Lag screw insertion is routinely done under arthroscopic control.
Prognosis is dependent on accurate articular reduction, minimal articular defects, and in turn, minimal postoperative osteoarthritis development.
Reduction of the fracture (if necessary) is achieved by flexion of the carpal region. Frontal fractures of the radial facet of the third carpal bone are repaired with cortex screws of either 3.5 mm or 4.5 mm diameter. This is determined principally by overall fragment size, but generally fragments measuring less than 10 mm in the dorsopalmar plane will require smaller implants.
In acute fractures there is frequently marked hemarthrosis and lavage will be necessary to permit visibility. This is performed by placing a egress cannula through the medial instrument portal.
The dorsal compartment of the middle carpal joint should then be evaluated completely and any additional lesions noted. Non-displaced fractures have variable amount of cartilage disruption.
Any loose cartilage and bone debris is debrided.
This picture shows the defect after removal of an osteochondral wedge at the proximal limit of fracture.
4. Needle placement
Insertion of needles
The proximal medial and lateral margins of the fracture are defined by arthroscopically guided percutaneous insertion of hypodermic needles.
It is important that these needles are placed perpendicular to the dorsal surface of the carpus to accurately delineate the fracture width.
An 18 gauge spinal needle is subsequently placed midway between these two needles close and parallel to the proximal articular surface and directed across the midpoint of the fracture as close to 90 degrees as possible. This needle is the most important directional guide for implant placement.
The configuration of most frontal plane slab fractures of the radial facet are such that the tip of the needle usually lodges in the palmar fossa of the bone and can be implanted in the non-articular surface to stabilize the spinal needle sufficiently.
Once the spinal needle has been placed, a further 18 gauge needle is inserted parallel to it into the carpometacarpal joint.
A flexed LM and ...
…skyline radiographinc views are taken to provide the proximal-distal location of the lag screw.
5. Screw fixation
Based on the radiographs a stab incision is placed over the dorsal aspect of the third carpal bone half way between the spinal needle and the needle in the carpometacarpal joint.
Drilling the glide hole
The glide hole is drilled across the fragment until it enters the fracture line (the distance is predetermined by measurement by the radiographs).
The drill depth can be verified using an intraoperative radiograph.
Drilling the thread hole
The 3.2 mm drill guide is inserted into the glide hole and the 3.2 mm thread hole is prepared through the remaining third carpal bone. It is recommended that the hole is drilled completely through the entire bone to avoid the screw being longer than the hole which can result in screw breakage.
The hole is lightly countersunk at the dorsal surface and debris is flushed.
Determining screw length
Correct screw length is determined using the depth gauge. The screw is generally 32-36 mm long.
Tapping the hole
The hole is tapped with a 4.5 mm tap protected by the 4.5 mm drill guide. This should be done by hand.
The 4.5 mm screw of appropriate length is inserted. The screw is tightened under arthroscopic visualization. After the screw is tightened any debris or elevated cartilage is removed from the fracture line.
Intraoperative radiographs are take to confirm screw length and correct positioning.
Pearl: Use two fingers and thumb when doing final tightening, thereby avoiding excessive torque and potential breaking of screw head, especially if 3.5 mm implants are used.
Intraoperative radiographs are taken to confirm screw length and correct positioning.
In the repair of fractures with small dorsopalmar thickness (using a 3.5 mm screw) there is the potential for breaking the fracture fragment. This is rare, but if it occurs, removal of the fracture fragments is necessary and athletic soundness can still result.
All incisions are sutured (only skin sutures are necessary). A sterile bandage is placed on the carpus. Perioperative antibiotics are indicated for 24 h and the horse should be placed on anti-inflammatory medication for 5 – 7 days. The horse should be confined to a stall (box) but hand walking is commenced at 2 weeks.
The horse can usually go back into race training at 4 months but only after clinical and radiographic examination and clearance by veterinary surgeon.