Introduction
Interfragmentary screws, also known as lag screws, are specialized screws used in the fixation of bone fractures. As the name suggests, they work by causing compression between bone pieces at the fracture site. This compression brings the shattered ends together, giving rigidity and high chances for healing.
Interfragmentary screws get their name from the lag effect they create. The word "lag" refers to the delay between the screw head advancing and the far bone fragment moving. The close and far fragments are drawn together by the axial compression created by this brief separation. As the screw slides through the near fragment, its threads grab hold of the far fragment.
The lag effect of these screws allows for compression and stability over oblique, spiral, and some transverse fracture patterns. Interfragmentary screws promote healing and anatomical alignment, and bone integrity restoration by fully contacting and retaining the ends of the bones.
While other fixation techniques like bone plates, intramedullary nails, tension bands, or external fixators may be utilized, interfragmentary screws are often a key component of the overall construct. Their compression promotes a bony union and adds stability. Their function is particularly important in fracture patterns that are inherently unstable and need compression across the break to promote the best possible healing.
What Are the Types of Fractures That Require Interfragmentary Screws?
Fractures with straight or slightly angled lines can heal with just stabilization, as the bone ends meet neatly when set. However, spiral and long oblique fractures over 30 degrees have an inherent instability. The axial and rotational forces cause the jagged ends to shift and overlap. Muscle pull adds distraction. This prevents direct contact with the bone ends needed for optimal healing.
Compression across the fracture is required in certain comminuted fractures, as well as in unstable spiral and oblique fractures. The dislocated, overlapping ends are forced to reunite by compression. It opposes the forces of rotation, axial force, and muscle that are attempting to break the pieces apart. The compression is maintained by rigid fixing. Because of its resilience, the fracture can heal and recover its original anatomy.
Special screws called interfragmentary or lag screws provide this key compression. Lag compression results from the threads engaging the far fragment and the shaft gliding through the close fragment. Therefore, interfragmentary screw compression is necessary for the successful union of unstable spiral, oblique, and comminuted patterns, whereas uncomplicated fractures merely require stabilization.
What Are the Functions of an Interfragmentary Screw?
Interfragmentary screws work by threaded engagement in the far bone fragment, with a gliding mechanism in the near fragment. Compression is created as the screw advances and draws the pieces together. Their operation relies heavily on this lag effect. To create a rigid construct, axial force and rotational control are also offered.
To uniformly distribute compressive stresses, installation must be done perpendicular to the fracture line. Concentrated stress could cause distraction at the opposite cortex or loss of reduction.
What Are the Types of Interfragmentary Screws?
Lag screws are mostly made in two designs:
1. Fully Threaded Screws:
The threads on these screws extend the full length of the screw. To use them for compression, the near bone cortex is drilled larger than the screw. This lets the threads glide through the near fragment without catching.
The far cortex has a regular-sized hole, so the threads grip and pull that end toward the screw head. This creates compression across the fracture.
2. Partially Threaded Screws:
These screws have a smooth, unthreaded center portion. The ends of the screws are threaded.
The screw can pass through the nearby fragment thanks to the smooth section. When tightened, the threads at the far end grip and crush the fracture. No special drilling is needed with partial threads. But they can only be used for lagging, not regular screw fixation.
What Is the Surgical Technique Involved in the Placement of Interfragmentary Screw?
When inserting interfragmentary screws, careful surgical technique is essential to produce sufficient compression across the fracture. The actions consist of:
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Reduce the fracture and use K-wires or clamps to create a temporary repair. Using fluoroscopy, confirm anatomic alignment.
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Determine the appropriate screw vector perpendicular to the fracture. Drill through the near cortex with a drill bit equal to the outer screw diameter. This is the glide hole.
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Make a big hole in the nearby bone cortex first. This is where the screw's "glide hole" is located.
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After that, insert a drill guide into the bone. In the distant cortex, create a "threaded hole" with a smaller drill bit. The screw threads should fit this hole securely.
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Determine the depth required for the screw to grasp the distant end of the bone firmly. Select a screw length that can hold a minimum of four cortices in total.
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Insert the lag screw through the near cortex glide hole. It will slide freely and then grab the far cortex, compressing the fracture.
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Examine the imaging's compression. If more stability is required, add plates or more implants.
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Drilling with caution avoids damaging the cortex of the bone. Appropriate screw placement and fracture alignment are aided by imaging and guides.
What Are the Clinical Applications of Interfragmentary Screws?
Interfragmentary screws can be used for a variety of purposes in fracture treatments, such as:
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Spiral and oblique diaphyseal fractures of long bones.
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Metaphyseal fractures with instability.
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Selected multifragmentary fractures.
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Osteotomy fixation.
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Small articular fractures with subchondral bone support.
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As part of plate constructs for added stability.
Articular fractures with a significant attached subchondral bone component can be fixed with lag screws. During installation, care must be taken to prevent cartilage damage or joint penetration. Screws with full threading enable adjusting as needed.
Conclusion
The use of interfragmentary screws is crucial when repairing fractures that tend to separate. In order to promote better healing, they compress the fragments of shattered bone together. The screws work by pulling the far bone end toward the near end. With good surgical skills and using other implants, too, these special screws can help patients regain use of the injured bone. More research is still needed on the best ways to put the screws in and new screw designs. In general, by compressing the ends, interfragmentary screws are very helpful in correctly repairing unstable fractures. When applied appropriately, they can enhance recovery and results. More study is needed to keep improving how they are used.
