1 Principles topenlarge
Internal fixation with cortex screws, inserted in lag fashion, reduces the shear forces across the fracture plane by interfragmentary compression and neutralizes the inherent torsional forces caused by the anatomical configuration of the sagittal groove of the proximal phalanx.
Internal fixation also shortens the time to healing and increases the quality of healing, frequently preventing arthritis of the fetlock joint.
Take care to thread the entire transcortex, while not endangering the soft tissues beyond the bone, and pay close attention to adequate countersinking. Technique errors may lead to complications and possibly implant failure.
Intraoperative radiographic control is essential for proper placement of the interfragmentary screws across the proximal phalanx. Either a real-time imaging system or anatomic references, such as 2 mm drill bits, can be used.
2 Approach top
This procedure is performed with the patient in lateral recumbency.
Multiple stab incisions can be used as surgical approach to the bone. If detailed identification of the anatomic landmarks is desirable or necessary, a longer incision along the lateral aspect of the proximal phalanx is preferred.
The lateral incision for multiple screw insertion begins at the fetlock joint and extends to the distal end of the fracture.
3 Fixation top
Incomplete fractures require no reduction.
Since the fracture originates at the proximal articular surface, the fixation always starts immediately below the fetlock joint.
The screws can be configured either in a dorsal plane or in a triangular configuration.
The most proximal screw(s) in either configuration is/are the most important and critical one/s for a successful result.
Dorsal screw placement
In the dorsal screw placement, a single screw is placed distal to the mid-sagittal groove of the proximal articular surface and centered in the dorsal two thirds of the palpable width of the bone. In long incomplete fractures, a minimum of 2 and most commonly 3 screws will be used.
A 4.5 mm cortex screw is typically used for this procedure. The 4.5 mm glide hole is drilled immediately past the fracture plane using preoperative planning and/or intraoperative radiographic control.
The 3.2 mm thread hole is prepared using the 3.2 mm insert drill sleeve. The thread hole is drilled through the entire transcortex of the proximal phalanx.
Pitfall: blind hole
Not exiting the drill hole through the transcortex leads to a blind hole and may result in serious complications and potential lack of interfragmentary compression of the fracture.
The 4.5 mm countersink is used to prepare uniform seating of the screw head and to ensure concentric loading of the screw head.
This is especially important at an oblique bone surface relative to the screw axis. In this case special attention has to be given to the proximal half circle of the hole. However, care must be taken to prevent penetration of the screw head through the near cortex.
Measuring screw length
The depth gauge is used to measure the proper length of the screw. The depth gauge should always be inclined proximally to measure the maximum length of the cortex screw appropriate for the bone.
Inclining the depth gauge distally measures a screw length that does not engage all of the far cortex and encourages stripping of the screw during tightening, resulting in inadequate compression of the fracture plane.
Insertion of the first screw
The 4.5 mm tap, protected by the 4.5mm tap (drill) sleeve, is used to prepare the thread hole for screw insertion. A screw of the proper length is chosen and solidly tightened.
The number of the screws used depends on the length and the configuration of the fracture. When screws are placed in the dorsal plane, they are typically placed 20-25 mm apart.
If the fracture plane spirals medially or laterally, the orientation of the screws is rotated as well to keep them perpendicular to the fracture plane.
Triangular screw configuration
Triangular screw configuration represents another option of screw placement.
The initial screw in the triangular configuration is placed slightly more dorsal than in the dorsal configuration.
A second screw is placed palmar/plantar to the initial screw, parallel to the first screw, again just distal to the joint.
A third screw is placed in the dorsal plane 25 mm distal to the second screw in the center of the proximal phalanx.
If the fracture plane spirals medially or laterally, the orientation of the screw is rotated as well to keep it perpendicular to the fracture plane.
A fourth screw can be used if the fracture length dictates it.