15. Anatomy of the knee Flashcards
Functions of the knee
Factors strengthening the joint
Several factors help improve stability and strength:
Bony factors
- Bony expansions
- Locking mechanism
- Femoral angle
Soft tissue factors
- Ligaments
- Menisci
- Muscles
Bony expansions
Locking mechanism
Reduces amount of energy required when extended
3 components:
- Shape of femur
- Rotation
- Centre of gravity
- Shape of femur
- Rotation
Medial rotation of femur on tibia in extension
- Tightens ligaments of the knee
- Centre of gravity
Centre of gravity in front of knee
- Maintains extension
Femoral angle
Varus deformity (Genu varum)
Deformity in the angle between femur and tibia
Medial displacement of the tibia
- Common in children under 2, rickets
Pushes knees apart - ‘Bow-legged’ = Decrease in Q angle
Valgus deformity
Lateral displacement of the tibia
- Common in children aged 2-4, rickets, arthritis
Brings knees together - ‘Knock-kneed’ = increase in Q angle
Ligaments of the knee
Provide stability
Two groups of strong ligaments:
Extracapsular – outside capsule
- Medial collateral
- Lateral collateral
Intracapsular – inside capsule
- Anterior cruciate
- Posterior cruciate
Lateral/fibular collateral ligament
Medial/tibial collateral ligament
Intracapsular ligaments
Function of cruciate ligament
Anterior cruciate
- Prevents anterior displacement of tibia on femur
Posterior cruciate
- Prevents posterior displacement of tibia on femur
Maintain femur against tibia
- Always one ligament tense
Cruciate ligaments
Anterior cruciate ligament
Weaker - can become injured
- Common sports injury
- Caused by sharp twisting of knee
- Immediate decreased range of movement
Lachman test (variation of Drawer test):
- Patient in supine position with knee bent 20-300 of flexion
- Move tibia anteriorly and posteriorly while maintaining position of femur
- Laxity during this manoeuvre indicates anterior cruciate ligament injury
Posterior cruciate ligament
Stronger - Rarely injured
- Principle stabilizer when knee flexed
(Especially when walking down hill)
Menisci of joint
Crescent-shaped plates of fibrocartilage
- Deepen the articulating surfaces/stability
- Shock absorbers
- Provides smooth viscous film for joint
Menisci attachments
Horns of menisci attached to intercondylar area of tibia
Mobile - Accommodates rolling of femoral condyles
Medial meniscus less mobile
- Attached to medial collateral ligament
Unhappy triad
Twisting on a flexed knee/blow to lateral side
Contact sports (rugby tackle)
Locking of the knee
Rupture:
- Anterior cruciate ligament
- Medial collateral ligament
- Medial meniscus (attached to MCL)
Poor blood supply to intracapsular structures
Doesn’t repair easily
Muscles acting on knee joint
Knee reinforced by tendons from surrounding muscles and iliotibial tract
Iliotibial tract
- Reinforces joint capsule
- Stabilizes extended knee
- (gluteus maximus, tensor fascia lata)
Extensors of leg
Quadriceps extends leg
Major stabilizing muscle of the knee
4 heads:
- Rectus femoris
- Vastus lateralis
- Vastus intermedialis
- Vastus medialis
Quadriceps femori
Extensor mechanism
Extensor mechanism injury
Rupture of quadriceps tendon or patellar ligament
Fracture of the patella
- Due to fall or blow to knee
- Results in loss of active extension
Dislocation of patella common
- Due to sudden twisting/jumping or ligamentous laxity
Flexors of leg
Hamstrings and gastrocnemius flex leg
Hamstrings actually three muscles:
- Biceps femoris
- Semimembranosus
- Semitendinosus
Also medially and laterally rotates leg when knee flexed
And extends thigh
Hamstrings
Unlocking of extended leg
Popliteus unlocks knee joint
- Laterally rotates femur on tibia when foot is on ground
Synovial membrane
From margins of articular surfaces of femur to tibia
Attached to patella
Extends superiorly behind quadriceps tendon
Cuffs anterior surface of cruciate ligaments
Bursae
Synovial fluid filled sac lined by synovial membrane
- Protection
- Reduce friction
Many bursae associated with the knee
occur at tendon insertions
Posteriolateral and posteriomedial aspects of knee
Bursitis
Inflammation of bursae
- Due to repetitive movements or direct pressure
Prepatellar bursitis (‘housemaid’s knee’)
- Seen in carpet fitters
Leaning forward on the knees brings the prepatellar bursa in contact with floor
Infrapatellar bursitis
(clergyman’s knee’)
After prolonged periods of prayer clergymen sat back on their heels bringing infrapatellar bursa in contact with floor
Popliteal/Baker’s cyst
Abnormal fluid filled sacs in popliteal fossa
- Due to herniation of synovial membrane/bursa
Common in patients with chronic inflammatory joint disease (e.g. arthritis)
- Presents as swelling in the popliteal fossa
- Can affect joint movement
Treat by aspiration and cortisone injection
Vascular supply
Anastomosis around knee
- Femoral artery
- Popliteal artery
Limited blood supply to intracapsular structures
- Poor repair following injury
