The components of a ring fixator system are categorized into 2 categories: main and secondary. The main parts are the standard elements utilized to correct skeletal deformities: rings, wire-fixation bolts, wires, and buckles, pin clamps, and pins.
The secondary parts of the system comprised of the elements essential for the assembly of the fixator: rods, plates, posts, supports, hinges, washers, bushings, sockets, nuts and bolts. To assemble the several pieces of equipment various kinds of wrenches and wire tensioner are required. There are no screws in this system and screwdrivers aren’t required.
A ring with multiple holes and a flat surface is the main component of a circular external fixator (Ilizarov).
The ring encloses a limb segment: 2 or more rings are connected to make a frame. The ring’s flat surface supports the heads of the nuts and bolts. The surface-nut interface or surface -bolt guarantees firm fixation of the wires, threaded rods, and bolts while treatment. The flat surface of the rings is essential for attaining a secure wire inclination and plane orientation. All rings in a frame are aligned perpendicular to bone’s long axis. A ring is made of carbon fiber or stainless steel provides strong support for the frame and is intended to bear high stresses of the tensioned wire, up to 150 kg. The ring’s internal diameter measures from 80 to 240 mm. A whole set has rings of 12 different diameters to suit several limb thicknesses.
A ring fixator set has several half and full rings. A full ring is lighter, has more holes than 2 connected half-rings, and doesn’t need connecting bolts and nuts. The holes in the ring are utilized for introduction of a threaded rod, a connector plate or a hinge. On the negative side, a full ring must be placed before the introduction of wires. If clinical state demands the removal of a full ring during the treatment tenure, then it must be cut with specific instruments.
Every half-ring, depending on its size, has eighteen to twenty-eight holes in the mid-segment of the flat surface. The standard holes are equally distance threaded (4 mm apart) and are of same size (8 mm in diameter). Threaded rods or bolts are affixed in the holes. 2 half-rings are joined by bolts and nuts to make a full ring. The ends of the plate don’t have standard-sized hole, are offset as well as ledged to fit together on an even plane to form a full ring.
The half-rings may also be connected to make an oval ring, three- and four-leaf clover rings and another specialized construct with the help of additional devices to make more space between the ring and the limb.
A five-eighths ring enables joint motion and is usually deployed near knee and elbow joints. Besides motion, these rings enable the introduction of cross wires, a distinct benefit near these joints. This ring can be used in the middle of a regular frame to offer access for management of soft tissue. Though, 5/8 ring is weaker than a full ring; a three-point connection to a full ring reinforces as well as strengthen it. Wires attached to a 5/8 rings are tensioned only after such stable connection is established. These rings can be accessible in three sizes from 130 to 160 mm.
From the mechanical viewpoint, it’s not essential to remove an orthopaedic nail in a weight-bearing limb and dissimilar from a plate, it can be left indefinitely in the body. Removal initiated by request of the patient should be delayed for eighteen months. Intramedullary devices sometimes induce local changes which can be irritative either to the bone or to the patient and require removal. Swelling and local pain secondary to backing out of the implant is another indication for removal; confirmed bone union on radiological examination is a prerequisite for such a removal.
A sharp angled deformation seeming in the follow-up roentgenogram is an indication of appliance failure. A sharply bent device is necessary to be removed and replaced as it has undertaken plastic deformation and is expected to fail with further weight bearing. Bent nails may be removed forcefully straightening and extraction.
Nail removal shouldn’t be undertaken lightly. Specialized extraction equipment fitting the exact nail must be available. Although removal is often a straightforward process, mismatching equipment, nail breakage, damage to the threads in the proximal end of the nail and distortion in the bony anatomy preventing removal are common causes of difficulty.
Difficult situations in nail removal
- Extraction hook fails to grasp.
- Stack the canal round the hook with ball-tipped guide wires. Hold the bunch in a vice grip and hammer out to remove the nail.
- Bone growth along the track of the locking bone screws removed some time ago.
- Re-drill the holes with an appropriately large drill bit.
- The broken nail in a united fracture; extraction hook fails to regulate the alignment as well as pulls the nail tight against the opposite cortex rather than out of the canal.
- Take away the proximal half of the nail. Pass a guide wire that is ball-tipped into the proximal canal till the ball is near the broken end of the nail. Ream the proximal canal up to the nail’s broken end to facilitate easy removal. Pass 2 or more guide wires into the distal nail fragment and impact them with a hammer. Grasp the bunch in a vice grip as well as hammer out to remove the nail.
- The broken solid nail in an ununited fracture.
- Take away the proximal half. Create a window in the bone distal to the tip of the nail as well as hammer the nail out. In the femur, arthrotomy of the knee joint can be needed. In the tibia, if the nail is short, make an opening in the medial malleolus to arrive till the nail.
- The protruding nail in a healed fracture and nail doesn’t move.
- Use specialized metal cutting equipment to remove the offending nail length.
- Impacted nail.
- Find the point of maximum hold under fluoroscopy. Create a longitudinal cut in the bone with the help of an oscillating saw or an osteotome at that point. Extend the cut either way till the bone springs open enough to allow removal of the nail. The method is suited best for a tibial nail and is employed only as a final resort.
Full weight bearing may start immediately after the removal of an intramedullary nail, but a coming back to vigorous sporting activity should be delayed until rehabilitation is attained.
Pins are versatile and are usually helpful for internal fixation. A pin has a comparatively small diameter and is inserted through the soft tissue and bone with comparatively little trauma. A Kirschner wire is inserted with a power drill as well as a guidance system. The soft tissues tend to wind round the Kirschner wire at the time of insertion and must always be protected. Several types of guides are in use: a telescoping guide linked to the drill, or an external guide with a handle are common orthopedic Instruments. A power drill with a quick locking and release mechanism save considerable time and is also suitable for insertion of Kirschner wire from the barrel of the drill, that doubles as a guide.
Kirschner wire insertion
- Always deploy a power drill to insert a Kirschner wire.
The wire bends once it’s inserted on a hand drill.
- To protect soft tissues during insertion, always use a guide to direct the pin.
- Support the wire.
- Place the cutter jaws at right angles to the wire.
- Bend the wire up after cutting.
- Tip of wire must not touch the plaster cast.
Fashion a point- 2 oblique cuts to fabricate a sharp tip
Kirschner wires are helpful for provisional fixation of a comminuted fracture. They help in accurate placement of the fragments and orthopedic implants, especially the plates and the bone screws. Multiple wires are inserted without any added trauma. When provisional stability has been attained, X-ray pictures can be made to visualize the strength as well as weakness of the construct.
Planning is necessary while inserting Kirschner wires for provisional fixation. Pins are introduced in such a way that they don’t obstruct the final placement of the definitive implants. For instance, wires are passed parallel to each other in the same direction in which the lag screw is to be introduced in order that there’s no obstruction to compression of the fracture with the lag screw.
Whenever a Kirschner wire is used for definitive fixation, the pin’s end should be cut a centimeter under the skin and may be bent; a long wire end will protrude through the skin, either because of the pressure from within or from outside. If a wire is cut too near the bone, it’s difficult to locate it at the time of removal. The jaws of a wire cutter are always positioned at right angles to the wire. If they’re positioned at any other angle, a twisting force develops whereas snapping the wire. This force is transferred to the bone and due to this unintentional fracture may occur in a small bone, specifically in the cancellous area. A sharp point at the pin trip may be fabricated by making an oblique cut and then rotating the pin to make the 2nd oblique cut. The free end of the wire must always be bent with an orthopedic instrument. The bending force, if transmitted to the bone, may cause an inadvertent fracture; this complication is avoided by gripping and stabilizing the pin at the time of bending. There’s a genuine danger of migration of a straight pin into the soft tissues or into the bone; a pin is known to travel a long distance across the planes of soft tissue. When using a pin around the shoulder, it’s imperative that the trip be bent.
A pin enclosed in a plaster cast shouldn’t touch the cast but move freely inside. There’s considerable movement between the cast and the bone because of the presence and the thickness of the soft tissues. If any pin touches the plaster, forces are transferred to the bone, resulting in pin loosening as well as loss of fixation. A pin must be removed after it has served its purpose.
The orthopedic implants manufacturers deploy many of the metal-working methods. A brief account of the fabrication process of some implants follows.
An orthopedic bone screw is manufactured by very precise procedures from titanium or steel rod with a diameter of at least the width of a cannulation at this step, which is added by gun drilling. Gun drilling is an exact process with minimum variation from the required dimensions. The special drill contains a central channel for cooling fluid delivery as well as for removal of metallic debris. The next step in screw production includes machining of the threads. The cutting flutes are cut on a Lathe/ CNC Turning Centre or a SPM. In the bone head drilling and punching with a hexagonal tool is done to make recess for Screw Driver.
Electropolishing as well as passivation are done on the screw to clean the material after machining and increase the resistance of material to the corrosion.
A bone plate is produced from a sheet from which its profile is cut by laser Cutting. Different variety of plates, shaped to suit the bone profile on which it must be fitted, are cut out of steel or titanium sheets. After facing, holes are drilled or milled according to the design (self-compressing or round).
The plate for a sliding hip screw is produced from a bar which is forged and bent to the desired angle (e.g. 135⁰, 140⁰, 145⁰, 150⁰). The plate’s barrel segment is turned on a lathe and the barrel hole is drilled as well as broached. The plate is than milled to attain the correct profile. The holes of plate are milled to obtain the correct profile. The counter on the holes of plate are milled and the product is finally finished. Finishing is labour intensive job, which needs advanced manual skills.
AO/ASIF nail is an example of combined metal working techniques. This is made from a tube which is cold drawn as well as annealed several times to decrease the wall thickness and diameter of the tube.
Then the longitudinal slot is cut into the tube, and the drive is threaded. The locked Universal nail isn’t made from a sheet of stainless steel but from a stainless-steel tube which is cold drawn and annealed many times to decrease the wall thickness and diameter. A longitudinal slot is then cut into the tube and, finally, the threads are cut in the drive. A non-slotted nail is manufactured from a solid bar of stainless steel. The central cannulation is produced through the method of gun drilling.
It is evident from above that machining of implants utilizes many machining and metal working techniques, which are essentially in ‘cold condition’ i.e. at a temperature at which its microstructure is not affected.
An ideal trauma implant material should be inert, non-toxic to the body, and completely corrosion-proof. It should be inexpensive, simply worked, and mouldable in various shapes without expensive manufacturing techniques. It should have high resistance to fatigue and great strength too. Such a material isn’t available at present.
There are at least fifty alloys and grades of alloys known as commercial stainless steel. Only a few are helpful as implant biomaterial in fracture surgery.
Stainless steel designated as ASTM F-55, -56 (grades 316 and 316L) is utilized extensively for fracture fixation implants. Stainless steel of type 316L is an iron-based alloy. Alloying with chromium generates a self-regenerating, protective chromium oxide layer which offers a major protection against corrosion.
The addition of molybdenum reduces the rate of slow, passive dissolution of the chromium oxide layer by up to thousand times. Molybdenum further protects against pitting corrosion. Nickel imparts any further corrosion resistance and enables the production process, while limited quantities of silicon and manganese are added to control some problems related to manufacturing.
The carbon component raises the strength, but its existence in the alloy is undesirable. Under certain conditions created as a results of improper heat treatment, the carbon segregates from the main elements of the alloy, taking with it a considerable amount of chromium in the form of chromium carbide precipitates. Carbides formed at grain boundaries, whereas corrosion selectively happens. Moreover, the carbides degrade the material’s mechanical properties. Mixing of small quantities of titanium or niobium lessens the creation of intergranular carbides by competing for carbon.
Stainless steel of type 316L has a very low permissible level of carbon to diminish this problem. To rise the resistance to fatigue failure, 316L stainless steel can be accessible in a special grade with smaller as well as more widely spaced inclusions. This grade, designated as AISI 316LVM and specified by ASTM F-138, is made by a special method known as ‘vacuum-melting’, which results in a cleaner metal. The strength of 316L may be greatly altered by the method of production, and it may be made extremely ductile (up to 55 percent strain to failure). The alloy selected F-745 is a high strength casting material, whereas 22-13-5 has much more yield strength than 316L for the same treatment method.
Though it is a stiff, strong, and biocompatible material, 316L stainless steel has a slow nevertheless finite corrosion rate. Concerns regarding the long-term effects of nickel ions, still, prevail. Stainless steel is best fitted for short-term implantation within the body as in fracture fixation. Stainless steel is often used because the base materials are cheap, the alloy may be formed using common techniques, as well as its mechanical properties may be controlled over a wide range for ductility and strength. The elastic modulus of stainless steel is approx. 12 times higher than the elastic modulus of cortical bone.
Titanium is the 9th most common element in the crust of earth, where it forms oxidic minerals (ilmenite, rutile). The pure element is very reactive; it’s the sole element that burns in nitrogen. Though, the metal quickly becomes coated with an oxide layer, making it resistant to most chemicals and physiologically inert. Titanium is utilized for making orthopaedic implants in 2 forms: commercially pure and a range of alloys.
Titanium-aluminum-vanadium alloy (ASTM F-136) is generally known as Ti6A14V. This alloy is broadly used to manufacture implants. Impurities such as hydrogen, oxygen and nitrogen tend to make it brittle, which describes why only minimal amounts are acceptable in titanium alloys utilized in surgical implants. ASTM F-136 limits the oxygen concentration to a particularly low level of 0.13%, referred as the ELI (extra low interstitial) grade. Limiting the extent of dissolved oxygen improves the material’s mechanical properties, mainly increasing its fatigue life. Aluminium stabilizes the alpha material’s form while vanadium stabilizes the beta form. Combination of both components forms a two-phase alloy with good strength properties and one that may be heat treated. Ti6A14V ELI is often used for making orthopedic implants.
Commercially pure titanium isn’t a single chemical element however is alloyed by a level of oxygen dissolved into the metal. It also has traces of nitrogen, iron, hydrogen and carbon. The presence of these trace elements effects the mechanical properties of the titanium. International standard ISO 5832/2 defines the extent of permissible impurities.
Titanium has an elastic modulus approx. half that of the cobalt-chromium alloys and stainless steel. The lower stiffness of bone plate made of titanium decreases the severity of stress shielding and cortical osteoporosis. Another benefit of lower stiffness is that a titanium plate is less susceptible to fatigue failure than a stainless-steel plate. The elastic properties of titanium need that contouring be attained by slight over bending; it’s important however, that no metal should be twisted or bent repeatedly at the same location. The titanium’s modulus of elasticity is still roughly 6 times that of cortical bone. The ductility of titanium alloy is considerably less than that of most stainless steels. Because of this difference a surgeon needs some adaptation of his feel when knowing the optimal amount of torque to be applied to the bone screws. The feel should be acquired prior starting to use these bone screws in clinical practice.
The corrosion resistance of pure titanium is outstanding as a very dense and stable layer can be destroyed mechanically at the time of implantation by instruments such as bending pliers. The passive layer is restored spontaneously, quickly and effectively (re-passivation). In the presence of unstable fixation, the components of titanium of an internal fixation system are exposed to fretting conditions and make metal debris. Such debris causes black or grey coloration of the surrounding tissues. This discoloration, which isn’t a result of corrosion, is harmless. Special surface treatment of the ortho implant lessens such discoloration.
Successful use of a bone plate relies on the properties of the plate, the bone screws, the bone and on the accurate application of biomechanical principles.
Plate related factors
The strength of a plate relies on its cross-sections; thickness is the most significant contributing factor. The strength differs with the cube of the thickness. The plate should be made from a material of suitable strength and stiffness of the plate should be close to that of the bone. The titanium’s stiffness is closer to that of bone, while stainless steel is stiffer than titanium. Very stiff bone plates can weaken the bone after fracture healing is done. The plate’s contact surface is also a significant factor. The conventional plate’s surface causes reduction of blood supply under the plate, resulting in immediate post-fixation osteoporosis.
In the new design featuring decreased contact self-compressing plate to lessen contact surface, substantial material is removed from the plate’s undersurface between the holes, giving it an arched appearance. A conventional orthopedic plate is weak at the screw holes. The excavation lessens the stiffness of the plate to the similar level as of the screw hole area. The bone plate with this design now has even stiffness in both the areas. Such a plate may be bent in a constant curvature with a good fit of the screw head in the plate hole and still preserve the mechanical features that consistently distribute bending as well as torsional stresses over a long distance along the plate. The fatigue life of plate is prolonged because its holes are protected from localized high stresses. The holes of plate are symmetrical, consistently distributed along the plate, and have oblique undercuts on the plate’s lower side. This undercut enables unhindered inclination of a lag screw up to 40⁰ longitudinally, as well as 7⁰ in the transverse plane, in both directions through the plate. Usage of a shaft screw as a lag screw through the plate in position of a fully threaded cortical screw provides superior static compression force. The plate’s even distribution holes allow the place of the plate to be adjusted or for the plate to be replaced for a longer plate without conflicting with any previously drilled holes.
The plate’s length is another important factor. A too short plate can make a construct unstable, while application of a very long plate can cause needless damage to the soft tissue.
Screw related factors
The bone screws fasten a plate to the bone. The effectiveness of a screw largely depends on the design of its head. Well-made threads are easy to insert and hold well in all conditions. A well-designed slot for the screwdriver’s placement ensures ease of screw insertion. Suitable number of screws is necessary to hold the plate. A minimum of 2 in each fragment are essential to prevent rotation. The total number of screws required for a fixation relies on the site and kind of fracture. The ratio of the pilot hole to screw’s depth depends on this ratio. Strength of the plate fixation relies on turn on the holding power of the bone screws. The screws should be made of strong material which may withstand heavy loads. Plates and screws should be of the same material to diminish corrosion.
The success of bone plate fixation relies on:
- Plate thickness, geometry, dimensions, material used
- Screw design, number, material, and hold in the bone
- Bone-mechanical properties as well as the health of the bone
- Construct- direction of load and placement of plate
- Compression between the fragments
Bone related factors
The health of the bone is a factor which is often overlooked while fixing a fracture. A young bone is dense in consistency and a screw holds good in such a bone. As the holding power of a screw is dependent on the elastic force offered by the bone, it’s obvious that the denser the bone, the stronger the hold. In the elderly people, the bone is porotic, being less dense than young bone. The holding elasticity of porotic bone is a lower magnitude as well as leads to inferior screw hold. Therefore, the mechanical properties of bone and health are of importance in this context.
The bone’s interaction and the plate are important, since the 2 are combined in a composite structure that becomes a vital entity in the strength of a fixation. The strength of a plate-bone construct is its capability to withstand load without structural failure. This entity may be described as a torsional strength or a bending strength, depending on the load application. An orthopedic plate is a load sharing device. Loads may be transmitted between plate as well as bone through the bone screws and through friction kind of forces between the plate surface and the bone. Some of the load is supported by the plate and some load passes between the fragments of bone. The reconstructed bone must support a certain load.
Cortical and Cancellous Screws
A cancellous bone screw has larger threads and a higher pitch in comparison to the cortical screw. It resembles a modified wood-type screw. Its tip isn’t tapered. The core diameter, that is smaller than that of the shaft, offers a greater surface area for purchase of the screw threads on bone. A rise in the thread diameter of a cancellous screw raises its pull-out strength. A cancellous screw is inserted into a pilot hole that is untapped; the dimensions of the pilot hole equals the screw’s core diameter. Its big threads form companion threads in the bone by compression as well as by deforming the bone trabeculae. The spring reaction originates from the cancellous bone as it’s deformed during the thread forming procedure.
The cortical bone screw is a machine-type screw. The threads are smaller (in diameter) and have lower pitch. The core diameter is relatively large as well as offers the necessary strength. The smaller pitch rises the holding power of the screw. Threads cut in the pilot hole prior this screw is inserted; this is attained by a separate tool (tap) or by the screw’s self-taping tip. The elastic reaction, vital to grip the bone surfaces together, comes from bone’s elastic deformation and not the screw. This happens as screw is stiffer than the cortical bone. The modulus of elasticity of the screw is more than ten times that of bone; thus, much of the elastic deformation happens in the bone.
Whenever a screw is positioned in a bone, the bone deforms and offers the elastic binding force, irrespective of the kind of screw on the type of bone into which it is inserted.
The term ‘self-tapping screw’ refers to a screw that is inserted directly into a pre-drilled hole without 1st tapping a thread. Self-tapping screw can further be subdivided into thread-cutting and thread-forming screws. The thread-forming sort moulds (i.e. forms) its own elastic-plastic deformation or by bone’s local destruction. The thread-cutting bone screw cuts its thread by the bone over which it advances.
The cancellous orthopedic screw is a thread-forming self-tapping screw. The screw thread makes its own mating bone thread through compressing the soft cancellous bone. A tap shouldn’t be used to insert a cancellous screw- a cancellous bone screw inserted in a tapped hole has lower pull-out strength than 1 inserted in an untapped hole, and as tapping removes cancellous bone from the hole, as well as effectively enlarges it. The amount of bone removed by tapping rises as the density of bone reduces the mean volume increase is about 25 percent. When a cancellous bone screw is to be inserted first hard cortical it’s essential to tap the cortical bone. Cancellous taps are offered for this reason alone. The smooth shaft of the cancellous bone screw offers the lag effect without the need for over-drilling. This becomes important in the larger 6.5 mm screws, where a very large hole would require to be drilled in the near cortex to produce a lag effect. Changing lengths of partly threaded and fully threaded cancellous screws are utilized as indicated in different clinical situations.
A cortical bone screw can be either self-tapping or non-self-tapping.
A self-tapping cortical screw is a thread-cutting screw as well as has the cutting lip of a tap essentially milled into its tip. This device cuts a thread in the sense cortical bone on that the screw advances.
The self-tapping screws was criticized for a long period since it was supposed that a self-tapping screw would offer a poor hold in the bone as implanted in fibrous tissue rather than in the bone. Later research has shown this view to be not correct. Size for size, a self-tapping and a non-self-tapping screw have nearly the same holding power. The cutting flutes’ design of a self-tapping screw is significant. A poorly designed tip encounters significant resistance, mainly in thick cortical bone. At times the resistance can be such that the torque essential to drive in the screw is greater than the screw’s tolerance, and the screw can break. The resistance to insertion can interfere with the accuracy of placement, mainly if one is trying to insert the bone screw obliquely to lag 2 bone fragments together.
Self-tapping screws are widely utilized in bone surgery as they can be inserted quickly, decreasing the operation time for internal fixation. Additionally, relatively fewer steps and instruments are needed. The screw tip must offer from the far cortex so that the flutes are clear and at least 1 complete thread engages the cortex. Reinsertion of a self-tapping bone screw into the same threaded hole cannot always be a cause for concern; reinsertion, though, should be manual and a drill machine ought to be avoided as power insertion can inadvertently create a new track.
The main benefit of this screw is simplicity, as pre-tapping is not necessary. Moreover, a very tight fit of screw thread to bone is ensured because the bone screw cuts its own precise thread.
Fully and Partially Threaded Screws
Cortical and cancellous screws are available as fully as well as partially threaded screws. A cortical bone screw is often fully threaded. In plate fixation, a bone screw must purchase firmly on both the cortices. The purchase on the near cortex contributes 80 percent of the grip and the distal cortex contributes around 20 percent. A fully threaded cortical screw may function as lag screw only once the near cortex is over-drilled. A fully threaded cortical screw can be either self-tapping or non-self-tapping.
A partially threaded cortical screw is known as shaft screw. The shaft diameter resembles to the outer diameter of the thread. This screw has better strength as well as stiffness than a fully threaded screw which is a benefit when it’s used as a lag screw and as an axial compression screw. It’s a non-self-tapping screw.
A cancellous screw can be fully threaded or partially threaded. A fully threaded cancellous screw is utilized as a placement screw to fix a bone plate in metaphyseal as well as epiphyseal regions. A partially threaded cancellous screw is utilized as a lag screw.
A cannulated screw is utilized for precise insertion in epiphyseal or metaphyseal site over a guide wire reducing the problem of having to remove and reposition an incorrectly placed screw. A guide wire correctly visualizes the path of the screw. Moreover, the guide wire maintains the reduction as well as controls the fracture fragments. If position of guide wire must be changed, it may be done without enlarging the hole and sacrificing bone’s purchase strength. Final placement of the screw needs use of cannulated drill as well as occasional use of a cannulated tap. The screwdriver is cannulated too. Cancellous cannulated screws come in small and large sizes. Large cannulated cancellous screws are utilized to fix fractures of the femoral neck, tibial plateau and femoral condyle. Small cannulated cancellous screws are used for fixation of the distal radius, distal humerus, carpal scaphoid, distal and proximal tibia. In comparison with an equivalent non-cannulated screw the greater root diameter required to accommodate the central bore of the cannulated screw efficiently reduces holding power to some extent.
The cannulated screw head has either an inner hexagonal recess to work with a cannulated screwdriver, or an associate external hexagonal or square head as well as a cannulated wrench. The internal recess design enables use of a slim screwdriver and allows a spherical outer shape to the screw head. This may be important in removal of screw. Bone growing around a head of screw with an external hexagonal head makes removal hard, since bone must be removed to enable engagement of the wrench. If 2 external hexagonal screw heads touch, they will lock. The benefit of using the external hexagonal head is the strength offered to the coupling with the driving wrench. The round head with an internal recess puts more demand on the tip of screwdriver. The screwdriver’s hexagonal tip must be small sufficient to fit within the recess in the screw head, yet itself must be cannulated, leaving slight material in it. Moreover, strength of the screw head-shaft junction is essential. The internal hexagonal recess removes head material. If this recess is very deep, strength can be lost at the head-shaft junction.
To maintain strength, the shaft’s diameter of a cannulated screw is usually designed slightly larger than that of a solid screw of size that is comparable. Since the cylinder’s stiffness in bending is a function of the third power of its radius, a small rise in the shaft’s outer radius will compensate for the cannula. Cannulation doesn’t appear to be a problem in the greater screws, but in a smaller screw leaving a cannulation big enough for stiff guide wire can require the shaft diameter to be significantly increased or the screw will be significantly weaker. There is a misapprehension that a solid screw is stronger than a cannulated screw of an equal outer thread diameter. The 6.5 mm cannulated screw often has a a little larger shaft (root) diameter than its solid 6.5 mm counterpart. A solid 6.5 mm can have a 3.0 mm thread root diameter, whereas the 6.5 mm counterpart has a root diameter of 4.8 mm. If the cannula is 2 mm in diameter, the area as well as polar moments of inertia of the cannulated screw would be 102.6 mm and 514.8 mm in comparison with 27 mm and 81 mm, i.e. 3.8 and 6.4 times larger, respectively, than those of the solid screw. A cannulated screw must have a bigger root diameter than the solid screw to enable room for the cannula. This does reduce its holding power, as the thread depth is smaller. Clinically cannulated screws appear to function fine and have more than enough holding power.
A cannulated screw for cancellous bone should be self-tapping and self-cutting. The screw tip cuts only when turned clockwise and is blunt when rotated counterclockwise (removal direction). A self-cutting, self-tapping tip is beneficial in percutaneous procedures. After the guide pin is positioned, the screw is advanced through the soft tissue during turning it in a counterclockwise direction. The tip doesn’t cut or wind the soft tissues. When cortex is reached, the rotation is reversed to clockwise, enabling it to cut into the bone.
The Herbert Screw
The Herbert screw is a specialized orthopedic implant to achieve interfragmentary compression. There is no head in this unique device and threads are present at both ends of the bone screw, with a pitch differential between the trailing and leading threads. The intention is for the bone screw to be buried under a bony surface. Interfragmentary compression is attained by the difference in thread pitch: the coarser pitch moves the orthopedic screw a greater distance through bone with every turn than does the finer pitch. As the screw is twisted, the bone surfaces come together creating compression.
A screw head is thus not needed. In absence of a screw head it’s possible to insert this bone screw through articular surfaces without the head being prominent. A cannulated version is accessible: Cannulated Herbert screw enables percutaneous scaphoid fracture fixation, avoids prolonged cast immobilization and enables a more rapid return to work or sport.
A cannulated Herbert screw is easier to install than an uncannulated version and provides better protection to the bone spicules that interdigitate between the screw’s threads in comparison with cases in which the bone is originally tapped, the tap is removed, and the bone screw is then installed during a 3rd pass through the bone threads. Placing a guide wire first enables for correct visualization of the path, position, as well as length of the screw. The guide wire also helps in reduction and control of fractured fragments. Current indications include capitellar, fractures of the carpal scaphoid, fractures, radial head fractures, osteochondritis dissecans, osteochondral fractures, and small joint arthrodesis. The Herbert screw may be quite tough to remove.
Wound Care & Dressing:
- Post operation, all care should be taken such that chances of infection are minimized.
- Following surgery, care should be taken to keep the wound dry and clean.
- The dressing should be changed at regular intervals as advised by the Doctor.
- Unless allowed by the Doctor, dressings should be done by skilled and trained medical personnel.
- Unless otherwise permitted by the Doctor, the shower should be avoided to prevent the wound & dressing getting wet.
- Likewise, the wound shouldn’t be submerged in a pool or bathtub.
- Application of ice helps in reducing swelling and pain, but it should be carried out strictly on advice of treating surgeon.
- Application of ice, if allowed by the Doctor, should be done in a manner so that the wound does not get infected.
- Care must be taken with ice to prevent frostbite.
- Strictly follow weight bearing instructions, which are given at time of discharge. No overstraining of bones at injury site should be done.
- A cane or crutches may be essential to assist walking in case of injury to bone of the leg.
- Elevating the operated limb elevation may help to reduce swelling. However, same should be done in consultation with treating surgeon.
- The anesthetic drugs utilized during your surgery can cause nausea during for the first twenty-four hours.
- If vomiting and nausea become severe or the patient shows symptoms of dehydration (lack of urination) please consult the Doctor.
- A low-grade fever (100.5) isn’t uncommon in the 1st twenty-four hours. However, doctor should be consulted in case of temperature build up or continued temperature hike.
- Please call the doctor with any temperature over 101.0 degrees.
- Self-medication for pain relief MUST be avoided.
- Local anesthetics are prescribed by Doctor for pain relief after surgery.
- During 1st or 2nd day after surgery, swelling may peak.
- Taking pain medicines before bed time will assist in sleeping.
- It’s important not to drink alcoholic drinks or drive while taking medicine.
If you are taking other medicines, you should check with Doctor for its continuance and dosage along with medicines prescribed for the surgery.