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.

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

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Mechanical Axis

Anatomic Axis

CORA Determination

The CORA method handles angulation, translation and length; rotation (torsion) is the axial deformity it does not capture on the coronal film and must be assessed separately - a high-yield omission in many answers.

Deformity is classified by plane, by source segment and by the Paley osteotomy rule that the chosen cut will obey.

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Patient Assessment

Indications for Correction

Imaging

Intraoperative Technique

Once planning is complete, correction is executed and confirmed intra-operatively. A temporary external fixator (rail fixator with swivel clamps) or a guide-wire construct holds the planned correction while an alignment rod (cautery cord or long metal rod from femoral head centre to ankle centre) is screened across the knee to confirm the mechanical axis passes through the desired point before definitive internal fixation (plate or nail) is applied.

Stress Radiographs

Varus/valgus stress views:

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

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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.

Paley's Osteotomy Rules

Beyond locating the CORA, the examiner wants the opening-versus-closing decision and its trade-offs, plus a feel for the wedge size:

Wedge magnitude (rule of thumb): each degree of coronal correction shifts the weight-bearing line roughly 3 to 4 mm across the tibial plateau (Dugdale), and any joint-space opening from lateral laxity must be subtracted to avoid over-correction. The opening-wedge gap height is approximately the osteotomy width multiplied by the tangent of the correction angle.

So choose the wedge by the desired length change and the cortex you wish to preserve, protect the far hinge in an opening wedge, and always remember the common peroneal nerve in a lateral closing-wedge HTO.

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

Rehabilitation protects the osteotomy hinge and fixation while restoring motion.

• Weight-bearing: opening-wedge osteotomies (a tension construct relying on the intact far hinge) are usually protected with toe-touch/partial weight-bearing for ~6 weeks; closing-wedge and compressed constructs (more inherently stable) allow earlier loading at surgeon discretion.
• Hinge protection: avoid premature full load that can fracture or collapse the far cortex (loss of correction).
• Motion: early knee range of motion to prevent stiffness; quadriceps activation and patellar mobilisation, especially after opening-wedge HTO where patella baja can develop.
• Graft / gap: opening wedges filled with autograft, allograft or substitute; monitor for delayed union at the gap.
• Frame care (gradual correction): pin-site care, scheduled strut/ring adjustments per the planned schedule, and serial alignment radiographs.
• Imaging: check correction and union with standing long-leg films; confirm the mechanical axis has reached the planned target rather than just "looks straight".
• Thromboprophylaxis per unit protocol; remove temporary external fixation once internal fixation/union is secure.

• Alignment is the dominant survival factor. After valgus tibial osteotomy, achieving at least ~8° valgus (slight overcorrection) gave ~90% 5-year and ≥65% 10-year survival; under-correction in an overweight patient dropped 10-year survival to ~19% (Coventry 1993).
• Disease modification: because frontal-plane malalignment multiplies the odds of compartmental OA progression 4-5 fold (Sharma 2001), restoring the mechanical axis genuinely slows medial (or lateral) wear, not just symptoms.
• Valgus correction is different: distal femoral osteotomy should aim for neutral, since over-correction into varus sharply overloads the medial compartment (Wu 2022).
• Joint-line obliquity left uncorrected (single-level fix of a double-level deformity) is associated with shear and less durable results - the rationale for double-level osteotomy in the young.
• Conversion to arthroplasty: a prior osteotomy does not preclude later TKA but can modestly increase technical difficulty; osteotomy is best viewed as an arthroplasty-sparing, time-buying procedure in the young active patient.
• Prognostic modifiers: obesity, smoking, large corrections, advanced cartilage loss and inaccurate planning all worsen outcome; accurate CORA-based planning and adequate (not excessive) correction optimise it.

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.

• 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.