Special attention is given to place the screw in the center of the cross-section of the bone. In abaxial fractures the screw is inserted through the smaller fragment.
Size of the screw
These fractures are usually treated with 3.5 mm cortex screws. While it is possible to insert a 4.5 mm screw along the bone, the 8 mm screw head is wider than the bone’s width causing impingement on the articular surface/deep digital flexor tendon.
Left: Top a 3.5 mm screw inserted into an intact distal sesamoid bone; Bottom: a 4.5 mm cortex screw inserted into a distal sesamoid bone. Note: the screw head of the 3.5 mm cortex screw does not protrude over the edges of the bone, whereas the 4.5 mm cortex screw does.
Details on appropriate preparation and draping for this procedure can be found here.
3. Screw fixation
Application and adjustment of the aiming device
The aiming device with the 4.5 mm drill guide is applied over the marked entry point for the screw and fixed in a symmetric position relative to the mid-sagittal plane.
An exact lateral radiograph is taken along the axis of the aiming device.
Intraoperative fluoroscopy is a significant advantage and reduces operating time.
In a clinical case the fluoroscope is covered with sterile plastic drapes. In this cadaveric study this step was omitted to facilitate visualization.
Adjustments are made in positioning of the aiming device and lateral radiographic beam until the projection of the x-ray beam assures placement of the aiming device allowing complete intraosseous preparation of the screw hole. This step is greatly facilitated if intraoperative fluoroscopy or CT-imaging is available. The use of fluoroscopy is also time-saving.
Top: the lateromedial flouroscopic view of the distal sesamoid bone shows the aiming device not properly applied to the foot; Bottom: the aiming device was reseated but still needs adjustment
Top: the aiming device is properly seated; Bottom: dorsopalmar view of the properly seated aiming device.
Preparation of the hole through the hoof wall
The 3.5 mm drill bit is used to prepare a hole across the hoof wall. At that point the hole is flushed and the drill bit exchanged with a new 3.5 mm drill bit.
Attention: Application of the aiming device to the surface of the hoof and drilling of the glide hole from the hoof surface to the collateral cartilage and subsequently to the surface of the distal sesamoid bone provides additional support for the drill bit and prevents accidental slippage of the drill bit on the slanted surface.
The new (clean) 3.5 mm drill bit is used to prepare a hole across the distal sesamoid bone to avoid accidental contamination of the screw hole.
Note: the collateral cartilage may be ossified which leads to greater resistance during drill penetration.
Preparation of the glide hole
Subsequently, the glide hole is prepared along the smaller fragment with the new drill bit.
Progression of drilling is frequently checked radiographically.
The intraoperative view shows the glide hole being drilled across an intact distal sesamoid bone.
Note: ideally the drill hole should be located a drill bit width further proximal than depicted here.
Preparation of the thread hole
The 2.5 mm drill sleeve is inserted into the aiming device followed by preparation of the thread hole along the remaining portion of the bone.
Note: to allow drilling of the thread hole across the entire trans fragment a long 2.5 mm drill bit is needed. If not available, the aiming device is removed, the hole across the hoof wall concentrically enlarged with a 6-8 mm drill bit down to the proximal sesamoid bone, and the insert drill sleeve inserted directly into the thread hole.
The aiming device is removed and a 6 mm drill hole is prepared through the hoof wall and across the collateral cartilage following the 3.5 mm glide hole.
Care is taken to apply only minimal force while drilling because there is limited resistance to drill progress.
The intraoperative lateromedial radiograph or fluoroscopic view depicts the transluminal screw hole across the entire distal sesamoid bone.
A countersink depression is carefully prepared at the entry point of the glide hole into the distal sesamoid bone for ideal seating of the screw head.
Determining screw length
The approximate screw length required is determined by measuring the length of the distal sesamoid bone on a dorsopalmar/plantar radiographic view.
The measurement must be divided through the approximate factor 1.1 to correct for the magnification effect. Usually a 46 to 56 mm long screw is adequate for the distal sesamoid bone.
Note: it is better to insert a slightly shorter screw than one that protrudes into the hoof wall.
A screw 4 mm shorter than the measured length is inserted and solidly tightened.
An immediate postoperative dorsopalmar/plantar radiograph is taken to assess screw length and fracture fixation. The screw should reach the opposite end of the bone, but ideally not protrude beyond it.
Left: all three immediate postoperative radiographs demonstrate intraosseous location of the 3.5 mm cortex screw.
4. Alternate technique: computer assisted surgery
Computer assisted surgery
The use of computer assisted surgery provides excellent precision during preparation of the screw hole. The general approach is described in Module ?? P3 fractures
Left: Screen shot of the VetGate System showing three orthogonal views of an intact distal sesamoid bone. The green bar represents the preplanned orientation of the screw in its proportional diameter; Red represents the 3.5 mm drill bit penetrating the hoof wall and the ossified collateral cartilage. The red dotted line represents the trajectory of the drill bit. The bottom right picture represents the drill and drill bit as well as the preplanned screw hole in space.
This screen shot depicts the moment the 3.5 mm drill reaches the sagittal plane of the distal sesamoid bone (imaginary location of the fracture line). The dotted red line is slightly off axis but still within the distal sesamoid bone. The rest of the procedure is identical to the technique described above.
This screen shot represents completion of drilling of the thread hole. The computer identifies the drill bit exit point from the distal sesamoid bone and lists the length of the screw hole (56.9 mm – blue bar on the screen shot).
5. Closure of the hoof wall
Gentamycin-impregnated collagen sponge is inserted in the hole to fill the space between the surface of the distal sesamoid bone and the hoof wall.
Here shown for a dorsal hole in the hoof wall. The same technique applies.
The rest of the hole is filled with antibiotic-impregnated polymethylmethacrylate (PMMA) or artificial hoof wall resin (left) (again shown of a dorsal hole in the hoof wall).
The edges are sealed with cyanoacrylate glue or like shown here with an additional artificial hoof wall patch protruding over the edges of the hoof wall hole (again shown of a dorsal hole in the hoof wall).
Horses are usually in stall rest and hand-grazing only for at least 60 days, followed by hand walking or machine walking exercise for an additional 60 days before allowing turnout in a very small paddock. Most horses get about a minimum of 6 months before returning to training. Follow up radiographs are usually taken 60 days postoperatively. If necessary, additional radiographs are taken at a later stage. Return to work depends on the healing progress. Intra-articular medications depend on surgeons preference and the degree of articular damage seen.
The 5 months follow up radiographs revealed good healing of the fracture. The horse was sound and resumed its racing career successfully.
The prognosis for return to work is good to guarded. There is usually little articular damage acquired at the time of injury.