Lower Limb Alignment




Deformity causing alteration of mechanical axis from knee centre


Tibial Malunion APTibial Malunion Lateral




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


Mechanical Axis Lower Limb


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


Femoral Mechanical AxisMechanical Axis Tibia


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

- 6o valgus

- increased in shorter femurs

- decreased in longer femurs


Anatomic Axis FemurTibio-femoral Anatomic Axis


Femoral AA 

- piriformis fossa to centre knee joint

- 6o from mechanical axis

- to knee joint is 81° on lateral side




Femoral Neck-shaft angle 

- 125-131°


Femoral Neck Shaft Angle


mLPFA (mechanical Lateral Proximal Femoral Angle)

- line tip GT to centre femoral head

- 90o to MA




Knee joint

- 3o valgus relative to MA

- 6o valgus distal femur

- 3o varus proximal tibia


Post Slope 

- 9°


Proximal Tibial Slope


MPTA (medial proximal tibial angle)

- 87o or 3o varus


Medial Proximal Tibial AngleAnatomic Lateral Distal Femoral AngleMechanical Lateral Distal Femoral Angle


aLDFA (anatomical lateral distal femoral angle)

- 81o


mLDFA (mechanical lateral distal femoral angle)

- 87o


Mechanical Axis Deviation


Distance from centre of knee to MA in mm






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



- 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

- > 10o


Valgus malalignment knee / ankle

- > 15o


Mechanical axis deviation

- > 20 mm