High Yield Overview

DEFORMITY ANALYSIS - CORA AND MAD

Center of Rotation of Angulation | Mechanical Axis Deviation | Osteotomy Planning

CORACenter of Rotation of Angulation

MADMechanical Axis Deviation

87°Normal LDFA (lateral distal femoral angle)

87°Normal MPTA (medial proximal tibial angle)

OSTEOTOMY RULES (PALEY)

Rule 1

PatternCORA at osteotomy = angulation only

TreatmentIdeal - no translation

Rule 2

PatternCORA not at osteotomy = angulation + translation

TreatmentTranslation may be acceptable

Rule 3

PatternTranslation osteotomy

TreatmentNo angular correction, just axis shift

Critical Must-Knows

CORA = intersection of proximal and distal anatomic/mechanical axes
MAD = distance from mechanical axis to center of knee (normal 0-10mm medial)
Osteotomy at CORA corrects deformity without translation
LDFA and MPTA both normally 87 degrees
Joint line obliquity must be assessed and corrected if abnormal

Examiner's Pearls

"
Proximal tibial osteotomy corrects up to 15 degrees safely
"
Distal femoral osteotomy for valgus greater than 12-15 degrees
"
Medial opening wedge HTO changes tibial slope posteriorly
"
CORA method allows precise osteotomy planning

Clinical Imaging

Pre and Post-Operative Results

Pre and post-operative X-rays showing bilateral genu valgum correction with mechanical axis lines — Full-length standing AP radiographs: (a) Pre-operative bilateral genu valgum (knock-knee deformity) with lateral mechanical axis deviation, (b) Post-operative correction with bilateral distal femoral osteotomies and plate fixation - yellow lines demonstrate restored mechanical axis alignment through knee centerCredit: Ganjwala D et al., Indian J Orthop (PMC4175864)

Pre and post-operative clinical photographs showing correction of genu valgum — Clinical photographs demonstrating correction of bilateral genu valgum: Left panel shows pre-operative knock-knee deformity, right panel shows post-operative neutral limb alignment following deformity correction surgeryCredit: Ganjwala D et al., Indian J Orthop (PMC4175864)

Critical Deformity Analysis Exam Points

CORA Concept

CORA is where the proximal and distal axes intersect. This is the apex of the deformity. An osteotomy at CORA corrects angulation without creating translation. Away from CORA, you get angulation AND translation.

Mechanical Axis

Mechanical axis runs from hip center to ankle center. Should pass through or just medial (0-10mm) to knee center. MAD measures deviation from this ideal. Positive = lateral; negative = medial.

Joint Orientation Angles

LDFA (lateral distal femoral angle) = 87° ± 3°. MPTA (medial proximal tibial angle) = 87° ± 3°. JLCA (joint line congruence angle) = 0-2°. Deviations localize the deformity to femur, tibia, or joint.

Osteotomy Selection

Tibial osteotomy for tibial deformity (MPTA abnormal). Femoral osteotomy for femoral deformity (LDFA abnormal). Choose level to correct CORA. May need combined osteotomies for biplanar or oblique JLCA.

Normal Joint Orientation Angles

Angle	Normal Value	Location
LDFA (Lateral Distal Femoral Angle)	87° ± 3°	Lateral angle between femoral mechanical axis and knee joint line
MPTA (Medial Proximal Tibial Angle)	87° ± 3°	Medial angle between tibial mechanical axis and knee joint line
LPFA (Lateral Proximal Femoral Angle)	90° ± 5°	Lateral angle between femoral mechanical axis and femoral neck axis
LDTA (Lateral Distal Tibial Angle)	89° ± 3°	Lateral angle between tibial mechanical axis and ankle joint line
MAD	0-10mm medial	Mechanical axis deviation at knee

Mnemonic

LLMM 87-87Joint Orientation Angles

LDFA

Lateral Distal Femoral Angle = 87°

LPFA

Lateral Proximal Femoral Angle = 90°

MPTA

Medial Proximal Tibial Angle = 87°

MAD

Mechanical Axis Deviation = 0-10mm medial

Memory Hook:LDFA and MPTA are both 87 degrees - easy to remember!

Mnemonic

CORAOsteotomy Planning Steps

Calculate joint orientation angles

LDFA, MPTA, JLCA

Outline mechanical axes

Draw proximal and distal axes

Recognize CORA location

Where axes intersect

Apply osteotomy at CORA

Or accept translation if away

Memory Hook:CORA method = systematic deformity analysis!

Mnemonic

AT-AT-TPaley Osteotomy Rules

At CORA

Angulation only correction

Translation zero

No secondary deformity

Away from CORA

Angulation PLUS translation

Translation created

Secondary deformity occurs

Translation osteotomy

Axis shift without angulation

Memory Hook:Rule 1 = AT CORA no translation; Rule 2 = Away from CORA = translation!

Overview and Epidemiology

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

CORA Method

The CORA method revolutionized deformity correction by providing a mathematical basis for osteotomy planning. The key insight: an osteotomy at CORA produces pure angular correction without translation, while an osteotomy away from CORA creates both angulation and translation.

Pathophysiology

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:

Draw proximal anatomic or mechanical axis (line along proximal segment)
Draw distal anatomic or mechanical axis (line along distal segment)
Point where these lines intersect = CORA
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

ACA vs CORA

ACA (Angulation Correction Axis) is perpendicular to the plane of deformity at CORA. When the osteotomy is made at CORA and rotated around ACA, perfect correction occurs. The ACA is a theoretical axis essential for 3D deformity correction.

Clinical Presentation

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

Investigations

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

External fixator with rail and swivel clamps for deformity correction — Intraoperative photograph showing external fixator application with rail fixator and two swivel clamps for temporary deformity correction. The external fixator maintains alignment during the correction procedureCredit: Ganjwala D et al., Indian J Orthop (PMC4175864)

Intraoperative alignment verification with alignment rod — Intraoperative alignment confirmation using an alignment rod to verify mechanical axis correction during deformity surgery. The external fixator maintains position while alignment is checkedCredit: Ganjwala D et al., Indian J Orthop (PMC4175864)

Internal plate fixation while external fixator maintains alignment — Combined external and internal fixation: Plate fixation is performed while the external fixator maintains the corrected limb alignment. This technique ensures precise deformity correction is maintained during internal fixationCredit: Ganjwala D et al., Indian J Orthop (PMC4175864)

Stress Radiographs

Varus/valgus stress views:

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

Management

CORA Method Planning

Step 1: Draw mechanical axes

Proximal mechanical axis from hip to knee
Distal mechanical axis from knee to ankle
Note intersection point (CORA)

Step 2: Measure joint orientation angles

LDFA: Should be 87°
MPTA: Should be 87°
JLCA: Should be 0-2°
Identify which angle is abnormal

Step 3: Localize the deformity

Abnormal LDFA = femoral deformity
Abnormal MPTA = tibial deformity
Abnormal both = combined deformity
Abnormal JLCA = joint line obliquity

Step 4: Plan osteotomy

Ideal: Osteotomy at CORA (angulation only)
Alternative: Osteotomy away from CORA (accept translation)
Calculate correction angle

Step 5: Determine correction magnitude

Target MAD: 0 to 10mm medial for neutral alignment
For medial compartment OA: 3-5mm lateral (slight overcorrection)
For lateral compartment OA: Neutral to slight medial

This section covers CORA method planning.

Surgical Management

Paley's Osteotomy Rules

Rule 1: Osteotomy at CORA

Principle:

When osteotomy is made at CORA and angulated
Result is pure angular correction
No translation created

Application:

Ideal scenario for deformity correction
Osteotomy level matches apex of deformity
Hinge at CORA allows rotation around ACA

Advantages:

Clean angular correction
No secondary deformity
Predictable outcome

Limitations:

May not always be practical (CORA in joint, periarticular)
May require intra-articular osteotomy

This section covers Rule 1.

Complications

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

Evidence Base

CORA Method Original Description

Paley D, Tetsworth K • Clin Orthop Relat Res (1992)

Key Findings:

CORA = intersection of proximal and distal axes
Osteotomy at CORA prevents translation
Mathematical basis for osteotomy planning

HTO Mechanical Axis Correction

III

Dugdale TW et al. • Clin Orthop Relat Res (1992)

Key Findings:

Overcorrection to 3-5mm lateral MAD recommended
Undercorrection associated with failure
Mechanical axis through lateral compartment unloads medial

Joint Orientation Angles Normal Values

III

Moreland JR et al. • Clin Orthop Relat Res (1987)

Key Findings:

LDFA = 87° ± 3°
MPTA = 87° ± 3°
Normal mechanical axis passes through or medial to knee center

Computer-Assisted Deformity Correction

Paley D • Orthop Clin North Am (2014)

Key Findings:

Computer planning reduces errors
3D analysis for multiplanar deformities
Navigation assists intraoperative accuracy

Exam Viva Scenarios

Practice these scenarios to excel in your viva examination

VIVA SCENARIOStandard

Scenario 1: Varus Knee Analysis

EXAMINER

"You are shown a long-leg standing radiograph of a 55-year-old with medial compartment osteoarthritis. The LDFA is 87°, MPTA is 82°. Analyze this deformity."

EXCEPTIONAL ANSWER

Thank you. Analyzing this deformity systematically: The LDFA is 87°, which is normal, indicating the femur is not contributing to the varus. The MPTA is 82°, which is abnormal - it should be 87°. This 5° difference indicates the tibial mechanical axis is tilted 5° into varus. The CORA for this deformity would be at the proximal tibia at the level of the abnormal MPTA. Since the deformity is tibial, a proximal tibial osteotomy (HTO) is appropriate. My target correction would be to restore MPTA to 87-90°, aiming for a MAD of 3-5mm lateral to the knee center to unload the medial compartment. I would plan an opening wedge HTO, understanding that Rule 2 applies - my osteotomy is below CORA (which is at the joint) so I will create some lateral translation, which is acceptable.

KEY POINTS TO SCORE

LDFA normal (87°) = femur not contributing

MPTA abnormal (82°) = tibial varus

CORA at proximal tibia

HTO appropriate, target overcorrection

COMMON TRAPS

✗Blaming femur when LDFA is normal

✗Not calculating correction amount

✗Aiming for neutral instead of slight overcorrection for OA

LIKELY FOLLOW-UPS

"What if LDFA was 82° instead?"

"How would you perform the HTO?"

"What are the complications of HTO?"

VIVA SCENARIOAdvanced

Scenario 2: Double Deformity

EXAMINER

"A young adult has valgus malalignment with LDFA of 93° and MPTA of 93°. How do you analyze and plan correction?"

EXCEPTIONAL ANSWER

Thank you. This is a complex double-level deformity. The LDFA of 93° indicates distal femoral valgus - the normal is 87°, so there is 6° of femoral contribution to valgus. The MPTA of 93° indicates proximal tibial valgus - the normal is 87°, so there is also 6° of tibial contribution. Both bones are contributing equally to the overall valgus malalignment. In this case, I have two CORAs - one at the distal femur and one at the proximal tibia. I could correct this with a single osteotomy at one level accepting residual deformity at the other, or I could perform double-level osteotomy. For a young patient, I would consider double-level correction to normalize both LDFA and MPTA. This could be staged or simultaneous depending on patient and surgical factors.

KEY POINTS TO SCORE

LDFA 93° = 6° femoral valgus contribution

MPTA 93° = 6° tibial valgus contribution

Two CORAs - double-level deformity

May need double osteotomy for full correction

COMMON TRAPS

✗Correcting only one level

✗Not recognizing double-level deformity

✗Over-correcting at single level creating secondary deformity

LIKELY FOLLOW-UPS

"Would you do staged or simultaneous osteotomies?"

"What is the maximum safe correction per level?"

"How would you approach the distal femur?"

VIVA SCENARIOStandard

Scenario 3: Osteotomy Rules

EXAMINER

"Explain Paley's Rule 1 and Rule 2 for osteotomy planning."

EXCEPTIONAL ANSWER

Thank you. Paley's osteotomy rules describe the relationship between osteotomy location and CORA. Rule 1 states that when an osteotomy is made at CORA - the apex of the deformity where proximal and distal axes intersect - and the bone is angulated, the result is pure angular correction without translation. This is the ideal scenario. Rule 2 states that when an osteotomy is made away from CORA, angular correction necessarily creates a translational secondary deformity. The magnitude of translation is proportional to the distance from CORA and the magnitude of angular correction. This is clinically important because many times we cannot make an osteotomy exactly at CORA - for example, in a varus knee where CORA is within the joint. An HTO made below the joint will correct angulation but also create lateral translation of the tibia, which is usually acceptable.

KEY POINTS TO SCORE

Rule 1: Osteotomy at CORA = angulation only

Rule 2: Osteotomy away from CORA = angulation + translation

Translation proportional to distance from CORA

HTO is a Rule 2 osteotomy - CORA at joint, osteotomy below

COMMON TRAPS

✗Confusing the rules

✗Not understanding why translation occurs

✗Thinking all translation is unacceptable

LIKELY FOLLOW-UPS

"What is Rule 3?"

"Give an example of when translation is acceptable"

"What is the ACA?"

Australian Context

In Australia, deformity analysis and correction is performed by orthopaedic surgeons with subspecialty training in limb reconstruction. The CORA method is the standard approach taught in Australian fellowship programs and is aligned with international practice.

Imaging requirements:

Long-leg standing radiographs available at most radiology centers
CT scanograms for accurate length measurements
EOS imaging available at some tertiary centers

Computer-assisted planning and navigation are increasingly used for complex deformity correction, with software systems (TraumaCad, Bone Ninja) facilitating CORA analysis and osteotomy planning. Multidisciplinary discussion is standard for complex cases.