MSK 8a: the knee Flashcards
What type of joint is the knee?
Synovial hinge (flexion and extension allowed)
Articulations of the knee
Lateral femorotibial: between the lateral femoral and tibial condyles
Medial femorotibial: between the medial femoral and tibial condyles
Femoropatellar: between the patella and femur
Fibula DOES NOT articulate
Describe the bony structures involved in the knee
Femoral condyles:
- medial larger than lateral as takes more weight
- between the two, anteriorly there is a shallow depression for articulation with the patella, and posteriorly a deep notch known as the INTERCONDYLAR FOSSA
Femoral epicondyles:
- superior to the condyles
- provide a site of attachment for the collateral ligaments
- medial side has an ADDUCTOR TUBERCLE
Tibial surface:
- TIBIAL PLATEAU
- medial (slightly concave) and lateral (slightly convex) surfaces, separated by an intercondylar eminence
- femoral condyles rest on top: not very stable due to mismatch of shapes
What are menisci and what is their function?
Crescent-shaped plates of fibrocartilage on the articular surface of the tibia. Wedge-shaped as thicker at the edges. Attach to intercondylar areas and the joint capsule via coronary ligaments.
Function:
- deepen surface and act as shock absorber
- distribute weight through the femur
Why is the knee considered a mechanically-weak joint?
Incongruent articular surfaces
What type of bone is the patella? What is its function?
Sesamoid
Triangular, with a superior base and inferior apex. Medial and lateral facets covered in hyaline cartilage to articulate with the femoral condyles.
Functions:
- attachment for quadriceps tendon superiorly and patellar tendon inferiorly; therefore helps with LEG EXTENSION
- PROTECTS anterior aspect of knee from trauma
Describe the medial and lateral menisci
Medial:
- C-shaped
- less mobile on the tibial plateau than the lateral
- points of attachment further apart so less movement possible
- may be torn if medial collateral ligament is overstretched
Lateral:
- nearly circular, smaller
- more freely moveable than medial meniscus
- has a better blood supply so tears on the periphery have a higher chance of healing
How is knee stability determined?
- The strength and actions of the surrounding muscles and their tendons: especially vastus medalis and vastus lateralis
- Ligaments connecting the femur and tibia (intracapsular, extracapsular and that strengthen the capsule)
Describe the joint capsule of the knee
External fibrous capsule and inner synovial membrane
Surrounds the sides and posterior aspect:
-is absent anteriorly: replaced by the quadriceps tendon, patella and patellar ligament (continuous with the sides of these)
-strengthened laterally by inferior fibres of VM and VL
-strengthened posteriorly by the oblique popliteal ligament (an expansion of semimembranosus)
Which ligaments are intracapsular?
Cruciate ligaments and menisci
Describe the cruciate ligaments
Criss-cross within the joint capsule but outside of the synovial membrane, in the centre. The crossing serves as a pivot for rotatory movements at the knee. In every position, one or parts of one/both CL is tense. They maintain contact with the articular surfaces.
ANTERIOR CRUCIATE LIGAMENT:
- weaker so more injuries than PCL + poor blood supply
- from anterior intercondylar area of tibia to posterior part of medial side of lateral femoral condyle
- role: limits hyperextension, limits posterior rolling of femoral condyles on tibial plateau during flexion, and prevents posterior displacement of femur on tibia
POSTERIOR CRUCIATE LIGAMENT:
- stronger. From posterior intercondylar area of tibia to anterior part of lateral surface of medial femoral condyle
- role: limits hyperflexion, anterior rolling of femur on tibial plateau during extension and anterior displacement of the femur on tibia/posterior displacement of tibia on femur
Mnemonic to remember cruciate ligaments
PAMs APpLes
Posterior passes anterior inserts medially, anterior passes posterior inserts laterally
What is the main stabilising facet for the femur in the weight-bearing flexed knee?
Posterior cruciate ligament
Why can the tibia not be pulled anteriorly when the knee is flexed at a right angle?
The anterior cruciate ligament holds it
What is the transverse ligament of the knee?
A slender fibrous band that joins the anterior edges of the menisci, crossing the anterior intercondylar area
Describe the 5 extra capsular ligaments of the knee
FIBULAR/LATERAL COLLATERAL: cord-like and strong, from lateral femoral epicondyle to lateral fibular head. Splits the tendon of biceps femoris into two and is reinforced by the iliotibial tract
TIBIAL/MEDIAL COLLATERAL: strong and flat, from medial femoral epicondyle to medial tibial condyle. Attaches to the medial meniscus at its midpoint: both commonly torn in contact sports
PATELLAR: distal part of quadriceps femoris tendon (anterior), attaches to tibial tuberosity
OBLIQUE POPLITEAL: expansion of semimembranosus, reinforces joint posteriorly, arises posterior to medial tibial condyle and passes superolaterally to lateral femoral condyle where it blends with the posterior joint capsule
ARCUATE POPLITEAL: strengthens posterolaterally; from fibular head to posterior surface
What are the movements of the knee, and which muscles produce them?
FLEXION: 160 deg passive, 140 deg with flexed hip, 120 deg with extended hip. Produced by gastrocnemius, sartorius and hamstrings. Inhibited by calf contractual and length of hamstrings
EXTENSION: to 0 degrees (straight alignment of tibia and femur). By quadriceps femoris, some of tensor fasciae latae. Inhibited by the lateral meniscus and ACL. Hip flexion reduces extension of knee
MEDIAL ROTATION: 10 deg with knee flexed, 5 deg with extended. By semimembranosus and semitendinosus when knee flexed, and popliteus when extended. Inhibited by collateral ligaments. When weight bearing popliteus rotates laterally
LATERAL ROTATION: 30 deg. By biceps femoris when knee flexed. Inhibited by collateral ligaments
What is locking and unlocking of the knee?
When knee fully extended with foot on ground, knee passively ‘locks’ due to medial rotation of femoral condyles on tibial plateau: making lower limb good for weight-bearing
To unlock, the popliteus contracts, rotating the femur laterally 5 deg on the tibial plateau to allow flexion to occur
How might the knee joint be investigated in a patient?
Clinical examination
Imaging: plain films/MRI
Arthroscopy
Aspiration
Blood supply to the knee
10 vessels that form peri-genicular anastomoses: branches of the femoral, popliteal, anterior tibial recurrent and circumflex fibular arteries
Nerve supply to the knee
Branches from the femoral, tibial, common fibular, obturator and saphenous nerves
Bursae in the knee
At least 12
E.g. subcutaneous pre patella, intrapatella, supra patellar
Look at diagram so know where main ones are
How does the “unhappy triad” knee injury occur?
Occurs due to a blow to the side of the knee or lateral twisting of a flexed knee
TCL is firmly attached to the medial meniscus, so injury to one is likely to injure the other; and since the ACL is also taught during flexion it can also be torn
What is the most common problem with the knee joint?
Ligament sprain: when foot is fixed in the ground and a force is applied against the knee
Hyperextension of the knee
Damages the ACL
May rupture e.g. in skiing
Causes free tibia to slide anteriorly under the fixed femur: test with anterior draw test
Tears usually along midpoint of ACL
How might the PCL be damaged?
Fall onto tibial tuberosity with a flexed knee, e.g. when knocked to the floor in basketball. Tibia is pushed back against femur: test with posterior draw test
Menisci injuries
Usually medial (lateral moveable)
Indicated by pain on lateral rotation of the tibia on the femur
Usually in conjugation with MCL or ACL tears
Peripheral tears heal well as good blood supply; if don’t heal meniscus is removed (so knee becomes less stable)
Patellar dislocation
Usually a result of direct trauma or twisting
Dislocates laterally
Patellar fractures
Direct trauma can shatter patella, or severe force from extensor muscles
A transverse mid-patellar fracture is usually comminuted, hard to heal as fractures continually pulled apart so need a tension band to prevent movement
Popliteal (Baker’s) cyst
Abnormal fluid-filled sacs of synovial membrane in the popliteal fossa
Sign: chronic knee effusion
Can connect to synovium of knee
What are the main forms of bursitis of the knee?
Pre-patellar: i.e. housemaids knee. Friction between skin and patella, or inflammation from injury
Subcutaneous infra-patellar: i.e. Clergyman’s knee. Friction between skin and tibial tuberosity, causing oedema over proximal tibia
Supra-patellar: due to abrasions or penetrating wounds (infection; may spread to rest of knee)
Deep infra-patellar: between patellar ligament and tibia. Inflammation usually removes the dimples that are normally seen either side of the patellar ligament in extension
How would you perform McMurray’s test, and what is it used for?
Testing the medial and lateral menisci
Flex knee, palpate and check for pain. Then test medial meniscus by extending knee and laterally rotating tibia, and the lateral meniscus by extending knee and medially rotating tibia
What is Cooper’s sign?
Patient complaining of joint line pain in affected knee when turning over in bed at night to change leg position
Caused by meniscal tear pulling on joint capsule. Can be used to differentiate knee pain from arthritic pain
