Trochanteric Lag Screw Undercuts for Migration and Cutout Resistance
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Solution Overview
Problem
Traditional trochanteric nailing systems suffer from complications such as lag screw migration and cutout due to cranial migration, which can lead to varus collapse of the neck-shaft angle, necessitating improvements in fixation devices.
Innovation Solution
A trochanteric nailing system featuring a lag screw with helical threads having undercuts and variable width, along with helical flutes, to enhance bone engagement and resistance to lateral forces, combined with a set screw groove for increased fixation and resistance to migration.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If a single large diameter lag screw is used for trochanteric fixation, then the device is simple and easy to implant, but the screw is prone to migration and cutout during patient activity
Solution Approach 1:
The thread is divided into multiple courses with varying pitches, where each course engages bone at different depths. This segmentation allows the screw to distribute fixation forces along its length rather than relying on a single thread engagement point, thereby preventing migration while maintaining ease of implantation.
Solution Approach 2:
Different portions of the thread have different local properties - the first course has a finer pitch for initial engagement, while subsequent courses have progressively coarser pitches for deeper bone compression. This local variation optimizes both the ease of insertion and the resistance to migration at each depth level.
2Manufacturing precision
If the thread pitch is made fine for precise bone engagement, then fixation accuracy is improved, but the screw requires more torque to insert and may cross-thread
Solution Approach 1:
The thread pitch is made dynamic rather than static - it varies along the length of the screw from finer at the tip to coarser toward the head. This dynamic pitch configuration allows the screw to easily insert at the tip with fine pitch for precision, then progressively engage deeper bone with coarser pitch, avoiding cross-threading while maintaining precision.
Solution Approach 2:
The pitch parameter of the thread is changed progressively along its length. The first course has a fine pitch for precise initial engagement, while subsequent courses have increasingly coarser pitches. This parameter change allows the screw to achieve precise bone engagement without requiring excessive insertion torque that would cause cross-threading.
3Strength
If the lag screw is made longer to engage more bone, then fixation strength is improved, but the risk of varus collapse increases
Solution Approach 1:
The screw is segmented into multiple courses that engage bone at different depths and orientations. Each course contributes to fixation strength independently, distributing the load along the screw length rather than concentrating stress at a single engagement point, thereby preventing varus collapse while maintaining strength.
Solution Approach 2:
Instead of increasing fixation strength by extending the screw length in one dimension, the invention engages bone in multiple dimensions through multiple thread courses at different radial and longitudinal positions. This multi-dimensional engagement provides equivalent or superior fixation strength without increasing the risk of varus collapse.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The system effectively resists lag screw migration and cutout, distributing stress more evenly across the bone, reducing the risk of complications and promoting healing by maintaining fixation during patient activity.
Implementation Method 1
The thread has a threadform into which at least one undercut has been formed. The presence of the undercut enables the lag screw to retain, grab or otherwise engage bone matter within or adjacent the undercut.
Implementation Method 2
the width of the thread increasing from the tip toward the head of the shaft. This configuration results in increased compression of the bone matter as the lag screw is inserted.
Data Source
AI summary
A trochanteric nailing system makes use of a lag screw operatively connectable to a trochanteric nail. The lag screw has a helical thread disposed over a suitable surgical length on the outer surface of the shaft of the lag screw. The helical thread includes undercuts formed therein to improve resistance to cutout or lag screw migration. The lag screw may also include a variable-width thread to improve fixation and resistance to migration or cut out. Helical flutes reduce torque forces exerted on bone during insertion, while maintaining the improved fixation of other features about the entire circumference of the lag screw. The lag screw may also be equipped with tapered set screw grooves which engaging opposing portions of a set screw of the trochanteric nail when received therein to resist migration or cutout.