Knee Flashcards
what condition predisposes to medial compartment knee OA
genu varum
what condition predisposes to lateral compartment knee OA
genu valgus
What procedure may younger patients with medial compartment OA benefit from rather than TKR?
Osteotomy - shifts the load to the lateral compartment
or
UKR
What are the 3 types of knee replacement
- TKR - replacement of all 3 compartments
- Unicompartmental knee replacement
- Patellofemoral replacement
Disadvantages of UKR for isolated compartment OA
Higher failure rates than TKR
materials used in TKR
Femoral component - cobalt chrome, stainless steel
Tibial component - cobalt chrome, stainless steel, titanium
High density polyethylene bearing surface
What happens to the ACL and PCL in TKR
The ACL is usually excised
The PCL can be left, but is usually sacrificed in most designs
What happens to the collateral ligaments in TKR
Any tight ligaments need to be lengthened or released for a symmetrical balance as unequal tension of the collateral ligaments leads to stress and eventual loosening.
What is there a higher chance of with TKR compared to THR
unexplained pain
?complexity and reliance on soft tissue tension
Why do the menisci have limited healing potential
Only a blood supply on the outer third - the rest is avascular
Role of the knee menisci
Distribute load from convex femoral condyles to relatively flat tibial articular surfaces
Compare the fixing of the medial and lateral menisci
Medial is more fixed whereas lateral is more mobile
Why are tears more common in the medial menisci
Because it is more fixed, so is under a greater amount of shear stress
Classic mechanism of injury for meniscal tear
Twisting force on a loaded knee e.g. football, squatting
What % of ACL ruptures also have a meniscal tear
25%
Presentation of meniscal tear
localised pain to medial (majority) or lateral joint line
Steinmann’s test +ve - pain on tibial rotation localising to affected compartment
effusion develops day after injury
catching sensation or knee ‘locking’ - difficulty straightening the knee
instability - if a loose fragment is caught in the knee when walking
define true knee ‘locking’
a mechanical block to full extension caused by a torn meniscus flipping over and becoming stuck in the joint line
define ‘pseudo-locking’
not a sign of meniscal injury - temporary difficulty in straightening the joint which will either resolve spontaneously or there is a ‘trick manoeuvre’ which relieves the issue
Ix for suspected meniscal tear
MRI
Patterns of meniscal tear
Parrot beak tear
Longitudinal tear
Radial tear
Bucket handle tear
Describe a radial meniscal tear
Extends radially from the free edge of the meniscus
Describe a longitudinal meniscal tear
Tear is parallel to the circumference of the meniscus
Describe a parrot beak tear
Type of oblique tear - the tear gap has a curved V shape
Describe a bucket handle tear
A large longitudinal tear is able to flip out of its normal position and displace anteriorly or into the intercondylar notch where the knee locks and is unable to fully extend due to mechanical obstruction from the trapped meniscal fragment
What are degenerative meniscal tears?
Tears that occur as the meniscus weakens with age - probably represents primary OA
How are acute meniscal tears and degenerate ones distinguished from each other?
From Hx and examination -
Degenerative tears will be Steinmann’s -ve
Associated symptoms of OA
Why is it important to distinguish acute meniscal tears from degenerative ones?
They are treated differently - degenerative tear symptoms will not improve with resection so shouldn’t be treated with arthroscopy
Tx of meniscal tear
Most - arthroscopic menisectomy
In a younger patient with a fresh tear, could consider repair, but these have a 25% failure rate.
What % of meniscal tears are not suitable for meniscal repair
90%
Why do degenerative meniscal tears not improve with arthroscopy?
They’re due to OA in the joint, so removing the meniscus will increase the stress in the already worn and damaged surfaces
Role of the ACL
resists anterior subluxation of the tibia
Classic mechanism of injury for ACL tear
Turning of the upper body laterally on a planted foot, leading to internal rotation of the tibia
Presentation of ACL rupture
“pop” is heard
haemarthrosis develops within a hour of injury
rotatory instability
Anterior drawer test +ve and Lachmans test +ve - excessive anterior translation of the tibia
Rule of thirds in ACL rupture
1/3 can compensate and able to function well
1/3 can avoid instability by avoiding high impact sports
1/3 do poorly with frequent giving way in normal activities
Tx of ACL rupture
ACL reconstruction NOT repair
involves tendon graft passed through tibial and femoral tunnels and secured to bone
common locations for tendon graft to be taken for ACL reconstruction
patellar tendon
semitendinosis
gracilis
Role of the PCL
resists posterior subluxation of the tibia
Classic mechanism of injury for PCL rupture
Direct blow to anterior tibia with the knee flexed
Presentation of PCL rupture
Recurrent hyperextension
Instability in descending stairs
Popliteal knee pain and bruising
Tx of PCL rupture
If isolated - won’t require reconstruction
If part of multi-ligament injury - PCL reconstruction
role of MCL
resists valgus stress
presentation of MCL tear
knee laxity
pain on valgus stress
tenderness over origin or insertion of MCL
Tx of MCL tear
hinged knee brace
Why is surgery for MCL tears not common
MCL has a good bloody supply and is thin so is always expected to heal
What other ligament in the knee is usually damaged with MCL tear
ACL
Role of LCL
Resists varus stress
What other ligament in the knee is usually damaged with LCL tear
PCL
Why is LCL harder to heal than MCL
It is much thicker than MCL
What nerve can be damaged with LCL injury and why
Common peroneal nerve
The hyperextension and varus giving rise to the injury, and from excessive stretch.
Tx of LCL tear
LCL reconstruction with tendon graft
Why are complete knee dislocations a surgical emergency
They involve rupture of all 4 knee ligaments, so high chance of neurovascular injury
What neurovascular structures are usually damaged in complete knee dislocation
popliteal artery
common peroneal nerve
+/- leading to compartment syndrome
Tx of complete knee dislocations
emergency reduction and external fixation for temporary stabilisation
multiple ligament reconstruction
rechecking of vascular status
When should you suspect osteochondral or chondral injury after knee injury
if there is ongoing pain or effusion after the knee injury
Ix for suspected osteochondral knee injury
Xray
MRI
Arthroscopy
Tx of osteochondral knee injury
Depends if the bone is weight-bearing area and how large the fragments are
Large - fix with pins
Non-weight bearing - remove arthroscopically
What does the extensor mechanism of the knee consist of
tibial tuberosity patellar tendon patella quadriceps tendon quadriceps muscle
Causes of patellar tendon or quadriceps tendon rupture
rapid contractile force to the muscles e.g. after lifting heavy weight, after a fall
Are patellar tendon ruptures more common in younger or older age groups?
Younger (<40)
Are quadriceps tendon ruptures more common in younger or older age groups?
Older (>40)
Predisposing factors to extensor mechanism rupture
Hx of tendonitis (Quinolone antibiotics) Chronic steroid abuse Diabetes RA Chronic renal failure
Why should steroid injections for tendonitis of the extensor mechanism be avoided?
High risk of tendon rupture
Presentation of extensor mechanism rupture
Unable to straight leg raise
Obvious palpable gap in extensor mechanism
Ix for suspected extensor mechanism rupture
Xray
USS
Tx extensor mechanism rupture
tendon to tendon repair or reattachment of the tendon to the patella
Define patellofemoral dysfunction
disorders of the patellofemoral articulation resulting in anterior knee pain
In what direction does the patella always dislocate
Laterally
