1 Introduction topenlarge
In short oblique fractures, only a single lag screw can be inserted. To guarantee for sufficient stability, a protection plate must be used.
Independent lag screw
As the screw should be inserted perpendicularly to the fracture plane, in most cases it has to be inserted independently of the plate.
Lag screw through the plate
In selected cases, when the fracture geometry permits, the lag screw may be inserted through the plate.
AO teaching video: Short oblique fracture of metacarpal III treated with a 2.0 mm lag screw and 2.0 adaption plate
2 Approaches top
For this procedure the following approaches may be used:
3 Open reduction topenlarge
Reduction by traction
Traction is exerted by an assistant, while the surgeon reduces the fracture, using pressure from a periosteal elevator, or a dental pick.
Secure the reduction with pointed reduction forceps and confirm using image intensification. Make sure that the reduction forceps does not interfere with planned screw position.
It is essential to confirm that the apex of each fracture fragment has been properly reduced.
Check rotational alignment
Turn the hand over and flex the fingers passively to check for correct rotational alignment.
The image on the right shows rotational malalignment of the middle finger (“scissoring”).
4 Preparations for fixation topenlarge
Plan correct screw placement
Lag screws should be inserted perpendicularly to the fracture plane. Tightening a screw that is not perpendicular to the fracture plane risks fracture displacement.
Pitfall: Beware of fissure lines
Often there are short fissure lines that are not apparent on the x-rays. Check for these under direct vision and make sure that the screws are not placed in these fissure lines.
Pitfall: Screw too close to fracture
Do not insert the screw too close to the fragment apex. A minimal distance between the apex and the screw head, equal to the screw head diameter, must be observed.
5 Drilling topenlarge
Drilling for lag screws
There are two options to prepare the gliding hole and the thread hole:
1) Gliding hole first
Drill the gliding hole in the near cortex. Check and maintain perfect fracture reduction and then insert a drill guide. Drill the thread hole in the far (trans) cortex through the drill guide.
This method ensures that the thread hole is perfectly in line (coaxial) with the gliding hole.
This is the preferred method.
2) Thread hole first
Drill a hole through both cortices, using the drill bit for the thread hole. Then use the corresponding larger drill bit to overdrill the near cortex to create the gliding hole.
This technique is useful for small fragments. The disadvantage, however, is that the holes may not be centered in relation to each other.
If the near cortex is tapped prior to overdrilling for the gliding hole, eccentric passage of the second drill is less likely. This can be achieved by inserting the chosen self-tapping screw through the near (cis) cortex and then removing it. The drill will now follow exactly the threaded axis.
Pitfall: drilling through far cortex
Make sure that the drill bit for the gliding hole does not reach and damage the far cortex, as the purchase of the screw will be compromised.
Countersinking in diaphyseal bone
There are two important reasons for countersinking:
1)The risk of soft-tissue irritation is greatly reduced by ensuring that the the screw head protrudes only minimally from the bone surface.
2)Countersinking ensures that the screw head has a maximal contact area with the bone, distributing the forces from the screw head more widely than an unsunk head.
Pitfall: breaking the cortex in diaphyseal bone
Do not advance the countersink too deeply into the cortex: the cortical thickness will determine the depth of countersinking. Excessive penetration risks break-through of the screw head when tightened and loss of fixation. Countersinking is, therefore, done by hand and not with a power tool.
Pitfall: Damaging the cortex in metaphyseal bone
Avoid countersinking in the metaphyseal region as the cortex is very thin.
6 Measuring topenlarge
Pitfall: Oblique measurement
When measuring for screw length in oblique drill holes, the measurement to the acute angle is different from the measurement to the obtuse angle. This problem increases with the degree of obliquity.
Always measure both angles and use the longer measurement. However, keep in mind that too long a screw can protrude to the extent that it puts the soft tissues at risk.
Screw length pitfalls
Ensure that a screw of the correct length is used.
-Too short screws do not have enough threads to engage the far cortex properly. This problem increases when self-tapping screws are used due to the geometry of their tips.
-Too long screws endanger the soft tissues, especially tendons and neurovascular structures. With self-tapping screws, the sharp cutting flutes are especially dangerous, and great care has to be taken that the flutes do not protrude beyond the cortical surface.
7 Screw insertion topenlarge
Insert the lag screw in the center of the fracture. Carefully tighten the screw.
The fracture is now compressed. The reduction forceps may now be removed.
Confirm reduction and correct screw position using image intensification.
8 Protection plate topenlarge
A 2 mm adaption plate is cut to the correct length, so that it allows two screws to be inserted into each fragment. Usually the plate will have 5 holes.
The plate must be adapted perfectly to the bone surface. Since the plate will not exert axial compression, there is no need for overbending.
If the plate is not well adapted to the bone surface, secondary displacement may occur when the screws are tightened.
If the fracture extends into the metaphysis, a T-, Y- or L-shaped adaption plate may be preferable, as it allows for two screws in short fragments.
Where possible, the plate should be placed strictly dorsally on the metacarpal, as this is the tension side of the bone.
The center hole is placed over the fracture. Make sure that the plate screws do not interfere with the lag screw.
If the plate is not properly centered on the dorsal surface of the metacarpal, the screws may not penetrate the medullary canal and may not have sufficient purchase. Secondary fractures may result.
Drill a thread hole through the most proximal plate hole and insert the screw without completely tightening it.
Align the plate correctly in the longitudinal axis of the metacarpal and repeat the procedure for the most distal screw, using neutral drilling.
Insert the two screws close to the fracture, also with neutral drilling. Now tighten all screws alternately.