+967-774203774 [email protected]
Partner Portal Staff Login
العربية
Part of the Master Guide

Intramedullary Nails and External Fixators: Advanced Biomechanics, Design Principles, and Clinical Performance

Master All Types of Screws in Orthopedics: FRCS Exam Guide

23 Apr 2026 96 min read 178 Views
Types of Screws

Key Takeaway

Your ultimate guide to Master All Types of Screws in Orthopedics: FRCS Exam Guide starts here. Common types of screws in orthopedics include cortical, cancellous, fully/partially threaded, locking, non-locking, cannulated, and Poller screws. Design features differentiate these types of screws, focusing on thread pitch, tip design, and core to outer diameter ratio. Cortical screws are ideal for dense bone, while cancellous screws suit less dense bone. Locking and cannulated screws offer specific stability and guidance functions.

Comprehensive Exam


00:00

Start Quiz

Question 1

A 55-year-old male presents with a displaced intra-articular calcaneal fracture. You plan for open reduction and internal fixation. Which screw characteristic is most crucial for achieving interfragmentary compression across the fracture fragments?





Explanation

Interfragmentary compression, the hallmark of lag screw technique, is achieved when the screw threads purchase only in the far fragment, while the near fragment is allowed to 'lag' or slide along the smooth, unthreaded portion of the screw shaft. This requires a partially threaded screw and a gliding hole in the near cortex, which must be larger than the major (thread) diameter of the screw, allowing the screw head to draw the near fragment towards the far fragment as it tightens. A fully threaded screw, without a gliding hole, would fix both fragments equally and not generate compression.

Question 2

In the context of fracture fixation, what is the primary biomechanical advantage of using a locking screw in a locking plate system compared to a non-locking cortical screw?





Explanation

Locking screws thread into the plate, creating a fixed-angle construct. This effectively turns the screw-plate interface into a 'fixed-angle internal fixator', where the strength of the construct is not dependent on screw purchase into the near cortex, but rather on the angular stability created by the locked threads. This provides significantly improved pullout strength, especially beneficial in osteoporotic bone where traditional screw purchase is compromised. While reduced need for precise contouring is an advantage, the primary biomechanical benefit is the enhanced stability and pullout resistance due to the fixed-angle construct, rather than direct interfragmentary compression (which locking screws often limit).

Question 3

A surgeon is fixing a comminuted humeral shaft fracture with a neutralization plate. What is the primary role of the screws when used in a neutralization plate construct?





Explanation

A neutralization plate is used to protect a primary lag screw or screws providing interfragmentary compression. Its main function is to shield the fracture from bending, torsional, and shear forces, allowing the lag screws to maintain compression without being subjected to disruptive stresses. The plate itself bears the majority of the external loads applied to the bone, thus 'neutralizing' these forces from reaching the fracture site. The screws fix the plate to the bone, but their primary biomechanical role in this construct is to hold the plate securely to provide the shielding effect, not to generate compression or share axial load equally in the immediate fracture zone.

Question 4

You are performing an arthrodesis of the subtalar joint. Which type of screw is typically preferred for maximizing compression across the joint surfaces and why?





Explanation

For arthrodesis, maximizing compression across the fusion surfaces is paramount to promote fusion. Fully threaded cortical screws, when used in a lag fashion (requiring overdrilling the near cortex for a gliding hole and appropriate screw length), can generate significant compression. While partially threaded cancellous screws are excellent lag screws, fully threaded screws (appropriately used in a lag technique) offer consistent purchase along their entire length in the cortical bone of the calcaneus and talus, which can be advantageous for robust compression and stability in an arthrodesis setting. The key is applying lag technique, which can be done with either partially or fully threaded screws (with proper overdrilling). However, cortical screws offer better purchase in the cortical bone of the tarsals. Headless screws are good for compression but their primary advantage is being buried; cancellous screws are good but the question specifies 'maximizing compression' across joint surfaces, and cortical bone is prevalent here. The most reliable method for sustained robust compression across an arthrodesis, especially in dense tarsal bone, often involves fully threaded cortical screws used as lag screws.

Question 5

Regarding screw design, what distinguishes a cancellous screw from a cortical screw in terms of thread characteristics and typical application?





Explanation

Cancellous screws are designed for optimal purchase in soft, cancellous bone. They typically have a larger core diameter relative to their outer thread diameter, and a coarser thread pitch, meaning fewer threads per unit length. This design maximizes the bone-screw interface in porous bone. Cortical screws are designed for dense cortical bone, featuring a smaller core diameter relative to thread diameter, and a finer thread pitch (more threads per unit length). This allows them to cut effectively into hard bone and provide strong purchase. Therefore, option D correctly describes cortical screws, and option B describes cancellous screws. The key distinction is finer pitch for cortical and coarser pitch for cancellous.

Question 6

A surgeon is performing an anterior cruciate ligament (ACL) reconstruction using a soft tissue graft. Which type of screw is most commonly used for femoral or tibial fixation of the graft and why?





Explanation

Bioabsorbable interference screws are the most common choice for ACL graft fixation (both femoral and tibial tunnels) when using a soft tissue graft. They provide excellent interference fit and rigid primary fixation, compressing the graft against the tunnel wall. The advantage of bioabsorbability is that it avoids a permanent implant, which can be beneficial in case of revision surgery or future imaging. While other screw types could theoretically be used, interference screws are specifically designed for this application to achieve strong primary fixation and are often made from bioabsorbable materials like PLLA, PLDLA, or TCP composites.

Question 7

What is the primary function of a position screw in fracture fixation?





Explanation

A position screw is used to hold two bone fragments together in a fixed position without actively compressing them. Unlike a lag screw, which specifically generates interfragmentary compression by threading only the far cortex (or having a gliding hole in the near cortex), a position screw threads into both the near and far cortices, or into both fragments, thus holding them at a fixed distance from each other. This is often used in syndesmotic fixation (e.g., tibiofibular syndesmosis) where excessive compression could lead to cartilage damage or loss of motion.

Question 8

Which of the following scenarios is most appropriate for the use of a fully threaded cortical screw in a lag fashion?





Explanation

A fully threaded cortical screw can be used in a lag fashion to achieve interfragmentary compression. To do this, a gliding hole (larger than the major diameter of the screw) must be drilled in the near cortex, and a smaller thread hole (matching the core diameter) drilled and tapped in the far cortex. This technique is commonly used for oblique or spiral diaphyseal fractures where strong compression is desired. For example, a spiral tibial fracture often benefits from lag screw fixation across the fracture line. Syndesmotic injuries typically use position screws, locking plates are often used for comminuted fractures, and specific headless or cannulated screws are preferred for malleolar or tendon anchor fixation.

Question 9

What is the consequence of 'overtapping' a screw hole in cortical bone?





Explanation

Overtapping refers to cutting threads that are too deep or too wide for the chosen screw. This effectively reduces the amount of bone in contact with the screw threads, diminishing the screw's purchase in the bone. The result is a weaker screw-bone interface, leading to reduced pullout strength and an increased risk of screw loosening. Proper tapping ensures an optimal fit between the screw threads and the bone, maximizing stability.

Question 10

A surgeon is considering the use of headless compression screws for a scaphoid fracture fixation. What is the primary advantage of a headless design in this application?





Explanation

Headless compression screws are particularly advantageous in articular and periarticular fractures (like the scaphoid) because their design allows them to be completely buried beneath the cartilage or cortical surface. This eliminates prominence of the screw head, preventing soft tissue irritation, damage to articular cartilage, and making them suitable for intra-articular placement. They also provide compression across the fracture line due to differential pitch (distal threads have a coarser pitch than proximal threads, pulling the fragments together).

Question 11

When applying a dynamic compression plate (DCP), the 'load' or 'eccentric' hole is designed to achieve what specific biomechanical effect?





Explanation

The dynamic compression plate (DCP) utilizes a specific elliptical screw hole design. When a screw is inserted eccentrically (at one end of the ellipse) and tightened, the spherical undersurface of the screw head slides along the inclined plane of the hole. This causes the bone fragment attached to that segment of the plate to translate towards the fracture site, generating interfragmentary compression across the fracture. This mechanism converts the tightening of the screw into axial compression at the fracture.

Question 12

A patient presents with a severe osteoporotic distal radius fracture requiring plate fixation. Which type of screw-plate construct would offer the most stable fixation against pullout forces?





Explanation

In osteoporotic bone, the bone quality is compromised, significantly reducing the pullout strength of traditional screws. Locking plates with locking screws create a fixed-angle construct, essentially acting as an internal fixator. The screws lock into the plate, and the strength of the construct comes from the rigidity of this screw-plate interface, rather than solely relying on the purchase of the screw threads in the often poor-quality bone. This provides superior stability and pullout resistance in osteoporotic bone compared to non-locking constructs.

Question 13

What is the primary function of the flutes on the tip of a self-tapping screw?





Explanation

Self-tapping screws have a fluted tip, which acts like a tap, cutting threads into the bone as the screw is advanced. This eliminates the need for a separate tapping step, streamlining the surgical procedure. The flutes also help in clearing bone debris, preventing bone compaction at the screw-bone interface, which could otherwise impede proper thread engagement.

Question 14

When utilizing a screw in a 'buttress' fashion, what is its main biomechanical role?





Explanation

A buttress plate (or screw acting as a buttress) is typically placed on the tension side of a fracture or used to prevent collapse of metaphyseal fragments under axial load. Its primary role is to resist shear or compressive forces that would otherwise cause a fragment to displace or collapse. It 'buttresses' the fragment, preventing it from migrating. This is distinct from providing interfragmentary compression or acting as a tension band.

Question 15

A 30-year-old active patient sustains a midshaft clavicle fracture with significant shortening. You decide to fix it with a plate and screws. What is the appropriate drill bit size for the thread hole for a 3.5 mm cortical screw?





Explanation

For a standard 3.5 mm cortical screw, the thread hole (pilot hole for the screw threads) requires a 2.5 mm drill bit. This matches the core diameter of the 3.5 mm cortical screw. If a lag screw technique is employed using a 3.5 mm cortical screw, the gliding hole in the near cortex would be 3.5 mm, while the far cortex thread hole remains 2.5 mm. The options are 0-indexed, so 2.5mm is at index 1.

Question 16

What is the primary concern when implanting screws into cortical bone at high rotational speeds?





Explanation

High rotational speeds during screw insertion, especially without adequate cooling, can generate significant heat. This heat can cause thermal necrosis (death) of the surrounding bone tissue, compromising the screw-bone interface, leading to aseptic loosening, and potentially impeding bone healing. While stripping threads or breakage are possible with excessive torque, thermal necrosis is a specific concern related to high speed and lack of cooling during drilling and screwing.

Question 17

Which factor has the LEAST impact on the pullout strength of a bone screw?





Explanation

All factors listed influence pullout strength. Screw thread pitch, core diameter, and major diameter directly determine the amount of bone engaged by the threads and the screw's resistance to stripping. Bone mineral density directly relates to the quality of the bone in which the screw is inserted. However, the material of the screw (stainless steel vs. titanium) primarily affects its fatigue strength, corrosion resistance, and biocompatibility, but has a relatively minor direct impact on the initial pullout strength of the bone-screw interface when compared to the geometric factors and bone quality itself, assuming sufficient strength of the screw material. The interface quality is dictated more by geometry and bone properties.

Question 18

A surgeon uses a cannulated screw system for a femoral neck fracture. What is the primary advantage of cannulation?





Explanation

Cannulated screws have a hollow central channel that allows them to be inserted over a pre-placed K-wire or guide wire. This is a significant advantage, particularly in fractures where precise screw placement is critical (e.g., femoral neck, scaphoid, malleoli). The K-wire is first inserted under fluoroscopic guidance to ensure optimal position, and then the cannulated drill and screw are advanced over it, ensuring accurate screw trajectory without repeated attempts that can compromise bone quality.

Question 19

In a tension band wiring construct for patella fracture, what is the biomechanical role of the K-wires and the figure-of-eight wire?





Explanation

In a tension band construct (e.g., for patella or olecranon fractures), the K-wires (or sometimes small screws) act as intramedullary fixation, preventing displacement and rotation of the fragments. The figure-of-eight wire, placed anteriorly on the tension side, converts the distractive (tensile) forces that would otherwise open the fracture on the tension side into compressive forces across the articular (compression) side during joint movement. This dynamic compression promotes healing.

Question 20

When fixing a lateral malleolus fracture with a standard one-third tubular plate, what is the recommended minimum number of cortices that should be engaged by screws distal and proximal to the fracture?





Explanation

For most plate fixation of fractures, especially in non-locking constructs, the general principle is to have at least two screws engaging a minimum of four cortices on each side of the fracture. For example, two bicortical screws (each engaging two cortices, near and far) distal to the fracture and two bicortical screws proximal to the fracture provides 4 cortices of purchase distal and 4 cortices proximal. Therefore, a minimum of 4 cortices (2 screws) distal and 4 cortices (2 screws) proximal should be engaged. The question asks for 'number of cortices', not 'number of screws'. So 4 cortices distal and 4 cortices proximal.

Question 21

A surgeon is evaluating screw lengths for a transcervical femoral neck fracture fixation using three cannulated screws. Which of the following is a critical principle for optimal screw placement and length?





Explanation

For femoral neck fractures, it is crucial that the screws gain purchase in the dense subchondral bone of the femoral head but do not violate the articular surface. Optimal placement involves extending the screw tips into the femoral head to within 5-10 mm of the subchondral bone, or just engaging it, but certainly not breaching it. The goal is to maximize purchase without causing articular damage or future hardware prominence. Option A states 'without protruding subchondrally' which implies 'not breaching the articular surface' which is correct for optimal purchase and avoiding joint irritation. Option B implies avoiding subchondral bone completely, which would compromise purchase. Option C explicitly states breaching the subchondral bone, which is incorrect as it implies violating the articular surface.

Question 22

What is the main advantage of titanium over stainless steel for orthopedic screws?





Explanation

Titanium (and its alloys) offers superior corrosion resistance and excellent biocompatibility compared to stainless steel. This reduces the risk of adverse tissue reactions or allergic responses. While stainless steel is stiffer (higher modulus), titanium's lower modulus of elasticity is often considered advantageous as it is closer to that of bone, potentially reducing stress shielding. Titanium also has good fatigue strength but is generally more expensive and technically more challenging to manufacture than stainless steel. Radiopacity for both is adequate, but titanium can cause less artifact on MRI.

Question 23

A 4.5 mm cortical screw has a major diameter of 4.5 mm and a core diameter of 3.2 mm. For a lag screw technique, what size drill bit is typically used for the gliding hole in the near cortex?





Explanation

To create a gliding hole in the near cortex for a lag screw, the drill bit size must be equal to or slightly larger than the major (outer) diameter of the screw. This allows the screw threads to pass freely through the near cortex without engaging, thus enabling the screw head to compress the near fragment against the far fragment where the threads engage. For a 4.5 mm cortical screw, the gliding hole should be 4.5 mm. The thread hole in the far cortex would be 3.2 mm (matching the core diameter).

Question 24

In the context of fracture fixation, what is the role of a 'pre-tapped' hole?





Explanation

Pre-tapping involves using a separate tapping instrument to cut threads into the bone after drilling the pilot hole but before inserting the screw. This is typically done in dense cortical bone where self-tapping screws might struggle or risk excessive torque, which could lead to bone necrosis or screw breakage. Pre-tapping ensures precise thread formation, reduces insertion torque, and minimizes stress on the bone, leading to better screw purchase and reducing the risk of stripping or thermal injury.

Question 25

Which of the following statements regarding bioabsorbable screws is FALSE?





Explanation

Bioabsorbable screws are designed to degrade over time, eliminating the need for removal. They are indeed commonly used in soft tissue fixation (e.g., ACL reconstruction). However, their strength is generally lower than that of metallic screws, and they lose strength over time as they degrade. They can also cause inflammatory reactions (sterile effusions) and their degradation products can alter local pH. Therefore, the statement that their strength typically exceeds metallic screws is false.

Question 26

A patient undergoes fixation of a proximal humeral fracture with a locking plate. Postoperatively, you notice a lucency around several locking screws. What is the most likely cause of this lucency in the absence of infection?





Explanation

Lucency around locking screws in a locking plate construct, in the absence of infection, is often attributed to stress shielding. Locking plates create a very rigid construct, which can shield the underlying bone from physiological loads. This lack of stress can lead to localized bone resorption around the screws, known as stress shielding osteopenia, or can be a sign of inadequate load transfer through the bone, rather than through the implant. In non-locking screws, lucency typically indicates loosening due to micromotion. However, locking screws are designed to prevent micromotion at the screw-plate interface, so lucency around locking screws suggests a different biomechanical phenomenon, often related to stress shielding.

Question 27

What is the main biomechanical difference between a variable angle locking screw and a fixed-angle locking screw?





Explanation

Variable angle locking screws allow the surgeon to insert the screw at a chosen angle (within a certain conical range) relative to the plate, and then lock it in that position. This offers greater flexibility in screw trajectory, allowing the surgeon to optimize screw placement for specific fracture patterns, avoid joint surfaces, or capture small fragments. Fixed-angle locking screws, in contrast, must be inserted at a predetermined, fixed angle to the plate. Both types, once locked, provide angular stability.

Question 28

When performing syndesmotic screw fixation following an ankle fracture, which principle is MOST important to adhere to?





Explanation

Syndesmotic screws are typically used as position screws. The goal is to reduce the syndesmosis and hold it in its anatomical position, not to compress it excessively. Excessive compression can lead to restricted ankle motion, pain, and potentially chondrolysis. Therefore, a fully threaded screw inserted through three or four cortices (depending on technique) acts as a position screw, maintaining the reduction without generating undue compression. While a fully threaded cortical screw is commonly used, the principle of it acting as a position screw is paramount.

Question 29

What surgical technique is crucial to prevent screw stripping when inserting a cortical screw into dense bone?





Explanation

Screw stripping occurs when the threads cut into the bone are damaged or fail to engage properly, often due to excessive torque during insertion or an improperly sized pilot hole. To prevent this in dense cortical bone, it is crucial to use a pilot drill bit that accurately matches the core diameter of the screw. Additionally, pre-tapping the hole with a tap of the correct size before inserting the screw significantly reduces the torque required for insertion and ensures precise thread formation, thereby preventing stripping.

Question 30

A 60-year-old female with osteoporosis sustains a displaced fracture of the proximal humerus. Which screw design would provide the best purchase in her compromised bone quality?





Explanation

In osteoporotic bone, the bone stock is poor, making traditional screw purchase unreliable. Locking screws, when used with a locking plate, do not rely on direct compression of the plate to the bone or on the screw's purchase in the near cortex. Instead, they lock into the plate, creating a fixed-angle construct that acts as an internal fixator. This angular stability significantly improves pullout strength and overall construct rigidity in poor bone quality, making it the superior choice for osteoporotic fractures.

Question 31

What is the primary purpose of 'countersinking' a screw head?





Explanation

Countersinking involves reaming a small conical depression in the bone surface around the pilot hole. This allows the screw head to sit flush with or slightly below the bone surface. The primary purpose is to reduce soft tissue irritation and prominence of the hardware, particularly in superficial locations or near joints. It does not primarily affect screw purchase or fatigue strength directly, though a well-seated screw head contributes to overall construct stability.

Question 32

Which type of screw is most appropriate for a unicortical fixation in the diaphyseal segment of a long bone?





Explanation

Unicortical fixation, while generally less stable than bicortical, is often employed with locking plates, particularly in situations where bicortical fixation is not feasible (e.g., risk to neurovascular structures on the far cortex) or not necessary due to the fixed-angle stability of the locking construct. Locking screws, by threading into the plate, provide angular stability even with unicortical purchase, as the strength comes from the screw-plate interface, not solely from the bone-screw interface. Other screws primarily rely on bicortical purchase for optimal stability in diaphyseal bone.

Question 33

What is the mechanical advantage of a screw having a larger major (outer) diameter compared to a smaller one, assuming all other factors are equal?





Explanation

A larger major (outer) diameter screw, with appropriate thread engagement, will have a greater surface area of contact between its threads and the bone. This increased bone-screw interface directly translates to greater resistance to pullout forces and potentially greater torsional strength, provided the bone quality is adequate to support the larger diameter. Ease of insertion, compression, and thermal necrosis are affected by other factors like thread design, pilot hole size, and insertion speed, not solely the major diameter.

Question 34

In the context of internal fixation, what does 'stress shielding' refer to?





Explanation

Stress shielding occurs when a rigid implant (like a plate or intramedullary nail) bears a significant portion of the physiological load that would normally be carried by the bone. This reduction in stress on the bone (in accordance with Wolff's Law) can lead to localized bone resorption and reduced bone density around the implant. While sometimes unavoidable, excessive stress shielding can weaken the bone, increasing the risk of refracture upon implant removal.

Question 35

Which screw type is most suitable for fixing a small, intra-articular osteochondral fragment where the screw head must not protrude into the joint space?





Explanation

Headless compression screws are specifically designed for applications where the screw needs to be fully buried within the bone, particularly in articular or periarticular fractures. Their lack of a prominent head prevents irritation to overlying cartilage or soft tissues. They also provide compression across the fracture due to their differential thread pitch. This makes them ideal for fixing osteochondral fragments.

Question 36

What is the primary advantage of a 'self-drilling' screw over a 'self-tapping' screw?





Explanation

Self-drilling screws combine the drilling and tapping steps into a single instrument. They have a drill bit tip that creates the pilot hole, followed by flutes that cut the threads as the screw advances. This eliminates the need for a separate drilling step, saving time and reducing the number of instruments used in surgery. Self-tapping screws still require a pilot hole to be drilled first, but then cut their own threads.

Question 37

Which type of screw is typically color-coded in orthopedic sets to indicate its compatibility with a specific plate system?





Explanation

Locking screws are almost always color-coded to match specific locking plate systems. This is critical because locking screws must be perfectly compatible with the threaded holes of their respective plates to ensure proper engagement and the creation of a fixed-angle construct. Using an incompatible locking screw would compromise the angular stability of the system. While some other screws might have color codes for diameter, locking screws and their corresponding plates are very commonly matched by color to ensure system integrity.

Question 38

A surgeon is repairing a tibial plateau fracture with a buttress plate. What is the primary role of the screws placed distally and proximally to the fracture in this construct?





Explanation

In a buttress plate construct for a tibial plateau fracture, the plate is placed to prevent the collapse or displacement of articular fragments under axial load. The screws fix the plate securely to the bone, effectively transmitting the axial loads through the plate rather than allowing the fragments to collapse. While lag screws might be used intra-articularly for primary reduction, the screws holding the buttress plate are primarily serving to anchor the plate and provide the buttressing effect, supporting the articular surface and preventing secondary collapse.

Question 39

What is the function of the differential thread pitch in some headless compression screws (e.g., Herbert screw design)?





Explanation

Headless compression screws with a differential thread pitch (e.g., the Herbert screw principle) have a coarser pitch on the distal threads and a finer pitch on the proximal threads. As the screw is advanced, the distal threads engage the far fragment and pull it, while the proximal threads engage the near fragment more slowly. This differential advancement rate effectively draws the two fragments together, generating interfragmentary compression across the fracture. This compression is maintained as the screw is fully seated.

Question 40

Which type of screw is typically preferred for metaphyseal fractures in pediatric patients where growth plate sparing is critical?





Explanation

In pediatric metaphyseal fractures requiring screw fixation near the physis, partially threaded, small-diameter cannulated screws (often K-wires or similar small pins if minimal fixation is needed) are preferred. The small diameter minimizes damage to the growth plate. The partially threaded design allows for compression across the fracture without violating the physis if the threads are placed distal to it, and cannulation aids precise placement to avoid critical structures. Larger diameter screws or fully threaded screws crossing the physis are generally avoided due to the higher risk of growth arrest or deformity.

Question 41

When using a fully threaded screw to achieve interfragmentary compression (lag screw technique), what specific step is required that is NOT needed for a partially threaded lag screw?





Explanation

For a fully threaded screw to act as a lag screw and generate interfragmentary compression, a gliding hole must be specifically drilled in the near cortex. This gliding hole must be equal to or slightly larger than the major (outer) diameter of the screw. This allows the screw threads to pass freely through the near fragment without engaging, while the threads then engage the far fragment, drawing the near fragment towards the far fragment as the screw is tightened. A partially threaded lag screw inherently has a smooth shaft for the near fragment to slide along, eliminating the need to overdrill the near cortex for a gliding hole.

Question 42

Which of the following is a potential complication specifically associated with the long-term presence of bioabsorbable screws?





Explanation

While bioabsorbable screws avoid permanent implant presence, a known complication is the potential for an inflammatory response, leading to aseptic osteolysis (bone resorption) or sterile effusion (fluid collection) as the material degrades. This reaction is usually benign and self-limiting but can sometimes require intervention. Stress shielding, prominence, fatigue fracture, and corrosion/metal ion release are typically associated with metallic implants.

Question 43

What is the significance of the 'pitch' of a screw thread in relation to its mechanical properties?





Explanation

The pitch of a screw thread is the axial distance advanced by one complete turn of the screw, or more simply, the distance between two adjacent threads. A finer pitch (more threads per unit length) provides greater purchase in dense bone but requires more turns to insert. A coarser pitch (fewer threads per unit length) offers less purchase in dense bone but greater purchase in cancellous bone and faster insertion. It directly influences both the pullout strength (more threads engaged per length equals more strength) and the insertion torque required.

Question 44

A surgeon is attempting to reduce and stabilize a severely comminuted diaphyseal fracture using a bridge plating technique. What is the primary role of the screws in this context?





Explanation

In bridge plating for comminuted diaphyseal fractures, the plate acts as an extramedullary splint. The screws are inserted bicortically into the main proximal and distal fragments, bypassing the comminuted zone. The primary role of the screws is to securely fix the plate to these main fragments, effectively 'bridging' the comminution. The goal is not anatomical reduction of the comminuted fragments or interfragmentary compression within the comminution, but rather maintenance of length, alignment, and rotation while providing a stable environment for indirect healing. The screws fix the plate to the main fragments to achieve this splinting effect.

Question 45

Which type of screw design minimizes the risk of vascular or neurological injury when used near delicate structures, particularly in unicortical fixation?





Explanation

Locking screws, particularly when used unicortically with a locking plate, minimize the risk of injury to structures on the far side of the bone. Since locking plates derive their stability from the fixed-angle screw-plate construct rather than bicortical purchase or screw compression, unicortical screw placement often provides sufficient stability, thus avoiding penetration of the far cortex and protecting adjacent neurovascular structures. Other screw types often require bicortical purchase for optimal stability, increasing the risk.

Question 46

What is the main reason a small fragment system (e.g., 3.5 mm screws) might be preferred over a large fragment system (e.g., 4.5 mm screws) for certain fractures, even in adults?





Explanation

Small fragment systems are designed for smaller bones (e.g., forearm, hand, foot) or smaller fragments of larger bones (e.g., periarticular fractures). Their smaller screw and plate sizes require less bone removal, which is critical when dealing with limited bone stock, intricate anatomy, or numerous small fragments. They allow for more screws to be placed in a confined area and reduce the stress on small bone fragments. While they can lead to smaller incisions, the primary advantage stems from matching the implant size to the bone size and fragment dimensions.

Question 47

In an anterior cervical discectomy and fusion (ACDF), what type of screws are typically used to fix the plate to the vertebral bodies?





Explanation

In ACDF, anterior cervical plates are typically secured with self-tapping locking screws. These screws thread into the plate, creating a fixed-angle construct that provides angular stability. This is crucial in the cervical spine to maintain reduction, prevent toggling, and provide a stable environment for fusion. While unicortical purchase is often sufficient due to the locking mechanism, some designs allow for bicortical fixation. The self-tapping feature facilitates insertion in the dense cortical bone of the vertebral bodies.

Question 48

What is the primary risk associated with placing a screw too close to a previous drill hole or screw track?





Explanation

Placing a new screw too close to an existing drill hole or screw track significantly compromises the bone quality and integrity in that area. The previous hole effectively creates a stress riser and reduces the amount of intact bone available for the new screw's threads to engage. This leads to diminished purchase, reduced pullout strength, and an increased risk of stripping the new screw or creating a bone defect that can contribute to non-union or refracture.

Question 49

Which of the following describes the 'neutral' position for screw placement in a Dynamic Compression Plate (DCP) hole?





Explanation

When a screw is inserted directly in the center of the elliptical hole of a DCP, it acts as a 'neutral' screw. In this position, the spherical undersurface of the screw head does not slide along the inclined plane, and thus no axial compression is generated across the fracture. This position is typically used when interfragmentary compression is already achieved by lag screws, or when the plate is used primarily for neutralization or bridging, maintaining length and alignment without adding further compression.

Question 50

A T-plate is being used for a distal tibial pilon fracture. What is the main purpose of the multiple screws directed into the epiphysis/metaphysis through the plate's 'head'?





Explanation

T-plates (or similar periarticular plates) used for pilon fractures have a broad 'head' designed with multiple screw holes to capture and stabilize small, often comminuted, articular fragments of the epiphysis/metaphysis. The primary purpose of these screws is to buttress these fragments, prevent their collapse under axial load, and maintain the reduction of the articular surface. They essentially act as 'fixed-angle' or 'buttressing' screws to reconstruct the joint block.

Question 51

What is the major mechanical disadvantage of using a fully threaded screw as a position screw in a high-stress environment, such as a long bone fracture?





Explanation

A fully threaded screw, when used as a position screw (i.e., threading both cortices/fragments equally without a gliding hole), holds fragments in a fixed position but does not generate interfragmentary compression. In many long bone fractures, interfragmentary compression is a highly desirable biomechanical principle that promotes primary bone healing by reducing motion at the fracture site. The disadvantage is not that it's inherently weaker or causes soft tissue irritation, but that it misses the opportunity to apply active compression, which is often biomechanically superior for healing.

Question 52

Which screw characteristic is most crucial for maintaining reduction of an osteotomy in cancellous bone without causing bone resorption around the threads?





Explanation

For optimal purchase and stability in cancellous bone, screws are designed with a larger outer diameter and a coarser thread pitch (fewer threads per unit length). This design maximizes the surface area of bone engaged by the threads, distributing forces over a larger area and providing superior pullout strength in soft bone, thereby minimizing the risk of bone resorption due to localized stress or micromotion around the threads. A finer pitch is for cortical bone, and a smaller core implies a larger thread height relative to core, good for cortical bone.

Question 53

A surgeon is performing an acetabular fracture fixation. What type of screw is generally considered safest and most effective for anterior column fixation where the inner table of the pelvis is the far cortex?





Explanation

In complex pelvic and acetabular fracture fixation, particularly anterior column fractures, the proximity of vital structures (e.g., obturator nerve, external iliac vessels) makes bicortical fixation risky. Locking plates with locking screws offer a significant advantage here. Their ability to provide angular stability even with unicortical purchase minimizes the risk of far cortical perforation and associated neurovascular injury, making them safer while still providing adequate stability for fixation. This approach significantly reduces iatrogenic complications.

Question 54

What is the primary disadvantage of using stainless steel screws compared to titanium screws in situations where magnetic resonance imaging (MRI) may be required post-operatively?





Explanation

Stainless steel (specifically 316L stainless steel, a commonly used orthopedic alloy) is ferromagnetic, meaning it can create significant artifacts on MRI scans. These artifacts appear as large signal voids or distortions, which can obscure critical anatomical details and hinder diagnostic interpretation of soft tissues or bone healing around the implant. Titanium, being paramagnetic or diamagnetic depending on the alloy, produces significantly less artifact on MRI, making it the preferred material if post-operative MRI is anticipated.

Question 55

When performing a syndesmotic fixation, which statement regarding screw removal is generally accepted as standard practice?





Explanation

Syndesmotic screws are typically removed once the primary fracture has healed and the syndesmosis has had time to stabilize, usually around 8-12 weeks post-operatively. This is done to prevent screw breakage (due to cyclical loading as the bone heals and moves) and to allow for normal physiological motion of the ankle mortise during full weight-bearing, which can be restricted by the rigid position screw. While removal can be considered if symptomatic earlier, routine removal is preferred prior to full, unprotected weight-bearing. Bioabsorbable screws, if used, would obviate the need for removal.

Question 56

Which of the following scenarios is LEAST likely to benefit from the use of a locking screw?





Explanation

Locking screws excel in situations where bone quality is poor, or when a fixed-angle construct is desired (e.g., comminuted fractures, bridge plating, osteoporotic bone, periarticular fractures). In a simple, transverse diaphyseal fracture, the primary goal is often to achieve strong interfragmentary compression, which is best accomplished with a lag screw. A neutralization plate with non-locking cortical screws provides adequate stability by protecting the lag screw. Using locking screws in this specific scenario wouldn't offer a significant advantage over a well-executed lag screw + neutralization plate, and in fact, locking plates generally limit interfragmentary compression compared to traditional DCPs used in compression mode.

Question 57

What is the consequence if the drill bit used for the pilot hole for a self-tapping cortical screw is significantly smaller than the screw's core diameter?





Explanation

If the pilot hole is significantly smaller than the core diameter of the screw, the screw will encounter excessive resistance during insertion. This dramatically increases the insertion torque required. Such high torque can lead to several problems: the screw itself might break, the drill bit could break during tapping, or the bone threads could strip due to the excessive force trying to cut into insufficient space. This compromises the stability of the fixation and can damage the bone. Proper pilot hole sizing, matching the core diameter, is crucial.

Question 58

A surgeon is planning to fix a distal femoral fracture with a long plate. What is the advantage of using 'far cortical locking' screws in the diaphysis for bridge plating?





Explanation

Far Cortical Locking (FCL) screws are a specialized type of locking screw that engage only the far cortex in a diaphyseal bridge plating construct. They provide flexible stability, allowing for controlled micromotion at the fracture site. Unlike traditional rigid locking screws that create a stiff, fixed-angle construct, FCL screws permit slight physiological loading and movement, which is believed to promote secondary bone healing (callus formation) in comminuted fractures by reducing stress shielding while still maintaining alignment and length. They are not designed for interfragmentary compression or superior angular stability in the same way traditional locking screws are.

Question 59

Which biomechanical principle is primarily applied when using screws in a 'tension band' construct?





Explanation

The tension band principle is designed to convert distractive (tensile) forces acting on one side of a bone into compressive forces on the opposite side (the compression side) during functional loading. This is typically achieved with a wire (often supplemented by K-wires or screws) placed on the tension side of a fracture (e.g., patella, olecranon, medial malleolus). As the joint moves or load is applied, the tension band resists the distraction, thereby compressing the fracture fragments on the opposite side, promoting healing.

Question 60

What is the term for the smooth, unthreaded portion of a partially threaded screw?





Explanation

The smooth, unthreaded portion of a partially threaded screw between the screw head and the threaded portion is referred to as the 'shaft' or 'run-out' (though run-out more technically describes the transition zone where threads fade out). In a partially threaded lag screw, this smooth shaft is critical as it allows the near bone fragment to slide along it without engaging the threads, enabling the screw head to draw the near fragment towards the far fragment and achieve interfragmentary compression.

Previous ChapterHow to Maximize Orthopedic Screw Pullout Strength Next Chapter Grasp the Biomechanics of IM: Essential Insights for Exams
Dr. Mohammed Hutaif
Medically Verified Content by
Prof. Dr. Mohammed Hutaif
Consultant Orthopedic & Spine Surgeon
Chapter Index
    Back to Master Guide