant Method
L t l Tilt
Lateral∠ABC = +45°

C
meddial (−°)+°+laterral (+°)

RDAE
B

Lateral patellar tilt:∠ABC > +16°
ClinicalAnatomic Factors Translational Forces

Checklist

Trochlear Dysplasia: Crossing Sign
Lateral radiograph▪ Draw outline of the trochlear groove = trochlear floor▪ Draw outline of the ventral surface of lateral femoral condyle

Normal: trochlear floor outline nevercrosses lateral condyle outline

Clinical

Trochlear dysplasia: trochlear floor
outline crosses lateral condyle outline

Anatomic Factors Translational ForcesChecklist

Trochlear Dysplasia: Lateral Trochlear Inclination
NormalL∠ABlateralcartilaginous trochlea is demonstrated▪ Draw Line A: line tangent to posterior
femoral condyles▪ Draw Line B: line tangent to lateraltrochlear facet▪ Measure lateral trochlear inclination
anglel (∠AB)( AB)

Line BmedialAxial MRI/CT technique▪ Axial image
g at most p
proximal level

Line A

Normal: ∠AB > 11°
ClinicalAnatomic Factors Translational Forces

Checklist

Trochlear Dysplasia: Lateral Trochlear Inclination
T hlTrochlear
DDysplasial iRLine B

lateralmedialm∠ABLine A
Trochlear Dysplasia:∠AB < 1111°
ClinicalAnatomic Factors Translational Forces

Checklist

Trochlear Dysplasia: Facet Asymmetry
Normalfemorotibial joint space▪ Measure length of medial
trochlear facet ((Line A))▪ Measure length of lateral trochlear
facet (Line B)▪ Measure ratio (A:B) of medial
facet length (A) to lateral facet
length (B)RLine BLine Amedial▪ Axial image 3 cm above thelateral

Axial MRI/CT technique

Normal ratio: A:B > 40%

ClinicalAnatomic Factors Translational Forces

Checklist

Trochlear Dysplasia: Facet Asymmetry
Trochlea Dysplasia
LLine B
laterallmediallLine A

Trochlear Dysplasia: A:B < 40%

ClinicalAnatomic Factors Translational Forces

Checklist

Trochlear Dysplasia: Depth

Trochlear Depth =Clinical

A+B-C
2

medialeA
Line

Line C

femorotibial joint space▪ Draw reference Line D tangent toposterior femoral condyles▪ Measure the maximal anteroposterior
distance of the:✓ Medial femoral condyle (Line A)✓ Lateral femoral condyle (Line B)▪ Measure the anteroposterior distance(Line C) between the deepest point of
the trochlear groove and Line D▪ Calculate the trochlear depth:RlateralAxial MRI/CT technique▪ Axial image
g 3 cm above theLine B

NormalLine
Li D
Normal: trochlear depth > 3 mm

Anatomic Factors Translational Forces

Checklist

Trochlear Dysplasia: DepthLine BLine C

Trochlear Depth =A+B-C2

laterallmediall

LLine A

Trochlea Dysplasia

Line DTrochlear dysplasia:
Trochlear depth < 3 mmClinicalAnatomic Factors Translational ForcesChecklist

Quadriceps (Q) angle∠AB∠ABRadiograph technique

Normal Q angle:
Males: ∠AB < 15°
FFemales:l∠AB
AB < 20°

Clinical

Linne ALinne B

Linne Bsuperior iliac spine to the centerof the patella▪ Draw Line B: line from the centerof the patella to the tibial
tuberosity▪ Measure the Q angle (∠AB)

Linne A▪ Draw Line A: line from the anterior

Abnormal Q angle:
Males: ∠AB > 15°
FFemales:l∠AB
AB > 20°

Anatomic Factors Translational ForcesChecklist

Tibial Tubercle-Trochlear Groove DistanceSingle image technique• On axial image through deepest portion oftrochlear groove, draw:✓ Line tangent to posterior condyles (posteriorcondyle line)✓ Line perpendicular to posterior condyle linethat passes through deepest point of
trochlear groove (trochlear groove line)• Transpose:✓ Posterior condyle and trochlear groove lines
onto axial image through tibial tuberosity• On axial image through tibial tuberosity, draw:✓ Line perpendicular to posterior condyle linethat passes through tibial tubercle (tibialtubercle line)• Measure:✓ Shortest distance between tibial tubercle andtrochlear groove lines (TT-TG distance)

ClinicalLevel oftibialtubercle

Normal TT-TG distance: < 20 mm
Abnormal TT-TG distance: > 20 mm▪ Associated with pathologic
patellar instability

Anatomic Factors Translational Forces

Checklist

Tibial Tubercle-Trochlear Groove Distance
Double image technique• Superimpose axial images through (1)deepest portion of trochlear groove and (2)tibial tuberosity, then draw:
 Line tangent to posterior condyles(posterior condyle line)
 Line perpendicular to posterior condyleline that passes through deepest pointof trochlear groove (trochlear grooveline)
 Line perpendicular to posterior condyleline that passes through tibial tubercle(tibial tubercle line)

Superimposed images• Measure:✓ Shortest distance between tibial tubercle
and trochlear groove lines (TT-TGdistance)

ClinicalNormal TT-TG distance: < 20 mm
Abnormal TT-TG distance: > 20 mm▪ Associated with pathologic
patellar instability

Anatomic Factors Translational Forces

ChecklistChecklist for Patellofemoral Instability
Anatomical Factors

Translational Forces

Measurement

NormalMeasurement

NormalPatellar Height

Insall Salvati & CatonInsall-Salvati
CatonDeschamps ratios = 1.0 ± 0.2Quadriceps Angle

Males: < 1515°
Females: > 20°

Patellar Tilt - radiographIndex ∠ open laterally

Tibial Tubercle-Trochlear
Groove Distance< 2 cmPatellar Tilt - CTIndex ∠ < 2020°

Patellar Displacement

Patella intersects reference line

Patellar Congruence-28 ° < index ∠ < +16°

Crossing sign

Trochlear floor outline nevercrosses lateral femoral condyleoutlineTrochlear Inclination

Index ∠ > 11°

Trochlear Facet
Asymmetry

medial to lateral facet ratio >40%

Trochlear Depth> 3 mmClinicalAnatomic Factors Translational Forces

ChecklistReferences
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2. Caton, J., et al., [Patella infera. Apropos of 128 cases]. Rev Chir Orthop Reparatrice Appar Mot,
1982. 68(5): p. 317-25.
3 Laurin,
3.
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Am,, 1974. 56(7):( ) p
p. 1391-6.
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11.Koeter, S., et al., A new CT scan method for measuring the tibial tubercle trochlear groove distancein patellar instability. Knee, 2007. 14(2): p. 128-32.