Knee Anatomy

Knee Anatomy

Reza Omid, M.D.Assistant Professor Orthopaedic

SurgeryShoulder & Elbow Reconstruction

Sports MedicineKeck School of Medicine of USC

Bony Anatomy

Tibiofemoral joint

Patellofemoral joint

Femoral Condyles

– A – Lateral Condyle»Smaller radius of

curvature»Smaller in all

dimensions»Extends more anteriorly

– B – Medial Condyle»Larger radius of

curvature»Extends more distally

– C – Intercondylar notch

Tibial Plateau– D – Medial Plateau

»Greater surface area»Concave»Circular shape

– E – Intercondylar Eminence

– F – Lateral Plateau»Smaller surface area»Convex»Oval shape

Patella

–Sesamoid bone in quadriceps

–Dividing central ridge

–Comprised of seven facets

»Medial and Lateral facets divided into 3rds

»7th facet is most medial (odd facet)

–Medial half usually smaller

–Thick hyaline cartilage (5.5mm)

Femoral Sulcus

–Lateral wider and higher

–Both with sagittal convexity

Screw Home Mechanism

• Knee achieves terminal extension via the “screw home mechanism

• The tibia externally rotates in relation to the femur.

• When the knee needs to flex, the popliteus contracts which causes internal rotation of the tibia and in essence unlocking the knee and allowing it to bend

Bony Alignment

Popliteal Artery Originates at the adductor hiatus and passes through the

popliteal fossa, then deep to the fibrous arch over the soleus muscle

Divides into the anterior and posterior tibial arteries at the distal aspect of the popliteus muscle

Popliteal Artery

• The popliteal artery is 9mm posterior to the posterior cortex of the tibia at 90° of flexion and even closer in extension.

• Place retractors biased to the medial side when possible.

Skin Blood Flow

• If two longitudinal incisions are present, the more lateral incision should be used (if allows adequate exposure) because most of blood supply comes in medially.

• The lateral skin edge is more hypoxic than the medial skin edge so keep this in mind when placing sutures.

Tibial Nerve•Initially lateral to the popliteal artery

•Crosses at midpoint to end medial to the artery at soleus arch

Common Peroneal Nerve

•Lateral aspect of the popliteal space

•Medial and posterior to the biceps femoris tendon

Infrapatellar Branch of Saphenous

Patellofemoral Biomechanics

Joint Reactive Force– In flexion, patella compressed onto femur creating joint reactive force

–Stair climbing – 3.5 X BW

–Deep bends – 7-8 X BW

Menisci• Primarily type I collagen with fibers

arranged obliquely, radially, and vertically

• Outer 10% to 30% has blood supplied from the perimeniscal capillary plexus off the superior and inferior medial and lateral genicular arteries

Meniscus FunctionLoad Transmission

– 50% load transmitted in extension– 85% load transmitted at 90 degrees flexion– Resection of 15-34% increases pressure 350%

Secondary Stabilization– Medial meniscus provides anterior restraint

»Especially in ACL deficient knee

Lateral Meniscus• Loose peripheral attachment

allows greater translation during motion– Average excursions of the

menisci with knee flexion»5.2 mm for the medial»11 mm for the lateral

• Bare area anterior to popliteus tendon

• Two highly variable meniscofemoral ligaments attach it to medial femoral condyle:

• Anterior – Humphrey• Posterior – Wrisberg’s

Ligaments

Tensile strengths of various knee ligaments:•MCL ~ 4400-5000N•PCL ~2500-3000N•ACL ~ 2200-2500N•LCL ~750N

Anterior Cruciate Ligament

– 26-38 (33) mm in length» ACL graft selection you aim for at

least 100-110mm graft length because it needs about ~33mm for the tibial tunnel, ~33mm for the femoral tunnel and ~33 for the graft itself

– 11 mm in width– Primary restraint

» Anterior translation of tibia (74-85%)

» Normal 3-5mm of translation– Secondary restraint

» Internal rotation» Varus/Valgus» Hyperextension

Anterior Cruciate Ligament

Two bands–Anteromedial band taut in flexion

–Posterolateral band taut in extension

Anterior Cruciate Ligament

Femoral Attachment– Posterior portion of medial surface of

LFC– Oriented in line of axis of femur in

extension– Footprint in shape of circular segment– Posterior convexity 4 mm anterior to

articular surface– Surface area measures 16-24 x 11 mm– Lateral to midline on AP view– Posterosuperior on lateral view

Anterior Cruciate Ligament

Tibial Attachment– Anterolateral to medial spine– Insertion has oval shape– Sections attach to bone, AHLM,

PHLM– 15 mm posterior to anterior tibia– 17-30 x 11 mm surface area– Just lateral to midline on AP– 40% back on lateral view

Lateral Bifurcate Ridge

–Running perpendicular to the lateral intercondylar ridge) seperates the origins of the anteromedial and posterolaterla bundles.

Lateral Intercondylear Ridge

• Resident’s ridge on the lateral femoral condylar wall denotes the lateral intercondylear ridge and marks the most anterior and superior extent of the femoral origins of the ACL.

Anterior Cruciate Ligament

Blood Supply– ACL completely ensheathed in fold of synovial membrane– Although intraarticular, technically extrasynovial– Main supply is middle geniculate with smaller contribution from

both inferior geniculates

Innervation– Branches of tibial nerve– Very few pain receptors in substance of ACL

Posterior Cruciate Ligament

–38 mm in length–13 mm in width–Narrowest diameter at midsubstance

–Anterolateral band»More robust, Taut in flexion

–Posteromedial band»Thinner, Taut in extension

Posterior Cruciate Ligament

Femoral Attachment– Lateral surface MFC– Shape of circular segment– Distal margin 3 mm proximal to articular

surface

Posterior Cruciate Ligament

Tibial Attachment– Depression between tibial plateaus– 1 cm distal to tibial articular surface– Can have contributions to PHLM as well as

meniscofemoral ligaments– Average width 13 mm

PCL BiomechanicsFunction

–Primary restraint»Posterior translation of tibia (90-95%)

»Greatest translation occurs at 75 degrees flexion

–Secondary restraint»Varus/valgus»External rotation

Medial Structures

Layer 1: Deep fascia and Sartorius

Layer 2: Superficial MCL, MPFLLayer 3: Joint capsule, Deep MCL

Pes Anserinus

Medial Ligaments• Superficial MCL (Medial Collateral Ligament)• Originates on medial epicondyle

– avg: 3.2 mm proximal and 4.8 mm posterior to medial epicondyle

• Tibial insertions (2) distal and proximal– Proximal: anterior arm of the semimembranosus tendon– Distal: broad-based, just anterior to the posteromedial

crest of the tibia, most located within the pesanserine bursa

• Posterior Oblique Ligament (POL)• superficial, central (main component), and capsular arms

• Deep MCL– Divided into meniscofemoral and meniscotibial ligaments

MCL

MCL BiomechanicsStability – Most

important in flexion when

posterior structures relaxed– Valgus rotation– External rotation– Medial/Lateral translation

Superficial MCL most important for stability (57-78%)Sectioning Deep

MCL does not result in instability if Superficial MCL

intact

Medial Patellofemoral Ligament

Runs transversely in Layer 2

Originates from adductor tubercle, femoral epicondyle, and

superficial MCL

Proximal fiber inserts on undersurface of VMO and vastus

intermedius

Distal fibers insert on superomedial patella

Width averages 1.3 cm

MPFL

MPFL BiomechanicsSoft tissue restraint of

extensor mechanismPatella subluxes most

easily at 20° knee flexionMPFL resists patellar

lateral subluxation greatest in extensionPrimary stabilizer

followed by patellomeniscal,

patellotibial, and medial retinaculum

Lateral StructuresLayer 1

– IT band– biceps tendon

Layer 2– Lateral retinaculum– patellofemoral ligaments

Layer 3– Joint capsule– LCL– arcuate ligament– fabellofibular ligament– popliteofibular ligament

Lateral Structures Layer 1

IT band biceps tendon

Layer 2 Lateral retinaculum patellofemoral ligaments

Layer 3 Joint capsule LCL arcuate ligament fabellofibular ligament popliteofibular ligament

Iliotibial BandCoalescence at greater

trochanter of tensor fascia lata, gluteus medius and

gluteus maximus

IT band continues distally to form the:

– IT tract» Inserts distally on Gerdy’s

tubercle and on distal femur through intermuscular septum

– Iliopatellar band» Inserts on lateral patella

resisting medial directed forces

IT Band Biomechanics

Functions– Stabilizes against varus

opening– Knee extensor in extension– Knee flexor in flexion– External rotator of tibia in >40

flexion

Lateral Collateral LigamentArises in fovea slightly proximal (1.4 mm) and

posterior (3.1 mm) to lateral epicondyleAttaches to V-shaped plateau of fibular head

(8.2mm distal to anterior edge)Surrounded by biceps femoris tendon distally

Average length 59-71 mmAP diameter 3.4 mm ML diameter 2.3 mm

LCL Origin

• Posterior (4.6 mm) and proximal (1.3 mm) to the lateral femoral epicondyle

• Posterior and superior to the insertion of the poplitieus (18mm away from each other)

LCL BiomechanicsTightest in extension, 0-30 degrees

Becomes looser in flexion >30 degreesPrimary restraint to varus

Secondary restraint to ER and posterior translation

Posterolateral Corner

• FCL• Popliteus tendon• Popliteofibular lig

Posterolateral CornerStatic Stabilizers (highly variable)– LCL– Fabellofibular ligament– Short lateral ligament– Popliteofibular ligament– Arcuate ligament– Posterolateral capsule– Posterior horn lateral

meniscus– Lateral coronary ligament

Posterolateral Corner

Dynamic Stabilizers– IT band– Lateral

gastrocnemius– Biceps femoris– Popliteus

Popliteus Complex

Dynamic– Popliteus muscle

Static– Popliteofibular ligament– Popliteotibial fascicle– Popliteomeniscal fascicle

Popliteus Muscle• Originates from

posteromedial surface of proximal tibia

• Tendon passes in hiatus of coronary ligament, crosses under LCL, inserts on lateral femoral condyle 3-5 mm proximal to articular surface

Popliteofibular ligamentAverage length 42 mm

Descends from popliteus muscle (at

musculotendinous junction) to posterosuperior fibular

head

Composed of anterior and posterior fascicle

Functions as pulley to the popliteus

Arcuate LigamentFibers oriented in various directions

Y-shaped configuration over popliteus

Medial limb terminates into oblique popliteal

ligament

Lateral limb invariable present, and is less

distinct

Fabellofibular vs Short Lateral Ligament

Fabellofibular ligament– Present when fabella present (8-16%)– Courses from fabella to fibular head

Short lateral ligament– Present when fabella absent– Courses from lateral femur to fibular head– Represents a homologue of the

fabellofibular ligament