1 Principles topenlarge
Anatomy of the distal femur
The distal femur has a unique anatomical shape. Seen from an end-on view, the lateral surface has a 10° inclination from the vertical, while the medial surface has a 20–25° slope. A line drawn from the anterior aspect of the lateral femoral condyle to the anterior aspect of the medial femoral condyle (patellofemoral inclination) slopes approximately 10°. These anatomical details are important when inserting screws, or blade plates. In order to avoid joint penetration, these devices should be placed parallel to both the patellofemoral and femorotibial joints planes.
The muscle attachments to the distal femur are responsible for the typical displacement of the distal articular block following a supracondylar fracture, namely shortening with varus and extension deformity. Shortening is due to the pull of the quadriceps and hamstring muscles, while the varus and extension deformity is caused by the unopposed pull of the adductors and gastrocnemius, respectively.
The popliteal vessels, the tibial nerve and the common peroneal nerve lie in close proximity to the posterior aspect of the distal femur. Because of this, vascular injuries occur in about 3% and nerve injuries in about 1% of fractures of the distal femur.
There are no significant arteries, veins, or nerves on the lateral side of the knee.
There may be bleeding from the lateral genicular arteries, which will need to be controlled using diathermy.
At the posterior aspect of the knee lie the popliteal artery, nerve and vein. It must be borne in mind that these structures can be damaged by the injury, or can be damaged by the surgeon during the reconstruction.
Consideration must be given to fracture reduction in:
- Internal/external rotation
Reduction can be performed with a single reduction tool (e.g., large distractor), or by combining several steps (for example fracture table +/- external fixator, +/- reduction via the implant, etc.) to achieve the final reduction.
The preferred method depends on the fracture and soft-tissue injury pattern, the chosen stabilization device and the experience and skills of the surgeon.
If a large fragment has separated from the fracture zone and impaled the adjacent muscle, direct reduction of that fragment may be required.
It is very important to restore the biomechanical axis of the lower limb. The normal biomechanical axis follows a line from the center of the femoral head, through the center of the proximal tibia and then through the center of the ankle joint. This axis can be checked intraoperatively by using a piece of cable, such as the diathermy cord, to give an approximate estimate of the axis, as follows.
The biomechanical axis must be restored and care should be taken to ensure that there is no malrotation of the distal femur on the proximal femur.
If no traction table is used (i.e., using the freehand technique) the cable method may be used. In this technique, the electrocautery cord is held from the iliac spine across the patella to the cleft between the first and second toes. If rotation is correct, this cord will pass over the midline of the patella, and slightly medial to the tibial eminence.
Another method of assessing rotational reduction is to compare the cortical thickness above and below the fracture. If a shaft fracture is multifragmentary, the image intensifier cannot be used to compare cortical diameters on each side of the fracture.
This illustration shows the longitudinal axes of the lower limb.
Choice of implant
For retrograde femoral nailing to achieve adequate fracture stabilization, the fracture should be at least 6 cm from the joint line to achieve distal locking with two transverse screws or a screw and a spiral blade. In contrast, more distal fixation can be achieved with plates, or locked fixators. For example the distal most screws in a LISS plate, or a condylar plate, may be subchondral.
The distal-most fixation for various implants are:
- LISS plate: subchondral
- Condylar plate: subchondral
- 95° angled blade plate: 1.5 – 2 cm
- 95° dynamic condylar screws: 2 cm
- Retrograde intramedullary nail: 6 cm (for 2 locking screws, or one locking screw and a spiral blade)
2 DCS fixation to the distal fragment topenlarge
Preparation for correct positioning
The DCS is a two part device with a stable linkage, used to attach the distal femoral condylar mass to the diaphysis. Correct orientation of the knee joint plane requires meticulous placement of the screw device.
Determine the correct position for the DCS with the help of guide wires around the joint. Under image intensifier control, pass one guide wire lateral to medial along the tibio-femoral joint line (red). Pass a second guide wire over the anterior surface of the knee to indicate the plane of the patello-femoral condyles (green).
The ideal position of the DCS is shown by the yellow wire. Note that it is inserted parallel to both the red wire in the frontal plane and is parallel to the green line on the end-on view on the femur. This latter orientation ensures that the plate comes to lie flush with the lateral cortex.
The ideal entry point for the DCS is shown on the diagram. The guide wire for the DCS is positioned at 2 cm proximal to the distal end of femur. On the lateral view, the distal femur is divided into thirds and the DCS entry site is located at the junction of the anterior and middle thirds.
Insert the guide wire at the chosen entry site of the DCS. Insert the guide wire under image intensifier control all the way across the femur. Check the position of the guide wire carefully to ensure it has been correctly positioned, with the parallelism already described.
Correct depth of guide-wire insertion
The depth of guide-wire insertion is crucial. Remember that the cross section of the distal femoral condylar mass is trapezoidal and slopes markedly on the medial side. The tip of the guide wire should just engage the medial cortex, and so will appear short of the medial condylar cortex on the AP intensifier image.
Pitfall: too long a guide wire
It is important to remember that the distal femur tapers from the posterior to the anterior. Therefore, if a straight AP view is obtained, the guide wire can appear to be inside the bone. If it appears to be outside the bone, it is most likely too long and the DCS will cause pain and possibly heterotopic ossification. In order to assess the exact length of the guide wire obtain an AP view with 30° internal rotation of the lower extremity.
In this illustration, internal rotation by 30° reveals that the guide wire length was chosen inappropriately.
Screw length measurement
Next, slide the direct measuring device over the guide wire and determine guide-wire insertion depth and, thereby, the length of the DCS required.
After assembling the DCS triple reamer and setting the reamer to the correct depth, ream the hole for the DCS over the guide wire.
After tapping, insert the DCS over the guide wire, so that its outer end is still visible 2-3 mm outside the lateral cortex of the distal femur.
In order for the plate barrel to slide over the screw, the T-handle should be parallel, on the lateral view, to the long axis of the distal fragment.
Pearl: omit tapping in osteoporotic bone
In osteoporotic bone, tapping should be omitted.
Detach the T-handle and pass the plate barrel over the screw shank.
Alternative: reconnect T-handle to screw
Some surgeons reconnect T-handle to the screw to help to adjust the position the plate. However, this maneuver is not absolutely necessary and some surgeons do not perform it.
Use the impactor to bring the plate down to the bone, with the barrel sliding over the screw shank.
The compression screw may be utilized to couple the screw to the plate.
Pearl: do not use compression screw in osteoporotic patients
Do not use the compression screw in osteoporotic patients – it can cause the DCS thread to strip out from the soft cancellous bone of the medial femoral condyle.
Final screw placement in distal fragment
A cancellous screw can then be inserted into the most distal screw hole of the plate to prevent rotation of the distal femoral articular block around the axis of the DCS.
3 Plate fixation to the proximal fragment topenlarge
The key concept in fracture reduction when utilizing a DCS is that proper insertion in the distal femur allows the surgeon to use the plate to assist the reduction. The surgeon must control for length, rotation and sagittal plane deformity (hyperextension/hyperflexion). When performed in an open manner, length can be restored by manual traction. The sagittal plane deformity correction can be aided by a supracondylar bolster.
Closed reduction is aided by:
Complete anesthetic muscle relaxation of the patient
A bolster posterior to the supracondylar region to prevent hyperextension
Use of the femoral distractor, or external fixator
- Colinear clamp (as illustrated)
In some circumstances it may not be possible to achieve a perfect anatomical reduction initially. Indeed, anatomical reduction of all fracture segments may not be desired except insimple extraarticular fractures. As long as the mechanical axis is restored this should be adequate in most situations.
Loosely secure the plate to the proximal femur with a Verbrugge clamp.
Take care to restore the mechanical axis of the femur in all planes. Give
consideration to fracture reduction in:
- Internal/external rotation
Use of the articulated tension device
Secure the articulated tension device to the proximal femur with a bicortical screw.
The articulated tension device is very useful to apply controlled compression to the fracture site. It should be used when possible in fracture patterns where there is contact between the proximal and distal main fragments. It may not be used in situations of severe metaphyseal comminution and/or osteoporosis.
Insertion of first screw into proximal fragment
Tighten the articulated tension device with the spanner so that the indicator on the tension device is in the green zone, checking the fracture site carefully to ensure that no unwanted displacement occurs.
Insert a screw through the plate close to the compression device to secure the fixation. Insert the screw eccentrically in the plate hole to maintain the fracture compression.
Complete the fixation of the plate to the femur with sufficient screws, using neutral insertion of the screws in the plate holes.
Additional screw insertion
Lastly remove the articulated tension device and complete the fixation by inserting additional screws according to the preoperative plan.
In oblique, single-plane metaphyseal fractures, anatomical reduction and interfragmentary compression are essential goals and an interfragmentary lag screw should be inserted.