1 Indications topenlarge
Intramedullary pin fixation with cerclage wires is an option for small breed dogs for long oblique or spiral simple fractures in which at least 3 cerclage wires can be applied. The fracture must be completely reconstructible and the bone must share the axial load. It is not recommended for older patients or for those dogs that cannot be adequately confined postoperatively. Failure to choose the correct fracture type and patient for this repair will likely lead to loss of reduction of the fracture.
2 Principles topenlarge
The dimensions of the pin should be 70-80 % of the isthmus of the medullary canal. If the pin is too small it will not have adequate strength or bending stiffness to maintain stability.
3 Approach topenlarge
The skin incision is made along the craniolateral border of the femoral shaft from the level of the greater trochanter to the level of the patella. The subcutaneous fat and superficial fascia are incised or bluntly dissected directly under the skin incision.
The superficial fascia is retracted. The junction between the fascia lata and the biceps muscle is carefully identified by looking at the direction of the fibers.
A detailed demonstration can be found under "Open approach to the shaft".
The vastus lateralis and intermedius muscles, on the cranial surface of the femoral shaft, are retracted cranially by freeing the loose fascia between the muscle and the bone. Take care to avoid unnecessary detachment of the adductor magnus muscle as it attaches on the fascies aspera of the femur and is the major blood supply to the fractured bone.
4 Surgical technique topenlarge
Bone holding forceps are applied to the proximal and distal fragment for distraction. This is necessary to counteract the strong muscles surrounding the bone, particularly in large breed dogs.
Once distraction is achieved, the bone fragments are pulled, toggled or levered along the fracture line into perfect anatomical reduction with the help of one or two pointed reduction forceps placed across the fracture line.
The fracture is preliminary fixed with one or two pointed reduction forceps while the cerclage wires are applied.
Note: take care to avoid placing the forceps at the planned cerclage sites.
Anatomical reduction and the stability of preliminary fixation are carefully checked.
The distance between the cerclage wires needs to be at least 1/2 diameter away from the tip of the fragment and ½-1 diameter away from each other.
A detailed demonstration of cerclage wires technique can be seen under "Additional material".
Approach to the trochanteric region
When it is necessary to expose the greater trochanter for implant placement, the attachment of the vastus lateralis on the neck of the femur may be partially incised.
A 1 cm skin incision is made at the level of the most proximal part of the greater trochanter.
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.
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 pin is driven into the cancellous bone of the distal metaphysis until the tip contacts but does not penetrate the cortex.
Gently slide the pin down into the trochanteric fossa.
Validation of pin placement
The location of the tip of the pin is assessed by comparing with a pin of the same length held on the outside of the bone. When the pin reaches the level of the distal pole of the patella, the pin is correctly placed in the medullary canal.
Note: Pin placement can be checked with intraoperative radiography if available.
Pitfall: In some breeds (such as chondrodystrophic breeds), the femur has a significant cranial curvature. Avoid penetration of the cranial cortex by orienting the pin from cranial at the trochanteric entrance to caudal at the distal metaphysis. Carefully evaluate the position of the tip of the pin using a 2nd pin.
Cutting the pin
The protruding part of the pin is cut as close as possible to the level or below the level of the greater trochanter to avoid trauma to the muscle and sciatic nerve. This level will allow pin removal if necessary in the future.
Note: If the pin is difficult to cut close with this method, it can be cut prior to seating into the distal femur.