Dynamic hip screw or Sliding Screw Fixation is an implant assembly consisting a lag screw, a sideplate and cortical screws which fix the side plate to the proximal femoral shaft.
Dynamic hip screw is used in fixation of proximal hip fractures mainly intertrochanteric fractures but can also be used in selected cases of fractures of femoral neck and subtrochanteric fractures.
The implant is most commonly used in adults. Pediatric dynamic hip screw is also available for use in selected cases of pediatric hip fractures.
Biomechanics of Dynamic Hip Screw
To understand the principle of fixation behind dynamic hip screw, we need to go to their history of development.
These angled blade plates had a fixed angle to match the neck shaft angle. One part of these plates was inserted into lateral cortex of proximal femur and pass along the neck to take final purchase in the head.
The distal part was fixed to the femoral shaft by screws.
Apart from other issues like failure of purchase and frequent need for osteotomies to accommodate the plate, a major concern was that these plates did not allow any compression across the fracture site as the rigid unibody assembly did not allow any movement.
It led to stress failures of the implant and frequent non unions as no compression was allowed after surgery was done.
Dynamic hip screw or sliding hip screw is a unique assembly. The screw can slide in the barrel of the plate. Therefore when the person bears weight, the screw slides and along it comes the proximal fragment compressing on to the distal fragment.
Thus idea behind the dynamic compression is that the femoral head component is allowed to move and fracture fragments come together for better healing.
The side plate via its barrel provides strong support to the sliding screw and allows it to collapse in a controlled manner.
Indications for Dynamic Hip Screw Fixation
- Intertrochanteric Fractures
- Basicervical fracture of femoral neck
- Subtrochanteric fractures [selected cases]
Parts of Dynamic Hip Screw
Lag screw is a special screw after which the DHS gets its name of dynamic hip screw or sliding hip screw.
The screws are available in various lengths of 50 to 145 mm to match the neck length across the population. It has a thread diameter of 12.5 mm and thread length of 22 mm.
Shaft is slotted to fit the barrel and has a length of 8.0 mm. The shaft and barrel slots fit each other so that the screw is able to slide inside the barrel but cannot rotate.
On the distal aspect of shaft of the lag screw, inner side is threaded. This is for placing compression screw after the screw and plate assembly has been fixed.
The purpose of the compression screw is to pull the lag screw towards barrel as the compression screw is tightened.
Short barrel dynamic hip screw side plate is preferred when when screw < 80 mm . In such cases, the screw prevents disengagement of assembly as well
The lag screw length to be used is measured with gauge. This measurement allows for 5 mm of compression. If more compression is desired, use a shorter screw. A screw 5 mm shorter permits 10 mm of compression.
Side plates are available with a plate barrel angle 130°–150°.
The barrel plate angle matches the neck shaft angle. Therefore variable angles are designed tp match the angle in different persons.
However, 135 degrees and 150 degrees are most commonly used sizes. 135 degrees is most commonly used.
The plates are available in sizes varying from two holes to 16 hole.
The plate is 5.8 mm thick and 19mm wide. Holes are spaced at 16mm.
Barrel lengths are 25–38 mm.
Technique of Dynamic Hip Screw
For this surgery, a C-arm image intesnsifier is required to check for guide wire and screw positioning.
Usually the surgery is performed for intertrochanteric fractures. Most of them could be treated closed.
The majority of intertrochanteric hip fracture can be reduced closed on a fracture table.
Occasionally, however, an open reduction may be necessary to achieve adequate fracture alignment.
The size of hip screw measured preoperatively on the x-ray to get an idea about probable size to be used.
A urinary catheter is inserted before surgery.
There should be enough padding into groin and genitals need to be protected.
Ipsilateral arm is taped over the chest.
After preparation of the part, the proximal femur is exposed through an incision extending from the greater trochanter for approximately 8-10 cm distally.
The angle between the wire and the femoral shaft must equal the angle of the proposed fixation device (usually 135°). The tip of the guide wire must lie in the center of the femoral head and 1 cm from the subchondral line on both the AP and lateral views.
Tip Apex Distance
A helpful concept regarding this center-center position is Baumgaertner’s tip apex distance, the sum of the distances from the tip of the lag screw to the apex of the femoral head, measured on both the anteropsterior and lateral radiographs
This is the tip apex distance. The TAD must be less than 2.5 cm for a minimal screw cutout.
After the guide wire is confirmed to be in place, the cannulated reamer [also called triple reamer] is used to drill over the already placed guide wire till the tip of the wire.
The reamer is set the to the correct depth as measured on table by direct measuring device.
The lag screw is inserted into the femoral head after tapping of the drilled channel.
The side plate and barrel are placed over the screw and attached to the femoral shaft with the appropriate screws. Fluoroscopic images are taken throughout the repair to ensure the maintenance of the reduced fracture position and the proper positioning of the fixation device.
Depending on the bone strength, two hole to six hole plate is used in intertrochanteric fractures. Longer plates are required in case of subtrochanteric fractures.
It is desirable to obtain compression at fracture site. For this, traction on the affected limb is released and compression screw is inserted.
Wound is closed in layers.
Medoff’s Biaxial Compression Plate Fixation
Medoff modified the standard compression hip screw, recognizing the need for axial compression until stability is obtained, especially in intertrochanteric fractures with a significant subtrochanteric component. His modification includes the traditional proximal screw in a barrel, a two-part sliding plate, and a distally positioned compression screw to compress the proximal femoral fracture fragment to the shaft in the subtrochanteric region. The plate that is applied to the lateral part of the femur is in two parts, with the barrel and proximal part sliding into a slot on the distal part that is fixed to the femur with oblique screws.
It usually is recommended for unstable intertrochanteric fractures and subtrochanteric fractures.
The Medoff plate comes in two versions: six-hole and four-hole plates. The six-hole plate has an axial compression screw at the end of the plate and a proximal blocking screw to prevent barrel back-out in highly comminuted fractures and to provide for more axial and anatomical compression.
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