GB2369867A

GB2369867A - External fixation pins

Info

Publication number: GB2369867A
Application number: GB0030009A
Authority: GB
Inventors: John Radwell
Original assignee: Individual
Current assignee: Individual
Priority date: 2000-12-08
Filing date: 2000-12-08
Publication date: 2002-06-12
Anticipated expiration: 2020-12-08
Also published as: GB0030009D0; GB2369867B

Abstract

An external fixation pin (1) for fixing into a bone comprises a drilling point (2) at the leading end of the pin and a first threaded section (3) of constant diameter immediately succeeding the drilling point; the first threaded section including at least one thread cutting flute (4) at the leading end thereof for cutting a thread into the bone. A second threaded section (5) immediately succeeds the first threaded section and has a diameter which increases from its leading end to its trailing end. The tapered section (5) has at least one cutting flute for cutting a bore having a diameter substantially equal to the nominal diameter of the pin. A smooth cylindrical section (7) immediately succeeds the tapered section and has a diameter which is the nominal diameter of the second pin.

Description

EXTERNAL FIXATION PINS

The invention relates to external fixation pins which are used to re-align broken bones of humans and animals. These pins are generally about 15cm long with the first 3-4cm being screwed into the bone and the remainder of the pin extending outwardly of the body to be joined via clamps to an externally mounted stabilising bar.

The leading end of a conventional pin is provided with a drilling point followed by a threaded section of constant diameter equal to the nominal diameter of the pin. The threaded section is long enough to extend right through the bone to be engaged in both the distal and proximal cortices.

Several problems occur with this type of pin.

When inserting pins by hand there is a natural tendency for the operator to allow the"approach angle"to wander, thereby'fudging'the hole in the proximal cortex. The effect is to create a larger hole in that area than is required for firm fixation. This tends not to be a problem in the distal cortex, the"approach angle" being controlled by the proximal cortex.

Most pins are manufactured with a thread designed to create resistance to side-slip or pull out forces. With self-drilling pins, a problem arises when the self-drilling point makes contact with the internal surface of the distal cortex. At this point, the pitch of its thread naturally drives the pin forward but the distal cortex resists forward progress. Since the pin is rotating without adequate forward motion, there is a tendency to strip the thread-form in the proximal cortex, leading to a loosening in that area.

In brittle bone the risk exists that the proximal thread will hold, creating a

"burst through"effect, fracturing the distal cortex.

One post-operative problem which frequently occurs is pin tract infection ; the origins of which include but are not limited to loose pins and local irritation of soft tissues. Indeed, pin tract infection may in itself lead to pin loosening and thereby a spiral of infection and loosening will continue until the cycle is interrupted. The incidence of loose pins may be largely reduced by adopting a careful pin insertion technique. It is difficult for the operator to select a pin with a thread length exactly equal to the diameter of the bone into which it is to be inserted. As a consequence, an over-length thread will generally be selected. The effect of this will be that an excess of thread will obtrude into the surrounding soft tissue, creating local irritation and inflammation of the soft tissues. This inflammation can prove to be the focus for infection which then invades the pin tract.

Solutions to the problem of thread stripping have been offered in several ways including but not necessarily confined to pre-drilled pilot holes and slip thread design. Pre-drilled pilot holes reduce the problem of thread stripping by providing a path of lesser resistance in both cortices. It constitutes, however, an additional procedure to the surgical operation that is not always desirable. Slip thread designs have been confined to self-drilling and self-tapping pins. They were designed to remove the threaded portion of the pin from the proximal cortex whilst the tip of the pin is advanced through the distal cortex. This was not entirely successful in practice due to the need to select accurately from a necessarily wide range of thread patterns that were not always available to the surgeon.

Pre-drilled pilot holes and the use of non-drilling self-tapping pins largely overcome the problem of'fudging'. It is however an additional procedure to the surgical operation that is not always desirable.

Non-drilling self-tapping tapered pins provide a larger diameter to fit the proximal cortex. The perceived drawbacks of this otherwise popular design are several. In particular, these pins require a pre-drilled pilot hole as described above. The pilot hole will have a diameter relevant to the diameter of only one spot along the length of thread: in consequence, the leading part of the thread will always be too small for the hole and, most importantly, the trailing part of the thread will be too large for the pilot hole. The clinical effect of this is that the proximal hole will be filled with a thread of ever increasing diameter, above that for which it has been drilled, producing a concomitant increase of pressure between pin and bone at that point. Such excessive pressure is considered to induce"pressure necrosis"which creates late pin loosening. Advocates of this pin design acknowledge the problem of pin loosening and some recommend a late continued insertion of the tapered pin to regain a mechanical hold.

The invention provides an external fixation pin which sets out to overcome or mitigate the problems referred to above.

Accordingly the invention provides an external fixation pin for fixing into a bone comprising: a drilling point at the leading end of the pin; a first threaded section of constant diameter immediately succeeding the drilling point, the first threaded section including at least one thread-cutting flute at the leading end thereof for cutting a thread into the bone; a second threaded section immediately succeeding the first threaded section, the second section having a diameter which increases from its leading end to its trailing end and including at least one cutting flute for cutting a bore in the bone having a diameter substantially equal to the nominal diameter of the pin; and a smooth cylindrical section immediately succeeding the second threaded section and having a diameter equal to the nominal diameter of the pin.

Embodiments of the invention are described below with reference to the

accompanying drawings, in which : Figure 1 is a side view of an external fixation pin ; Figure 2 is an enlarged view of the forward part of the pin; Figure 3 is a view of the cutting point of the pin taken at 90'to the view of Figure 2; and Figure 4 is a view of the forward part of the pin embedded in a bone.

A self-drilling, self-tapping, fixation pin 1 is shown in the drawings.

A self-drilling point 2 is provided at the leading end of the pin and this is immediately succeeded by a first threaded section 3 having a diameter greater than the nominal (maximum) diameter of the pin. The threaded section 3 includes at least one thread-cutting flute 4. The self-drilling point initiates the hole in which the self-tapping flute 4 cuts a hole for the relatively short length of threaded section 3 that follows.

The short first threaded section 2 is immediately succeeded by a shorter tapered threaded section 5 having a diameter which increases from its leading end to its trailing end and rises to the nominal pin diameter. This tapered section contains a cutting flute 6 designed to enlarge the hole in the proximal cortex to a diameter substantially equal to the nominal diameter of the pin.

The tapered section 5 is immediately succeeded by a generally smooth cylindrical section 7 whose diameter is the nominal diameter of the pin. This smooth section extends to the rear of the pin which is squared off to enable it to be rotated by the drilling tool.

The length of the threaded sections is such that the smooth section 7 enters the proximal cortex before the pin engages the distal cortex. The pin is then progressed to the distal cortex into which it will be located.

At this time, the unthreaded proximal cortex will be filled with a plain non-threaded section of pin offering a clean hole, a substantial fit and no surplus thread.

In one example, a pin having a nominal diameter of 5mm has a first threaded section of 4mm diameter.

The axial length L2 of the second threaded section 5 may be about 20% to 40% of the axial length Li of the first threaded section 3.

As is known, the smooth section 7 of the pin may be provided with markings every centimetre for guidance of the surgeon.

Claims

1. An external fixation pin for fixing into a bone comprising : a drilling point at the leading end of the pin; a first threaded section of constant diameter immediately succeeding the drilling point, the first threaded section including at least one thread-cutting flute at the leading end thereof for cutting a thread into the bone; a second threaded section immediately succeeding the first threaded section, the second section having a diameter which increases from its leading end to its trailing end and including at least one cutting flute for cutting a bore in the bone having a diameter substantially equal to the nominal diameter of the pin; and a smooth cylindrical section immediately succeeding the second threaded section and having a diameter equal to the nominal diameter of the pin.

2. An external fixation pin as claimed in Claim 1, wherein the axial length of the second threaded section is from 20% to 40% of the axial length of the first threaded section.

3. An external fixation pin substantially as described herein with reference to the accompanying drawings.