Executive Editor: Jörg Auer

Authors: Jörg Auer

Phalanges Sesamoid bone, Distal

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Glossary

1 Principles top

Fracture of the distal sesamoid bone - screw fixation enlarge

Position of the screw

Special attention is given to place the screw in the center of the cross-section of the bone. In abaxial fractures the screw is inserted through the smaller fragment.


Fracture of the distal sesamoid bone - screw fixation enlarge

Size of the screw

These fractures are usually treated with 3.5 mm cortex screws. While it is possible to insert a 4.5 mm screw along the bone, the 8 mm screw head is wider than the bone’s width causing impingement on the articular surface/deep digital flexor tendon.

Left: Top a 3.5 mm screw inserted into an intact distal sesamoid bone; Bottom: a 4.5 mm cortex screw inserted into a distal sesamoid bone.
Note: the screw head of the 3.5 mm cortex screw does not protrude over the edges of the bone, whereas the 4.5 mm cortex screw does.

2 Screw fixation top

Fracture of the distal sesamoid bone - screw fixation enlarge

Application and adjustment of the aiming device

The aiming device with the 4.5 mm drill guide is applied over the marked entry point for the screw and fixed in a symmetric position relative to the mid-sagittal plane.


Fracture of the distal sesamoid bone - screw fixation enlarge

An exact lateral radiograph is taken along the axis of the aiming device.


Fracture of the distal sesamoid bone - screw fixation enlarge

Intraoperative fluoroscopy is a significant advantage and reduces operating time.

In a clinical case the fluoroscope is covered with sterile plastic drapes. In this cadaveric study this step was omitted to facilitate visualization.


Fracture of the distal sesamoid bone - screw fixation enlarge

Adjustments are made in positioning of the aiming device and lateral radiographic beam until the projection of the x-ray beam assures placement of the aiming device allowing complete intraosseous preparation of the screw hole.
This step is greatly facilitated if intraoperative fluoroscopy or CT-imaging is available. The use of fluoroscopy is also time-saving.

Top: the lateromedial flouroscopic view of the distal sesamoid bone shows the aiming device not properly applied to the foot;
Bottom: the aiming device was reseated but still needs adjustment


Fracture of the distal sesamoid bone - screw fixation enlarge

Top: the aiming device is properly seated;
Bottom: dorsopalmar view of the properly seated aiming device.


Fracture of the distal sesamoid bone - screw fixation enlarge

Preparation of the hole through the hoof wall

The 3.5 mm drill bit is used to prepare a hole across the hoof wall. At that point the hole is flushed and the drill bit exchanged with a new 3.5 mm drill bit.


Fracture of the distal sesamoid bone - screw fixation enlarge

Attention: Application of the aiming device to the surface of the hoof and drilling of the glide hole from the hoof surface to the collateral cartilage and subsequently to the surface of the distal sesamoid bone provides additional support for the drill bit and prevents accidental slippage of the drill bit on the slanted surface.

The new (clean) 3.5 mm drill bit is used to prepare a hole across the distal sesamoid bone to avoid accidental contamination of the screw hole.

Note: the collateral cartilage may be ossified which leads to greater resistance during drill penetration.


Fracture of the distal sesamoid bone - screw fixation enlarge

Preparation of the glide hole

Subsequently, the glide hole is prepared along the smaller fragment with the new drill bit.


Fracture of the distal sesamoid bone - screw fixation enlarge

Progression of drilling is frequently checked radiographically.

The intraoperative view shows the glide hole being drilled across an intact distal sesamoid bone.

Note: ideally the drill hole should be located a drill bit width further proximal than depicted here.


Fracture of the distal sesamoid bone - screw fixation enlarge

Preparation of the thread hole

The 2.5 mm drill sleeve is inserted into the aiming device followed by preparation of the thread hole along the remaining portion of the bone.

Note: to allow drilling of the thread hole across the entire trans fragment a long 2.5 mm drill bit is  needed.
If not available, the aiming device is removed, the hole across the hoof wall concentrically enlarged with a 6-8 mm drill bit down to the proximal sesamoid bone, and the insert drill sleeve inserted directly into the thread hole.


Fracture of the distal sesamoid bone - screw fixation enlarge

The aiming device is removed and a 6 mm drill hole is prepared through the hoof wall and across the collateral cartilage following the 3.5 mm glide hole.

Care is taken to apply only minimal force while drilling because there is limited resistance to drill progress.


Fracture of the distal sesamoid bone - screw fixation enlarge

Fracture of the distal sesamoid bone - screw fixation enlarge

The intraoperative lateromedial radiograph or fluoroscopic view depicts the transluminal screw hole across the entire distal sesamoid bone.


Fracture of the distal sesamoid bone - screw fixation enlarge

Countersinking

A countersink depression is carefully prepared at the entry point of the glide hole into the distal sesamoid bone for ideal seating of the screw head.


Fracture of the distal sesamoid bone - screw fixation enlarge

Determining screw length

The approximate screw length required is determined by measuring the length of the distal sesamoid bone on a dorsopalmar/plantar radiographic view.

The measurement must be divided through the approximate factor 1.1 to correct for the magnification effect. Usually a 46 to 56 mm long screw is adequate for the distal sesamoid bone.

Note: it is better to insert a slightly shorter screw than one that protrudes into the hoof wall.


Fracture of the distal sesamoid bone - screw fixation enlarge

Screw insertion

A screw 4 mm shorter than the measured length is inserted and solidly tightened.


enlarge

An immediate postoperative dorsopalmar/plantar radiograph is taken to assess screw length and fracture fixation.
The screw should reach the opposite end of the bone, but ideally not protrude beyond it.

Left: all three immediate postoperative radiographs demonstrate intraosseous location of the 3.5 mm cortex screw.


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3 Alternate technique: computer assisted surgery top

Fracture of the distal sesamoid bone - screw fixation enlarge

Computer assisted surgery

The use of computer assisted surgery provides excellent precision during preparation of the screw hole. The general approach is described in Module ?? P3 fractures

Left: Screen shot of the VetGate System showing three orthogonal views of an intact distal sesamoid bone. The green bar represents the preplanned orientation of the screw in its proportional diameter; Red represents the 3.5 mm drill bit penetrating the hoof wall and the ossified collateral cartilage. The red dotted line represents the trajectory of the drill bit. The bottom right picture represents the drill and drill bit as well as the preplanned screw hole in space.


Fracture of the distal sesamoid bone - screw fixation enlarge

This screen shot depicts the moment the 3.5 mm drill reaches the sagittal plane of the distal sesamoid bone (imaginary location of the fracture line). The dotted red line is slightly off axis but still within the distal sesamoid bone.
The rest of the procedure is identical to the technique described above.


Fracture of the distal sesamoid bone - screw fixation enlarge

This screen shot represents completion of drilling of the thread hole. The computer identifies the drill bit exit point from the distal sesamoid bone and lists the length of the screw hole (56.9 mm – blue bar on the screen shot).

4 Closure of the hoof wall top

Fracture of the distal sesamoid bone - screw fixation enlarge

Gentamycin-impregnated collagen sponge is inserted in the hole to fill the space between the surface of the distal sesamoid bone and the hoof wall.

Here shown for a dorsal hole in the hoof wall. The same technique applies.


Fracture of the distal sesamoid bone - screw fixation enlarge

The rest of the hole is filled with antibiotic-impregnated polymethylmethacrylate (PMMA) or artificial hoof wall resin (left) (again shown of a dorsal hole in the hoof wall).


Fracture of the distal sesamoid bone - screw fixation enlarge

The edges are sealed with cyanoacrylate glue or like shown here with an additional artificial hoof wall patch protruding over the edges of the hoof wall hole (again shown of a dorsal hole in the hoof wall).

v1.3 2013-11-20