Abstract
BackgroundCanine ventral atlantoaxial (AA) stabilization is most commonly performed in very small dogs and is technically challenging due to extremely narrow bone corridors. Multiple implantation sites have been suggested but detailed anatomical studies investigating these sites are lacking and therefore current surgical guidelines are based upon approximate anatomical landmarks. In order to study AA optimal safe implantation corridors (OSICs), we developed a method based on computed tomography (CT) and semi-automated three-dimensional (3D) mathematical modelling using OsiriX™ and Microsoft®Excel software. The objectives of this study were 1- to provide a detailed description of the bone corridor analysis method and 2- to assess the reproducibility of the method. CT images of the craniocervical junction were prospectively obtained in 27 dogs and our method of OSIC analysis was applied in all dogs. For each dog, 13 optimal implant sites were simulated via geometrical simplification of the bone corridors. Each implant 3D position was then defined with respect to anatomical axes using 2 projected angles (ProjA). The safety margins around each implant were also estimated with angles (SafA) measured in 4 orthogonal directions. A sample of 12 simulated implants was randomly selected and each mathematically calculated angle was compared to direct measurements obtained within OsiriX™ from 2 observers repeated twice. The landmarks simulating anatomical axes were also positioned 4 times to determine their effect on ProjA reproducibility.ResultsOsiriX could be used successfully to simulate optimal implant positions in all cases. There was excellent agreement between the calculated and measured values for both ProjA (ρc = 0.9986) and SafA (ρc = 0.9996). Absolute differences between calculated and measured values were respectively [ProjA = 0.44 ± 0.53°; SafA = 0.27 ± 0.25°] and [ProjA = 0.26 ± 0.21°; SafA = 0.18 ± 0.18°] for each observer. The 95 % tolerance interval comparing ProjA obtained with 4 different sets of anatomical axis landmarks was [−1.62°, 1.61°] which was considered appropriate for clinical use.ConclusionsA new method for determination of optimal implant placement is provided. Semi-automated calculation of optimal implant 3D positions could be further developed to facilitate preoperative planning and to generate large descriptive anatomical datasets.Electronic supplementary materialThe online version of this article (doi:10.1186/s12917-016-0824-3) contains supplementary material, which is available to authorized users.
Highlights
Canine ventral atlantoaxial (AA) stabilization is most commonly performed in very small dogs and is technically challenging due to extremely narrow bone corridors
Methods of stabilization have evolved from simple dorsal AA sutures to ventral transarticular screw fixation (TSF) and more complex constructs composed of multiple ventral implants embedded in polymethylmethacrylate cement [2,3,4,5,6]
A diagnosis of Atlantoaxial instability (AAI) was reached if dens separation, agenesis, or hypoplasia was identified in conjunction with clinical signs consistent with a cranial cervical myelopathy or if unequivocal AA subluxation was visible in the computed tomography (CT) study
Summary
Canine ventral atlantoaxial (AA) stabilization is most commonly performed in very small dogs and is technically challenging due to extremely narrow bone corridors. The small size of affected dogs and extremely narrow bone corridors used to position stabilizing implants are often considered major technical limitations of these procedures [9, 10]. These technical difficulties have led neurosurgeons to develop novel techniques to either improve accuracy of implant placement or to multiply the number of implants to better distribute the load applied on the stabilizing construct [2,3,4,5,6, 9, 10]. It can be hypothesized that vertebral canal violation in AA stabilization is likely as common and underestimated by clinicians given that radiographs have a low sensitivity to detect vertebral canal violation and that postoperative CT is not commonly performed in veterinary medicine [13]
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