An external skeletal fixator with an intramedullary pin tie-in configuration is an alternative to bone plating in small cats.
The IM pin that is protruding from the gluteal region is tied in with a type 1 ESF frame applied on the lateral side of the femur. The pin counteracts the bending forces and the external fixator protects the fracture from collapse, shearing and torsional forces.
Note: This will be a less rigid stabilization than a bone plate. It is important to access 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 recommended pin diameter of 75-80% at the isthmus of the medullary canal must be reduced to allow placement of the bicortical ESF pins. The optimal exact IM pin diameter is unknown but a pin diameter of 30-50% at the isthmus is a practical guideline.
A 1 cm skin incision is made at the level of the most proximal part of the greater trochanter.
Approach to the fracture site
A small approach at the level of the fracture site can be performed for direct visualization of the fracture reduction.
5. Surgical technique
The tip of the pin is placed against the medial aspect of the greater trochanter.
The pin is started at a 20 degree angle relative to the axis of the bone to minimize slippage in the trochanteric fossa.
Gently slide the pin down into the trochanteric fossa.
As soon as the pin penetrates the fossa, the pin is redirected to align with the medullary axis of the bone. The pin is directed slightly caudally and medially to avoid the pin engaging the cranial cortex at the level of the proximal metaphysis.
The fracture is aligned or reduced by moving the distal bone segment with a point reduction forceps applied percutaneously and the pin is advanced in a close fashion into the distal main segment until the tip of the pin seated in the distal metaphyseal area at the level of the patella
The same procedure can be done with a small approach at the level of the fracture site with direct visualization of the fracture reduction.
Validation of alignment and rotation
Once the bone length has been restored, it is necessary to check for correct alignment and rotation. Since this technique is done primarily closed, rotational alignment can be judged by palpation of landmarks or by direct visualization of the entire limb alingment. Intraoperative fluoroscopy can be used if available. If the femur is correctly aligned in the axial plane the greater trochanter should be slightly caudal compared with long axis of the bone.
The proximal part of the IM pin, protruding from the gluteal region, is connected with a connecting bar to a type one external fixator frame applied on the lateral side of the bone. Preferably, the ESF pins are placed in the proximal and distal parts of the femur to avoid interference with the muscular bellies. The most distal pin is inserted at the level of the trochlear region. The most proximal pin is inserted at the base of the greater trochanter directed towards the lesser trochanter, because this is the thickest portion of the proximal femur. The use of positive profile threaded pins is recommended.
The type one frame is secured to the femur with at least two ESF pins for each major segment
Note: in a simple fracture,of a young animal, less than 7kg, one ESF pin per segment may be sufficient.
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.
Disassembly or staged disassembly
When there is evidence of good callus formation, staged disassembly of the construct can be considered to minimize complications associated with the transfixation pins.
Complete removal of ESF and the IM pin is indicated once the fracture is healed.