Plate fixation/transarticular lag screws

Complete cartilage removal is important to provide for maximum contact between the opposing subchondral bone plates. With compression, generated by the fixation technique, frictional forces are generated between the subchondral bone plates, which maximizes strength and stability of the fixation, providing maximum patient comfort and allowing early removal of postoperative cast support.

The implant is positioned over the proximal and middle phalanx in the sagittal plane. The distal end of the plate is placed in the proximal region of the middle phalanx. The distal end of the plate should not impinge on the proximal attachment of the distal interphalangeal joint capsule as well as the extensor process (A). The palmar/plantar position of the transarticular screws and distal plate screw should lie within the palmar/plantar eminences (B) to avoid the palmar/plantar aspect of the distal interphalangeal joint capsule and navicular apparatus.

Arthrodesis of the PIP joint using a 3-hole 4.5 DCP and lag screws

This procedure is performed with the patient placed in lateral recumbency and through the dorsal approach.

A proximal interphalangeal locking plate (PIP) (right) is advocated as it is specifically designed for pastern arthrodesis. Other plate constructs have been used successfully, including narrow, 4.5 mm, 3 and 4-hole DCPs and LC/DCPs.
The technique for fixation with DCP or LC/DCP is described for uniaxial eminence fractures as an example. For more details click here.

Extending the digit realigns the articulation into normal anatomic configuration.
The plate is contoured to approximate the sagittal dorsal surface of the distal proximal phalanx and proximal middle phalanx with the stacked combi-hole of the plate positioned over the proximal end of the middle phalanx.
The distal end of the plate is then bent slightly to ensure that the distal plate screw is inserted directly below the subchondral plate and into the proximal palmar/plantar aspect of the middle phalanx.

If realignment of the joint cannot be achieved manually, it may be necessary to attach the pre-contoured plate to the middle phalanx first and then use the plate as a lever to bring the joint into anatomic alignment.
The hole for the first screw is drilled just distal to the subchondral plate and the length of the screw hole is measured. Now the 5.0mm locking head screw is inserted and provisionally tightened.

Note: It may be advantageous to prepare the glide holes and their countersinking for the two abaxial transarticular cortex screws prior to applying the plate to ensure that the plate does interfere with their preparation at a later point of time.

In early mild cases, where the palmar/plantar support structures are not markedly contracted and fibrosed, reduction of the luxation may be accomplished manually.
In more advanced cases, anatomic realignment may not be possible by manual manipulation and may require using the plate as a lever arm…

….or in more severe cases performing a corrective ostectomy.

A 4.5 mm cortex screw is placed in load position in monocortical fashion. The screw should not be fully tightened, so the plate is not yet fully compressed to the bone and dynamic compression is not engaged.

Two 5.5 mm cortex screws are now placed transarticularly in lag fashion extending from the dorsal distal proximal phalanx into the proximal palmar/plantar middle phalanx. They cross the joint at the junction of the palmar/plantar ¼ and the dorsal ¾.
It is important that first one screw hole is finished and the screw inserted before the thread hole for the second screw is prepared.
The screws are placed medial and lateral to the plate and parallel to the sagittal plane.
Real time imaging is useful to direct screw positioning and intraoperative imaging of some nature is necessary to confirm proper implant position.

It is very important that the glide holes for the transarticular lag screws in the dorsal distal aspect of the proximal phalanx are appropriately countersunk to ensure the screw heads to engage the dorsal cortex in an appropriate fashion and thereby avoid bending, weakening and potentially breaking the screw head.

It is also important, that screws of appropriate length are used; they should not penetrate beyond the palmar/plantar cortex of the middle phalanx. For the transarticular lag screws, a screw measuring 4-6 mm less than the measured length is appropriate and prevents placing a screw which protrudes beyond the palmar/plantar cortex of the middle phalanx.

Once the transarticular lag screws are fully tightened, the proximal plate screw is fully tightened which compresses the plate to the bone and provides additional compression between the subchondral plates of the proximal phalanx and the middle phalanx.

Finally, the middle hole of the plate is filled using an 5.0 mm locking screw.

The final construct configuration is checked with appropriate imaging to ensure appropriate length and positioning of all implants.

The horse is placed in a half-limb cast for 2 weeks post surgery. That cast is removed with the horse standing and sedated and replaced by a bandage.
The horse is confined in the stall for 3 months. In the second 6 weeks a progressive program of hand-walking exercise is introduced.
Clinical and radiographic follow-up is performed at 3 months post surgery. If complications such as excessive lameness or implant-associated complications are not evident, the horse is allowed increasing amounts of paddock exercise.
Lameness should dissipate by 6-8 months postoperatively to a level consistent with reintroduction to intended use.