ESF Nomenclature

The ESF System is comprised of many different types of devices including linear, ring and acrylic ESF devices. Among the most popular of the current generation of linear devices is the IMEX SK™ device and it will be the device used in our discussions and laboratories. Regardless of specific manufacturer, linear devices have 3 basic elements (Figure 1):

1.  Fasteners (percutaneous fixation pins)

2.  Connecting rods

3.  Linkage devices (clamps linking the fixation pins to the connecting rods)

Fixation pins are termed either half- or full-pins (Figure 2) based upon how they are inserted.

• Half-pins penetrate the near-skin surface and both the near- and far-cortex of bone.
• Full-pins penetrate the near-skin surface, both cortical surfaces of the bone, and exit the far-skin surface of the limb.

The connecting rod(s), fixation pins and clamps define an ESF frame (Figures 1 & 2). Frame configuration is described by the number of distinct sides of the limb from which it protrudes [“unilateral” or “bilateral”] as well as the number of planes it occupies [“uniplanar” or “biplanar”].

• Unilateral-uniplanar (Type Ia) frames protrude from just 1 side of the limb and are restricted to one plane. Type Ia frames (Figure 2 – left) are formed by connecting 1 or more half-pins of each main fracture segment.
• Bilateral-uniplanar (Type II) frames protrude from 2 distinct sides of the limb, but are restricted to just one plane (typically the mediolateral plane). Type II frames (Figure 2 – right) are formed by connecting 1 or more full-pins of each main fracture segment.
• Bilateral-biplanar (Type III) frames protrude from 2 distinct sides of the limb and occupy 2 planes. Type III frames are formed when both a Type Ia and Type II frames are applied to a bone. These are seldom used with modern ESF devices.
• Unilateral-biplanar (Type Ib) frames occupy 2 planes, but because these frames do not protrude from 2 distinct sides of the limb (180° to each other) they are thought of as “unilateral.” If the frame occupies more than 1 plane, but does not connect a full-pin in the proximal segment to a full-pin in the distal segment, it is a Type Ib. Standard type Ib frames (Figure 3) are formed when two Type Ia frames are applied to a bone.

The frame classification described above fosters accurate communication between colleagues and also provides a basic sense of frame stiffness under axial loading (Type III>Type II>Type I).

Type Ib frames are sometimes applied for the purpose of enhancing this multiplanar stiffness. Uniplanar frames are most able to resist bending forces that are applied in the plane of the frame (versus those in the plane perpendicular to the frame). As an example, a frame occupying the mediolateral plane is less able to resist bending forces in the craniocaudal plane. Application of multiplanar frames, therefore, imparts better multiplanar stiffness. In theory, this is best accomplished by placing the frames 90° to one another. Clinical practicality, however, dictates that frames be applied as regional soft tissue anatomy and bony cross-sectional structure warrant. As an example, Type Ib frames applied to the radius typically consist of a frame in the craniomedial plane and a second frame in the craniolateral plane (Figure 3).

Type II frames are not as frequently employed as they once were. When the frames are comprised entirely of full-pins, they are called “maximal” Type II frames. A “minimal” Type II frame is comprised of one full-pin in the proximal main fracture segment and one full-pin in the distal segment, and the remaining positions are filled in with half-pins.

ExFix Pins

Smooth (non-threaded) Steinmann pins were originally used with ESF; however, premature pin loosening was a major problem and their use has been replaced by use of various threaded pin designs. Positive- versus negative-thread profile, cortical versus cancellous thread form, length of the threaded portion, and pin size must all be considered for optimal pin selection for a given bony insertion site.

Thread Profile

Negative profile threads are cut into the pin at the expense of the core diameter. When threads are cut into the pin over its entire length, the pin loses its stiffness and is subject to bending or breakage. When threads are conventionally cut only into the end of the pin (end-threaded pin), the abrupt change in pin diameter is a “stress-concentrator” and these pins are predisposed to breakage at the junction of the threaded and non-threaded portions. Historically, SCAT™ pins were designed such that threads engaged only the far-cortex of bone and the breakage-prone thread-shaft junction was located within the intramedullary canal, theoretically, protecting it from cyclic bending forces. In reality, these SCAT pins are seldom used with modern devices such as the IMEX SK™ device. The weaknesses of negative profile pins were initially overcome by introduction of fixation pins with a positive thread profile.

Positive profile threads are raised above the core diameter of the pin. This thread profile offers secure pin-to-bone fixation without having a breakage-prone stress-concentrator. These pins were technically difficult to apply with older KE devices, but application is greatly simplified with the clamp design of the IMEX SK™ device. The disadvantage of these pins is their large “footprint” in available bone stock; this can be problematic in small bones or where ESF is used in combination with an intramedullary pin. Most recently, IMEX has introduced its Duraface™ pins that have a negative thread profile with a tapered thread run-out design at the thread-nonthreaded transition zone. This design feature eliminates the stress-concentrator issue of conventional negative-thread profile pins while adding pin stiffness when compared to a positive-profile pin of equivalent thread-diameter.

Cortical versus Cancellous Thread Form

A cortical thread form is used in most locations. The cortical thread form has a finer thread pattern and a greater number of threads per unit of pin length when compared to cancellous pins. Cancellous pins use their relatively coarse thread pattern and few threads per unit length to maximize purchase of very soft cancellous bone in locations where there is little cortical bone for purchase (proximal tibial metaphysis, proximal humeral metaphysis, and, in some instances, distal femoral condyle, pelvis and vertebral body). The notion that all metaphyseal bone is soft is not true; cancellous pins should not be used in the humeral condyle, distal tibia or in the radius.

Regular versus Extended Length Pins

Extended-length pins are available in end-threaded designs (for use as half-pins) and centrally threaded designs (for use as full-pins). Extended-length pins have both an increased overall pin length and increased span of threads. These pins are useful when standard-thread length is insufficient to span the full diameter of bone or the soft tissue envelope is so extensive that standard-length pins will not protrude sufficiently from the limb.

Linkage Devices (ExFix Pin Clamps)

The SK™ clamp offers a significant improvement over the KE clamp in terms of both mechanical performance and “user-friendliness.” The mechanical performance is enhanced by its adaptation to use of relatively large diameter connecting rods as compared to the old KE clamp. The connecting rods are made of carbon fiber composite or titanium instead of stainless steel to reduce their weight. The SK™ clamp design also allows simplified introduction of a variety of pin sizes and designs including positive profile pins.

SK™ clamps have a two-piece aluminum body, a pin-gripping bolt (also known as the primary bolt) with a slotted washer and tightening nut, and a secondary bolt. The rod-gripping channel is formed by the hemi-circular cut-out shape of each half of the aluminum body. Tightening of the primary and secondary bolts allows the SK clamp to tightly grip the connecting rod without deforming the shape of the clamp body. The clamps can either be pre-positioned on the connecting rod or can be assembled (or disassembled) on the rod at any desired location during surgical application. The gliding washer upon the primary bolt has a meniscus (slot) that enables the bolt to effectively grip a range of different pin diameters; the curvature of the meniscus corresponds to the smallest diameter pin shank that can be securely gripped by the clamp (Table 1). The back of the slotted washer has serrated teeth that engage the outer surface of the aluminum clamp body when the primary bolt is tightened. This provides a rigid lock between the fixation pin and the connecting rod. Finger tightening of the secondary bolt allows the clamp to be stabilized on the connecting rod during pre-drilling and pin insertion, but still permits the clamp to swivel slightly as the primary bolt is tightened so that the orientation of the clamp can “self-correct” to the fixation pin.

VIN editor: Figures 1, 2, 3 and Table 1 were not provided at the time of publication.