Knee Complex: Functional Anatomy, Biomechanics, Assessment, and Injuries

Knee Complex: Functional Anatomy

Learning Objectives

  • Understand the functional anatomy of the tibiofemoral and patellofemoral joints.

Knee Complex

  • Consists of:
    • Tibiofemoral joint
    • Patellofemoral joint

Tibiofemoral Joint – Functional Anatomy

  • Joint Type: Double condylar joint with approximately 2 degrees of freedom.
  • Articulating Surfaces: Large rounded femoral condyles on a relatively flat tibial plateau.
  • Lined with hyaline cartilage.
Stability/Mobility Features
  1. Bone
    • Large femoral condyles on a shallow tibial plateau.
    • Plateau slopes posteriorly.
    • Longer medial femoral condyle (MFC) and larger medial tibial plateau.
    • Intercondylar eminences.
  2. Passive Elements
    • Capsule
    • Menisci
    • Collateral ligaments (MCL, LCL)
    • Intra-articular ligaments (ACL, PCL)
  3. Active Elements
    • PFJ complex (Patellofemoral Joint)
    • Hamstrings/Calf
    • Popliteus
  4. Biomechanical Elements
    • Closed/open chain considerations.
Articulating Surfaces - Hyaline Cartilage
  • Avascular and aneural.
  • Rounded chondrocyte cells embedded in an extracellular matrix.
  • Extracellular matrix contains:
    • Proteoglycans (negatively charged).
    • Collagen Type II.
  • Four distinct layers with collagen fibers oriented in different directions to withstand large shear and tensile forces (up to 65 times body weight).
Menisci
  • Enhance joint congruence.
  • Increase contact area, protecting cartilage.
  • Provide shock absorption.
  • Aid in controlling rotation.
  • Shape: Medial meniscus is a semi-circle (larger), while the lateral meniscus is a three-quarter ring.
  • Thicker peripherally.
  • Medial meniscus is attached to the MCL, making it less mobile and more prone to injury.
  • Blood supply only to the outer 1/3.
  • Horns are richly innervated.
Collateral Ligaments
  • Medial Collateral Ligament (MCL)
    • Large and long, blending with the capsule and medial meniscus.
    • Primary restraint to valgus forces.
    • Assisted by the pes anserinus tendons (SGT - Sartorius, Gracilis, Semitendinosus).
  • Lateral Collateral Ligament (LCL)
    • Smaller/thinner/chord-like and no attachment to the capsule or meniscus.
    • Primary restraint to varus forces.
    • Assisted by the iliotibial band (ITB).
Cruciate Ligaments
  • General
    • Intracapsular, extrasynovial.
    • Provide anterior and posterior stability.
    • Assist with femoral sliding motion.
  • Anterior Cruciate Ligament (ACL)
    • Spirally arranged fibers with anteromedial (AMB) and posterolateral (PLB) bands.
    • PLB is taut towards extension.
    • AMB tightens gradually from 30° flexion, with maximal tightness between 70°-90°.
    • Vulnerable at 30 degrees of flexion.

Stability Summary Key Points

  • Mediolateral stability primarily provided by the collateral ligaments, with contribution from the cruciate ligaments and some contribution from the menisci.
  • Rotary stability is contributed by both collateral and cruciate ligaments along with some contribution of the menisci.
  • The LCL is not supported by the ITB laterally.
  • Injury to the medial meniscus will result in altered loading of the hyaline cartilage. The menisci support healthy cartilage by acting as shock absorbers and controlling articular motion.

Knee Complex: Clinically Relevant Biomechanics

Learning Objectives

  • Understand clinically relevant biomechanics and muscle function principles of the knee complex.

Arthrokinematics

  • Sagittal Plane: Flexion and Extension.
  • Transverse Plane: Adjunct and Conjunct Rotation. Evolute represents the changing axis of rotation during knee motion.
Open Kinetic Chain (OKC)
  • Concave tibial plateau on convex femoral condyles.
  • Roll and slide occur in the same direction.
Closed Kinetic Chain (CKC)

  • Convex femoral condyles on concave tibial plateau.

  • Roll and slide occur in opposite directions.

  • During weight-bearing knee flexion, the femoral condyles roll posteriorly and slide anteriorly to maintain the knee axis centered over the tibial plateau.

  • Anterior sliding during weight-bearing knee flexion is caused by tension in the anterior cruciate ligament (ACL); wedge-shaped menisci assist this anterior glide.

  • Posterior gliding of the femur on the tibia during weight-bearing knee extension is caused by tension in the posterior cruciate ligament (PCL).

Screw Home/Locking Mechanism
  • Occurs during the final 30 degrees of extension.
  • A result of the longer/larger medial femoral condyle.
  • OKC: Relative external rotation (ER) of the tibia on the femur.
  • CKC: Relative internal rotation (IR) of the femur on the tibia.
Unlocking
  • Initiation of flexion requires relative external rotation (ER) of the femur on the tibia.

Patellofemoral Joint

Stability/Mobility Features
  1. Bone
    • Teardrop-shaped patella.
    • Two posterior facets articulate with the anterior surfaces of the medial and lateral femoral condyles (MFC and LFC).
  2. Passive Elements
    • Retinaculum (medial and lateral).
    • Hyaline cartilage.
  3. Active Elements
    • Quadriceps.
    • Hamstrings/Calf.
  4. Biomechanical Elements
    • Anatomical pulley system.
    • Least contact area in extension and most in flexion.
PFJ Tracking and Joint Forces
  • Joint reaction force (R) is the resultant force due to tension in quadriceps muscle (Fq) and joint surface contact area (Fp).
    R=F<em>q2+F</em>p2R = \sqrt{F<em>q^2 + F</em>p^2}
Patella Posture
  • Alta vs. Baja: Patella height relative to the femur.
  • Tilt: Medial or lateral tilt.
  • Glide: Medial or lateral glide.
  • Orientation: Combination of tilt and glide.
Patella Posture – Q Angle
  • Angle between the line from the anterior superior iliac spine (ASIS) to the central patella and the line from the central patella to the tibial tubercle.
  • Normal: Approximately 17°±3°17° \pm 3° for women and 13°±3°13° \pm 3° for men.
  • Increases with:
    • Genu valgus.
    • Femoral anteversion.
    • External tibial rotation.

Knee Complex: Subjective and Objective Assessment

Learning Objectives

  • Understand subjective and objective assessment principles.

The Process

  • Subjective Assessment → Physical Examination → Analysis → Treatment → Reassessment.
Knee Subjective Assessment
  • Current History (CHx)
    • Body chart (pain behavior) - WOCSSNOR
      • Where (PA, PB - Primary Area, Problem Area)
      • Other
      • Constant (vs. intermittent)
      • Superficial (vs. deep)
      • Severity (/10)
      • Nature (ache, sharp, throb?)
      • Other (symptoms - giving way, P&Ns)
      • Relationship (between PA and PB)
    • Contact vs. Non-contact
    • Direction of force
    • Position of knee (which way did it go?)
    • Foot planted?
    • Immediate weight-bearing (WB) response
    • Ability to play on
    • Pop/crack/noise
    • Swelling response?
      • Joint effusion vs. Hemarthrosis
        *Joint effusion: gradual onset, worse the next morning
        *Hemarthrosis: rapid swelling and heat within 1-2 hours
      • Locking?
      • Feel anything slip out?
      • Giving way?
  • Aggs/Eases
    • Stair Ascent vs. Descent
    • Weight acceptance
    • Pull-up
    • Stance phase
    • Forward continuance
    • Foot clearance
    • Foot placement
    • Swing phase
    • Controlled lowering
    • Leg pull through
Physical Assessment
  1. Observation
  2. Palpation
  3. Active Range of Motion
  4. Passive Range of Motion (Physiological and Accessory)
  5. Manual Muscle Tests (Strength and Length)
  6. Neurological Tests
Ligamentous Special Tests
  • Series of ligamentous tests in different planes.
  • End feel examination:
    • Excursion
    • End Feel
  • Muscle guarding can limit the examination; compare with the other side.

Knee Complex: Common Injury Considerations

Learning Objectives

  • Understand common injury considerations for the region.

Mechanisms of Injury

  • MCL: Valgus force (Grades I-III).
  • LCL: Varus force (Grades I-III, very low prevalence).
  • ACL
    1. Forced hyperextension (PLB)
    2. Pivoting/deceleration on a flexed knee
      • 20-30 degrees: PLB
      • 60-70 degrees: AMB
  • PCL: Forced hyperflexion.

Ligament Injury Grading

GRADEDEGREE OF TEARPAINLAXITYPOTENTIAL ASSOCIATED INJURY
IMinimal TearingTender to Palpation+ve stress test, EF -Meniscal
IIModerate TearDefinitely Tender to Palpation
  • stress test +, but EF present | |
    | III | Full Thickness | Severe Pain | No EF | Meniscal, ACL |

Patellofemoral Pain Syndrome (PFPS)

Local Factors
  • Patella shape/size
  • Femoral condyle shapes
  • Laxity of passive structures
  • Neuromotor control of the vasti muscles.
Remote Factors
  • Lower limb alignment
  • Trunk control

PFJ Key Points

  • A tight quadriceps muscle is a risk factor for PFJ pain because it increases retropatellar force.
  • A laterally tilted patella would result in greater contact on the lateral facet of the patella.
  • Hip abductor weakness can contribute to knee valgus and a greater resultant Q angle, leading to a lateral pull on the patella.