NOTES 32 Thursday 8th September n Hall 1A 16.25–16.50 CT assisted fracture repair (cheating with CT) Dean W.
Richardson New Bolton Center, University of Pennsylvania, 382 West Street Road, Kennett Square, Pennsylvania 19348-1692, USA.
Internal fixation of equine fractures has advanced over the last few decades primarily due to the improving expertise of practicing surgeons and correct application of implants according to proven mechanical principles.
The unfortunate reality of fracture repair in horses, however, is that failures still occur commonly because of infection, delayed union and implant failure.
Delayed union in species other than the horse is a lesser issue because the consequences of severe lameness and overload of the contralateral limb are less and the expectations for return to function are usually lower.
In horses, we still generally recognise that immediate comfort (ie stability) is an important element of successful fracture repair and that delayed union is a major complication because of implant failure and contralateral limb problems.
Equine surgeons have naturally tended towards aggressive open fracture repairs in order to minimise mechanical ‘errors’ that might reduce stability.
Arthroscopy and fluoroscopy/intraoperative digital radiography are already routine adjuncts to many internal fixation procedures but intraoperative CT has enormous potential to minimise the errors that are inherently more likely with minimally invasive techniques.
In particular, CT is especially valuable in complex fractures that are difficult to ‘figure out’ from plane films (eg comminuted P1), fractures that do not allow adequate fluoroscopic projections that can assure accurate screw placement (eg tarsal slab fractures) and bones with a complete lack of direct exposure (P3 and navicular).
One obvious advantage of a preoperative CT is that it can help make complex fractures more completely visualised as one looks at plane films (Fig 1).
The greatest advantage of CT assistance, however, involves using radiodense markers and measurements to confirm exact positioning of implants.
For example, a displaced articular wing fracture of the distal phalanx is exceedingly difficult to ‘hit’ accurately with a fluoroscopically guided screw.
The surface geometry of the distal phalanx is complex, hidden by hoof wall and the fracture plane cannot be directly observed.
Furthermore, the ‘target’ of the wing of P3 is quite narrow.
CT assistance makes the surgery feasible and relatively easy: • Meticulous cleaning of the entire hoof (not yet necessarily sterile). • Estimate the predicted locations of screw entry through the hoof wall AND the (imagined) extension of the line of the screw. • Place a bleb of barium sulphate paste in both locations. • Perform the CT. • Based on the first CT, correct the location of the barium spots. • Repeat the CT until there are perfect entry and ‘exit’ marks. • Mark the proposed entry and ‘exit’ sites by drilling shallow (easily recognised ~1mm deep) depressions. • Re-prep the foot for aseptic surgery. • Place a pneumatic tourniquet at the fetlock. • Using the marked spots for orientation, drill a large (7–10 mm) hole through the hoof wall. • If available, use an aiming device for the drill with the back end of the aiming device at the marked site of the linear extension of the exit site. • Drill a 2.5 or 3.2 mm hole to the fracture.
This can be done by measurement or feel depending on the fracture. (It is safer to measure but be sure that you are correctly identifying the surface of the near fragment.) Repeat CT to check length and direction of pilot hole • Correct (if necessary) and drill glide hole. • (Ideally) check once more with CT. • Complete the lag screw technique routinely. • (Ideally) repeat CT.
As with other uses of intraoperative imaging, some ‘shortcuts’ are possible with increasing experience. Other fractures The same principles apply when using the CT for intraoperative guidance in other types of fractures or arthrodeses.
Use a skin marker to help define the intended site of implant insertion.
Use a skin marker on the extended line for the drill bit.
Take post insertion CT to triple check placement (Figs 2 and 3). Fig 1: An articular wing fracture of the distal phalanx transverse CT image taken a few mm distal (below) the articular surface.
Top right: small blebs of barium sulphate paste are placed in depressions in the hoof to help aim the drill.
Middle left: Approximate measurements can be taken to help make the glide hole the correct length and to help estimate correct screw length.
Middle right: With CT assistance, the screw can be accurately placed in a small target.
Bottom: Lateral and DP post operative views. 33 Hall 1A n Thursday 8th September CT assistance is certainly not ‘essential’ for most fractures but it provides one more level of guidance to help assure accurate fixations.
Although the machine used in these cases (CereTom made by NeuroLogica Corporation, Danvers MA) has limitations because of its size, the portability of the unit and its speed make it very practical for intraoperative use in the distal limb of horses. Fig 3: Dorsopalmar (A) and palmar tangential (B) views of an acute navicular bone fracture in an 11-year-old Thoroughbred gelding.
In (C) a 2.0 mm hole is made to check drill placement.
D: A 3.5 mm glide hole is made to the fracture plane.
E: The 2.5 mm thread hole is continued through the larger fragment of the navicular bone.
F: The final placement of the 3.5 mm screw.
Orthogonal dorsopalmar (G) and lateromedial (H) projections confirm central placement of the screw. Fig 2: Computed tomography can also help accurate placement of screws in small fractures in locations such as the distal tarsal bones where tangential views are unavailable.
A: Scintigraphic study showing focal intense radiopharmaceutical uptake in dorsolateral T3.
B: DMPLO radiograph demonstrating displaced slab fracture of T3.
D: Barium paste blebs and/or steel staples can be used as skin markers to help define the exact centre of the fracture fragment.
F: Post operative CT showing exact central placement within the small fragment.
G: Post operative DMPLO projection.
H: The fracture was radiographically healed in less than 90 days.
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