External skeletal fixation configurations

An external skeletal fixator has pins in bone, clamps and connecting bars outside the limb. Describe both the side of the limb used and the planes occupied by the pins.

Type Ia is unilateral and uniplanar. Half pins connect to one bar on one side. Type Ib is unilateral and biplanar. Half pins connect to two bars in different planes.

Type II is bilateral and uniplanar. Full pins cross the limb and attach to a bar on each side. Type III is bilateral and biplanar. It adds fixation in a second plane. The pin-plane maps are cross-sectional memory cues; pins in different planes can sit at different levels along the bone.

A half pin still engages both bone cortices. It enters through one skin surface and does not exit the opposite skin. A full pin passes through both skin surfaces and can connect to bars on both sides.

Mixed frames explain the extra names. A type I-II hybrid combines half pins with one full pin and two connecting bars, often with a diagonal. A modified type II combines half pins and two full pins with two bars. It remains bilateral and uniplanar; some unilateral half pins do not make the whole frame unilateral.

The second-edition modified type III example uses half and full pins with three bars. It is still bilateral and biplanar. The side-and-plane description defines the group; four bars are not required for every type III variant.

Choose pin paths from the local safe corridors. A more complex frame can increase stiffness but also increases soft-tissue compromise. Bilateral frames are unsuitable around the humerus and femur because the body limits access to the far side.

References

Chris Wood's original study note; Johnston & Tobias, Veterinary Surgery: Small Animal, 2nd ed., Chapter 42, Fig.42.5 and Table42.2 (supplied PDF pp.1711-1713). Classification cross-check: AO Surgery Reference, external skeletal fixators.