Alignment in TKR

Question 1

Describe the normal anatomical alignment of the femur and tibia?

Answer 1

• Femur is in 5-7^o of Valgus
• tibia is in in 2-3 ^o of varus

Question 2

What are the technical goals of TKR?

Answer 2

1. Restore mechanical alignment
2. Restore joint line
3. Balance ligaments
4. maintain normal q angle - maintain patella-femoral tracking

Question 3

Can you draw the mechanical and anatomical axis of the limb?

Answer 3

• 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

Question 4

How is femoral alignment achieved?

Answer 4

• 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 <5^o or short >7^o

Question 5

How is tibial alignment achieved?

Answer 5

• 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

Question 6

How is patellofemoral alignment achieved in TKR?

Answer 6

• 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

Question 7

What axis are use to ensure good femoral prosthesis placememt to avoid increasing the q angle?

Answer 7

• 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***

Question 8

What is the Q angle?

Why is increasing this in TKR a problem?

Answer 8

• 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

Question 9

What happens if the femoral component is placed in Internal rotation?

Answer 9

• you bring the trochlear groove and patella medially
• Increase Q angle to the tibial tuberosity
• also make the medial component tight in flexion

Question 10

What happens if the femoral component is medialised?

Answer 10

• 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

Question 11

What is the preferred rotation of the tibia?

Answer 11

• 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

Question 12

What happens if the tibial component is IR and medialised?

Answer 12

• IR=> external rotation of the tibial tuberosity -> increase Q angle
• medialisation will also increase Q angle

Question 13

What position of the patella prothesis wll increase the q angle?

Answer 13

• lateralisation of the patella

Question 14

What is the preferred position of the patella prostheis?

Answer 14

• Centred over patella or medialised
• to decrease the q angle
• remove lateral osteophytes

Question 15

What are the complications of patella non resurfacing cf resurfacing?

Answer 15

• 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

Question 16

How is joint line preserved in TKR?

Answer 16

• 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

Question 17

What is the goal in varus deformity of the knee?

Answer 17

• release tight medial structures
• tighten the lax lateral ligaments

Question 18

Describe the intraop release in a varus knee?

Answer 18

tight medial ( concave), loose lateral ( convex side)

1. Osteophytes, meniscus and capsule
2. **Deep MCL and capsule **
3. Posteriomedial corner
   
   • semimembranosus and capsule
4. 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
5. ​PCL- rarely released

Question 19

How are the lateral structures tightened in varus knee deformity?

Answer 19

• use a prothesis that is the size to fill up the gap and make the stretched lateral ligaments taut

Question 20

What is the goal in valgus knee deformity

Answer 20

• Tight lateral components ( concave), Loose Medial (convex)
• Release lateral components correct valgus
• tighten loose medial components

Question 21

What is the order of lateral release to the knee?

Answer 21

1. Osteophytes, meniscus, capsule
2. Lateral capsule
3. Iliotibial band- tight in extension
   
   • z plasty or release of Gerdy’s tubercle
4. popliteus- tight in flexion
   
   • release ant part of insertion
5. LCL
   
   • some release before iliotbial band/popliteus
   • if do release consider constrained prothesis

Question 22

How is flexion/ contraction deformity released?

Answer 22

• Concave side is posterior- needs to be release

1. Osteophytes
2. Posterior capsule
3. 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

Question 23

What is the complication of releases?

Answer 23

• Peroneal nerve palsy
  
  • correction of valgus and flexion deformity has highest risk of injury

Question 24

What is the tx of a pt with peroneal n palsy post op?

Answer 24

• 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

Question 25

What is the goal of gap ( sagittal ) balancing in TKR?

Answer 25

* 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**

Question 26

What do you do if tight in flexion and tight in extension?

Answer 26

* didn't cut enough tibia so * Cut more tibia

Question 27

What do you do if balanced in flexion but tight in extension?

Answer 27

* didn't take enough off distal femur or didn't release enough post capsule so 1. **Release post capsule** 2. **cut more distal femur**

Question 28

What do you do if loose in flexion and tight in extension?

Answer 28

* _distal femur too long_ 1. **resect more distal femur or use thinner distal augmentation wedge** 2. **upsize femoral component**

Question 29

What do you do if tight in flexion and balanced in extension?

Answer 29

* _not enough removed from post femur/ PCL scarred, too tight_ 1. **decrease size of femoral component** 2. **recess and release PCL** 3. **resect post slope in tibia** 4. **resect more posterior femoral condyle** 5. **release posterior capsule**

Question 30

What do you do if loose flexion and balanced in extension?

Answer 30

* resected too much off posteror femur 1. **increase size of femoral component** ( ap only) 2. **Posteriorize femoral component** ( use augment post femur)

Question 31

What do you do if tight in flexion and loose extension?

Answer 31

* too much distal femur removed and too little posterior 1. **Downsize femur** 2. **use thicker tibial insert until balanced**

Question 32

What do you do if loose in flexion and loose in extension?

Answer 32

* cut too much tibia 1. use thicker tibia PE 2. use thicker tibial metal insert