Principles of Deformity Correction in Orthopaedic Surgery
Deformity correction is a three-dimensional geometric and biomechanical problem. Successful correction requires precise understanding of mechanical and anatomical axes, angular relationships, and the relationship between the CORA (Centre of Rotation of Angulation) and the plane of deformity.
Modern monolateral fixation systems rely on accurate pre-operative planning to ensure correction occurs in the correct plane and avoids secondary translation or malalignment.
Mechanical Axis
The mechanical axis represents the line of axial load transmission in static weight-bearing.
- In the tibia, it closely approximates the anatomical axis
- In the femur, it extends from the centre of the femoral head to the centre of the knee joint
- In the full lower limb, it runs from the centre of the femoral head to the centre of the tibio-talar joint
In a normally aligned limb, this line passes near the centre of the knee joint.
Normal alignment reference values:
- MPFA = 84° (80–89°)
- MPTA = 87° (85–90°)
- LDFA (aLDFA) = 81° (79–83°)
- MNSA = 130° (124–136°)
- JLCA = 0–2°
- LDTA = 89° (86–92°)
- PPTA = 81° (77–84°)
- ADTA = 80° (78–82°)
- PPFA = 90°
- PDFA = 83° (79–87°)
- ANSA = 170° (165–175°)
Deviation of the mechanical axis from the centre of the knee represents Mechanical Axis Deviation (MAD).
Anatomical Axis
The anatomical axis is defined as the mid-diaphyseal line of a long bone.
In deformity:
- Each segment has its own anatomical axis
- These axes intersect at or near the apex of deformity
- The intersection helps define angular deformity magnitude and location
CORA (Centre of Rotation of Angulation)
The CORA is the point where the anatomical axes of proximal and distal segments intersect.
It defines:
- The true apex of angular deformity
- The ideal point for correction without inducing translation
Key principle:
Correction performed through the CORA corrects angulation without creating secondary translation.
However, when the CORA does not coincide with the physical apex of the bone deformity, translation becomes part of the deformity complex.
Bisector Line
The bisector line divides the angle formed at the CORA equally.
It is critical in monolateral fixation systems because:
- The hinge of correction should lie on the bisector line
- This allows simultaneous correction of angulation and translation
- Placement away from the bisector results in residual translation or mechanical mismatch during correction
Translation
Translation is present when:
- The CORA is offset from the apex of the deformity
- The deformity includes a combined angular + translational component
If the hinge is placed at the apex rather than the CORA bisector line:
- Angulation may correct
- Translation will persist
Correct hinge positioning ensures both components are addressed simultaneously.
True Plane of Deformity
Angular deformity exists in a single oblique plane, even when it appears in multiple projections.
- AP radiographs define coronal plane deformity
- Lateral radiographs define sagittal plane deformity
- Most deformities are combined oblique plane deformities
Vector-based planning method:
Using orthogonal radiographs:
- Example coronal deformity: 21° valgus
- Example sagittal deformity: 38° recurvatum
Plotting these values on orthogonal axes defines:
- True plane of deformity: ~29° from sagittal plane
- Resultant deformity magnitude: vector sum of angular components
This confirms that deformity correction must occur in the true plane of deformity, not in the independent AP and lateral planes.
Mechanical Axis vs Anatomical Axis
In deformity analysis:
- Mechanical axis defines load transmission
- Anatomical axis defines bone morphology
In the femur:
- Mechanical axis runs from femoral head centre to knee centre
- Anatomical axis runs along diaphyseal midline
- These axes are not parallel, increasing complexity in deformity planning
Key Planning Principles
Effective deformity correction requires:
- Identification of mechanical axis deviation (MAD)
- Definition of anatomical axes of each bone segment
- Determination of CORA location (anatomical axis method preferred)
- Identification of translation component (CORA vs apex mismatch)
- Calculation of true plane of deformity (vector method)
- Alignment of correction hinge to CORA bisector line
Clinical Implication for Monolateral Fixation
With monolateral systems:
- Hinge position determines correction behaviour
- Convex side placement may induce distraction at osteotomy
- Concave side placement may cause compression and potential jamming
- Correct hinge alignment ensures controlled angulation + translation correction
Summary
Deformity correction is fundamentally governed by:
- Mechanical axis restoration
- CORA-based angular correction
- Recognition of translational components
- Accurate identification of the true plane of deformity
A structured understanding of these principles allows predictable correction of complex multiplanar deformities using monolateral fixation systems.