Principles of Orthopaedic Screws | Orthopaedic Academy

[00:00] is firstly to describe the screw as a mechanical device, secondly to identify its physical attributes and its functions in surgery, and thirdly to outline the techniques for insertion in bone. After studying this guide, you should be able to identify the basic screw forms and sizes of the screw.

[00:20] of the AO armamentarium and outline the principles underpinning the surgical techniques of using screws to achieve a variety of outcomes. Asked the question, what is a screw? Surprisingly few can describe a generic screw's basic function.

[00:40] All these are screws, aircraft and ships' propellers, a corkscrew, and surgical screws. What have they in common as mechanical devices? A screw is a device for converting rotational forces into linear motion. Due to the helical morphology of the thread, as it turns out,

[01:00] turns in a material, the slope of the helix causes the screw to move along the longitudinal axis of its shaft. With the exception of some corkscrews, a generic screw has a solid central core, about which is wrapped to helical surface. In the surgical context, most

[01:20] screws have a screw head, the function of which will be described later. One of the basic attributes of surgical screws is the material of which they are made. Most are metal, either stainless steel or titanium, they're being inert or virtually inert in the body tissues. Of course, biodegradable screws

[01:40] are made of a variety of materials that are not inert in the body tissues as they are designed to be absorbed slowly. Some screws are threaded throughout the length of their cores, fully threaded screws. Others are only partially threaded over a portion of the core furthest from the screw head, so-called partially threaded

[02:00] shaft screws. They were also referred to as lag screws in the past, but the term lag is now reserved for describing the function of a screw, not its thread form. This slide uses as an example the thread of a screw designed to gain a hold in cortical bone.

[02:20] cortical screw. A surgical screw is a device manufactured to high specifications and is to be used with care and precision. In order to select the correct instruments and technique for insertion of any screw, the surgeon needs to be familiar with its dimensions. The symbol of a circle

[02:40] cut through with a slash represents diameter. The diameter of the core determines the minimal hole size for the screw to be accommodated in the bone and so determines the drill used to create the pilot hole for the screw. The outside thread diameter, as illustrated,

[03:00] is the minimal side of any hole through which the screw will glide without the threads purchasing in the bone. This is sometimes known as the nominal diameter of the screw as the screws are often named by this dimension. For example, the standard cortical screw has an outside diameter of 4.5.

[03:20] millimeters and is called a 4.5 cortical screw. The effective thread depth is the maximum depth of the helix theoretically available for gaining purchase in the bone and thereby driving the screw forward when it's rotated. It is half the difference between the core diameter

[03:40] and the outside thread diameter. An appropriate length of screw needs to be chosen. Too long and it may cause problems by irritating the soft tissues or protruding subcutaneously. Too short and it will not gain full purchase in the bone. The techniques of measuring the correct screw length

[04:00] will be considered shortly. The pitch of the screw is the length travelled by the screw with each 360 degree turn of the helix. The shorter that distance the finer the pitch. The longer that distance the coarser the pitch. The finer the pitch the more turns of the helix engage in the pitch.

[04:20] in a given depth of cortex. The screw head has two functions. One is to permit the attachment of a driver in order to produce the necessary rotation. This is achieved either by slotting the head, as in wood screws, but no longer in surgery, or by producing a shaped recess.

[04:40] This is usually hexagonal, but may be a crosscut or a star shape. The second function of the screw head is to arrest motion when the head contacts the surface of the bone or plate hole, or washer, which is effectively a one-hole plate. For this reason, the diameter

[05:00] of the head must be greater than the outside thread diameter. To introduce a screw into a firm and relatively unyielding material such as cortical bone, certain steps are essential. A core diameter pilot hole is drilled. As already described, this is determined by the diameter

[05:20] of the core of the screw. For example, the core diameter of a standard cortical screw is 3.1mm and so a 3.2mm drill bit is used for the pilot hole. If the screw is inserted so that the head will directly contact the cortical bone surface.

[05:40] the bone is countersunk to receive the head. This will be discussed shortly.

[06:00] thread diameter. The female helix may be created by using a dedicated tap in the pilot hole or by using a self-threading or self-tapping screw. The self-threading screw has two or more flutes at its tip, specially designed both

[06:20] to cut an accurate thread by removing cortical bone and to avoid jamming of the resultant bone debris in the helix. As indicated the selection of the correct length of screw is vitally important. For a standard cortical screw the tip should just project beyond the file.

[06:40] or trans cortex, so that the thread bites in the full cortical thickness. For the self-cutting screws, the length must be chosen so that the flutes pass clear of the trans cortex. If they lie within that cortex, bone can grow into the flutes and make their later removal difficult. So measure the

[07:00] screw hole for length and add 2mm. How does tightening the screw create compression? When the screw is driven fully in, the head contacts the underlying bone and resists further longitudinal motion. If the screw is then rotated a fraction further, tightened,

[07:20] The threaded portion tries to advance and this creates a minute stretching of the screw, resulting in a tensile force in the core. This is balanced by an equal compression force at the screw head-bone interface. Why should we countersink the cortex beneath a cortical screw head, the cis-cortex?

[07:40] cortex. You will recall that stress is the force applied divided by the area over which the force is applied. The smaller the area, the greater the stress on the bone. If we fail to countersink the cortex, the area of compression is small and the stress high. This risks failure of the bone.

[08:00] If we countersink, the area is increased and the stress reduced. It is to be noted that, with cancellous screws, which are used largely in metafasil and epiphyseal sites, the underlying cortex is too thin to countersink. If we try to countersink, the head can

[08:20] presses the cancellous bone beneath and it may fail, being much less resistant to high stress than thick cortex. What we do in that case is not to countersink but to use a washer which increases the area over which the screw head compression is born, thereby reducing the stress.

[08:40] If we insert a screw across a fracture plane with a view to closing the fracture gap and applying a compressive force at the fracture plane, there are certain conditions to be met. If we thread both the near-SIS and far-trans-cortices, the compressional neae is present.

[09:00] not pass across the fracture plane. This can be seen in the polarized light model where the force lines generated by the grip of the screw threads in the fragments plus the compression forces generated by the screw head in the near fragment are visible. In this model the threads have been prevented from

[09:20] in the near fragment by drilling a gliding hole at least the diameter of the outside thread diameter. The head then generates compression forces which can be seen to pass across the fracture plane. This is known as the lag screw technique. The standard AO cortical screw has an outside

[09:40] thread diameter of 4.5 millimeters and is therefore referred to as the 4.5 cortical screw. Its dimensions are shown on this slide. The head diameter is 8 millimeters. To accommodate the core, a 3.2 millimeter pilot hole has to be

[10:00] drilled. If it is to be used for the lag technique, the gliding hole is made with a 4.5mm drill bit. The thread is cut using a 4.5-cortical tap. The screws are available in stainless steel or titanium. The smaller cortical

[10:20] has an outside thread diameter of 3.5 millimeters and is therefore referred to as the 3.5 cortical screw. Its dimensions are shown in the slide. To accommodate the call, a 2.5 millimeter pilot hole has to be drilled. If it is to be used for the glider technique, the glider

[10:40] The threading hole is made with a 3.5mm drill bit. The thread is cut using a 3.5mm cortical tap. They are available in stainless steel or titanium. As indicated here, to use a screw for the lag technique, three conditions need to be fulfilled.

[11:00] 1. The screw must not purchase in the near fragment and so a gliding hole must be drilled in the near cortex. 2. The thread of the screw must purchase in the far fragment. 3. There must be a screw head to arrest screw progression. The screw can be a fully threaded cortical screw.

[11:20] or a partially threaded cortical shaft screw. In the standard lag technique, the gliding hole is drilled first from outside in, using a drill equal to the outside thread diameter of the screw. In certain circumstances,

[11:40] In order to position the gliding hole optimally in the fracture plane, the gliding hole may need to be drilled from inside-outwards, using a drill equal to the outside thread diameter of the screw. This requires very careful rotation of the fragment to reach its endostial surface.

[12:00] biology of the fragment should not in any way be compromised by this maneuver. Very rarely, if there is a narrow spike at the tip of the far fragment, which would be difficult to drill optimally through a sleeve in the gliding hole after reduction, a technique can be used whereby the pilot hole in the

[12:20] far fragment is drilled first from inside out. The post of a specially curved drill guide is then engaged in the pilot hole and the fracture reduced. With gentle traction on the curved drill guide, the 4.5 drill sleeve is introduced through the sleeve of the curved guard.

[12:40] and the gliding hole drilled through the 4.5 sleeve coaxially with the pilot hole. This requires great skill and experience but is rarely necessary. Following the drilling of the gliding hole, diagram 1, anatomical reduction of the fracture, and then drilling the

[13:00] pilot hole, diagram 2, the near cortex is countersunk, diagram 3. The use of the depth gauge is critical for oblique screw tracks. It is tempting to hook the guide on the acute angle of the far hole, but as illustrated here, this will give a screw length measurement.

[13:20] that will be too short. It is the obtuse angle that must be engaged. If this proves impossible, then hook onto the acute angled side of the screw hole and then add 2 or 4 millimeters depending on the obliquity. Following depth measurement the correct length of screw is chosen.

[13:40] The final step is to tap the far cortex. This is a precision maneuver and the use of the tap requires great skill and surgical discipline. The tap must never be used in a power tool. The tap should only ever be used through a tap sleeve. Not only does this protect the soft tissues,

[14:00] it controls the alignment of the tap. The tap can cut a thread as it is withdrawn as well as when inserted. Therefore, if the tap is slightly angled as it is withdrawn, it will cut a second female helix and damage the thread that it cut on insertion. This can compromise the screw holes.

[14:20] Such a double helix is avoided by controlling the axis of the tap carefully, using the tap sleeve as the tap is gently unscrewed from the pilot hole. If a screw is inserted by the lag technique perpendicular to the long end

[14:40] axis of the bone, it produces maximum resistance to any shear forces generated by axial loading. On the other hand, if the screw is inserted perpendicular to the fracture plane, it produces maximum inter-fragmentary compression. These competing objectives can be managed in one of two ways.

[15:00] Firstly, two screws can be inserted, one perpendicular to the bone's long axis and the other perpendicular to the fracture plane. The other option is to produce maximal desirable inter-fragmentary compression with one or more screws perpendicular to the fracture plane and then achieve resistance to shear

[15:20] by using a plate to protect the primary screw fixation, the so-called protection or neutralisation function of the plate. It is clearly important that the holes in the near and far cortices be coaxial. If the holes are not coaxial, as the screw engages both fragments, the reduction

[15:40] of the fragments will be affected. For similar reasons, an inter-fragmentary screw should pass perpendicular to the fracture plane, with the holes in the centre of each fragment in a transverse section. So far we have concentrated largely on cortical screws, which are designed to gain thread purchase in hard cortical bone.

[16:00] Cancellous screws are designed to be used where the screw thread must gain a hold in cancellous bone. The cancellous screws have a deeper thread and a coarser pitch than their cortical counterparts. In addition, the tip is of a different design for reasons to be explained.

[16:20] of the helix increases from a point at the tip to the full thread diameter over two complete turns. Those cancella screws that are not fully threaded have a smooth shaft. The head design is similar to that of the equivalent cortical screw. Cancella screws create

[16:40] their own thread not by removing bone but by impacting it aside rather as a snow plow creates a path through the snow. The tip design achieves this. The standard large cancella screw has a 3.1 millimeter core but the unthreaded shaft has a diameter of 4.5 millimeter.

[17:00] The outside thread or nominal diameter is 6.5mm. Therefore it is referred to as a 6.5-cancellar screw. The drill bit used for the pilot hole is 3.2mm in diameter. The small cancela screw has a 1.9mm core.

[17:20] so a 2mm drill bit is used for the pilot hole. The unthreaded shaft is 2.4mm in diameter. The outside thread diameter is 3.5mm, hence the 3.5 cancella screw. This replaced the former 4mm cancella screw.

[17:40] Cancellous screws may be fully threaded for use in attaching plates to intact metafasial areas or partially threaded for using as lag screws through a fracture plane. The 6.5 cancellous partially threaded screws have either a 16mm or a 32mm threaded length.

[18:00] The small cancellous screws also have a fully threaded version or a partial thread, the length of which is proportional to the overall screw length. Let us take a distal femoral unicondylar fracture, such as this 3-3B1 fracture, as a model to demonstrate points of technique of inserting

[18:20] one or more 6.5 cancellous lag screws. The fracture is first reduced and held provisionally with one or more Kirchner wires. A 3.2mm pilot hole is then drilled across the fracture plane and its depth measured. A 6.5 cancellous tap is used under the pressure of the pressure.

[18:40] only to open the thin metafasyl cortex and the immediately underlying cancellous bone. The screw of chosen overall length and thread length is then inserted. If the cortex is anything other than strong, then a washer is used. If in doubt, use a washer. As the screw is driven home,

[19:00] the tip creates the thread in the bone by impacting it aside. There is no need to drill the cortex to 4.5 millimeters to accommodate the shaft. This will enter the bone with ease once the 6.5 millimeter thread has been created. As the screw is tightened the fracture plane is compressed.

[19:20] Canulated cancellous screws are available and can be used, a guide wire being inserted instead of one of the Kirchner wires at the stage of temporary stabilization after reduction. In conclusion, this guide has hopefully made you comfortable with the essential attributes of the common

[19:40] used surgical screws and the techniques for their insertion. This will equip you with a basic understanding of the use of one of the most fundamental surgical tools.