DHS and dynamic condylar screw design, indications and role in femoral fracture fixation
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10 questions
Q1What is a DHS and when is it used?▸
Sliding screw plate device for fixation of stable trochanteric fractures or fracture neck of femur with a high Pauwels angle
Material: 316L stainless steel
Q2Describe the components and key dimensions of a DHS.▸
Lag screw 12.5, 8; long barrel 38mm, short barrel 25mm, thread 22mm
Minimum sliding 10mm; recommended sliding 25mm (38+22+10=70, so 85mm is the cut-off; Gundel et al. Injury 1995)
Plate has a staggered array of holes to avoid stress concentration and bone splitting
Q3Why is the DHS plate screw angle 135 degrees?▸
130 degrees causes too much bending
150 degrees is difficult to insert and risks screw cut out
Q4What are the contraindications to a DHS?▸
No posteromedial support
Reverse oblique fracture pattern
No lateral buttress
Subtrochanteric extension
Q5What factors affect sliding of the DHS?▸
Plate barrel angle: decreasing the angle makes it less collinear with the joint reaction force, increasing friction at the screw-barrel junction and the bending moment
Screw length: a class 1 lever, so more screw protruding from the barrel generates more friction at the barrel, increasing bending and fatigue failure
Q6What are the 4 modes of failure of a DHS?▸
Cutting out of the compression screw from the femoral head
Pulling off of the side plate from the femoral shaft
Dissociation of the sliding compression hip screw from the barrel
Failure of the hip screw itself
Q7What are the advantages of a cephalomedullary nail and the registry evidence?▸
Biological: minimally invasive, does not disrupt the haematoma or rely on reaming graft
Mechanical: load sharing, more efficient load transfer, prevents uncontrolled collapse, intramedullary buttress for reverse oblique fractures
Norwegian hip registry (Injury 2013, 2000+ patients): DHS increase reop rate; cephalomedullary nail gives better pain, satisfaction, QoL and mobility
Long vs short nail: similar outcome but short nail has a higher peri-implant fracture rate
Q8What are the features of the dynamic condylar screw?▸
95 degree plate barrel angle
2 proximal round plate holes for independent screws to the calcar (6.5mm cancellous screw)
Q9What is the principle behind the dynamic condylar screw?▸
Tension band plating
Prerequisite: intact or well reduced calcar acting as a medial buttress
Need to lag the fracture through the plate for absolute stability
Q10Why was the dynamic condylar screw abandoned?▸
High rate of failure
Difficult to reduce the calcar and lesser trochanter from a lateral approach
Non locking, no angular stability
Calcar malreduction and loss of medial buttress: compression side of the tension band not restored, extra bending moment causing fatigue failure