High Yield Overview

TAYLOR SPATIAL FRAME

Hexapod System | Stewart Platform | 6-Axis Correction

6 strutsHexapod configuration

6 DOFDegrees of freedom (3 rotational, 3 translational)

14 paramsMounting parameters required

Web-basedSoftware planning platform

CORRECTION PARAMETERS

Translational

PatternAP, ML, axial (shortening/lengthening)

Treatment3 linear deformities

Rotational

PatternAngulation (varus/valgus), rotation, tilt

Treatment3 angular deformities

Reference Ring

PatternProximal or distal, orthogonal or oblique

TreatmentDefines coordinate system

Virtual Hinge

PatternCORA-based pivot point for correction

TreatmentMinimizes translation

Critical Must-Knows

Hexapod = Stewart platform: 6 telescopic struts connecting 2 rings = 6 DOF
Reference ring: Fixed ring (usually proximal) defines coordinate system
Moving ring: Ring that moves relative to reference ring during correction
Mounting parameters: 14 measurements (7 per ring) required for software
Total residual: Combined measure of all deformity components - correction complete when approaches zero

Examiner's Pearls

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Stewart platform originally designed for flight simulators (1965)
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TSF can correct all 6 axes simultaneously - unlike Ilizarov
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Virtual hinge at CORA minimizes unwanted translation during angular correction
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Frame mounting errors cause residual deformity - precision critical

Clinical Imaging

Imaging Gallery

Treatment of a posttraumatic multidimensional deformity with the Taylor Spatial Frame (TSF). Deformity correction included lengthening, correction of varus malalignment and translational deformity — Treatment of a posttraumatic multidimensional deformity with the Taylor Spatial Frame (TSF). Deformity correction included lengthening, correction of Credit: Manner HM et al. via J Child Orthop via Open-i (NIH) (Open Access (CC BY))

Critical TSF Exam Points

Hexapod Principle

Stewart platform kinematic mechanism: 6 telescopic struts connect 2 rings with universal joints. This configuration provides 6 degrees of freedom (3 translations + 3 rotations), allowing simultaneous multiplanar correction. Unlike Ilizarov, no hinge placement or frame rebuilding required.

Mounting Parameters

14 total parameters (7 per ring) must be measured accurately: ring diameter, ring-to-ring distance, frame offset (anterior, lateral), strut mounting positions (rotational orientation), and axial orientation. Errors in measurement cause residual deformity.

Reference vs Moving Ring

Reference ring = fixed ring (typically proximal) that defines the coordinate system. Moving ring = ring that moves during correction. Choice affects deformity prescription - be consistent. The reference ring is attached to the "stable" bone segment.

Total Residual

Total residual = composite measure of remaining deformity across all 6 axes. Calculated by software after inputting current deformity parameters. Correction progresses until total residual approaches zero. Used to monitor progress and endpoint.

Taylor Spatial Frame vs Ilizarov Circular Fixator

Feature	Taylor Spatial Frame (TSF)	Ilizarov Circular Fixator
Strut configuration	6 telescopic struts (hexapod)	Rods, hinges, motors (modular)
Degrees of freedom	6 DOF simultaneous correction	Sequential corrections required
Planning method	Computer software (web-based)	Mechanical/manual planning
Hinge placement	Virtual hinge (software)	Physical hinges required
Frame rebuilding	Not required during treatment	Often needed for direction changes
Learning curve	Software-dependent (steep initially)	Traditional mechanics (steep)
Cost	Higher (struts, software)	Lower (traditional components)
Residual correction	Easy re-prescription via software	Frame reconfiguration needed

Mnemonic

TAR-VAR6 Degrees of Freedom

Translation AP

Anterior-posterior displacement

Translation Axial

Shortening or lengthening

Translation ML (Right-Left)

Medial-lateral displacement

Varus/Valgus angulation

Coronal plane angulation

Apex rotation

Axial rotation (torsion)

Recurvatum/Procurvatum

Sagittal plane angulation

Memory Hook:TAR-VAR = 6 DOF: 3 Translations (AP, Axial, ML) + 3 Rotations (Varus/valgus, Axial rotation, Recurvatum)!

Mnemonic

DR FLOSSMounting Parameters (7 per ring)

Diameter

Ring internal diameter in mm

Ring-to-Ring distance

Distance between reference and moving rings

Frame offset AP

Anteroposterior offset from bone axis

Lateral offset

Mediolateral offset from bone axis

Orientation (rotational)

Rotational position of master tab

Strut positions

Numbered mounting holes for each strut

Segment orientation

Proximal or distal, left or right

Memory Hook:DR FLOSS your frame: Diameter, Ring distance, Frame offset (AP), Lateral offset, Orientation, Strut positions, Segment!

Mnemonic

MAD CORADeformity Parameters (CORA Analysis)

Mechanical axis deviation

Distance from mechanical axis to joint center

Angulation

Magnitude and direction of angular deformity

Displacement (translation)

Linear offset of bone segments

Center of Rotation of Angulation

Pivot point for correction

Orientation

Coronal, sagittal, or oblique plane

Rotation (axial)

Torsional deformity component

Axial length

Shortening or lengthening required

Memory Hook:MAD CORA analysis: Mechanical axis, Angulation, Displacement + CORA, Orientation, Rotation, Axial length!

Overview and Principles

The Taylor Spatial Frame (TSF) is a hexapod external fixator based on the Stewart platform mechanism, originally developed by Dr. J. Charles Taylor in Memphis, Tennessee. It enables simultaneous six-axis deformity correction through computer-assisted planning and graduated strut adjustments.

Key Principles:

The TSF consists of two rings connected by six telescopic struts arranged in a specific geometric pattern. This hexapod configuration creates a Stewart-Gough platform - the same kinematic mechanism used in flight simulators, robotic surgery, and precision positioning systems.

Six Degrees of Freedom:

Three translational: Anterior-posterior, medial-lateral, axial (shortening/lengthening)
Three rotational: Varus/valgus (coronal angulation), flexion/extension (sagittal angulation), axial rotation (torsion)

Advantages over Traditional Ilizarov:

Simultaneous multiplanar correction - no need for sequential corrections
No physical hinges required - virtual hinge calculated by software
No frame rebuilding - same construct throughout treatment
Residual correction capability - easily re-prescribe if deformity persists
Computer-assisted planning - reduces human calculation errors

Stewart Platform History

The Stewart platform was described by D. Stewart in 1965 for use in flight simulation. It provides 6 degrees of freedom through 6 linear actuators connecting two platforms. Taylor applied this mechanism to orthopaedic external fixation in the 1990s, revolutionizing deformity correction.

Indications and Contraindications

Indications

Deformity Correction:

Complex multiplanar deformities (congenital, developmental, post-traumatic)
Tibial and femoral malunion
Metabolic bone disease deformities (rickets, Blount disease)
Angular deformities with rotational and translational components

Limb Lengthening:

Limb length discrepancy requiring distraction osteogenesis
Combined lengthening with deformity correction
Congenital short femur, fibular hemimelia

Fracture Management:

Acute fracture reduction with gradual correction
Malunion correction with osteotomy
Nonunion with deformity

Infection and Bone Loss:

Bone transport for segmental defects
Infected nonunion management
Combined with Masquelet technique

Contraindications

Absolute:

Active sepsis (systemic)
Severe soft tissue compromise precluding pin/wire placement
Non-compliant patient (unable to perform daily adjustments)
Inadequate bone stock for wire/pin fixation

Relative:

Severe peripheral vascular disease
Uncontrolled diabetes mellitus
Osteoporosis (may require modified technique)
Morbid obesity (frame stability concerns)
Psychological unsuitability

Technical Principles

The Hexapod Mechanism

Stewart Platform Kinematics:

The TSF utilizes inverse kinematics - given a desired end position/orientation, the software calculates the required strut lengths. This is the opposite of forward kinematics (calculating position from joint angles).

Key Components:

Two rings: Reference ring and moving ring
Six struts: Telescopic struts with universal joints at each end
Universal joints: Allow multi-axis rotation at strut-ring connections
Strut mechanism: Threaded telescoping design for length adjustment

Why 6 Struts?

Six struts provide exactly 6 degrees of freedom - the minimum required for full spatial positioning. This is mathematically analogous to 6 linear equations with 6 unknowns, creating a fully determined (not over- or under-constrained) system.

Reference and Moving Rings

Reference Ring:

The "fixed" ring that defines the coordinate system
Usually placed on the proximal (stable) segment
All deformity parameters measured relative to this ring
Convention: Typically the ring closest to the trunk

Moving Ring:

The ring that moves during correction
Attached to the distal (deformed) segment
Moves to the corrected position as struts adjust

Reference Ring Selection

The choice of reference ring affects how deformity is prescribed to the software. If you select the proximal ring as reference, describe the deformity of the distal segment. Be consistent - errors in reference ring selection lead to correction in the wrong direction.

Virtual Hinge Concept

In traditional Ilizarov correction, physical hinges are placed at the Center of Rotation of Angulation (CORA) to achieve pure angular correction without translation.

The TSF creates a virtual hinge through software calculation:

The software calculates strut adjustments that simulate rotation about a hinge at any specified point
No physical hinge needed
Virtual hinge placed at CORA minimizes secondary translation
Can be adjusted/repositioned during treatment if needed

CORA (Center of Rotation of Angulation):

The intersection of the proximal and distal mechanical axes
Ideal pivot point for angular correction
Correction about CORA produces pure angulation without translation
Correction about a point away from CORA produces combined angulation and translation

Total Residual

Definition:

The total residual is a composite measure representing the magnitude of remaining deformity across all 6 axes after software calculation.

Calculation:

Software combines angular deformities (degrees) and translational deformities (mm)
Weighted combination into single value
Approaches zero as correction completes

Clinical Use:

Monitor correction progress
Endpoint determination (typically less than 5mm equivalent)
Identify incomplete corrections requiring adjustment

Surgical Technique

Preoperative Planning

Imaging:

Long-leg standing radiographs (AP and lateral)
CT scan for rotational deformity assessment
EOS imaging if available (low radiation, full-length)

Deformity Analysis:

Mechanical axis deviation (MAD)
Identify CORA(s) - may be single or multiple
Measure angular deformity (magnitude and direction)
Assess translational deformity
Evaluate rotational deformity (CT required)
Measure limb length discrepancy

Osteotomy Planning:

Level of osteotomy (ideally at CORA)
If CORA outside bone, plan for secondary correction
Corticotomy technique for lengthening

Ring Selection and Sizing

Ring Diameter:

Allow 2-3 finger breadths clearance circumferentially
Consider soft tissue swelling
Larger rings allow better access but reduce stability

Ring Configuration:

Full rings vs 5/8 rings vs foot plates
Proximal tibial: Consider knee ROM
Distal tibial: Consider ankle clearance
Femoral: May need half-rings for sitting

Frame Assembly

Steps:

Assemble rings with appropriate connectors
Attach 6 struts to designated mounting positions
Ensure universal joints move freely
Check strut length range adequate for planned correction
Record strut positions on mounting worksheet

Wire and Pin Placement

General Principles:

Minimum 2 fixation elements per ring (wires and/or pins)
Tensioned wires (1.5-1.8mm) for ring stability
Half-pins (5-6mm) for added stability
Safe corridors to avoid neurovascular structures

Tibial Application:

Proximal ring: Safe wire from fibular head to posteromedial tibia
Reference wire perpendicular to tibial axis
Avoid peroneal nerve
Distal ring: Safe zone anterior to posterior
Protect anterior tibial vessels

Wire Tensioning:

Olive wires: 130-150 kg tension
Smooth wires: 110-130 kg tension
Re-tension after 48 hours (settling)

This section covers frame application technique.

Complications

Frame-Related Complications

Pin Site Infection:

Most common complication (30-100% incidence)
Superficial: Local care, oral antibiotics
Deep: May require pin removal, IV antibiotics
Prevention: Regular pin care, good technique

Pin Loosening:

Presents as pain, instability, increased discharge
May require pin replacement
More common with half-pins in poor bone

Frame Instability:

Due to pin failure, ring breakage, strut malfunction
Requires urgent assessment and revision

Correction-Related Complications

Residual Deformity:

Due to mounting parameter errors
Incorrect deformity prescription
Patient non-compliance with schedule
Management: Re-measure, re-prescribe

Over-correction:

Deformity corrected beyond neutral
May be intentional (planned over-correction for rebound)
Unintentional: Revise prescription

Neurovascular Injury:

Stretch neuropathy with lengthening
Vascular compromise (rare)
Slow or stop distraction if symptoms develop

Bone-Related Complications

Premature Consolidation:

Regenerate consolidates before target length
Management: Speed up distraction, consider re-osteotomy

Delayed Consolidation/Nonunion:

At osteotomy site or regenerate
Risk factors: Smoking, diabetes, poor technique
Management: Bone graft, optimize biology

Regenerate Fracture:

After frame removal
Protect with cast/brace until remodeling complete

Soft Tissue Complications

Joint Stiffness:

Adjacent joints stiffen during prolonged treatment
Prevention: Aggressive physiotherapy
May require manipulation, release

Muscle Contracture:

Gastrocnemius (equinus) in tibial lengthening
Quadriceps in femoral lengthening
Consider prophylactic releases for large lengthenings

Complication Prevention

The most significant functional complications are joint stiffness and contracture. Physiotherapy from day one is essential. Monitor ROM at every visit. Consider prophylactic soft tissue releases (gastrocnemius slide, quadricepsplasty) for lengthenings greater than 4-5cm.

Evidence Base

Taylor Spatial Frame vs Ilizarov for Tibial Deformity

III

Rozbruch SR et al. • Clin Orthop Relat Res (2008)

Key Findings:

TSF achieved equivalent correction accuracy to Ilizarov (mean 98% correction)
TSF required significantly fewer frame adjustments during treatment
No difference in union rates or time to frame removal
TSF learning curve offset by software assistance

Mounting Parameter Accuracy in TSF

III

Simpson AL et al. • J Orthop Trauma (2009)

Key Findings:

Frame offset measurements most prone to error
5mm error in offset can produce 5 degrees residual angulation
Standardized measurement protocol reduces errors
Software residual mode allows correction of errors mid-treatment

Computer-Assisted Deformity Correction Outcomes

Eidelman M et al. • J Bone Joint Surg Br (2006)

Key Findings:

95% achieved planned correction
Mean correction time 3.2 months for angular deformity
Pin site infection rate 32% (all superficial)
No major neurovascular complications

Hexapod Frame for Fracture Reduction

Kucukkaya M et al. • Injury (2011)

Key Findings:

Gradual reduction achieved in all cases
Avoids risks of acute open reduction
Particularly useful for periarticular fractures with soft tissue compromise
Union achieved in 95% without bone grafting

Exam Viva Scenarios

Practice these scenarios to excel in your viva examination

VIVA SCENARIOStandard

Scenario 1: TSF Principles

EXAMINER

"Explain the Taylor Spatial Frame to me. How does it work and what advantages does it offer over traditional Ilizarov frames?"

EXCEPTIONAL ANSWER

Thank you. The Taylor Spatial Frame is a hexapod external fixator based on the **Stewart platform** kinematic mechanism. It consists of two rings connected by **six telescopic struts** arranged in a specific geometric pattern, with universal joints at each strut-ring connection. This configuration provides **six degrees of freedom** - three translational (anterior-posterior, medial-lateral, and axial) and three rotational (varus/valgus, flexion/extension, and axial rotation). The key advantages over traditional Ilizarov frames include: First, **simultaneous multiplanar correction** - the TSF can correct all six axes at once, whereas Ilizarov requires sequential corrections with hinge repositioning. Second, **no physical hinges required** - the software creates a virtual hinge at any specified point (ideally the CORA), eliminating the need for precise physical hinge placement. Third, **no frame rebuilding** during treatment - the same construct corrects the entire deformity without reconfiguration. Fourth, **computer-assisted planning** reduces calculation errors, and the residual mode allows easy re-prescription if initial correction is incomplete. The software requires input of 14 mounting parameters (7 per ring) and calculates daily strut adjustments for the prescribed correction.

KEY POINTS TO SCORE

Hexapod = Stewart platform with 6 telescopic struts

6 degrees of freedom (3 translational + 3 rotational)

Virtual hinge created by software (no physical hinges)

Simultaneous multiplanar correction capability

14 mounting parameters required for software

COMMON TRAPS

✗Confusing degrees of freedom with number of struts

✗Not understanding virtual hinge concept

✗Forgetting the reference vs moving ring concept

LIKELY FOLLOW-UPS

"What is the CORA and why is it important?"

"How do you measure the mounting parameters?"

"What causes residual deformity after TSF correction?"

VIVA SCENARIOStandard

Scenario 2: Complex Tibial Deformity

EXAMINER

"A 25-year-old man presents with a tibial malunion following conservative treatment of a fracture 2 years ago. Radiographs show 20 degrees varus angulation, 15 degrees procurvatum, and 2cm shortening. How would you plan correction using a TSF?"

EXCEPTIONAL ANSWER

Thank you. This is a complex multiplanar tibial malunion with angular deformity in two planes plus limb length discrepancy. The TSF is ideal for this case as it can address all three deformity components simultaneously. My approach would be as follows: **Preoperatively**, I would obtain standing long-leg radiographs (AP and lateral), CT scan to assess any rotational deformity, and use software to perform deformity analysis. I would identify the CORA for each plane of deformity. For a **malunion with significant angulation and shortening**, I would plan an **osteotomy at or near the CORA**. If the CORAs for the coronal and sagittal deformities are at different levels, I may need to accept correction about a compromise point (the software can calculate this). **Operatively**, I would apply the TSF with the reference ring on the proximal tibia (stable segment) and moving ring distally. I would use tensioned wires and half-pins in safe corridors. The osteotomy would be performed using low-energy corticotomy technique to preserve biology for the lengthening component. **Postoperatively**, I would accurately measure all 14 mounting parameters, input the deformity parameters to the software (20 degrees varus, 15 degrees procurvatum, 20mm shortening), and the software would calculate daily strut adjustments. For the **lengthening component**, I would allow a **5-7 day latency** then distract at **1mm/day in 4 divided doses**. The **angular correction** would proceed simultaneously at approximately 1 degree/day. Total treatment time would be approximately 3 months of correction plus consolidation phase.

KEY POINTS TO SCORE

TSF ideal for multiplanar deformity + lengthening

Identify CORAs for each plane of deformity

Osteotomy at or near CORA for optimal correction

Reference ring on stable (proximal) segment

Latency period before lengthening begins

COMMON TRAPS

✗Planning only angular correction and forgetting lengthening

✗Not considering CORA location for osteotomy planning

✗Forgetting to describe mounting parameter measurement

LIKELY FOLLOW-UPS

"What if the CORAs are at different levels?"

"How would you monitor regenerate quality?"

"What is your rehabilitation protocol?"

VIVA SCENARIOChallenging

Scenario 3: Residual Deformity After TSF

EXAMINER

"You applied a TSF for correction of a tibial deformity. At the end of the planned correction schedule, radiographs show a 10-degree residual varus deformity. What went wrong and how would you manage this?"

EXCEPTIONAL ANSWER

Thank you. Residual deformity after TSF correction is typically due to one of three causes: **mounting parameter errors**, **incorrect deformity prescription**, or **patient non-compliance**. My approach would be systematic investigation and correction. First, I would **verify patient compliance** - review the strut adjustment log and confirm the patient made all prescribed adjustments correctly. If compliance is questionable, I would re-educate and extend treatment. Second, I would **re-measure the mounting parameters** on current radiographs. Frame offset measurements are the most common source of error. A 5mm error in frame offset can produce 5 degrees of residual angulation. I would compare current measurements to initial prescription and identify any discrepancies. Third, I would review the **original deformity prescription** - was the initial deformity accurately characterized? Was the reference ring correctly identified? Fourth, I would use the **residual mode** in the TSF software. This is a major advantage of the TSF - I simply input the current residual deformity (10 degrees varus) and the software calculates new strut adjustments to complete the correction. No frame rebuilding or hinge placement required. I would then continue treatment with the new prescription until the total residual approaches zero. The key learning point is that mounting parameter accuracy is critical, and having a standardized measurement protocol reduces errors. The residual mode makes mid-treatment correction straightforward.

KEY POINTS TO SCORE

Three main causes: mounting errors, prescription errors, non-compliance

Frame offset errors most common source of residual

5mm offset error can cause 5 degrees residual

Use residual mode in software to prescribe correction

No frame rebuilding needed - major TSF advantage

COMMON TRAPS

✗Recommending frame removal and reapplication

✗Not systematically investigating the cause

✗Blaming the patient without checking parameters

LIKELY FOLLOW-UPS

"How do you accurately measure frame offset?"

"What is total residual and how is it calculated?"

"How would management differ if this were an Ilizarov frame?"

Australian Context

In Australia, the Taylor Spatial Frame is utilized at specialized limb reconstruction units within major metropolitan hospitals. The technology represents an important advance in managing complex limb deformities and is increasingly preferred for multiplanar corrections.

Device Availability and Training: The TSF is available through Smith and Nephew, with dedicated software platforms for case planning. Australian orthopaedic surgeons undertaking limb reconstruction fellowships receive training in TSF application, and the AO Foundation and ASAMI (Association for the Study and Application of Methods of Ilizarov) provide courses on hexapod frame application. The learning curve is significant, but computer-assisted planning reduces technical errors compared to traditional Ilizarov techniques.

Clinical Applications: Australian centres use the TSF for complex tibial and femoral deformity correction, limb lengthening, and fracture management. The ability to correct multiplanar deformities simultaneously is particularly valuable in post-traumatic malunion cases and congenital limb deficiencies. Combined with distraction osteogenesis principles, the TSF is used for limb length discrepancy management in conditions such as fibular hemimelia and congenital short femur.

Practical Considerations: Patient selection is critical, as TSF treatment requires significant compliance for daily strut adjustments and pin site care over prolonged periods (typically 3-6 months in frame). Multidisciplinary input including limb reconstruction surgeons, specialized physiotherapists, and orthotists is essential. The cost of the TSF system is higher than traditional Ilizarov components, but this is offset by reduced need for frame modifications and potentially shorter learning curve for multiplanar corrections.