Deformity Analysis - CORA and MAD

Deformity analysis is the systematic evaluation of limb alignment to identify the location and magnitude of angular and translational deformities. The CORA (Center of Rotation of Angulation) method, developed by Dror Paley, provides a mathematical approach to osteotomy planning.

Applications:

• Angular deformity correction
• Limb length discrepancy with deformity
• Malunion correction
• Developmental deformity
• Post-traumatic reconstruction

Importance:

• Accurate analysis prevents secondary deformities from osteotomy
• Guides osteotomy level selection
• Predicts outcomes of correction
• Essential for examination success

Understanding the geometry of deformity analysis is fundamental to correct application.

Mechanical Axis

Definition:

• Line from center of femoral head to center of ankle (talus)
• Represents weight-bearing axis of the limb
• Normally passes through or just medial to center of knee

Mechanical Axis Deviation (MAD):

• Distance from mechanical axis to center of knee
• Positive = axis lateral to knee (valgus)
• Negative = axis medial to knee (varus)
• Normal: 0 to 10mm medial to knee center

Anatomic Axis

Definition:

• Line through the center of the bone diaphysis
• Femoral anatomic axis is not collinear with mechanical axis
• Tibial anatomic axis approximately equals mechanical axis

Femoral anatomic-mechanical angle:

• Approximately 6 degrees
• Femoral anatomic axis is lateral to mechanical axis

CORA Determination

Method:

1. Draw proximal anatomic or mechanical axis (line along proximal segment)
2. Draw distal anatomic or mechanical axis (line along distal segment)
3. Point where these lines intersect = CORA
4. CORA represents the apex of deformity

Key principle:

• Single-plane deformity has one CORA
• Multiplanar deformity has multiple CORAs
• Oblique plane deformity appears different in AP and lateral views

Patient Assessment

History:

• Etiology of deformity (congenital, developmental, post-traumatic)
• Duration and progression
• Symptoms: pain, instability, functional limitation
• Previous surgery

Physical examination:

• Gait analysis
• Limb alignment (standing, supine)
• Joint range of motion
• Ligamentous stability
• Limb length measurement
• Rotational profile

Indications for Correction

Functional:

• Pain related to malalignment
• Gait abnormality
• Progressive deformity
• Accelerated compartmental wear

Prophylactic:

• Prevent arthrosis progression
• Improve joint preservation
• Optimize alignment before or instead of arthroplasty

Imaging

Long-leg standing radiographs:

• Full-length AP from hip to ankle
• Weight-bearing essential
• Single cassette or stitched images

Measurements required:

• Mechanical axis (hip center to ankle center)
• MAD (deviation at knee)
• LDFA, MPTA, LDTA
• JLCA (joint line congruence angle)
• Limb length

Lateral views:

• Assess sagittal plane alignment
• Posterior tibial slope (normal 10°)
• Recurvatum/procurvatum deformity

CT scanogram:

• Accurate length measurement
• Rotational profile assessment
• 3D deformity analysis possible

Intraoperative Technique

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Stress Radiographs

Varus/valgus stress views:

• Assess ligament integrity
• Determine reducibility of deformity
• Distinguish bony from ligamentous deformity

Paley's Osteotomy Rules

Planning Errors

• Incorrect axis drawing: Leads to wrong CORA location
• Measurement errors: Wrong correction magnitude
• Ignoring JLCA: Joint line obliquity persists
• Sagittal plane neglect: Changes tibial slope unexpectedly

Surgical Complications

• Under/over correction: Inadequate planning or execution
• Secondary deformity: Translation from osteotomy away from CORA
• Joint line obliquity: Uncorrected or created
• Slope changes: Particularly with opening wedge HTO

The single most examined skill is using joint orientation angles to decide WHERE the deformity sits. The same overall mechanical axis deviation can arise from very different sources, and each implies a different osteotomy. Work through the angles before naming a procedure.

• Target alignment - neutral vs constitutional varus. The traditional teaching is to overcorrect a varus knee to roughly 3-5mm lateral MAD (around 8 degrees valgus per Coventry). More recent debate around "constitutional varus" questions whether driving every patient to mechanical neutral is necessary or even ideal; correction should be individualised to the patient's native alignment and pathology rather than applied as a fixed formula.
• Overcorrection in valgus knees is different. Unlike varus HTO, distal femoral osteotomy for valgus should generally aim only for neutral or very slight varus. Biomechanical data show that pushing into varus sharply loads the medial compartment and may accelerate medial OA (Wu et al 2022).
• Single- vs double-level correction. When both LDFA and MPTA are abnormal, a single large correction restores the mechanical axis but leaves an oblique joint line. Double-level osteotomy normalises both joint orientation and avoids joint-line obliquity, at the cost of a bigger operation. The threshold for accepting residual obliquity versus operating at two levels is not firmly defined.
• Acute internal fixation vs gradual external correction. Acute correction with plates is faster and avoids prolonged frame time but is less forgiving of planning error; gradual correction with circular frames allows fine adjustment and large/multi-apical correction but carries pin-site and patient-tolerance burden. Choice is driven by deformity magnitude, soft tissue, and resources, not by a single right answer.
• Sagittal-plane consequences. Medial opening-wedge HTO tends to increase posterior tibial slope and can alter patellar height; these secondary effects are real and must be planned for, but the precise magnitude and clinical significance remain debated.

Deformity analysis using the Paley CORA framework is the internationally accepted standard, taught in FRCS (Tr & Orth), FRACS, EBOT/FEBOT, ABOS and DNB/MS curricula. Practice differs mainly in imaging access, fixation philosophy and resource availability rather than in the underlying geometry.

Global Epidemiology and Burden

• Symptomatic knee osteoarthritis affects an estimated 250 to 300 million people worldwide, and frontal-plane malalignment is one of the strongest modifiable mechanical risk factors for progression (Sharma et al, JAMA 2001).
• Realignment osteotomy volume is rising in younger, active patients (typically under 60) as an arthroplasty-sparing option, with the highest relative use in regions where joint preservation is prioritised.
• Post-traumatic and developmental (rickets, Blount disease, physeal injury, skeletal dysplasia) deformities form a larger share of the caseload in limited-resource and high-prevalence settings.

Society Guidance and Frameworks (side by side)

The named-society positions above are guideline-level statements rather than primary trial data.

Registry and Outcome Notes

• National arthroplasty registries (NJR UK, AJRR US, AOANJRR Australia, SHAR Sweden, NZJR New Zealand) record prior osteotomy as a factor in subsequent knee arthroplasty; prior osteotomy does not preclude later conversion but can modestly increase technical complexity.
• Long-term cohort data (Coventry et al, JBJS 1993) anchor the principle that adequate, slightly over-corrected alignment is the dominant survival factor after valgus tibial osteotomy.

High- vs Limited-Resource Practice Variation

Gradual correction with circular frames remains a globally important technique because it tolerates planning imprecision, corrects multi-apical and large deformities, and manages associated limb-length discrepancy and soft-tissue contracture.