Authors of section

Authors

Anton Fürst, Wayne McIlwraith, Dean Richardson

Executive Editor

Jörg Auer

Open all credits

Screw fixation in the standing horse

1. Principles

The major principle involved in standing lag screw fixation of these fractures is compression of the articular surface.The fixation of the distal portion of the bone may help prevent proximal propagation of the fracture, but definitely does not prevent it.

Standing lag screw fixation affords these benefits and specifically avoids the risks of recovery from general anesthesia. There remain risks for catastrophic failure for several weeks after fixation of the distal portion of the bone.

Medial incomplete condylar fracture - screw fixation

Screw placement

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. Fortunately, medial condylar fractures tend to be 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. Therefore, strong fixation can be achieved even though the repair is called “backward” or “reverse” lag screw.

Medial incomplete condylar fracture - screw fixation

2. Patient positioning

This procedure is performed with the patient standing.

screw fixation in the standing horse

3. Preparation of the stab incision

Location of the insertion

The site of the first screw is the center of the epicondylar fossa. The location of the insertion in the fossa can be estimated by bisecting an imaginary line extending from the palmar/plantar wing of the proximal phalanx to the palpable dorsal edge of the lateral condyle.

Displaced lateral condylar fracture - screw fixation

Needle placement

A needle should be placed at the selected site and with the help of a lateromedial radiographic/fluoroscopic view its correct location is verified. It is critical to remember that this view is the ONLY view that truly indicates the location for the incision/drill.

Note: The dorsopalmar/plantar view only provides information relative to the height of the needle but not whether is located in the center, at its edge or outside the fossa.

Displaced lateral condylar fracture - screw fixation

Incision

An incision is made with a #10 scalpel blade parallel to the fibers of the collateral ligament directly down to the bone surface. The epicondylar fossa is recognizable with the tip of the scalpel as it follows its contour.

Medial incomplete condylar fracture - screw fixation

4. Preparation of the stab incision

Preparation of the glide hole

In most cases, 4.5 mm cortex screws are used.

The glide hole must be drilled exactly to the fracture plane or just beyond it. It is absolutely essential in a non-displaced fracture, that the correct length and direction of the glide hole be verified with radiographs. Preoperative measurements on radiographs and careful measurements during drilling will help avoid any errors.

Medial incomplete condylar fracture - screw fixation

If generously draped, the overall alignment of the limb is easy to appreciate in a standing horse and it is actually easier to maintain correct orientation of the drill and drill bit because of this.

Medial incomplete condylar fracture - screw fixation

The 4.5 mm drill bit is removed, holding the drill guide in place. A 2.5 mm pin is placed through the drill guide into the glide hole.

Medial incomplete condylar fracture - screw fixation

The 4.5 mm drill guide is removed and the 3.2 mm drill guide is slipped over the 2.5 mm pin into the glide hole.
The pin is removed.

Medial incomplete condylar fracture - screw fixation

Ideally, one should check the positioning of the glide hole by taking a lateromedial radiographic view with the drill guide in place. If the hole in the drill guide is clearly visible and the proximal sesamoid bones are superimposed, the surgeon can be very confident that the screw will be properly directed across the condyle.

Medial incomplete condylar fracture - screw fixation

Preparation of the thread hole

The 3.2 mm drill bit is inserted into the corresponding drill guide and the thread hole is prepared across the remainder of the bone. The drill bit should be removed and cleaned frequently because this is very dense bone.
Caution should be used when approaching the far cortex to minimize damage and debris as the drill bit exits the bone.

Medial incomplete condylar fracture - screw fixation

The 3.2 mm bit is removed and the 2.5 mm pin re-inserted. This pin is valuable to minimize any difficulties finding the hole through the stab incision.

Medial incomplete condylar fracture - screw fixation

Tapping the hole

No countersink depression is prepared because the screw head comes to lie in the epicondylar fossa. Countersinking may add additional trauma to the collateral ligament.
Note: Some surgeons nevertheless prefer to prepare a countersink depression.

The 4.5 mm drill guide is subsequently placed over the 2.5 mm pin (A). The pin is removed (B) ...

Medial incomplete condylar fracture - screw fixation

... and the 4.5 mm tap inserted through the guide (C).

Medial incomplete condylar fracture - screw fixation

The hole is tapped routinely, keeping the very dense bone in mind. It is critical to tap completely through the far cortex (red circle). The bone is so dense that even a single millimeter of untapped bone may prevent complete insertion and tightening of the screw.
A long tap is strongly preferred to avoid impinging the end of the drill on the edge of the drill guide (yellow circle). This could result in stripping the threads in the hole or breaking the tap.

Medial incomplete condylar fracture - screw fixation

Screw insertion

In most horses, measurement of the hole is not performed in this location. Most race horses accommodate a 52 mm cortex screw distally.

The screw is inserted but not fully tightened.

Ideally, a fluoroscopic view is taken to estimate if the screw length is correct. If correct, the screw is fully tightened. Some surgeons prefer to have the horse take weight off the limb while the final tightening is performed.

Medial incomplete condylar fracture - screw fixation

A second screw is placed approximately 2 cm proximal to the first screw.

For this and all other additional screws a countersink depression is prepared for seating the screw head. Countersinking should only be deep enough to prevent screw bending. The goal is NOT to bury the screw head. That will make it more difficult to remove the screw and potentially weaken the bone.
It may be helpful to leave the power screw driver attachment in the head of the first screw to assist alignment of the second screw.

Medial incomplete condylar fracture - screw fixation

Additional screws may be added according to the radiographically visible length of the fracture. In standing horses, however, most surgeons place only 2-3 screws.

NEVER place a screw where you cannot clearly see the fracture line.

Medial incomplete condylar fracture - screw fixation

5. Aftertreatment

Horses are usually placed in a well-padded lower limb bandage.

Maintaining a horse in cross ties should be considered for horses with hind limb fractures to prevent them from lying down.

screw fixation in the standing horse

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 60 days before allowing turnout in a very small paddock. Most horses are rested for a minimum of 6 months before returning to training.

Intra-articular medications depend on surgeons preference and the degree of articular damage seen.

screw fixation

Radiographic evaluation

Follow up radiographs are usually taken 90 days postoperatively and again before returning to training.

Implant removal

Screw removal is generally recommended only if the screws extend up into the diaphysis.

Prognosis

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.

screw fixation in the standing horse