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Orthopaedic implants

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Abstract

An orthopaedic implant is a device designed to either restore the structural integrity of a damaged bone or to replace an absent bone or joint. The chapter covers implant materials; orthopaedic wire; intramedullary devices; bone screws; bone plates; external skeletal fixation.

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Figures

Image of 10.4
10.4 Tension-band wiring. The tension-band converts a distractive force to compression. One or two bicortical K-wires are inserted perpendicular to the fracture line. The tension-band wire is then applied in a figure-of-eight configuration; symmetrical twists in both portions of the wire are recommended to ensure even wire tension. The selected tension-band wire diameter should be relatively large, as the wire functions to oppose the major distractive force of the attached tendon or ligament, in this example, from the quadriceps muscle. This force is converted to a compressive force oriented approximately parallel to the K-wire. A = distractive force; B = distractive force; C = compressive force.
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10.6 Technique for making Rush pins. The desired length is calculated to be a minimum of three times the height of the physeal fragment. (a) The pin is secured between two chucks which are used to create a hooked end (b) at one end of the pin that can be trimmed to length. (c) The sledged end is created by cutting obliquely using a dedicated pin cutter. (d) A gentle bend is created along the remaining length of the pin. (e) Postoperative craniocaudal radiograph showing correct application of Rush pins in a mildly comminuted feline supracondylar femoral fracture.
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10.8 Techniques available for augmentation of intramedullary pin fixation of diaphyseal fractures. (a) Addition of a plate is known as ‘pin–plate’ or ‘plate–rod’ fixation. The pin diameter is usually reduced to 35–40% of the medullary canal diameter to allow purchase of at least four cortices in each fragment by the screws in the plate-and-screw construct. (b) A similar principle is followed using a tied-in external skeletal fixator. Morbidity is often significantly greater using this technique compared with pin–plate fixation.
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10.9 (a) The 3.5 mm titanium ILN pre-assembly (left) and post-assembly (right). A drilling guide is attached via a coupling system to the proximal end of the nail, which allows the drill guide to be perfectly matched to the cannulations in the nail. (b) Interlocking nail coding system.
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10.10 Glossary of screw morphology. Head = the design of the recess within the screw head varies. Screws may have a slot or a cruciate, hexagonal or star-shaped recess. Tip = self-tapping screws (shown here) have a thread-cutting tip. Thread diameter (A) = the widest diameter of the screw; this is the diameter that defines the screw size (e.g. 3.5 mm, 2.7 mm). Core diameter (B) = the difference between the thread diameter and core diameter is the thread height. Pitch (C) = the distance between adjacent threads. Screw working distance (D) = the length of bone traversed by the screw. Drawn by Vicki Martin Design, Cambridge, UK and reproduced with her permission.
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10.12 Screw length measurement. This 2.7 mm diameter 20 mm screw is measured from the head to the tip. The threads on the tip will not engage the trans cortex. If the head of the screw is countersunk into the bone, this screw will have a working distance of approximately 18 mm.
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10.13 A non-self-tapping 4 mm diameter cancellous bone screw (left) and a self-tapping 3.5 mm diameter cortical bone screw (right). Note the relatively larger core diameter (at the expense of thread height) of the cortical screw.
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10.14 Bone plate function. (a) Neutralization. (b) Dynamic compression. (c) Bridging. (d) Buttress.
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10.15 (a) A 6-hole 2.7 mm DCP. Plates are denominated according to the diameter of the screw that engages the plate holes. (b) If a screw is placed eccentrically, the screw head can move along the hole during tightening, causing compression at the fracture site. Drawn by Vicki Martin Design, Cambridge, UK and reproduced with her permission.
Image of 10.16
10.16 The DCP drill guide has ‘load’ and ‘neutral’ functions. Using the load guide, the screw is placed eccentrically in the hole; tightening the screws causes compression at the fracture site. Using the neutral guide, the screw is placed in the centre of the hole, and minimal compression is generated. Drawn by Vicki Martin Design, Cambridge, UK and reproduced with her permission.
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10.17 Use of the universal drill guide to produce compression in a DCP. (a) Dynamic compression. The universal guide is placed eccentrically (away from the fracture) without exerting pressure. (b) Neutral insertion. When the universal guide is pressed into the plate hole, it centres itself and allows neutral pre-drilling.
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10.18 A 2.7 mm reconstruction plate.
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10.19 A 2.0/2.7 veterinary cuttable plate. This plate can accept 2 mm or 2.7 mm screws.
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10.20 Various veterinary T-plates. From left to right: 3.5 mm 7-hole round-hole T-plate, 2 mm 8-hole DCP-type T-plate, 2 mm 6-hole DCP-type T-plate and 2 mm 5-hole DCP-type T-plate.
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10.21 Acetabular plates. From top to bottom: 3.5 mm 6-hole, 2.7 mm 6-hole and 2 mm 4-hole acetabular plates.
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10.22 Supracondylar plates. From left to right: 3.5 mm left, 2.7 mm right and 2 mm right supracondylar plates.
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10.23 (a) A 3.5/2.7 mm 9-hole hybrid pancarpal arthrodesis plate. (b) A 2.7/2 mm left 10-hole pantarsal arthrodesis plate.
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10.24 Locking screw technology. The Combi hole™ of the LCP allows cortical screws to be inserted on one side of the hole in either a neutral or compression mode, and threaded conical locking screws to be inserted on the other side of the hole. (a) A = threaded plate hole for locking screws; B = dynamic compression unit (DCU) for standard screws. (b) C = locking screw in the threaded side of the Combi hole™; D = cortical screw in the compression side of the Combi hole™. Note that the locking screw has a shallow thread profile because it does not need to generate a large frictional force between the plate and the bone. This allows a larger core diameter, which provides greater bending strength. (© DePuy Synthes UK)
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10.25 A 7-hole 2.7 mm LCP with a bending press (left). The locking drill guide (centre) ensures that the 2.0 mm drill bit is positioned perpendicular to the threaded section of the Combi hole™. Use of a threaded guide is important in fixed angle locking plates to ensure that the plate and screw threads match perfectly. The universal guide (right) can be used for preparation of non-locking screws. Non-locking screws must always be placed before locking screws because the non-locking screws would be unable to compress the plate against the bone after locking screw application.
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10.26 A 16-hole 2.7 mm SOP plate with bending irons attached and a guide for a 2.0 mm drill bit. Although the drill guide is not threaded, it can only couple perpendicularly with the screw hole.
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10.27 An 8-hole 2.7 mm VetLox plate with bending irons attached. The threaded locking screw guide that is shown in this image can be replaced with a non-locking guide allowing up to 30 degrees screw angulation in all directions.
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10.28 A 6-hole 3.5 mm Fixin plate with the second threaded bushing removed. The attached 2.9 mm drill guide has a Morse taper coupling system which ensures that the screw hole is drilled perpendicular to the plate.
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10.29 A dedicated bending press is available to facilitate contouring of the ALPS plate in multiple planes. (© KYON AG)
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10.30 External skeletal fixator configurations. (a) Unilateral, uniplanar (type 1a). (b) Unilateral, uniplanar with intramedullary pin tie-in. (c) Unilateral, biplanar (type 1b). (d) Modified bilateral, uniplanar (type 2b). (e) Bilateral, biplanar (type 3). (f) Ilizarov ring. Drawn by Vicki Martin Design, Cambridge, UK and reproduced with her permission.
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10.32 Negative-profile end threaded (Ellis) pins. The junction between the shaft and the threaded portion is a ‘stress riser’. The pins are designed so that this point is placed inside the medullary cavity with the negative thread seated in the far cortex. If this junction is outside the near cortex of the bone, there is a high risk of pin fracture at this site.
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10.33 Positive-profile end threaded pins are available with diameters from 0.9 mm to 4 mm.
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10.34 Centre-threaded positive-profile pins.
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10.35 Kirschner–Ehmer clamp.
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10.36 SK™ clamp. The primary bolt on the left is used to secure the transfixation pin; the secondary bolt on the right secures the connecting bar.
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10.37 Titanium (left) and carbon fibre (right) 6.3 mm connecting bars for use with the small SK™ system.
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10.38 (a) Temporary fracture stabilization using removable clamps and steel bars. Plastic tubing is pushed over pin ends following fracture reduction and bottom-plugged. Acrylic is mixed in self-contained packets and poured into tubing while still in the liquid phase. The steel clamps and bars are removed once the acrylic has set. (b) ESF putty. (a, Courtesy of JP Lapish)
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10.39 A partially threaded 6.3 mm connecting bar. One or more of these bars is used to attach a circular external skeletal fixator ring to a small SK™ linear ESF construct resulting in a hybrid linear–circular external skeletal fixator.
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10.40 A 6.3 mm SK™ hinge. One or more hinges can be incorporated into either linear or circular ESF constructs.
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10.41 CESF transfixation wire. A stopper or ‘olive’ is present on the wire.
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10.42 A wire clamp (left) and a small transfixation pin clamp (right).
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10.43 Various styles and sizes of rings serve as the basic structural element of CESF and hybrid ESF frames. These include full rings (right and left centre), sector rings (three-quarter) (bottom left), stretch rings (bottom right) and spinal arches (top left).
Image of 10.44
10.44 Fully threaded 6.3 mm connecting bar.

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