Should We Use Thicker K-Wires for Dynamic External Fixation Frames?

Management of the “pilon” fracture of the base of the middle phalanx upon the head of the proximal phalanx is challenging, and its outcomes are unpredictable. Stiffness is often a feature, with chronic functio laesa being a common problem.¹ A procedure frequently used to address these fractures is the use of a dynamic external fixator, which has the advantage of maintaining traction across the joint while allowing movement and preventing intrinsic tightness.

There are many devices used, including the Suzuki frame, proprietary ready-made ligamentotaxis, and the Grey-Giddins frame. One concern in these devices is that the traction seen on the operating table is lost over the 4 to 6 weeks before the removal of the device.

When considering engineering principles, the resistance of a cylindrical beam such as a K-wire is a function of both the constituent material and the diameter. Hooke’s law dictates that material only deforms when it reaches its elastic limit, which is what happens when the frame is applied. Just under this limit the relationship between stress and strain becomes non-linear—the proportionality limit. Here variegated traction will be applied; movement of the joint may exceed the elastic limit, permanently reducing traction.

The stiffness of the wire is the main determinant of the amount of traction that is provided, and it increases with the diameter of the wire. The most-used size is 1.1 mm, but thicker wires can be used in larger individuals. A frame built from 1.25-mm wires is significantly stiffer because the flexural rigidity of a solid rod increases by the fourth power of the diameter; an increase of .15 mm increases the amount of traction developed more than may be expected.² The author would be interested to hear the opinions of the readers on the use of thicker wires in distraction frames.

Affiliations: ¹Royal Devon and Exeter Hospital, Exeter, Devon, UK

Correspondence: Robert Thomas, MBChB, BSc, MRCS; rlt@doctors.org.uk

Disclosures: The author discloses no relevant financial or nonfinancial interests.