1 Principles top
Healing after LISS fixation relies on maintenance of the fracture viability in the metaphyseal / diaphyseal component of the fracture. This is made possible through closed reduction techniques and results in high union rates and low infection.
Malreductions (external rotation / valgus) can be common without meticulous intraoperative technique and radiographic control. The articular surface should be addressed under direct vision using standard techniques of interfragmentary compression.
If the single articular fracture line is not medial to the midline, a minimally invasive lateral/anterolateral approach will usually permit an adequate view of the joint fracture reduction.
If the articular fracture is more medial, the lateral parapatellar approach will provide a better exposure of the joint reduction.
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.
Two main steps
In the treatment of intraarticular distal femoral fractures, the surgeon first reconstructs the distal femoral articular block. A closed reduction of the metaphysis/diaphysis is then performed before the LISS plate is slid into the submuscular tunnel.
The gastrocnemius typically causes a hyperextension deformity of the distal femoral articular block.
Correction of hyperextension deformity
Hyperextension deformity must be corrected before fracture fixation. Aids to correcting this hyperextension deformity include:
- Muscle relaxation of the patient
- A bolster in the supracondylar region
- Flexion of the operating table leg segment
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.
Not only must the biomechanical axis be restored, but 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. The radiological landmarks of the center of the femoral head, the center of the knee and the center of the ankle joint should all be in line if the mechanical axis of the femur is correct.
Illustration of the longitudinal axes of the lower limb.
The LISS plate is an internal fixator used as an extramedullary splint, fixed to the two main fragments, leaving the intermediate fracture zone untouched. Anatomical reduction of intermediate fragments is neither sought nor necessary. Direct manipulation of intermediate fragments would risk disturbing their blood supply. If the soft-tissue attachments to these fragments are preserved, and the fragments are generally aligned, healing is unimpaired.
Alignment of the main shaft fragments can be achieved indirectly, using various aids.
Mechanical stability, provided by the bridging fixator, is adequate for gentle functional rehabilitation and results in satisfactory indirect healing (callus formation) of the diaphysis.
LISS fixation relies on the principle of bridge plating. It therefore works best in setting of a comminuted metaphyseal fracture. By contrast, standard compression plating (eg, blade plate, ORIF with LCP-DF) is used when the metaphyseal component of the fracture has one or two simple planes. This is an important point: a common failure of the LISS is seen when a simple plane fracture is treated with the relatively flexible LISS plate and left with a gap. This can result in a high rate of nonunion.
LISS plate insertion
LISS plate may be inserted using an open approach to the metaphysis/diaphysis, but are most commonly inserted using minimally invasive techniques, leaving the soft tissues intact around the fracture site. Incisions are made proximally and distally, and the LISS plate is inserted into a submuscular tunnel. This normally requires image intensifier monitoring.
It is important to restore axial alignment, length and rotation.
Reduction can be performed with a single reduction tool (eg, 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 may be required.
2 Choice of implant topenlarge
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)
3 Preoperative planning topenlarge
LISS plate length
Generally speaking, plates for the bridging technique should be longer than for conventional open plating techniques, in order to distribute the forces and to provide relative stability.
The preoperative x-ray planning template is useful in determining the length of the LISS plate and the positions of the screws.
Number of screws
In healthy bone, five well placed monocortical screws are inserted to secure the LISS to the main femoral shaft fragment.
The use of bicortical screws in severely osteoporotic bone may need to be considered.
4 Reduction and fixation of the articular block topenlarge
Reduction of the articular block
The chosen approach must adequately expose the articular surface of the distal femoral condyle. Reduction aids that are helpful include:
- A 5.0 mm or 6.0 mm Schanz pin in the medial and/or lateral femoral condyle to act as a joystick.
- Pointed reduction forceps, or large pelvic reduction clamps, to clamp from medial to lateral across the intercondylar split.
Pearl: combination of reduction aids
Attempts at reduction of the intercondylar split with the pointed reduction forceps alone are often unsuccessful, as rotational control of the femoral condyle is also needed. The use of the Schanz pin in conjunction with the pointed reduction forceps is therefore preferred.
Before definitive fixation is undertaken, more than one clamp is applied. Usually, one to two additional K-wires are inserted, either from medial to lateral, or lateral to medial.
If the K-wires are inserted from medial to lateral, they may either go through small stab incisions in the skin, or through the parapatellar retinaculum.
Definitive articular surface fixation
Screws may be placed along the periphery of the articular surface of the lateral femoral condyle going from lateral to medial to compress the intercondylar split.
These screws may be fully threaded 3.5 mm lag screws (shown with gliding hole), 6.5mm partially threaded lag screws, or 4.0/4.5 mm cannulated, partially threaded lag screws.
Insertion of screws in this manner leaves a „free zone“ of bone into which a laterally based plate system can be inserted (dotted circle).
This end-on view demonstrates the screw trajectories from lateral to medial.
On occasions, it is acceptable to insert screws through the articular surface, when no other option is available. These screws must be countersunk and recessed beneath the articular surface.
5 Reduction of the metaphysis/diaphysis topenlarge
Closed reduction is aided by:
- Early intervention
- Complete anesthetic muscle relaxation
- A bolster in the supracondylar region
- Manual traction
Reduction can also be aided by:
- Use of Schanz pins inserted into the medial, or lateral, femoral articular block to correct varus or valgus angulation of the femoral block.
- Insertion of a Schanz pin from anterior to posterior in the distal femoral articular block, which can be used to correct hyperextension.
Alternative: external fixator/femoral distractor
Some surgeons find it useful to use an external fixator (or femoral distractor) from the proximal femur to the proximal tibia.
Due to the pull of the gastrocnemius muscle, the distal fragment has a tendency to be displaced into extension at the metaphyseal fracture area, when distraction is applied.
To avoid this, the knee is brought into full extension, and the distal femoral fragment is stabilized in this position to the tibia using a temporary cerclage wire around a Schanz screw inserted in the distal femoral articular block.
Insert the proximal and distal fixator (distractor) pins carefully in order not to conflict with the later plating procedure. Safe positions would be anterolateral or anterior on the femur.
Alternative: fracture table
An additional alternative for closed reduction is the use of a fracture table. Note the support underneath the supracondylar region. Traction may be applied through either a traction boot, or a proximal tibial/calcaneal traction pin.
6 LISS plate insertion topenlarge
Assembly of LISS insertion instruments
Connect the two parts of the insertion guide with the connection bolt. Now place the fixation bolt into hole “A”. Then, place the insertion guide onto the LISS, engaging the three-point locking mechanism.
Screw the fixation bolt into the LISS by turning the knurled head of the bolt and slightly tighten it using the pin wrench. Next, screw the nut of the fixation bolt clockwise and tighten it slightly with the pin wrench, to stabilize the attachment between the insertion guide and the LISS plate.
For more stable fixation of the LISS plate to the insertion guide during insertion, introduce a second stabilization bolt with the drill sleeve into hole “B” and thread it into the LISS plate.
LISS plate insertion
Use the assembled insertion guide as a handle to insert the LISS plate into
the submuscular tunnel between the vastus lateralis muscle and the
Insert the LISS plate through the lateral incision.
Advance the LISS plate proximally under the vastus lateralis muscle, ensuring that its proximal end remains in constant contact with the bone. Position the distal end of the LISS plate against the lateral condyle. To identify the correct position, move the LISS plate proximally and then back distally until it fits the condylar contour.
Proper position check - position on the distal femur
When the LISS plate lies flat on the lateral surface of the condyle, it has been positioned correctly on the distal femur.
From the AP perspective, a 280 mm long guide wire is inserted through the fixation bolt in hole “A” and should be parallel to the plane of the tibiofemoral joint (green dashed line). At that point, the preshaped LISS plate is in the correct position, assuming normal anatomy. This helps to restore correct alignment in complex fracture patterns.
7 Preliminary LISS plate fixation topenlarge
Make a stab incision over the most proximal screw hole of the LISS plate and dissect bluntly down to it.
Insert an insertion sleeve with a 5 mm trocar through the appropriate hole (5, 9, or 13) in the insertion guide and down to the proximal LISS plate hole.
Insertion of the proximal stabilization bolt
Remove the trocar from the insertion sleeve.
Insert a stabilization bolt through the insertion sleeve and into the LISS plate. This creates a fixed parallelogram that facilitates further manipulation of the LISS plate.
Position on the proximal femur
It is very important to confirm the correct LISS plate position proximally. In the minimal invasive technique, this can be challenging. Even the use of an image intensifier does not guarantee an optimal position.
Pearl: palpating proximal end of LISS plate
To overcome this problem, the proximal incision is enlarged and the correct LISS plate position is controlled directly with the index finger, using the insertion guide to alter the plate position.
Proximal guide-wire insertion
If the length and rotation of the fracture fragments are correct, insert the
proximal guide wire after verification that the LISS plate lies on the
midlateral aspect of the femur.
It is extremely important to establish correct placement of the guide wire in order to ensure proper proximal insertion of the monocortical locking-head screws. After the proper length and rotation are assured, and appropriate positioning of the proximal portion of the LISS plate on the midlateral aspect of the femur has been established, a proximal guide wire is inserted through the stabilization bolt. It is still possible at this point to correct the sagittal plane alignment, as noted below. Small corrections of the adduction of the proximal fragment, or of the varus/valgus alignment of the distal femoral condyle, are possible.
Pearl: monocortical proximal guide-wire insertion
Insert the proximal guide-wire through one cortex only. This reduces the risk of guide-wire breakage
Once the reduction has been successfully completed and the LISS plate has been positioned correctly, the locking-head screws can be inserted.
8 Screw insertion into the distal fragment topenlarge
Length of screws
Determine the appropriate screw length by using a 280 mm long guide wire that has been passed through the fixation bolt in hole “A” and an indirect measuring device, or according to the preoperative plan.
Insert the 5.0 mm locking screws into the LISS plate through the insertion sleeves, which are passed through the insertion guide.
For the final locking of the screws, the use of the torque-limited screw driver is necessary.
9 Securing the proximal fragment topenlarge
Reduction of the proximal femoral shaft with the pull reduction instrument
With the help of the pull reduction instrument, secure the desired position of the shaft, in relation to the LISS plate. This is an important step because otherwise some displacement may occur during the insertion of self-drilling/self-tapping screws. (Note: this instrument has a 4.0 mm diameter which still allows for the insertion of a 5.0 mm locking-head screw into the same hole later on.)
Attach a syringe, filled with saline, to the drill sleeve to provide cooling during the bone drilling procedure.
The pull reduction instrument is self drilling and self tapping. It is inserted through a sleeve that has been placed in the appropriate hole in the insertion guide and has been advanced through a stab incision. The pull reduction instrument is inserted using a power tool but great care is needed, using an image intensifier, only to engage the threaded portion into the bone. Over-insertion will strip its thread in the bone.
The nut of the pull instrument is then tightened against the head of the insertion sleeve, and tightening continued until the bone has been drawn into the required position of reduction against the LISS plate.
10 Additional screw placement topenlarge
Insert additional locking-head screws (LHS) both proximally and distally. In general, a total of five proximal and five distal LHS are placed.
Bicortical self-tapping LHS can be used for shaft fixation in severe osteoporosis.
Final screw placement
After detachment of the insertion guide, insert a final screw into the distal fragment through the central hole in the distal portion of the plate (“A”).