Lower Limb Alignment

 

Definition

 

Deformity causing alteration of mechanical axis from knee centre

 

 

Aims

 

1.  Normal leg alignment

2.  Equal leg lengths

3.  Horizontal joint lines

 

Normal Anatomy

 

Mechanical Axes

 

A.  Coronal Mechanical Axis of Lower Limb

 

Line from centre femoral head to centre of ankle

- passes medial to the centre of knee 

- 10mm medial to the frontal plane centre

- medial tibial spine

- Llne of transmission of forces

 

 

Sagittal Plane

 

Centre of femoral head to centre of ankle

- just anterior to centre of knee joint

- aids in passive locking of knee joint in full extension

 

B.  Femoral Mechanical Axis

- centre of femoral head to centre knee

 

 

C.  Tibial Mechanical Axis

- same as tibial anatomic axis

- centre plateau to centre of talus / ankle

 

Anatomical Axis

 

Femoral - Tibial anatomic axis

- centre of femoral shaft to centre of knee, then centre of tibia

- 6^o valgus

- increased in shorter femurs

- decreased in longer femurs

 

 

Femoral AA 

- piriformis fossa to centre knee joint

- 6^o from mechanical axis

- to knee joint is 81° on lateral side

 

Hip

 

Femoral Neck-shaft angle 

- 125-131°

 

 

mLPFA (mechanical Lateral Proximal Femoral Angle)

- line tip GT to centre femoral head

- 90^o to MA

 

Knee

 

Knee joint

- 3^o valgus relative to MA

- 6^o valgus distal femur

- 3^o varus proximal tibia

 

Post Slope 

- 9°

 

 

MPTA (medial proximal tibial angle)

- 87^o or 3^o varus

 

 

aLDFA (anatomical lateral distal femoral angle)

- 81^o

 

mLDFA (mechanical lateral distal femoral angle)

- 87^o

 

Mechanical Axis Deviation

 

Distance from centre of knee to MA in mm

 

Malalignment 

 

Effect

 

Uncertain if malunion causes OA

- theoretical cartilage overload

- evidence in animals of cartilage changes

- evidence of increased forces in cadavers

 

Kristensen Acta Ortho Scand'89 

- <15° Malunion = °OA  

 

Known problems

 

Femoral Procurvatum

- MA passes posterior to knee joint

- prevents knee locking

 

Other joints stiff 

- proximal tibial varus malunion

- can be compensated if mobile STJ

- if stiff subtalar joint, will notice abnormal gait

 

Ligamentous laxity of convex side of deformity

- increasing instability

- i.e. varus malunion with posterolateral instability

 

Chondral damage on overloaded side

 

Defining site of malunion

 

CORA

- centre of rotation of angulation

- lines of intra-medullary canals

- intersection is CORA

- site of deformity

- best site for correction

 

 

Indications for Surgery

 

Varus malalignment knee / ankle

- > 10^o

 

Valgus malalignment knee / ankle

- > 15^o

 

Mechanical axis deviation

- > 20 mm