Hexapod Frame Deformity Correction

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Classical Ilizarov correction uses rings connected by threaded rods and physical hinges placed at the CORA. Each additional plane of deformity requires additional hinges, and any hinge malposition produces secondary deformity that requires the frame to be dismantled and re-built.

The hexapod concept (Taylor Spatial Frame and equivalents such as TL-Hex and Orthex) replaces rods and hinges with six oblique telescopic struts connecting two rings at special joints — a Stewart-Gough platform, the same kinematic architecture used in flight simulators.

• Because the six struts define the position of one ring relative to the other in all six degrees of freedom (coronal angulation and translation, sagittal angulation and translation, axial rotation, and length), any combination of deformities can be corrected simultaneously by an appropriate schedule of strut length changes.
• The surgeon enters the deformity into web-based software; the software outputs a daily strut adjustment prescription for the patient to perform.
• The mathematical hinge exists only in software — the "virtual hinge" — and can be placed anywhere in space (including outside the frame, e.g. at the ankle joint for foot correction) without any hardware change.

Chronic vs total residual programmes: the deformity can be entered as the original deformity (chronic correction from day one) or, after acute partial correction on the table, the residual deformity can be measured on post-operative radiographs and a total residual programme generated. The residual method is forgiving of mounting imprecision and is the workhorse technique in most units.

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Why hexapod over acute correction (plate/nail)? Gradual correction protects soft tissues and nerves in large or multiplanar deformities, allows simultaneous lengthening, permits weight bearing throughout, works in infected or previously infected bone without internal implants, and residual deformity is correctable without re-operation.

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Comparative series show hexapod frames achieve more accurate correction of complex multiplanar deformity than hinged Ilizarov constructs, at the cost of strut hardware expense; acute correction with internal fixation remains appropriate for uniplanar deformities of modest magnitude in healthy soft tissues.

Some degree of pin-site inflammation or infection is near-universal over a treatment course of many months — patients must be counselled and taught structured pin care. Two parts of the usual prescription are worth separating, because the evidence treats them very differently.

• Frequency: it probably does not matter. The only randomised comparison (W-Dahl, 50 patients) found no difference between daily and weekly pin-site care in infection frequency or severity, in pain, or in antibiotic use. Set the interval by what the patient will actually sustain across six months in a frame, not by reflex.
• Solution: it does. In the same unit's comparative study, 2 mg/ml chlorhexidine beat 9 mg/ml saline — grade 1 infection 8.5 versus 14 per cent, grade 2 infection 0.5 versus 3 per cent, a relative risk of 1.7 for a positive culture and 3.3 for Staphylococcus aureus with saline, plus fewer antibiotics and less pain at 6 and 10 weeks.

Dry dressings with gentle compression reduce skin-pin motion, which is the mechanical driver behind most of it.

Checketts-Otterburn classification:

Prevention. Hydroxyapatite-coated half pins reduce loosening and infection; meticulous insertion technique (sharp drills, saline cooling, no skin tension); release tented skin; minimise pin-soft tissue motion. Note where the trouble concentrates — in W-Dahl's randomised series every clinically loose pin at removal was a proximal pin.

• Global epidemiology and practice variation: post-traumatic tibial deformity is the dominant indication worldwide; in high-income settings hexapod frames increasingly replace hinged Ilizarov constructs for multiplanar corrections, while classical Ilizarov frames remain the mainstay in many low- and middle-resource settings because of hardware cost — strut sets are substantially more expensive than threaded rods and hinges, although the ring-and-wire skill base is shared.
• Society guidance: the BOA/BAPRAS open fracture standards (BOAST) frame the use of circular fixation in the reconstruction of open tibial fractures with bone loss within an ortho-plastic service; AO and limb reconstruction society (ASAMI/ILLRS) consensus material addresses distraction osteogenesis principles — latency, rate/rhythm, low-energy osteotomy — which apply directly to hexapod programmes. No major guideline mandates a specific hexapod system; principles, not brands, are examinable.
• Registries: no arthroplasty-style implant registry exists for circular frames; evidence is drawn from limb reconstruction unit series and multicentre cohorts. Reported accuracy of hexapod correction to within a few degrees and millimetres of target is consistent across series.
• Resource-setting adaptation: where software access or strut cost is limiting, hybrid strategies (acute partial correction plus simple frame, or classical Ilizarov) remain entirely legitimate; the deformity analysis (CORA, joint orientation angles) is identical whatever hardware is used.

Related pages. The frame it replaced and the biology they share: Ilizarov External Fixation, Distraction Osteogenesis and Limb Lengthening Principles.

• Latency duration: classic teaching is 5 to 10 days, but evidence for the optimal latency is weak; some units start correction (though not lengthening) almost immediately, particularly for pure angular correction without distraction.
• Rate and rhythm: 1 mm/day in four increments derives from Ilizarov's canine work; motorised or high-frequency adjustment devices question whether more, smaller increments improve regenerate quality — unproven clinically.
• Pin-site care protocols: no regimen (chlorhexidine vs saline, daily vs weekly, crust removal or not) has been shown clearly superior in randomised comparisons; consistency and patient education appear to matter more than the specific solution.
• Frame versus lengthening nail: for deformity correction with lengthening in intact soft tissues and non-infected bone, magnetically driven intramedullary lengthening nails increasingly compete with frames, avoiding pin sites at the cost of less multiplanar versatility and unsuitability in infection or small canals.
• Acute plus gradual hybrid: whether to acutely correct part of the deformity on the table (residual programme) or run the entire correction chronically is surgeon preference; acute partial correction shortens frame time but sacrifices some of the gradual-correction soft-tissue advantage.
• Cost-effectiveness: strut hardware cost versus re-operation savings from re-programmability has not been rigorously established; in resource-limited settings classical Ilizarov remains standard.