Tibial shaft fracture: Fixation with a Taylor Spatial Frame (TSF) circular external fixator (Smith and Nephew)
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The TSF was designed by the Taylor brothers in 1994 and has become one of the most widely used external fixation systems. It utilises two rings that are connected by six diagonal struts. The frame can be connected to the bone with an almost limitless combination of half pins and fine wires that allow it to be used in a wide variety of conditions. In the acute trauma setting, advantages include minimal soft tissue disruption and early weightbearing. It is particularly useful in cases at risk of deep infection or where residual deformity may need correcting e.g. bone loss or planned deformity to allow for soft tissue cover.
The Taylor Spatial Frame (TSF) is a modern hexapod external fixator that is able to correct six axes of deformity simultaneously using a virtual hinge (a hexapod is a six-legged structure). If you are unfamiliar with deformity correction, then the concept of six axes may need some explanation. We are used to thinking of structures in three dimensions with an x, y and z axis. However, rotation is possible around any of these axes, therefore six axes in total.
A deformity may be either an isolated angulation, translation, rotation in any plane; or length discrepancy; or a combination of them in more than one plane. For any deformity, it is important to identify the “Centre Of Rotation of Angulation (CORA)”. The CORA is the point around which a deformity can be corrected to resolve the deformity without incurring any translation. For example, with a tibial shaft malunion showing a valgus deformity in the coronal plane only (i.e. no sagittal or axial plane deformity), then to identify the CORA, the anatomic axis of both segments is drawn and where these lines intersect, this is the CORA. If an osteotomy is performed at this level and the deformity is corrected using an Ilizarov external fixator, then the hinges should be placed directly in line with the CORA (i.e. one in front and one behind and both should line up perfectly with the CORA when viewed in the coronal plane). When the deformity is corrected, the bone will become perfectly straight and the axes proximal and distal to the deformity will align with each other.
Now imagine you have the same deformity but there is also a rotational malalignment to the limb e.g. the foot is internally rotated. To correct the deformity using an Ilizarov construct, you would have to correct the valgus deformity first and then re-build the frame to correct the rotational deformity (or vice versa). This often requires another anaesthetic, as the Ilizarov frame will be destabilised in the process and this can be painful for the patient. With a hexapod fixator, both deformities can be corrected simultaneously by differentially altering the length of the six struts that connect the two rings of the fixator. There are therefore no hinges used in a hexapod system however, the software that calculates the “prescription / programme” for adjusting the struts uses a virtual hinge which is in fact the same as the CORA of the deformity.
For this patient’s fracture pattern, the majority of surgeons would probably use an intra-medullary nail. However, due to the patient sustaining a compartment syndrome, a fat embolism from an ipsilateral femoral fracture and then taking more than 2 weeks to recover from their other injuries, we felt a frame would be a safer option.
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Author:Ross Fawdington FRCS (Tr & Orth)
Institution: The Queen Elizabeth Hospital, Birmingham, UK.