Taylor Spatial Frame

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.

Indications

Contraindications

View larger

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:

1. Two rings: Reference ring and moving ring
2. Six struts: Telescopic struts with universal joints at each end
3. Universal joints: Allow multi-axis rotation at strut-ring connections
4. 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

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.

The topic states that mounting-parameter errors cause residual deformity — the specific culprit is the frame (ring) offset, and it is worth knowing how to get it right.

• What the offset is. The frame/ring offset is the perpendicular distance from the bone (reference) axis to the centre of the reference ring in the AP and lateral planes; together with ring diameter and ring-to-ring distance it locates the ring in space relative to the bone, so the software knows where the frame is.
• Why it dominates the error budget. Offset is the hardest parameter to measure and the most error-prone, and a small offset error translates almost directly into residual deformity — roughly a millimetre of offset error per degree of residual angulation in typical constructs; the error grows with the distance between the origin and the reference ring.
• How to measure it accurately. Use calibrated radiographs (a calibration marker of known size) or a calibrated image intensifier, with the bone axis drawn on true AP and lateral films and the perpendicular distance to the ring centre measured — non-calibrated films are the least accurate.
• The CT alternative. Basing the correction on the actual 3D frame-to-bone position on CT compensates for mounting error more completely (sub-degree and sub-millimetre in phantoms) but is reserved for complex cases because of cost and radiation.

Frame-Related Complications

Correction-Related Complications

Bone-Related Complications

Soft Tissue Complications

The software section refers to an "origin" without defining it — the origin/corresponding-point framework is the conceptual core of how a hexapod deformity is prescribed.

• Origin and corresponding point. The surgeon chooses an origin — a defined reference point on the reference (fixed) fragment, conventionally at the osteotomy level on the reference-ring side — and a corresponding point, the matching point on the moving fragment that should end up superimposed on the origin once the deformity is corrected. The deformity is described as the translations and angulations needed to move the corresponding point onto the origin.
• Structure at rest vs deformity parameters. The software needs two things: the mounting parameters (where the frame sits — the "frame at rest") and the deformity parameters (AP and lateral angulation, AP/lateral/axial translation, and axial rotation of the corresponding point relative to the origin). From these it computes the six daily strut lengths.
• Choosing the origin sensibly. Placing the origin at the CORA makes the correction a pure angulation (a virtual hinge there), minimising secondary translation; an origin away from the CORA produces combined angulation and translation that the software still resolves but which must be intended.
• Why this makes residual correction easy. If a residual deformity remains, the surgeon simply re-defines the current corresponding-point offset (the residual program) and the software issues a fresh strut schedule — with no frame rebuild, unlike an Ilizarov.

When a multiplanar deformity is identified, the exam question is usually "which device/technique?" Use the clinical features below to justify your choice rather than reflexively reaching for a hexapod.

The TSF (Smith & Nephew) is one of several hexapod systems worldwide; equivalent platforms include the TrueLok-Hex (Orthofix), the Ortho-SUV frame, and the MAXFRAME (Zimmer Biomet). All share Stewart-platform kinematics and web/desktop planning software, so the principles below are device-agnostic.

Global epidemiology and case mix:

• Hexapod frames are concentrated in tertiary limb-reconstruction units because of the planning, monitoring and patient-compliance demands.
• Commonest indications globally: post-traumatic tibial malunion/nonunion, congenital and developmental deformity (Blount disease, rickets, fibular hemimelia, congenital short femur), and limb-length discrepancy.
• In high-burden, limited-resource regions, neglected fractures and infected nonunions (often post-bonesetter) form a disproportionate share of the workload.

Societies, training and standards (side-by-side):

There is no single high-level guideline mandating hexapod over Ilizarov; selection is guided by deformity complexity, surgeon experience, software access and cost.

• Hexapod vs Ilizarov accuracy: Hexapods give superior 3D control and easier residual re-prescription, but comparative data are largely level III-IV single-centre series. Systematic review evidence suggests a potentially higher healing index with hexapods, and no high-level trial proves a union-rate advantage.
• Radiograph- vs CT-based mounting parameters: Frame-offset measurement is the dominant error source. Calibrated radiographs perform well, but CT/3D planning compensates for mounting error more completely. Routine CT planning is not universally adopted because of cost and radiation.
• Acute vs gradual correction: Gradual six-axis correction is more accurate for complex deformity, yet for simple uniplanar deformity acute one-stage correction avoids months in a frame; the threshold (often cited around 10 degrees of uniplanar varus) is convention, not high-level evidence.
• Latency, rate and rhythm of distraction: The classic 5-7 day latency and 1 mm/day in four divided increments derive from distraction-osteogenesis biology, but optimal rate varies with age, site and regenerate quality and is individualised, not protocolised.
• Pin-site care: Reported pin-site infection rates range widely (commonly 30 percent or more, mostly superficial). No single cleaning regimen is proven superior, which is why incidence figures must be quoted with caution.
• Registry gap: Absence of a dedicated hexapod registry means complication and survival figures rest on heterogeneous series, limiting generalisability.