Screw fixation

Precise anatomic reduction of the articular surface of the proximal sesamoid bone is the most critical and yet most difficult part of the surgical procedure.
The proximal fragment is subject to multiple twisting and sliding forces and can be very difficult to be maintained in alignment during the surgery. Frequent arthroscopic evaluation is required to ensure perfect reduction prior to screw placement.
Less than perfect reduction results, aside from the development of degenerative joint disease, in cyclic fatigue of the implant(s) and may lead to premature screw breakage.

The proximal sesamoid bone has a relatively small pyramidal shape that leaves little room for error with screw placement. Insertion of two screws would be ideal to increase the stability of the fixation and counteract torsional forces upon the fixation, but accurate placement requires significant skill and planning.
The author’s preference is a combination of a 4.5 mm and a 3.5 mm screw to optimize compression and counteract rotational forces within the space limitations of the bone.

Advanced imaging such as fluoroscopy is an advantage. If not available, frequent intraoperative radiographic monitoring and insertion of anatomical markers are used to help the surgeon to implant the screws accurately.
The following description of the procedure uses standard radiography as imaging modality.

Fixation of a midbody fracture of a proximal sesamoid bone with a 4.5mm cortex lag screw

Details on appropriate preparation and draping for this procedure can be found here.

A standard arthroscopic approach is made to the palmar/plantar pouch, taking care to place the arthroscope in the most proximal aspect of the pouch.
The fracture is visualized and any relevant comminution along the fracture line is debrided.

A stab incision is performed through the skin and the insertion of the suspensory ligament to achieve access to the apex of the proximal sesamoid bone. A second stab incision through the skin at the level of the distal sesamoidean ligaments provides access to the base of the bone. The arms of large pointed reduction forceps are inserted through these incisions to achieve and maintain reduction of the fracture.

Occasionally, an additional pointed reduction forceps is required between the abaxial margin of the proximal fragment and the axial margin of the base to help eliminate the sliding action that often occurs when trying to reduce the fracture. For the latter purpose additional stab incision are needed to apply the reduction forceps.

External rotation of the foot while holding the metacarpo/metatarsophalangeal joint in dorsiflexion is helpful in reducing the fracture.

Fracture reduction is confirmed arthroscopically.

If advanced imaging technologies are not available, 19 gauge 1.5 inch (3.8 cm) needles are used as radiographic markers to assist the surgeon with screw placement.
The needles are placed under arthroscopic supervision at the apex, base, and fracture line of the proximal sesamoid bone in both the frontal and sagittal planes.

Screw placement is performed either in proximal to distal or distal to proximal direction depending on the fracture configuration and the surgeon’s preference. Most commonly, the screws are inserted from distal to proximal, as described in the following steps.
A stab incision is placed palmar/plantar and median to the vascular bundle at the base of the proximal sesamoid bone. If two screws are inserted, two parallel stab incisions are placed.

Standard lag technique is applied. Intraoperative diagnostic imaging is strongly encouraged to determine when the glide hole reaches the fracture plane and to prevent inadvertent overdrilling of part of the proximal fragment, which would result in a weakened fixation. It is critical not to lose contact with the drill hole when the drill guides are changed. Therefore a K-wire is used to fill the hole once the large drill is removed and to facilitate placement of the small drill guide in the glide hole.

Countersinking is not recommended because the screw head comes to lie in the natural concavity located at the base of the proximal sesamoid bone and origin of the different distal sesamoidean ligaments in the area.

A longer drill bit (180 mm) is recommended to avoid interference of the drill with the bulbs of the heel and in doing so impede the ability to maintain the correct drilling angle.

It is helpful to have an assistant dorsiflex the foot to allow more room for the drill.

When two screws of different sizes are placed, the 4.5 mm screw is preferably inserted more axially.

Final tightening of the two screws is performed in an alternating pattern.

Proper placement of the screw(s) is confirmed both radiographically ...

... and arthroscopically.

Occasionally, the orientation of the fracture line does not allow routine screw insertion from the base of the bone. In these cases the screws are implanted in proximolateral to distomedial direction.

The same principles apply for surgical repair, but some difficulty with screw placement may be encountered at the abaxial margins of the proximal fragment, because of the uneven, slanted bone surface in this location.

A temporary half-limb cast is applied for recovery from general anesthesia and removed shortly thereafter.

Stall rest is recommended for 60 days, followed by hand-walking for 60 days. Subsequently paddock exercise is recommended for a minimum of 90 - 120 days. Ideally, horses should not be returned to active training prior to 10 months.

Radiographic evaluation is performed every 60 days until the fracture has healed (generally 4-6 months).

Prognosis is generally considered to be fair, primarily because of the less than optimal healing properties of the proximal sesamoid bone, which often results in delayed or incomplete healing, sesamoiditis, and/or failure of the fixation.