The combination of an intramedullary pin with an external fixator in a tie-in configuration is a treatment option for small patients, without enough proximal bone stock for plate fixation or with comminution at the level of the lesser trochanteric region.
The use of this technique is limited by the morbidity associated with external fixators in very muscular areas.
Note: This technique should be used with caution if there is not enough bone stock in the neck fragment or if the greater trochanter is fractured. At least two points of fixation are necessary for stability in the proximal fragment.
Biomechanics of the fixation construct
Tie-in ESF is a biological fixation technique when done in a closed or open but do not touch approach. The intramedullary pin is used as a reduction device to align the fracture and to restore bone length. The two implants work together; the pin protects the repaired fracture against bending forces, while the external fixator resists axial collapse, rotation and shear. Without the pin, the external fixator is subjected to high bending forces because the bone is not sharing any load in the calcar area.
Note: This will be a less rigid stabilization than a bone plate. It is important to assess the ability of this type of stabilization to counteract all the forces that will be placed on it. Safe zone ESF pin placement is difficult in the femur because of the large muscle mass on the lateral surface. In addition, the intramedullary pin will cause interference with the gluteal muscles.
The length of the incision will depend on the extension of the fracture.
Note: Tie-in ESF should be as biological as possible. An open but do not touch approach or MIO is best. The ESF pins on the femoral shaft should be placed through stab incisions in safe zones.
4. Surgical technique
The dimensions of the pin should be 30-50% of the isthmus of the medullary canal. This provides sufficient protection against bending forces and allows placement of positive profile bicortical ESF pins.
The pin is started at a 20 degree angle relative to the axis of the bone to minimize slippage into the trochanteric fossa.
Because of the very small fragment size, orientation of the pin is critical. A maloriented pin may cause significant coxa valga or coxa vara deformity.
The pin is passed through the fragmented section and driven into the distal fragment with a power or hand chuck until the tip of the pin reaches the bone of the distal metaphysis. The femur is distracted to its full length by pushing the pin against the distal femoral cortex.
Spatial alignment is achieved without disturbing the bone fragments within the zone of comminution.
Validation of alignment and rotation
Once the bone length has been restored, it is necessary to check for correct alignment and rotation. Rotational alignment can be judged by palpation or by direct visualization of the greater trochanter and femoral trochlea. The distal part of the femur is held in a true lateral position. The position of the greater trochanter and alignment of the adductor magnus muscle are then inspected.
If the femur is correctly aligned in the axial plane, the adductor magnus muscle should be aligned and the greater trochanter should be slightly caudal to the long axis of the bone.
The alignment can also be checked by confirming the orientation of the femoral neck relative to the plane of the femur. This is done by inserting a small pin alongside the femoral neck with the femur in a true lateral position. Orientation of the pin should be about 15°-25° in the cranial direction relative to the sagittal plane of the femur.
Correct alignment and rotation can be checked with intraoperative imaging. The lateromedial projection is used and the whole bone, including the proximal and distal joints, must be visible.
If the femur is properly aligned in the transverse plane, about 1/3 of the femoral head should be visible cranially to the cranial femoral cortex (Image A). Image B shows excessive internal rotation and C shows excessive external rotation of the distal femur.
Comparison with the contralateral unaffected limb can be useful.
Application of the external fixator
A positive profile threaded pin is applied in the proximal fragment. The most distal ESF pin is applied at the level of the trochlear region. The connecting bars and clamps are assembled and the remaining pins are positioned creating a type I external fixator. Two or three pins per fragment should be used. One of the proximal pins is anchored into the femoral head and neck.
Note: Care must be taken to avoid perforating the articular surface.
The proximal part of the IM pin, protruding from the gluteal region, is connected with the connecting bar of the external fixator creating a tie-in configuration and is the second point of fixation.
Validation of fixation
Postoperative orthogonal radiographs are taken to assess fixation.
5. Case example
2 year old domestic shorthair cat with an A3 fracture from being hit by a car.
The fracture was repaired using an external fixator combined with an intramedullary pin in a tie in configuration.
Postoperative radiographs at 1 month.
Postoperative radiographs at 4 months.
Activity restriction is indicated until radiographs indicate bone healing of the fracture.
The pin-skin interface should be cleaned and adequately protected.
Frequent rechecks are recommended in the early postoperative period to access stability of the frame and the pin-skin interface.
Phase 1: 1-3 day after surgery
The aim is to reduce the edema, inflammation and pain. Integrative medical therapies, anti-inflammatory (note in the cat that many are toxic; only use drugs labeled for cats) and analgesics medications are indicated.
Sterile sponges should be placed in between the skin and connecting bars to protect the pin tract wounds.
The pin-skin interface should be cleaned daily.
Phase 2: 4-10 days after surgery
The aim is to resolve the hematoma, the edema and control the pain, as well as to prevent muscle contracture. Analgesic medications may still be needed. Anti-inflammatory medications in the cat are not labeled for continued use after a few days and should be avoided. Rehabilitation and integrative medical therapies can be used. Once the pin tract wounds are closed, the bandage can be downsized to only cover the ESF clamps and bars.
If the cat is not starting to use the limb within a few days after surgery, a careful evaluation is recommended.
10-14 days after surgery the sutures are removed.
Phase 3: 10 day-bone healing
Radiographic assessment is performed every 4-8 weeks until bone healing is confirmed.
Patients with external fixators should be checked every 7-10 days for pin track infection and fixator stability.
Disassembly or staged disassembly
When there is evidence of good callus formation, staged disassembly of the construct can be considered.
Complete removal of ESF and the IM pin is indicated once the fracture is healed.