Alignment in TKR Flashcards
Describe the normal anatomical alignment of the femur and tibia?
- Femur is in 5-7o of Valgus
- tibia is in in 2-3 o of varus
What are the technical goals of TKR?
- Restore mechanical alignment
- Restore joint line
- Balance ligaments
- maintain normal q angle - maintain patella-femoral tracking
Can you draw the mechanical and anatomical axis of the limb?
- Tibial anatomical axis - a line that bisects intramedullary canal
- tibial mechanical- centre of prox tibia to centre of the ankle
- Femoral mechanical thru centre of femoral head to intercondylar notch
- anatomical line bisects the medullary canal of the femur
How is femoral alignment achieved?
- The difference between the anatomcial and mechanical axis ( 5-7 degrees)
- perpendicular to mechanical axis
- jig measures 6 degrees from femoral guide ( antomical axis)
- Will vary if tall <5o or short >7o
How is tibial alignment achieved?
- Usually mechanically and anatomical axis of tbia coincide so can usually cut the proximal tibia perpendicular to the anatomical axis
- if there is a tibia defomrity the tibial cut must be made perpendicular to the mechanical axis and an extra medullary guide is used
How is patellofemoral alignment achieved in TKR?
- Aim to preserve patella tracking
- abnormal patellar tracking is the most common complication of TKR
- aim to avoid increasing the q angle by not
- IR of femoral prosthesis
- Medialisation of femoral components
- IR of the tibial prosthesis
- placing the patellar prosthesis lateral on the patella
What axis are use to ensure good femoral prosthesis placememt to avoid increasing the q angle?
-
Transepicondylar axis
- line running from medial to lateral epicondyles
- the axis is parallel to the cut tibial surface
- a post femoral cut parallel to the epicondylar axis will create the appropriate rectangular flexion gap
-
Posterior condylar axis
- line running across tips of the 2 posterior condyles
- this is 3 degrees of IR from the transepicondlyar axis , the femoral prosthesis should be ER by 3 degrees from this axis to produce a rectangular flexion gap
- *****if the lateral condyle is hypoplastic use the posterior condyle axis -> IR of the femoral component***
What is the Q angle?
Why is increasing this in TKR a problem?
- the angle between the axis of the extensor mechanism ( ASIS to patella) and axis of patella tendon ( centre of patella to tibial tuberosity)
- increase Q angle-> increase in lateral subluxation forces over patella relative to trochlear groove
- -> pain and mechanical symptoms , accelerated wear and dislocation
What happens if the femoral component is placed in Internal rotation?
- you bring the trochlear groove and patella medially
- Increase Q angle to the tibial tuberosity
- also make the medial component tight in flexion
What happens if the femoral component is medialised?
- Bring the trochlear groove to a more medial position
- so bring the patella medial
- increasing the q angle
- so you want the femoral component to be slightly lateral
What is the preferred rotation of the tibia?
- Neutral
- best way to ahieve this is to centre over medial third of tibial tubercle
- this will leave a portion of the posteromedial tibia uncovered and some overhang of the prothesis over the tibia on the posterolateral tibia
What happens if the tibial component is IR and medialised?
- IR=> external rotation of the tibial tuberosity -> increase Q angle
- medialisation will also increase Q angle
What position of the patella prothesis wll increase the q angle?
- lateralisation of the patella
What is the preferred position of the patella prostheis?
- Centred over patella or medialised
- to decrease the q angle
- remove lateral osteophytes
What are the complications of patella non resurfacing cf resurfacing?
- Increased risk of anterior knee pain
- increased risk of secondary resurfacing
- no increased risk of revision surgery
- no increased risk of extension tendon complications
- no diff inpt satisfaction
How is joint line preserved in TKR?
- goal to restore joint line that is the same thickness as the bone and cartilage that was removed
- this preserves ligament tension
-
elevating the joint line >8mm ->
- Mid flexion instability
- patellofemoral tracking problems
- patella Baja
-
Lowering joint line
- Lack of full extension
- flexion instability
What is the goal in varus deformity of the knee?
- release tight medial structures
- tighten the lax lateral ligaments
Describe the intraop release in a varus knee?
tight medial ( concave), loose lateral ( convex side)
- Osteophytes, meniscus and capsule
- **Deep MCL and capsule **
-
Posteriomedial corner
- semimembranosus and capsule
-
Superifical MCL
- can find it as it blends into pes anserine complex
- cannot completely release-> valgus instability so only perform subperiosteal elevation
- if tight in extension = release post oblique portion
- if tight in flexion= release anterior portion
- PCL- rarely released
How are the lateral structures tightened in varus knee deformity?
- use a prothesis that is the size to fill up the gap and make the stretched lateral ligaments taut
What is the goal in valgus knee deformity
- Tight lateral components ( concave), Loose Medial (convex)
- Release lateral components correct valgus
- tighten loose medial components
What is the order of lateral release to the knee?
- Osteophytes, meniscus, capsule
- Lateral capsule
-
Iliotibial band- tight in extension
- z plasty or release of Gerdy’s tubercle
-
popliteus- tight in flexion
- release ant part of insertion
-
LCL
- some release before iliotbial band/popliteus
- if do release consider constrained prothesis
How is flexion/ contraction deformity released?
- Concave side is posterior- needs to be release
- Osteophytes
- Posterior capsule
- Gastronemius muscles ( medial to lateral)
- all releases are preformed with knee at 90 degrees of flexion
- allows popliteal artery to fall posteriorly to decrease risk of injury
- NB not to tx by more tibial resection-> change joint line -> patella alta
What is the complication of releases?
-
Peroneal nerve palsy
- correction of valgus and flexion deformity has highest risk of injury
What is the tx of a pt with peroneal n palsy post op?
- Immediately release dressing
- Flex knee
- watch for 3 months to see if function returns
- if function doesn’t return, consider nerve conduction studies or operative exploration to access for damage
What is the goal of gap ( sagittal ) balancing in TKR?
- Goal is to obtain a gap that is equal in flexion and extension
- this will ensure the tibial insert is stable throughout the arc of motion
- if asymmetric tightness= adjust femur
- distal femur= affect extension gap
- posterior femur= affect flexion gap
-
if symmetrical tightness (extension+ flexion)= adjust tibia
- tibia cuts affect flexion and extension
What do you do if tight in flexion and tight in extension?
- didn’t cut enough tibia so
- Cut more tibia
What do you do if balanced in flexion but tight in extension?
- didn’t take enough off distal femur or didn’t release enough post capsule so
- Release post capsule
- cut more distal femur
What do you do if loose in flexion and tight in extension?
- distal femur too long
- resect more distal femur or use thinner distal augmentation wedge
- upsize femoral component
What do you do if tight in flexion and balanced in extension?
- not enough removed from post femur/ PCL scarred, too tight
- decrease size of femoral component
- recess and release PCL
- resect post slope in tibia
- resect more posterior femoral condyle
- release posterior capsule
What do you do if loose flexion and balanced in extension?
- resected too much off posteror femur
- increase size of femoral component ( ap only)
- Posteriorize femoral component ( use augment post femur)
What do you do if tight in flexion and loose extension?
- too much distal femur removed and too little posterior
- Downsize femur
- use thicker tibial insert until balanced
What do you do if loose in flexion and loose in extension?
- cut too much tibia
- use thicker tibia PE
- use thicker tibial metal insert
