Knee Joint Biomechanics

Objectives

• Review the structures and alignment of the knee complex
• Describe arthrokinematics and kinematics of the knee
• Discuss stresses and loading patterns on the knee during weight-bearing (WBing) and non-weight-bearing (NWBing) conditions
• Identify passive restraint structures at the knee and their functions
• Describe the role of muscles influencing knee motion
• Identify and describe abnormal knee alignments and their functional impact

Knee Complex

• Consists of the tibiofemoral joint and the patellofemoral joint
• The tibiofemoral joint is a modified hinge joint
• The patellofemoral joint is a plane joint

Tibiofemoral Joint: Femoral Condyles

• Features convex surfaces
• It is separated by an intercondylar notch
• The notch is the attachment site for the anterior cruciate ligament (ACL) and posterior cruciate ligament (PCL)
• The medial condyle has a larger anterior-posterior (A-P) diameter
• The medial condyle has a larger medial articulating surface.
• Femoral condyles are located medially with respect to the femoral head.
• The lateral condyle aligns more directly with the femur shaft
• The medial condyle is larger and projects distally

Tibiofemoral Joint: Tibial Plateau

• Tibial plateaus have concave joint surfaces
• Menisci accentuate the concavity
• The intercondylar eminence separates the medial and lateral plateaus
• The eminence slightly increases joint congruity
• The surface area of the medial plateau is approximately 50% greater than the lateral plateau
• The tibial plateau overhangs the tibia shaft posteriorly and slopes slightly downward

Tibiofemoral Joint Alignment

• The femur's longitudinal axis is directed inferiorly and medially from proximal to distal
• The tibia's longitudinal axis is directed almost vertically
• It forms a medial angle between the femur and tibia of 180°-185°
• A slight physiologic valgus angle is present at the knee
• A measurement greater than 185° indicates genu valgum ("knock knees")
• A measurement less than 175° indicates genu varum ("bow legged")

Tibiofemoral Alignment: Normal

• The weight-bearing line passes through the center of the knee joint
• Loading is equally distributed between the medial and lateral compartments

Tibiofemoral Alignment: Genu Valgum

• Compressive loads increase on the lateral aspect of the knee
• Tensile stress increases on the medial aspect

Tibiofemoral Alignment: Genu Varum

• Compressive loads increase on the medial aspect of the knee
• Tensile stress increases on the lateral aspect

Tibiofemoral Joint During Single Limb Activity

• During single-limb activities, like gait, the line of force shifts medially
• Compressive stresses are increased medially

Tibiofemoral Joint Menisci: Shape and Composition

• Fibrocartilaginous discs with a semicircular shape
• The medial meniscus is C-shaped
• The lateral meniscus is nearly a complete circle (4/5 of a circle)
• Located on the tibial condyles
• Cover one half to two thirds of the tibial articular surface

Menisci Functions

• Enhances joint congruence
• Reduces friction between the tibia and femur
• Distributes forces during weight-bearing
• Provides shock absorption
• Menisci assume 50% - 70% of the load through the knee

Menisci Details

• Both menisci open toward intercondylar tubercles
• Thicker peripherally than centrally (wedge shaped)
• The lateral meniscus covers a greater percentage of the lateral tibial surface when compared to the surface area covered by the medial meniscus
• Articular cartilage of the medial tibial plateau is more susceptible to injury
• The Medial plateau is more exposed and has greater compressive loads during daily activities

Meniscal Attachments

• Anterior and posterior horns: anterior and posterior ends of menisci attach firmly to the tibia
• Transverse ligament connects menisci anteriorly
• Patellomeniscal ligaments: anterior capsular thickenings attach menisci to the patella (directly and indirectly)
• Coronary ligaments attach menisci to tibial plateaus peripherally

Meniscal Attachments - Medial MeniscusSpecific

• Firmly attached to the joint capsule via the deep portion of the MCL (Medial Collateral Ligament)
• The anterior and posterior horns attach to the ACL (Anterior Cruciate Ligament) & PCL (Posterior Cruciate Ligament) respectively
• The Semimembranosus is connected via capsular attachments

Meniscal Attachments - Lateral Meniscus Specfic

• The anterior horn shares a tibial attachment site with the ACL
• Attaches to the PCL & medial femoral condyle posteriorly via the meniscofemoral ligament
• The Popliteus is attached via capsular connections

Medial Meniscus vs Lateral Mensicus Attachments

• The medial meniscus has more ligamentous & capsular restraints than the lateral meniscus
• Less translational mobility in the medial meniscus
• This contributes to a higher incidence of injury in the medial meniscus

Menisci: Vascularity

• In the first year of life, blood vessels are contained throughout the meniscal body
• After weight-bearing begins, vascularity diminishes
• In adults, only the outer 25%-33% of the menisci is vascularized by capillaries from the joint capsule

Menisci: Zones of Vascularity

• Most peripheral: red zone
• Red-white zone
• Innermost zone: white zone
• The peripheral portion gets nutrition via blood vessels
• The central portion relies on the diffusion of synovial fluid for nutrition
• Intermittent loading from weight-bearing and muscle contraction optimize diffusion
• Meniscal horns and peripheral regions are innervated with nociceptors and mechanoreceptors

Joint Capsule

• Composed of a superficial fibrous layer
• Contains a thinner deep synovial membrane
• The medial patellar retinaculum is the anteromedial capsule
• The lateral patellar retinaculum = anterolateral capsule
• Extensor retinaculum is a reinforced capsule by capsular ligaments
• Highly innervated with nociceptors & mechanoreceptors
• Mechanoreceptors play a role in proprioception & reflex-mediated muscle responses

Extensor Retinaculum

• Reinforced medially by:
  • medial patellotibial ligament
  • Medial patellofemoral ligament (MPFL)
    • MPFL blends with the vastus medialis and functions as an important patellar stabilizer
• Medial reinforcements resist excessive lateral translation
• Lateral Patellofemoral Ligament and Lateral Patellotibial Ligament resist excessive medial glide of the patella

Ligamentous Reinforcements of the Joint Capsule

• A posterior oblique ligament (POL) reinforces capsule posteromedially
• An arcuate ligament reinforces capsule posterolaterally
• The oblique popliteal ligament runs obliquely across the posterior capsule

Joint Capsule: Synovial Layer

• Inner lining of the joint capsule
• Secretes and absorbs synovial fluid
• Synovium folds back between the femoral condyles
• ACL and PCL are located outside of the synovium but within the fibrous capsule
• Several fat pads exist within the fibrous capsule but outside the synovial layer

Major Ligaments of the Knee

• Collateral ligaments provide stability in the medial-lateral direction
  • Offer the greatest resistance to varus or valgus stress when the knee is at ~25° flexion
• Cruciate ligaments provide anterior-posterior stability

Medial Collateral Ligament

• Resists valgus forces
• Resists excessive knee extension
• Resists extremes of axial rotation, especially during external rotation of the tibia
• Injury Mechanism: Valgus-producing force with a planted foot, or severe hyperextension

Lateral Collateral Ligament

• Resists varus forces
• Resists excessive knee extension
• Resists extremes of axial rotation
• Injury Mechanism: varus-producing force with a planted foot, or severe hyperextension

Collateral Ligaments Details

• They have a relatively offset attachment on the femur with respect to the axis of motion
• Taut in knee extension and lax in flexion
• Provides stability against rotation during extension but allows rotation when the knee is flexed
• MCL has good blood supply and does not usually require surgical repair

ACL: General Info

• The ACL has two functional bundles
• Both bundles are under tension in full extension (the AM bundle is to a lesser degree)

ACL: Anteromedial Bundle

• Primarily resists the anterior translation of the tibia
• Most taut between 45°-60° of flexion
• Undergoes less change in length than the PL bundle during range of motion

ACL: Posterolateral Bundle

• It is lax in flexion from 60°-90°, allowing for the rotation of the tibia
• Most taut in full extension
• It limits anterior translation of the tibia at low angles of knee flexion

ACL - Structure and Function

• Most fibers resist excessive extension
• Resists excess anterior translation of the tibia and/or excess posterior translation of the femur
• Resists extremes of varus and valgus, and tibial rotation
• Injury Mechanism: Large valgus-producing force with a planted foot, Large axial rotation torque at the knee (in either direction) with the foot firmly planted, strong quadriceps contraction with knee in full or near-full extension, or severe hyperextension

PCL

• It is thicker and stronger than the ACL and is injured significantly less often
• It is comprised of two bundles: Anterolateral (AL) and Posteromedial (PM) bundles
• The PCL is taut with greater angles of knee flexion overall
• The AL bundle is most taut in flexion and the PM bundle is more lax
• The AL bundle is more lax in extension; the PM bundle is taut

PCL - Function and MOI

• Resists posterior translation of the tibia
• Limits extremes of knee flexion
• Resists extremes of varus & valgus forces and tibial rotation
• Common MOI include contact injuries/trauma, ''dashboard injuries'' during a car accident and/or hyperflexion

Other Ligaments

• Oblique Popliteal Ligament:
  • Proximal Attachment: Posterolateral femur near the lateral gastrocnemius head
  • Distal Attachment: Posteromedial tibia near semimembranosus insertion
  • Motions it Limits: Protects the posterior knee from hyperextension
• Popliteofibular Ligament:
  • Proximal Attachment: Musculotendinous junction of popliteus near the lateral femoral epicondyle
  • Distal Attachment: Tip of the fibular styloid process
  • Motions it Limits: Resists posterolateral tibial rotation and posterior tibial translation
• Ligament of Humphrey:
  • Proximal Attachment: Posterior horn of the lateral meniscus
  • Distal Attachment: Distal portion of the PCL attachment to the femur
  • Motions it Limits: Anchors the lateral meniscus
• Ligament of Wrisberg:
  • Proximal Attachment: Posterior horn of the lateral meniscus
  • Distal Attachment: Medial femoral condyle
  • Motions it Limits: Stabilizes the lateral meniscus
• Arcuate Ligament:
  • Proximal Attachment: Posterior capsule and popliteal tendon on the lateral femoral condyle
  • Distal Attachment: Forms a Y-insertion with one head on the posterior fibular head and the other on the oblique popliteal ligament
  • Motions it Limits: Protects the posterolateral capsule against hyperextension and rotational forces

Knee: Screw Home Mechanism

• Coupled motions occur during knee flexion/extension
• Open chain knee extension: In the last 30°, extension of the tibia is coupled with external rotation (ER) of the tibia
• Closed chain extension: Internal rotation (IR) of the femur occurs on the tibia during the last 30°
• During flexion, the mechanism is reversed to "unlock" the knee

Screw Home Mechanism: Open Chain Extension

• Femoral shape facilitates external tibial rotation
• Tension in the ACL
• Lateral pull of quadriceps

Knee: Arthrokinematics (Open Chain)

• During open-chain knee extension, the tibia rolls and slides anteriorly on the femoral condyles
• During open-chain knee flexion, the tibia rolls and slides posteriorly on the femoral condyles

Knee: Arthrokinematics (Closed Chain)

• During closed-chain knee flexion, the femoral condyles roll posteriorly and slide anteriorly on the tibia
• During closed-chain knee extension, the femoral condyles roll anteriorly and slide posteriorly on the tibia

Knee Joint Motion: Range of Motion

• Flexion: 130°–140°
• Extension: 0° (5-10° hyperextension)
• Internal tibial rotation: 15°
• External tibial rotation: 20°
• Tibial rotation is maximized at 90° knee flexion
• Open-packed position: 25° flexion
• Close-packed position: full extension

Patellofemoral Joint (PFJ)

• Articulation between the posterior patella & intercondylar (trochlear) groove of the femur
• This is the least stable joint of the lower limb
• The patella functions as an anatomic pulley that increases the moment arm (MA) of the quadriceps tendon
• In full extension, there is no bony contact between patella & femur
• As flexion increases, the contact area moves superiorly and then laterally on the patellar surface
• In full flexion, only the lateral & odd facets are in contact with the femur

Stabilizing Factors of the Patellofemoral Joint

• Tibiofemoral alignment
• Passive support from the retinaculum
• Distal portion of the ITB (iliotibial band)
• Shape of the femoral groove
• Ligamentous support
• Quadriceps muscles
• Neuromuscular control (timing of contraction)
• Kinematic chain (hip, foot, ankle)

Patella Stabilizing Forces

• Lateral directed forces are generated by the iliotibial band, bowstringing force on the patella, and lateral patellar retinacular fibers
• Medial directed forces are generated by the vastus medialis (oblique fibers) and medial patellar retinacular fibers

Key Measurements

• Q-Angle: represents the line of pull of the quadriceps
• Formed by the intersection of the line from the ASIS to the center of the patella, and a line from the center of the patella to the tibial tuberosity
  • Normal values
    • 8-14° for males
    • 15-17° for females

Patellar Motion: Curves

• Patella glides in a curvilinear path
• Flexion: patella initially shifts medially; then remains centrally or shifts slightly laterally as flexion increases
• Extension: patella moves in the opposite manner; shifts back laterally as full extension is approached

Patella - Shift vs Tilt vs Rotation

• Shift/Glide: Lateral and medial patellar shifts/glides are translations. Direction is defined by which femoral condyle the patella moves toward
• Tilt: Named for condyle the patella approximates with
  • Medial tilt: medial surface approximates the medial femoral condyle
  • Lateral tilt: the lateral surface approximates the lateral femoral condyle
• Rotation: Patellar rotation is described in reference to apex of patella - Medial rotation: Patella spins so that apex moves medially
  • Lateral rotation: Patella spins so that apex moves laterally
• Simultaneous actions during knee motion: translation (shift/glide), tilt, and rotation
• It keeps the patella between the femoral condyles with rotation of the femur during knee motion
• Early knee flexion causes the tibia & patella to rotate medially.
• Terminal knee extension causes tibia & patella rotate laterally

Vertical Patellar Position

• Insall-Salvati index: measurement of ratio of the length of the patellar tendon to the length of the patella
  • Typically ~ 1:1 (normal range on X-rays is 0.8 – 1.2)
• Patella baja: abnormally low position of the patella associated with a shortened patellar tendon
• Patella alta: abnormally high position of the patella, increases risk for patellar instability and alters the efficiency of the extensor mechanism

Patellofemoral Alignment & Motion: Dysfunctions

• Failure of the patella to glide, tilt, rotate, or shift appropriately during flexion/extension can lead to
  • Restricted ROM
  • Maltracking at the PFJ
  • Pain from abnormal compression of articular surfaces
  • Altered mechanics of the extensor mechanism

Patellofemoral Joint Stresses (PFJ)

• Joint stresses are very high with daily activities
• JRF is influenced by the magnitude of the quadriceps force & knee angle
• When the quadriceps contract, the patella is pulled superiorly by the quad tendon, and the patellar tendon resists that pull
• Combined forces create a posterior compression force of the patella on the femur
• Posterior compression varies with the amount of knee flexion

Patellofemoral Joint Stresses: Joint Angle

• In full extension, the posterior compressive force from the quadriceps is minimized, and the PF joint stress is low
• With knee flexion, the angle of pull between the quad tendon & patellar tendon decreases
  • This increases in Joint Reaction Force (JRF), producing greater Patellofemoral joint compressive forces
• Increased compression occurs regardless of quadriceps activity
• Passive tension in the tendons from passive flexion produces compressive forces
• When quads are active
  • Both active tension and passive elastic tension contribute to increasing compressive force

Knee Flexors

• Knee flexor group
  • Hamstrings: semimembranosus, semitendinosus, biceps femoris (long & short heads)
    • Function to resist valgus forces and provide stability to the knee joint
  • Sartorius
  • Gracilis
  • Popliteus
  • Gastrocnemius
  • Plantaris (when present)
• All the flexors except the short head of biceps femoris & the popliteus are 2 joint muscles.

Knee Flexors: Function in Transverse Plane

• Semimembranosus, semitendinosus, sartorius, gracilis, & popliteus all can produce medial (internal) rotation of the tibia on a fixed femur
• Biceps femoris produces lateral (external) rotation of the tibia

Hamstrings + Knee Flexion

Force production at the knee depends upon hip joint angle Greater hamstring force as a knee flexor is produced when the hip is flexed Produce less force with hip extended & knee flexed to 90° or > due to active insufficiency

• Semimembranosus assists in posterior movement of the medial meniscus during active knee flexion

Gastrocnemius

• Crosses both the knee & ankle joints
• Has greater ability to produce torque at the ankle than at the knee
• The ability to produce knee flexor torque depends on ankle joint position
• Produces greatest knee flexion torque when the knee is fully extended
• Often functions more as a stabilizing muscle at the knee rather than a mobilizing muscle

Sartorius & Gracilis

• Sartorius has the potential to produce knee flexion and medial tibial rotation and is more often activated during hip motion than knee motion
• Gracilis primarily functions as a hip flexor and adductor, and has the capability to produce weak knee flexion and slight medial rotation of the tibia

Popliteus

• Function: medially rotates tibia on the femur
• Is attributed with a crucial role in ''unlocking'' the knee for early flexion; however, unlocking would likely still occur wo/ activity of popliteus
• Attaches partly to lateral meniscus assisting in posterior movement of menisci during active flexion
• Reduces the risk of meniscal impingement

Knee Extensor Group

• Quadriceps: rectus femoris, vastus lateralis, vastus medialis, & vastus intermedius
  • Function is greatly influenced by patella
  • Not all fibers are oriented in the same direction
• VMO vs the VML is much more oblique distally as compared to proximally

Quad Function

• The patella increases the moment arm of the quadriceps tendon
• Deflects the line of pull of the quads away from the axis of rotation increasing the quads ability to generate extension torque
• Pulley function is decreased in full flexion due to the patella being fixed firmly in the intercondylar groove
• Patella is at the farthest distance from the axis of rotation at ~ 50° of knee flexion
  • Peak torques often observed between 45° - 60° of knee flexion(position of maximal MA & length-tension relationship)
  • MA diminishes again as knee moves further into extension