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Anatomy and Metabolism of Bone

Knee Extensors!!navigator!!

= quadriceps muscle consisting of

  1. Vastus medialis m.
  2. Vastus lateralis m.
  3. Vastus intermedius m.
  4. Rectus femoris m.

Insertion: combined as quadriceps tendon on patella

Pes Anserinus!!navigator!!

[pes, Latin = foot; anser, Latin = goose]

= tendinous configuration of 3 flexors + medial rotators of knee joint attaching inferomedially to tibial tuberosity

mnemonic:Say GraceSe before eating goose

  • Sartorius tendon (anterior)
  • Gracilis tendon (middle)
  • Semitendinosus tendon (posterior)

Patella Alta!!navigator!!

= high-riding patella as normal anatomic variant

  • Cause: patellar tendon too long → reduced patellar contact area → patellar dislocation
    • High-riding patella in 25% of patients with acute patellar dislocation
  • mostly asymptomatic

MR measurement:

  • patellar height ratio (Insall-Salvati index) = TL÷PL = patellar tendon length (TL) divided by superoinferior diameter of patella (PL) 1.1 ± 0.1 = normal; >1.3(M) or >1.5 (F) = patella alta; <0.74 (M) or <0.79 (F) = patella baja (infera)

Cruciate Ligaments!!navigator!!

  • Both cruciate ligaments are intracapsular but extrasynovial!

Anterior Cruciate Ligament (ACL)

  • Function: limits anterior tibial translation
  • Origin: inner face of lateral femoral condyle
  • Insertion: noncartilaginous region of anterior aspect of intercondylar eminence of tibia
  • Anatomy: several distinct bundles of fibers
    1. large posterior bulk = spiraling together at femoral origin
    2. small anteromedial bundle diverging at tibial insertion
  • thin solid taut dark band (sagittal MR with knee in extension) almost parallel to intercondylar roof (= Blumensaat line):
    • with knee extension posterolateral band taut
    • with increasing flexion:
      anteromedial band becomes more taut + posterolateral band more lax
  • thin hypointense band parallel to inner aspect of lateral femoral condyle + fanlike configuration toward tibial spine (coronal MR)
  • thin ovoid hypointense band proximally, elliptical configuration distally with higher intensity (axial MR)
  • greater SI than posterior cruciate ligament (due to anatomy)

Posterior Cruciate Ligament (PCL)

  • Function: limits posterior tibial translation
  • Origin: in a depression posterior to intercondylar region of tibia below joint surface
  • Insertion: most distal + anterior aspect of inner face of medial femoral condyle
  • thick dark band slightly posteriorly convex (arclike course on sagittal MR with knee in extension)
  • medial to ACL (coronal MR)

Collateral Ligaments of Knee Joint!!navigator!!

Medial (Tibial) Collateral Ligament

  • Origin: just distal to adductor tubercle of femur
  • Insertion: anteromedial face of tibia distal to level of tibial tubercle about 5 cm below joint line
  1. deep portion:
    • meniscofemoral ligament
    • meniscotibial ligaments
  2. superficial portion
    • vertical band from femoral epicondyle to pes anserinus
    • posterior oblique ligament = posterior oblique band from femoral epicondyle to semimembranosus tendon
  • deep and superficial dark bands separated by a thin bursa + fatty tissue (on coronal MR)

Lateral (Fibular) Collateral Ligament

  • Origin: lateral aspect of lateral femoral condyle
  • Insertion: styloid process of fibular head
  • bicipital tendon + iliotibial band join lateral collateral lig.

Arcuate Complex

  • Function: provides posterolateral stabilization
  • Consists of: lateral (fibular) collateral ligament + biceps femoris tendon + popliteus muscle and tendon + popliteal meniscal ligament + popliteal fibular ligament + oblique popliteal ligament + arcuate ligament + fabellofibular ligament + lateral gastrocnemius muscle

Posteromedial Corner of Knee

  1. Semimembranosus tendon
    Attachment: infraglenoid tubercle of posteromedial tibia; posterior joint capsule; posterior horn of medial meniscus
  2. Posterior joint capsule
  3. Posterior oblique ligament

Menisci!!navigator!!

= wedge-shaped semilunar fibrocartilaginous structures

  • Function: absorb shock, distribute axial load, assist in joint lubrication, facilitate nutrient distribution
  • Margin:
    1. superior concave surface conforming to femoral condyle → increase in contact area
    2. inferior flat base that attaches to central tibial plateau via anterior + posterior root ligament anchors → maintain normal meniscal position + biomechanical function
    3. thick peripheral portion
    4. tapered central free edge
  • Composition: collagen bundles oriented in
    1. circumferential (longitudinal) type I collagen bundles parallel to long axis of meniscus → hoop strength resisting axial load + preventing meniscal extrusion
    2. radial thin fibers perpendicular to longitudinal bundles forming a lattice → tying bundles together + providing structural support
  • Subdivision into thirds:
    • anterior horn
    • meniscal body
    • posterior horn
  • Attachment:
    • posterior root
    • anterior root
      • lateral meniscus (LM)
        • striated / comb-like appearance of anterior horn ← intimate association between anterior root of lateral meniscus + ACL insertion site
      • medial meniscus (MM)
        • anomalous insertion paralleling ACL → mimicks MM tear
        • anterior root may insert along anterior margin of tibia → mimicks MM subluxation
        • peripheral attachment to deep fibers of medial collateral ligament → meniscus less mobile
  • Imaging:
    • on sagittal images of menisci:
      • “bow-tie” structure peripherally
      • opposing triangles centrally
      • posterior horn larger than anterior horn for MM
      • anterior + posterior horns of similar size + shape for LM
    • on coronal images:
      • triangular shape through body of meniscus
      • wedge-shaped through horn of meniscus
    • on axial images:
      • open C-shaped configuration of medial meniscus
      • increase in width from anterior to posterior
  • Variants mimicking a tear:
    1. Transverse meniscal (geniculate) ligament (83–90%)
      = thin fibrous band that connects and stabilizes the anterior horns of the menisci
      • overrides superior aspect of menisci before completely fusing to menisci

      DDx: anterior root tear
      Dx: Trace cross section of transverse ligament through infrapatellar fat pad on more central SAG images!
    2. Meniscofemoral ligaments (MFL) in 89–93%
      1. Wrisberg ligament
        • posterior to posterior cruciate ligament
      2. Humphrey ligament
        • anterior to posterior cruciate ligament
          mnemonic:under the hump (of the PCL)

      Origin: superior + medial aspect of posterior horn of lateral meniscus
      Insertion: lateral aspect of medial femoral condyle
      Function: assist PCL + help control mobility of posterior horn of lateral meniscus during knee flexion + extension
      • demonstrated in ⅓ of cases on SAG images; usually limited to single most medial image!
    3. Popliteomeniscal fascicles (visualized in 90%)
      1. anteroinferior fascicle = floor of popliteal hiatus
      2. posterosuperior fascicle = roof of popliteal hiatus

      = synovial-lined fibrous bands that attach to posterior horn of lateral meniscus + help form popliteal hiatus
      Function: stabilize posterior horn
    4. Popliteal hiatus
      • = separates lateral meniscus from joint capsule
      • above posterior aspect of lateral meniscus on most superficial SAG slice!
      • popliteal tendon moves behind + inferior to meniscus on adjacent deeper SAG sections!
    5. Oblique meniscomeniscal ligament (1–4%)
      = connects meniscal horns in X-wise fashion on AXIAL image
      1. medial oblique meniscomeniscal ligament
        • anterior horn of medial meniscus to posterior horn of lateral meniscus
      2. lateral oblique meniscomeniscal ligament
        • anterior horn of lateral meniscus to posterior horn of medial meniscus
    • traverses intercondylar fossa between ACL and PCL

Anatomic Variants of Menisci

  1. Diskoid meniscus
    = abnormally shaped enlarged meniscus with further central extension onto tibial articular surface
    Prevalence: 1.5–3% for lateral meniscus; 0.12–0.3% for medial meniscus
    Side: lateral÷medial meniscus = 10÷1
    Age: children, adolescents
    • body of meniscus measures ≥15 mm on midline coronal image
    • ≥3 bow-tie shapes on contiguous sagittal (4-mm-thick) sections
    • diffuse intrameniscal signal intensity ← increased meniscal vascularity
  2. Meniscal flounce
    = rippled appearance of free nonanchored inner edge of medial meniscus
    Prevalence: 0.2–0.3% of asymptomatic knees
  3. Meniscal ossicle (rare)
    Cause: developmental, degenerative, posttraumatic
    Location: posterior horn of MM
    • calcification on radiograph mimicks loose body
    • increased signal intensity can mimic a tear
  4. Chondrocalcinosis
    Prevalence: 5– 15% (increasing with age)
    • calcifications on radiograph
    • increased signal intensity → lowers sensitivity and specificity for detection of meniscal tear

Posterolateral Corner Structures!!navigator!!

= arcuate complex

  1. Fibular collateral ligament
  2. Arcuate ligament
  3. Popliteus musculotendinous complex
  4. Popliteofibular ligament
  5. Fabellofibular ligament
  6. Posterolateral capsule

Popliteus Musculotendinous Complex

  • Origin: posteromedial tibial surface proximal to soleal line forming floor of popliteal fossa
  • Course: forms long strong popliteal tendon that passes underneath posterolateral joint capsule + arcuate ligament (extracapsular); enters knee through popliteal hiatus posteroinferiorly behind posterior horn of lateral meniscus; passes beneath lateral collateral ligament + tendon of biceps femoris
    = intracapsular – extraarticular – extrasynovial
  • Attachment: popliteal notch on lateral aspect of lateral femoral condyle; anteroinferior to proximal attachment of lateral collateral ligament on lateral epicondyle
  • Function: in non–weight-bearing state primary internal rotator of tibia on femur; in weight-bearing state external rotator of femur on leg
  • fluid-filled popliteus bursa surrounds popliteus muscle and tendon

Fibular Collateral Ligament

= (TRUE) LATERAL COLLATERAL LIGAMENT

  • Origin: lateral femoral epicondyle
  • Attachment: lateral aspect of fibular head + neck anterior and distal to fibular styloid process; often conjoined insertion with biceps femoris tendon
  • Function: simple passive restraint
  • extracapsular WITHOUT meniscal attachment

Arcuate Ligament

= Y-shaped inconstant thickening of posterolateral capsule

  1. lateral limb inserts into posterolateral joint capsule
  2. medial limb extends medially over popliteus muscle to oblique popliteal ligament

Popliteofibular Ligament

= attachment of popliteus tendon to fibular head

  • Origin: popliteus tendon near myotendinous junction
  • Attachment: posterior aspect of fibular styloid process posteromedial to the biceps insertion
  • Function: one of the strongest lateral stabilizers in knee

Fabella (present in 20%)

= sesamoid bone in lateral head of gastrocnemius muscle

  • Function: anchors fabellofibular ligament

Fabellofibular ligament (present in 40%)

  • Origin: fabella
  • Attachment: styloid process of fibular head posterior to arcuate ligament + lateral to popliteofibular ligament / to lateral femoral condyle (in absence of fabella)

Cysts and Bursae of the Knee!!navigator!!

  1. Suprapatellar bursa
  2. Popliteal bursa
  3. Pes anserine bursa
  4. Semimembranosus-tibial collateral ligament bursa
  5. Prepatellar bursa
  6. Infrapatellar bursa
  7. Tibial collateral ligament bursa
  8. Tibiofibular joint cyst
  9. Popliteus bursa
  10. Meniscal cyst
  11. Cruciate ligament cyst
  12. Ganglion (mucinous degeneration of ACL)

Outline