Arthrodesis: tension bad using wires or the sesamoid bones
Because the plate is placed on the dorsal aspect of the fetlock joint and would be subjected to severe bending if not protected, a tension band must be placed on the palmar/plantar aspect of the fetlock joint. The tension band can be created by two means:
Inserting cortex screws in lag fashion from the dorsal and distal aspect of McIII/MtIII into the proximal sesamoid bones uses the inelastic distal sesamoidean ligaments to create the tension band (left).
If the proximal sesamoid bones or distal sesamoidean ligaments have been disrupted by the injury, the tension band is created using a double strand of 1.5 mm wire (right).
The principles of fixation are the same. The technique varies slightly depending on the injury. A biaxial proximal sesamoid bone fracture will be used as the injury in the following illustrations.
2. Plate selection and preparation
A broad plate of 14-16 holes is normally selected based upon the size of the horse. A dynamic compression plate (DCP or LC-DCP) or a locking plate (LCP) can be used for stabilization. Some surgeons prefer a 10-hole LCP. In the following, the technique will be shown with an 14-hole DCP.
Maximum purchase needs to be obtained in the proximal phalanx, so as many screws as available should be inserted into the proximal phalanx. The proximal end of the plate is placed at the proximal diaphyseal-metaphyseal junction of McIII/MtIII.
Care should be taken to place the plate on the long axis of both bones, the proximal phalanx and McIII/MtIII.
The plate is contoured to place the fetlock in 15-20 degrees of dorsal extension.
If the wire is needed to create the palmar/plantar tension band, it is inserted with the joint luxated. A 3.2 mm hole is pre-drilled, with the joint reduced, in the dorsal plane in the mid-portion of the proximal phalanx prior to luxation of the joint. A second 3.2 mm hole is also created in the dorsal plane through the distal McIII/MtIII in similar distance proximal through the joint.
Note: care must be taken to place this hole in an area distant from the plate holes in the proximal phalanx. Therefore it may be advantageous to drill all four plates holes in the proximal phalanx prior to preparing the hole along the frontal plane.
Application of the wire
A 1.25 mm cerclage wire is threaded through the proximal phalanx with the joint luxated and then passed behind the proximal phalanx into the fetlock joint on its palmar/plantar surface. The other end of the wire is then crossed on the palmar/plantar aspect of the proximal phalanx, passed up the medial aspect of the distal McIII/MtIII, and threaded through the hole in the parent bone and the condylar fracture. A second 1.25 mm cerclage wire is threaded in reverse fashion through the two holes such that the wire ends come to rest at the medial aspect of the distal McIII/MtIII.
The iatrogenic condylar fracture is subsequently reduced and reattached to the parent bone with a cortex screw in lag fashion. The two figure-8 wire loops are twisted on the lateral an medial aspect of the bone with the fetlock joint in 10 degrees of flexion. (It is important to tighten the wire in slight flexion so that when the limb is again placed in extension, the wire comes under tension.)
The plate is reattached to the proximal phalanx.
5. Option B: Creating a tension band using the sesamoid bones
The iatrogenic condylar fracture is reduced and reattached to the parent bone with a cortex screw in lag fashion. The plate is then reattached to the proximal phalanx.
Placing the screws for the tension band
If the distal sesamoidean ligaments are being used to create the tension band, the screws are placed at this time. One cortex screw is inserted using lag technique through the distal aspect of McIII/MtIII into each of the proximal sesamoid bones to stabilize the sesamoid bones and the distal seamoidean ligaments to the back of McIII/MtIII creating the tension band. (It is important to place the screws in slight flexion so that when the limb is again placed in extension, the distal sesamoidean ligaments come under tension.)
6. Finalize plate fixation
The plate is then depressed to the surface of McIII/MtIII to create tension in the distal sesamoidean ligaments or the wire.
The tension device is attached to McIII/MtIII at the proximal end of the plate.
The tension device is tightened into maximum tension using the appropriate wrench.
The remaining screws are inserted in the plate.
Once all screws are in the plate one 5.5 mm cortex screw is placed in lag fashion medially and laterally through the proximal phalanx into the distal McIII/MtIII taking care not to interfere with the implants already in place in the limb. These two lag screws increase the stability and resistance to rotation at the fetlock joint.
Completed fixation with cerclage wires.
At this point, prior to closure, the surgeon may decide to use a bone graft, antibiotic containing implants and/or suction drainage depending on the injury.
Closure of the incision requires coaptation of the previously split extensor tendon, joint capsule and extensor branch of the suspensory ligament over the implants. The subcutaneous tissue and skin are then closed sequentially.
A sterile dressing is placed over the incision and a fiberglass cast placed from the proximal aspect of McIII/MtIII including the phalanges and the foot in the cast. The inclusion of a small wedge under the heal in the cast improves the comfort of the animal during the immediate postoperative period. Intravenous antibiotics are given for an appropriate period of time depending on the original injury and soft-tissue damage. Cast immobilization usually continues for 30 days with the cast being changed 2 weeks postoperatively. A bandage is then used until edema and swelling is no longer present. Stall rest with no free-choice exercise is maintained for 60 days. Follow up radiographs at that time will determine if gradually increasing exercise can be initiated. Implants are left in place and removed only if infection/drainage indicate the need for removal.
Note: Intravenous regional antibiotic perfusion can be performed during cast application on the table.