1 Principles topenlarge
Number and direction of screws
Interfragmentary compression is achieved through insertion of at least one screw placed in lag technique. Ideally, a second screw should be inserted to provide additional compression and rotational stability.
In these fractures there is adequate space for two 4.5 mm screws.
In perisagittal fractures, the screws may be inserted in a slightly oblique direction to acquire additional bone purchase in the fragment.
Screw insertion can be accomplished by applying routine technique or with the help of an aiming device.
Below the technique is described using the aiming device.
Intraoperative availability of direct radiography or a fluoroscope is a valuable asset and speeds up the procedure.
2 Screw fixation topenlarge
Determining screw length
Approximate length of screw(s) can be determined by measuring the width of the hoof capsule at the predetermined location(s) of screw insertion (orientated parallel to the joint and orthogonal to the fracture plane). From that width, the width of the hoof wall (as determined after performing the trephine- or drill hole down to the distal phalanx) is deducted twice, resulting in the approximate length of the screw. The glide hole measures approximately half the screw length.
Preparing the hole through the hoof wall
An 8-10 mm hole is drilled parallel to the joint surface in lateromedial direction across the hoof wall starting at the mark prepared preoperatively.
An alternative method involves the use of a trephine to prepare a 10-mm diameter hole across the hoof wall at the pre-determined site(s). The plug(s) is/are removed, stored in a saline-soaked sponge, and later re-inserted to fill the hole(s) after screw insertion.
Preparing the glide hole
The aiming device with the 4.5 mm drill sleeve secured in is inserted into the hole in the hoof wall down to the surface of the distal phalanx. The aiming device is oriented parallel to the joint surface and orthogonally relative to the fracture plane and solidly tightened. If the tip of the aiming device slips out of position during tightening, a small, shallow hole is drilled at the correct location. In mid-sagittal fractures the device is oriented orthogonally relative to the mid-sagittal plane of the distal phalanx (left).
The distal interphalangeal joint is distended and the arthroscope inserted using routine technique through a stab incision located 2 cm proximal to the coronary band and 2 cm lateral to the sagittal plane.
The joint is explored and the fracture identified with a hook probe.
The 4.5 mm glide hole is drilled to the fracture plane.
Frequent radiographic control of drill bit advancement relative to the fracture plane is encouraged.
Preparing the thread hole
The 3.2 mm drill guide is inserted through the aiming device into the glide hole. If necessary, the aiming device is somewhat loosened and using rotating motions any intraarticular malalignment of the fracture fragments is corrected under arthroscopic supervision. Once established the aiming device is tightened again.
The 3.2 mm thread hole is drilled across the remaining bone. Care is taken not the excessively penetrate the hoof wall on the opposite side.
Note: The long 3.2 mm drill bit must be used to prepare the thread hole; the standard 3.2mm drill bit is too short.
Tapping the hole
The thread hole is tapped using the 4.5 mm tap through the tap sleeve inserted into the aiming device.
Note: This step is out of routine order but necessary, as the countersink cannot be inserted through the aiming device.
A countersink depression is prepared in the surface of the distal phalanx. Care is taken to mainly engage the palmar/plantar rim of the glide hole in the depression.
The screw of predetermined length is inserted and solidly tightened.
Note: Protrusion of the screw tip into the hoof capsule can be detected by taking a tangential radiographic view relative to the exit point of the screw from the distal phalanx. If the screw is judged to be too long, it is exchanged by a shorted one.
Compression of the fracture is observed arthroscopically.
A second 4.5 mm screw is inserted parallel to the first screw using the technique just described.
Insertion of the power screw driver into the hexagonal/star-drive screw head aids in achieving a parallel axis for the second screw.
Radiographic views of a completed fixation
3 Closure of the hoof wall topenlarge
Filling the holes
A small portion of a gentamycin-impregnated collagen sponge is inserted in each hole to fill the space between the surface of the distal phalanx and the hoof wall.
The rest of the hole is filled with antibiotic-impregnated polymethylmethacrylate (PMMA) (left) or artificial bone (right). The edges are sealed with cyanoacrylate glue. If desired a Kevlar® strip can be glued over the top followed by non-adhesive tape covered.
Filling the hoof wall defect(s) with an antibiotic soaked sponge plug at the end of the surgical procedure and covering the hoof wall with a tight non-elastic, watertight bandage is an alternate option. In such a situation it is best to keep the horse in the hospital until the screw heads are covered by granulation tissue.
Once that has occurred the granulation tissue is painted with iodine solution every other day to dry it up.
All this prolongs the hospital stay and increases treatment costs significantly.
Once dry tissue covers the screw heads the hoof wall defect(s) is/are filled with hoof acrylic or artificial hoof.
Closing the trephine holes
In case trephine holes were prepared to access the distal phalanx, the plug(s) are reinserted into the hoof wall defects after insertion and tightening of the screw(s).
A Kevlar® strip can be glued over the top.
The edges of the Kevlar® strip are subsequently sealed with cyanoacrylate glue (super glue) to prevent access of bacteria to the surgery site(s).