The major principles involved in lag screw and plate fixation of these fractures are compression of the articular surface and the additional stability provided by a (locked) plate extending the length of the bone. A locking plate that is securely attached to the proximal and distal metaphyses of the bone should help prevent catastrophic failure even if the plate does not perfectly follow the perpendicular course of the fracture plane.
Although the glide hole is typically drilled through the smaller fragment and the thread hole across the larger fragment, this particular fracture is often repaired in the opposite manner. The major reason is that it is much safer and easier to approach the leg from the lateral side than the medial side during implant removal, which is routinely performed in the standing position. Fortunately medial condylar fractures tend to be very close to the midline so there is little difference between the width of the two fragments. The bone in this location is so strong, that 25 mm of thread engaged with a 4.5 mm cortex screw is already exceeding the strength of the screw. Although some term the lag screw technique “backward”, the strength of fixation is equally adequate.
AO teaching video
Minimally invasive fixation of a medial condylar fracture of the third metacarpal bone using lag Screws and an LCP
The distal two screws across the condyle are placed routinely through stab incisions under fluoroscopic or radiographic control. It is also desirable to examine the joint arthroscopically to confirm fracture reduction, evaluate joint surfaces and remove any loose fragments.
If the fracture is repaired through an open exposure, a dorsolateral approach - as described in the approach section - is indicated.
The dorsal fracture line is exposed with a periosteal elevator just enough to follow its proximal extent. The position of the plate is adjusted appropriately according to the configuration of the fracture. It may not be possible to completely avoid a fracture line under the plate, but it is nearly always possible to firmly secure the proximal and distal ends of the plate without risking entry into a fracture plane.
5. Plate preparation and application
Plate selection and preparation
A plate of appropriate dimensions is selected. In third metacarpal fractures, a 10-11 hole broad 4.5 mm plate is usually used, and in third metatarsal fractures, a 12-13 hole plate is used in an average sized race horse. Although traditional plates can be used successfully, locking compression plates (LCPs) are probably superior.
Using an aluminum template contoured to the dorsolateral surface of the bone the selected plate is prepared with a large bending press. If a traditional plate is used, the precision of the contouring is very important. The most common error is having the plate too far from the surface of the bone at its ends.
Insertion of a distal screw
The plate is held in position with fingers, a clamp or a push-pull device and one of the two most distal holes in the plate is used for placement of a 4.5 mm cortex screw in lag fashion. This presumes that the fracture plane is still visible at this level of the bone.
This distal screw may be inserted directly if exposure is adequate, but can also be inserted through stab incisions if the surgeon prefers not to retract the flap. When this screw is tightened, the plate is pulled against the surface of the bone.
Pitfall: plate placement Before drilling the second screw hole, the surgeon must be certain that the entire length of the plate is in the desired location. Once the second screw is placed, the longitudinal alignment of the plate cannot easily be changed.
Insertion of cortex screws
At least one additional cortex screw is placed through the proximal aspect of the plate to pull it down against the bone. Often, about 50% of the holes in the plate are filled with cortex screws, because they are less expensive than locking head screws. It is essential, that all cortex screws be placed and tightened before the first locking head screw is inserted.
If the configuration allows, cortex screws may be placed in lag fashion anywhere along the lengths of the bone to compress the fracture plane. 4.5 mm cortex screws are easier to place through the plate holes and generally strong enough, but 5.5 mm cortex screws may be used, especially in larger horses.
Pitfall: drilling into the splint bone Because the plate is positioned slightly dorsally on the lateral side, the drill path for all screws through the plate is towards the contralateral splint bone. A surgeon should be cautious to avoid unnecessary injury to that structure.
Insertion of locking head screws
The remaining screw holes are filled with appropriate length 5.0 mm locking head screws.
The guides must be carefully inserted so that the drill holes are perfectly perpendicularly aligned and the screws tightened properly into the plate.
Locking head screws are inserted using the torque limiting adapter followed by complete tightening by hand. When using stainless steel implants in equine bone it is common to far exceed the 4Nm torque.
If desired, antibiotic impregnated polymethylmethacrylate (left) or pieces of a collagen sponge (right) can be placed in any voids in the plate.
All screws are double-checked for complete tightness.
Wound closure is described in the approach section.
Alternative in plate placement
Another option that some surgeons prefer is to markedly contour the plate in an effort to “follow” the fracture as it spirals up the limb. This has the advantage of lessening the risk of screws being inserted along the fracture plane, but disadvantages of difficulties applying and removing the plate.
6. Implant removal
Plate removal is performed about 3 months postoperatively and should be done with the horse standing under sedation and local anesthesia.
It is usually possible to feel some of the screw heads in the plate through the skin. An incision is made directly over the screw head and the screw is backed out until its head protrudes above skin level. Do NOT remove the screw completely.
It is very helpful to have a sterile plate matching the one in the horse available. Once two screws in the implant have been identified the matching plate can be laid over the skin and used as a template to perform individual stab incisions over each screw head.
All screws are individually retracted just to skin level. The surgeon should be sure to fully insert the screw driver into the screw head recess, especially in hexagonal heads to avoid stripping. Screw drivers containing the star drive configuration are less prone to stripping, which is a major advantage.
The exposed screw heads must be counted to be absolutely certain that the number matches the screws visible on the radiographs! All screws are then removed.
The plate is loosened by using a carpenter’s nail set on the edge of a screw hole and striking with a mallet.
If necessary, the incision is enlarged at the proximal end of the plate. A 12 mm osteotome is used to pry the proximal end of the plate away from the bone. The plate is grasped with sterile locking pliers and pulled upward. If the plate is not easily removed, an osteotome is placed between the bone and the plate and with the help of the nail set and a mallet the plate is driven proximally. The osteotome placed between the plate and the bone acts as an inclined plane to ease removal.
All incisions are closed with simple interrupted skin sutures and a padded bandage applied. The distal two screws that were inserted before plate application are left in place.
Documenting radiographs are obtained.
Horses receive stall rest and hand walking for 30 days followed by paddock exercise for at least another 30 days before returning to any type of training. A final set of radiographs is advised before work at high speed begins.
Horses are usually kept under stall rest and hand-grazing only for at least 60 days followed by hand walking or machine walking exercise for an additional 30 days before removing the plate. The two distal screws are usually left in the bone.
The prognosis for medial condylar fractures is generally very good if catastrophic complications can be avoided. Medial fractures tend to have less preexisting joint pathology than lateral condylar fractures.