33rd Annual Scientific Meeting proceedings

This presentation focusses on the surgical management of spinal trauma, specifically those resulting in vertebral fracture and/or luxations. There have been a multitude of recent advancements in implant placement techniques, reduction techniques and spinal instrumentation systems. However, in terms of ‘have we gotten better’, it is important to appreciate that studies comparing the outcomes from newer techniques to other, older techniques do not exist – and therefore direct comparisons cannot be made. As vertebral fracture-luxation are often associated with traumatic disc extrusion and other systemic injuries, successful outcomes are also associated with appropriate management of secondary spinal cord injury (limiting hypotension, reducing anaesthesia time, etc.), the detail of advances in these areas is outside of the scope of this presentation.

Spinal instrumentation
Screw/pin and sculpted boluses of polymethyl methacrylate (PMMA) and screw plate constructs, placed dorsally in a bi-cortical fashion into the vertebral bodies, represent traditional spinal instrumentation systems in dogs and cats [1, 2]. Locking plate systems offer greater flexibility for implant insertion angles by a combination of plate contouring and/or allowing some angulation of screw head thread lock. Disadvantages of PMMA constructs includes difficulty in surgical revision, infection risk, challenges in wound closure and challenges in utilisation adjacent to laminectomy defects in relation to thermal injury and dead space which might increase the risk of re-compression secondary to seroma or haematoma formation. 

Titanium locking plates have recently been described for use in canine and feline spinal fractures [3]. Titanium implants have shown advantageous yield stress compared to stainless steel on an ex vivo bovine spine model [4]. These systems have the additional advantages of improved biocompatibility, resistance to infection and MRI compatibility. Ventral mono-cortical titanium plate fixation of the cervical spine is now a commonly reported technique.

Fluoroscopic-guided external fixation pins have the advantage of being minimally invasive and have recently been described to provide good outcomes when using a type 1a construct for the management of thoracolumbar fractures [5].  A trans-ilial technique has been described for L7 [6].

The vertebral pedicle is anatomically more robust in the caudal lumbar region and represents an attractive location for implant placement, given the medial and lateral cortices can be engaged by screw threads. The pedicles of L6 were recently described as suitable for pedicle screw insertion for the management of vertebral fractures [7]. In human medicine, poly-axial screws placed into the vertebral pedicles and fixed with rods into ‘tulip’ heads, has been a mainstay of vertebral stabilisation for several years. Pedicle screw rod fixation (PSRF) has the advantage of combining flexibility in implant insertion with ease of application, revision, and lack of exothermic reaction. PSRF have been reported for us in lumbosacral degenerative stenosis and now been reported for use in canine spinal fractures [8]. As these systems have been miniaturised for use in a range of screw sizes, their application is likely to become more popular with time.

Finally, there has been an increased interest in the use of ‘custom’ stabilisation implants, particularly in relation to stabilising vertebral malformations. These are patient-specific implants, often produced in titanium using 3D-printing and additive manufacturing techniques [9].

Implant placement and surgical planning
Traditionally, surgeons plan implant placement by a ‘free hand’ technique, that is planning from pre-operative imaging, experience of vertebral morphology and tactile feedback when drilling. Implant placement can be challenging in our patients due to the high variation in morphology and angled cortical surfaces. Planning from CT is standard practice and is known to be superior to radiography. Optimal safe implantation corridors can be determined using multi-planar reconstruction and open-access DICOM software.

3D-printed resin patient-specific drill guides can be utilised to aid implant selection by assessing patient specific anatomy, implant corridors, insertion angles and depth for safe placement, and optimal cortical bone purchase. They have been described for use in vertebral fractures [10]. Their use can be limited by the timeframe to manufacture where surgery is imminent. Alternatively, a free hand ‘probing’ technique can be used, assessing the cancellous bone for vertebral canal breaches after de-cortification using a burr. A recent study compared these techniques and found free hand probing to be a very versatile and safe method of insertion of spinal fixation pins that can be performed without delay [11].

Since the 1990s, navigated spinal surgery has been introduced in human medicine to improve accuracy when placing implants such as pedicle screws. Intra-operative fluoroscopy can be used to assist implant placement. In a recent cadaveric study, ‘end-on’ fluoroscopy, was accurate in identifying vertebral canal violation by bicortically placed Steinmann pins [12]. Minimally invasive percutaneous insertion of pedicle screws using neuronavigation has recently been described in a canine cadaveric proof of concept study [13].

In summary, recent advances in spinal instrumentation, surgical planning and intra-operative guidance have likely improved the ease, safety and success of managing spinal trauma.

References

1. Hettlich, B.F., Fosgate, G.T., and Litsky, A.S. (2017). Biomechanical comparison of 2 veterinary locking plates to Monocortical screw/polymethylmethacrylate fixation in canine cadaveric cervical vertebral column. Vet. Surg. 46 (1): 95–102. https://doi.org/10.1111/vsu.12581.
2. Hettlich, B.F., Allen, M.J., Pascetta, D. et al. (2013). Biomechanical comparison between bicortical pin and monocortical screw/polymethylmethacrylate constructs in the cadaveric canine cervical vertebral column. Vet. Surg. 42 (6): 693–700. https://doi.org/10.1111/ j.1532-950X.2013.12040.x.
3. Letesson J, Goin B, Trouillet JL, Barthez P. Long-Term Follow-Up of Dogs and Cats after Stabilization of Thoracolumbar Instability Using 2-0 UniLock Implants. Vet Med Int. 2022 Apr 26;2022:5112274. doi: 10.1155/2022/5112274.
4. Wedemeyer M, Parent S, Mahar A, Odell T, Swimmer T, Newton P. Titanium versus stainless steel for anterior spinal fusions: an analysis of rod stress as a predictor of rod breakage during physiologic loading in a bovine model. Spine (Phila Pa 1976). 2007 Jan 1;32(1):42-8. doi: 10.1097/01.brs.0000251036.99413.20.
5. Bitterli T, Mund G, Häußler TC, Farke D, Kramer M, Schmidt MJ, Peppler C. Minimal Invasive Fluoroscopic Percutaneous Lateral Stabilization of Thoracolumbar Spinal Fractures and Luxations Using Unilateral Uniplanar External Skeletal Fixators in Dogs and Cats. Vet Comp Orthop Traumatol. 2022 Jan;35(1):64-70. doi: 10.1055/s-0041-1736219.
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8. Özak A, Yardimci C, Nisbet HÖ, İnal KS: Treatment of traumatic Thoracal instability with pedicle screw-rod fixation system in a dog. Kafkas Univ Vet Fak Derg, 24 (4): 627-630, 2018. DOI: 10.9775/kvfd.2018.19663
9. Kimura S, Nakata K, Nakano Y, Nozue Y, Konno N, Sugawara T, Maeda S, Kamishina H. Case Report: Spinal Stabilization Surgery Using a Novel Custom-Made Titanium Fixation System for the Spinal Instability Caused by Vertebral Malformation in a Dog. Front Vet Sci. 2021 Nov 10;8:755572. doi: 10.3389/fvets.2021.755572. PMID: 34859088; PMCID: PMC8631319.
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11. Mullins RA, Espinel Ruperéz J, Bleedorn J, Hoey S, Hetzel S, Ortega C, Kraus KH, Guevar J. Accuracy of pin placement in the canine thoracolumbar spine using a free-hand probing technique versus 3D-printed patient-specific drill guides: An ex-vivo study. Vet Surg. 2023 Apr 18. doi: 10.1111/vsu.13958. Epub ahead of print. PMID: 37071824.
12. Goffart LM, Precht C, Fosgate GT, Maiolini A, Hettlich BF. Accuracy of end-on fluoroscopy in predicting implant position in relation to the vertebral canal in dogs. Front Vet Sci. 2022 Oct 20;9:982560. doi: 10.3389/fvets.2022.982560. PMID: 36337187; PMCID: PMC9630941.
13. Guevar J, Samer ES, Precht C, Rathmann JMK, Forterre F. Accuracy and Safety of Neuronavigation for Minimally Invasive Stabilization in the Thoracolumbar Spine Using Polyaxial Screws-Rod: A Canine Cadaveric Proof of Concept. Vet Comp Orthop Traumatol. 2022 Nov;35(6):370-380. doi: 10.1055/s-0042-1750056. Epub 2022 Jun 27. PMID: 35760365.