Knee - Structures & Function

Chapter 19: Structure and Function of the Knee Complex

Knee Complex

• Overview: The knee complex is the largest and most complex weight-bearing joint in the body.

• Type of Joint: It is classified as a biaxial modified hinge joint.

• Components: The knee complex consists of the tibiofemoral joint and the patellofemoral joint.

• Functional Role: It links the hips to the feet and functions in both open and closed-chain activities.

Bony Structures

• Femur:

  • Has a shaft that travels inferiorly and slightly medially.

  • Forms an angle with the tibia that averages between 170° and 175°.

  • Genu Valgum: Medial angulation of the femurs (less than 180°), commonly referred to as "knock-kneed".

  • Genu Varum: Lateral angulation of the femurs (more than 180°), often described as bow-legged.

• Distal Femur:

  • Includes medial and lateral epicondyles.

  • Contains the intercondylar groove, adductor tubercle, and popliteal fossa.

  • Features medial and lateral condyles.

• Medial and Lateral Condyles:

  • The medial condyle is longer than the lateral condyle.

  • This extension aids in locking the knee into full extension.

• Proximal Tibia and Fibula:

  • Features flat tibial plateaus and intercondylar eminence, which is a bump located between the two plateaus.

  • The tibial tuberosity provides attachment for muscles and ligaments.

  • The fibula does not articulate with the femur and serves primarily as an attachment point.

• Patella:

  • Noted as the largest sesamoid bone in the body.

  • Has a smooth hyaline cartilage on its posterior surface, which withstands large compressive and friction forces.

  • During squats, the patella is compressed on the condyles, increasing the internal moment arm and thereby allowing the quadriceps to produce more torque.

  • Functions as a pulley system.

• Patellofemoral Joint:

  • Formed by the posterior patella and the femoral intercondylar groove.

  • When involved in functional activities, it is subject to high loads of stress, experiencing forces up to 0.5 times body weight during walking and 7 times body weight during squatting.

  • Excessive body weight can increase stress on the articular cartilage of joint surfaces.

Patellar Motion

• Motion During Open-Chain Flexion: The patella moves inferiorly with the tibia.

• Motion During Open-Chain Extension: The patella moves superiorly and slightly laterally and posteriorly.

• Motion During Closed-Chain Movement: The patella remains fixed on the moving femoral condyles.

• Contact Points:

  • At 20° to 30° of extension, only the inferior pole contacts the joint, while between 90° and 60° of flexion, the patella is in maximal contact with the intercondylar groove.

Patellar Tracking

• Optimal Tracking: Necessary for efficient knee function.

• Factors Affecting Tracking:

  • Tight iliotibial (IT) band can pull the patella laterally.

  • Tight lateral patellar retinacular fibers contribute to lateral tracking.

  • Lax medial retinacular tissues cause medial instability and lateral movement of the patella.

  • A lax medial collateral ligament can exacerbate lateral tracking issues.

  • Shallow intercondylar grooves can affect stability.

  • Poor muscular control around the hip and knee can hinder proper patellar tracking.

Menisci

• Structure:

  • Semicircular fibrocartilage rings that create concave seats for femoral condyles.

  • Attached by coronary, transverse, and patellomeniscal ligaments.

  • Provides stability through muscle attachments.

• Medial and Lateral Menisci:

  • Asymmetrical fibrocartilaginous disks that are wider at the edges and taper towards the middle.

  • Function to distribute weight-bearing forces and reduce friction between joint surfaces, acting as shock absorbers.

• Meniscal Functions:

  • Distribute compression loads and provide joint congruency, guiding motion.

  • Reduce joint friction and help nourish articular cartilage.

  • Provide proprioceptive input through neural innervation.

Bursae

• Total Number: There are 14 bursae in the knee complex.

• Types of Bursae:

  • Suprapatellar synovial bursa: Between the distal femur and quadriceps tendon.

  • Subcutaneous prepatellar bursa: Between the patella and skin.

  • Deep (subtendinous) infrapatellar bursa: Between the proximal tibia and patellar ligament.

  • Subcutaneous infrapatellar bursa: Between the tibial tuberosity and the skin.

  • Posterior bursae:

  • Gastrocnemius bursae (lateral and medial): Between the gastrocnemius head and capsule.

  • Biceps bursa: Between the fibular collateral ligament and biceps tendon.

  • Popliteal bursa: Between the popliteus tendon and lateral femoral condyle.

  • Semimembranosus bursa: Between the semimembranosus tendon and head of the tibia.

Joint Capsule

• Structure: Encompasses the tibiofemoral and patellofemoral joints.

• Reinforcement:

  • Reinforced by medial and lateral patellar retinacular fibers.

  • Includes the medial and lateral collateral ligaments.

Pes Anserinus

• Muscle Group: Comprised of the sartorius, gracilis, and semitendinosus.

• Anatomy:

  • All cross the knee joint posteriorly and medially.

  • These muscles converge and attach on the anteromedial surface of the proximal tibia.

Lateral Collateral Ligament (LCL)

• Function: Reinforces the lateral capsule of the knee.

• Attachments: Attaches to the lateral femoral epicondyle and fibular head.

• Action: Resists varus forces acting on the knee.

Medial Collateral Ligament (MCL)

• Function: Fortifies the medial aspect of the joint.

• Attachments: Attaches from the medial femoral epicondyle to the proximal tibia.

• Integration: Blends with retinacular fibers, the medial meniscus, and the tendon of the semimembranosus muscle.

• Action: Protects against valgus forces that push the knee inward.

Functions of MCL and LCL

• Valgus and Varus Resistance:

  • MCL resists valgus forces.

  • LCL resists varus forces.

• Stability: Both ligaments stabilize the knee in the sagittal plane and resist extremes of medial and lateral rotation.

Anterior and Posterior Cruciate Ligaments

• General Characteristics:

  • Intracapsular ligaments with a vascular synovial lining.

  • Resist motion between the femur and tibia, particularly during movement between femoral condyles.

  • Contain mechanoreceptors relaying information about joint position and movement.

• Anterior Cruciate Ligament (ACL):

  • Attaches to the anterior tibial plateau and runs obliquely in a superior, lateral, and posterior direction, attaching to the lateral femoral condyle.

  • Becomes taut in full knee extension.

  • Opposes excessive anterior tibial translation of the tibia on the femur during extension to prevent the tibia from sliding forward.

• Posterior Cruciate Ligament (PCL):

  • Attaches from the posterior lateral tibia, running obliquely in a superior and medial direction to the medial femoral condyle.

  • Becomes taut in full knee flexion.

  • Opposes excessive posterior tibial translation during flexion, preventing the tibia from moving backward on the femur.

Osteokinematics

• Degrees of Freedom: The knee has two degrees of freedom:

  • Flexion and extension occur in the sagittal plane.

  • With the knee flexed, there is a small amount of lateral rotation.

Flexion and Extension Range

• The range for flexion and extension is from $0^ ext{o}$ to $150^ ext{o}$:

  • Open chain: at 90° flexion.

  • Closed chain: with full extension.

Genu Recurvatum

• Definition: Hyperextension of the knee beyond $10^ ext{o}$.

• Possible Causes:

  • Weak quadriceps muscles leading to eccentric movement, causing knees to snap back into extension.

  • Tight gastrocnemius muscles can pull posteriorly.

Q-Angle

• Definition: Clinical measurement reflecting the quadriceps line of pull.

• Measurement: Formed by a line connecting the anterior superior iliac spine (ASIS) and the midpoint of the patella to the long axis of the patellar tendon.

• Normal Range: Between $10^ ext{o}$ and $15^ ext{o}$ for healthy adults.

• Clinical Significance: An angle greater than $15^ ext{o}$ indicates increased lateral muscle pull on the patella, with women generally having larger Q-angles due to wider hips.

Medial and Lateral (Axial) Rotation

• Rotation Range: At $90^ ext{o}$ of knee flexion, there is an average of $45^ ext{o}$ of total axial rotation.

• Extension: No rotation occurs when the knee is in full extension.

Arthrokinematics

• Open-Chain Knee Extension: The tibia rolls and slides anteriorly; during flexion, the opposite direction occurs (posteriorly). This follows the rule of concave moving on concave.

• Closed-Chain Knee Extension: The femoral condyles roll anteriorly and slide posteriorly on the tibia; during flexion, the opposite action occurs (posteriorly), represented by convex moving on concave.

Screw-Home Mechanism

• Mechanism Description:

  • The medial epicondyle has a shape that is oriented with approximately a $30^ ext{o}$ lateral curve.

  • The medial epicondyle projects further anteriorly than the lateral condyle, influencing the tibia's motion during extension.

  • The tibia rotates laterally with terminal extension (during the last $20^ ext{o}$ of knee extension).

• Unlocking the Knee:

  • When moving from extension to flexion, the tibia rotates medially.

  • In closed-chain knee extension, the femur rotates medially on the fixed tibia.

Knee Extensors

• Main Muscle Groups:

  • Quadriceps Femoris: The primary muscles involved in knee extension.

  • Rectus Femoris: The only two-joint muscle that flexes the hip and extends the knee.

  • Vastus Group: Comprises three muscles generating approximately 80 ext{%} of extension force:

  • Vastus Lateralis

  • Vastus Medialis

  • Vastus Intermedius

• Force Production: The quadriceps can produce a force two-thirds greater than that of knee flexors, ideally eccentrically lowering the body from standing to seated positions, concentrically raising from seated positions, and isometrically absorbing shock during jumping.

  • Optimal length for the rectus femoris is when the hip is extended, while the vasti muscles generate peak extension at approximately $80^ ext{o}$ of knee flexion.

Knee Flexors: Hamstrings

• Muscle Composition: The hamstring group includes:

  • Semimembranosus

  • Semitendinosus

  • Biceps Femoris consisting of:

  • Long Head

  • Short Head (does not cross the hip)

• Function: Able to extend the hip and flex the knee, except for the short head, which only contributes to knee flexion.

Knee Flexors and Rotators

• Additional Muscles:

  • Sartorius

  • Gracilis

  • Medial and lateral heads of the gastrocnemius assist in knee flexion.

  • Popliteus: Plays a significant role in unlocking an extended knee through medial rotation of the joint.

Insufficiencies

• Active Knee Flexion in Prone Position:

  • The rectus femoris becomes passively insufficient due to being stretched over both the hip and knee.

  • The hamstrings are actively shortened, leading to a likelihood of active insufficiency.

  • The bulk of the hamstrings and gastrocnemius muscles also limit active knee flexion while in the prone position.

Influence of Hip Position on Knee Function

• Knee Extension with Hip Flexion:

  • Hamstrings can become passively insufficient, limiting further hip flexion, impacting activities like heel strike during walking.

• Knee Flexion with Hip Extension:

  • Hamstrings can become actively insufficient while the rectus femoris can become passively insufficient, limiting knee motion.

• Knee Flexion with Hip Flexion:

  • This position increases knee flexion and impacts leg movement during the swing phase.

• Knee Extension with Hip Extension: May also have influence on functional activities but needs further discussion.

Patellofemoral Pain Syndrome

• Description: A common condition characterized by diffuse anterior knee pain.

• Causes:

  • Alignment factors such as increased Q-angle.

  • Conditions like patella alta (high-riding patella) and quad tightness or weakness.

  • Excess foot pronation (collapse of the arches) and genu valgum.

  • Tightness of the IT band, weakness of the vastus medialis oblique (VMO), and tightness of the quadriceps.

Unhappy Triad

• Definition: A knee injury involving tears to the anterior cruciate ligament (ACL), medial collateral ligament (MCL), and medial meniscus.

• Cause: Resulting from a single blow to the outside of the knee, particularly with rotational force applied to a planted tibia.

Other Common Knee Pathologies

• Patellar Tendonitis: Inflammation of the patellar tendon, often seen in activities involving running and jumping.

• Osgood-Schlatter Disease: Overgrowth of the tibial tuberosity commonly seen in adolescents.

• Popliteal Cyst: Also known as Baker's cyst; a collection of fluid in the posterior aspect of the knee.

• Chondromalacia Patella: Degeneration or wearing away of cartilage in the posterior aspect of the patella.

• Prepatellar Bursitis: Inflammation of the prepatellar bursa located at the front of the patella.