The generated 3D models represent the equine sinonasal pathways, which connect the paranasal sinuses with the nasal cavity. The ability to display the sinonasal channels singly or in addition with the paranasal sinus compartments, the viewer is able to get a spatial impression of these complex anatomical structures. In literature, there are several studies published describing this structure by use of macroscopic preparation [7, 9, 11, 14] or CT imaging [15]. In contrast to traditional casting methods where the communication ways were closed to fill the paranasal cavities with plastoid [7], 3D models of the sinonasal channels could be generated by use of virtual casting methods. To the authors knowledge this is the first time that manual segmentation was used to generate 3D models of the equine sinonasal channels and that volumetric measurements for the evaluation of contralateral differences were performed.
In literature, there are varying descriptions of the equine sinonasal pathways. Most authors agree, that the nasomaxillary aperture provides a common entrance into the maxillary sinuses [8, 9, 11, 16]. The nasomaxillary aperture is a slit-like channel, dorsal and ventral limited by the lamellae of the dorsal and ventral conchal sinuses, positioned above the second (110/210) and third (111/211) molar [7, 15, 17]. In agreement with this, we found the entrance into the sinonasal channel as a slit-like opening, located in the middle meatus in all of the 38 examined head sides. The position of this entrance varied with age. The results of our study show, that in younger horses the opening was located more rostrally compared to descriptions in literature and compared to older horses (≥ 15 year old), where it was positioned above the second and third molar. This can be due to a so called age related rostral drift of the cheek teeth [12, 18].
Respecting the nomenclature for the openings between the sinus compartments [8, 19]: conchomaxillary aperture (Apertura conchomaxillaris), frontomaxillary aperture (Apertura frontomaxillaris), palatomaxillary aperture (Apertura palatomaxillaris), we redefined the opening into the sinonasal channel system, the nasomaxillary aperture (Apertura nasomaxillaris). This rostro-caudally and horizontal oriented, slit-like opening was limited by a typically seen ‘hook’ , which protruded from the lamella of the dorsal concha ventrally into the middle meatus. This was defined as the beginning of the sinonasal channel system. We concluded that from a critical point of view only this opening can be called nasomaxillary aperture (Apertura nasomaxillaris). The nasomaxillary aperture led into a common sinonasal channel and, in agreement with other authors [6, 8, 9, 11, 12], by this it formed a common entrance to the paranasal sinuses. In the following this common sinonasal channel was divided into two separate channels, as described by others [7, 9, 11], which were in communication with a rostral and caudal sinus system. Our results also show that the caudal sinonasal channel varied less in its path into the SMC compared to the rostral sinonasal channel. As described in literature [7, 17], there is some variation in the existence of this caudal sinonasal channel in relation to the caudal or dorso-caudal protrusion (Bulla septi sinuum maxillariuma) of the SCV. This protrusion forms the caudo-dorsal part of the maxillary septum and furthermore, the ventral limitation of the caudal sinonasal channel. When there is a large caudal protrusion of the SCV into the SMC, the intrasinuidal exit of the caudal sinonasal channel can be found directly rostral to the frontomaxillary aperture. In the current study in one case the dorsal protrusion of the SCV led to a separation of this caudal sinonasal channel into two parts.
In literature there are varying descriptions about the pathway leading into the rostral maxillary sinus. In the current study, in all cases a pathway between the nasal cavity and the rostral sinus system existed. In agreement with this, other authors [7, 9, 11] always found an entrance into the rostral sinus system. In the current study, this rostral sinonasal channel showed much variation in its communication with the two compartments of the rostral sinus system. In contrast, other authors describe, that the rostral pathway was always in communication with the SMR [7, 9, 11]. One author states [17], that there is a pathway into the SMR in cases, when the lamella of the ventral concha protrudes into the SMR. If this protrusion is absent, the lamella may be fused to the maxillary bone and the pathway is missing. We constantly found a fusion of the lamella of the ventral concha with the maxillary bone located between the rostral and caudal sinonasal channel. This separates both channels from each other, which is in agreement with other descriptions [11]. Others also analysed the communication ways between the nasal cavity and the paranasal sinuses by use of CT [15]. The nasomaxillary aperture was described as a connection between the SMC and the nasal cavity, whereas a connection between the SMR and the nasal cavity is not mentioned [15]. This can be due to the generated slice thickness. In the present study transversal slices with a thickness of 1.5 mm and 1024-image matrix were acquired, which resulted in high-resolution images. In contrast, Probst et al. (2005) examined transversal CT-datasets with a slice thickness of 5 mm and 512-image matrix [15].
From a clinical point of view, we state that for establishing physiologic conditions of the paranasal sinuses, the existence and functionality of these sinonasal channels is essential. In human medicine it is described that drainage of the paranasal sinuses depends on five factors: 1. the communication pathways, 2. the secretion, 3. the quality of the secrets, 4. the cilia activity and 5. the resorption [5]. In equine medicine some authors constitute that primary sinusitis leads to inflammation of the mucosa with following occlusion of the drainage pathways and accumulation of fluids in the paranasal sinuses [20]. Other authors describe, that the initial step in establishing paranasal sinus disease is the stagnation of mucociliary clearance, which can become chronically and lead to a mucosal hyperplasia. This hyperplasia contributes to the pathways occlusion [21]. In literature, mucosal hyperplasia in paranasal sinus disease up to 15 mm is described [22]. These authors concluded that the combination of mucosal swelling and lowered mucociliary clearance are predisposing for bacterial infections. Also the congenital missing of the natural openings is described to result in fluid accumulation within the paranasal sinuses and leading to a mucocele [23].
In human medicine functional endoscopic sinus surgery (FESS) is a minimal-invasive method to re-establish ventilation and drainage of the paranasal sinuses via the natural openings [24]–[26]. By re-establishing the natural pathways, there is no need for fenestration of the paranasal sinus at another area [25]. Moreover, in veterinary medicine balloon sinuplasty is described as a minimal-invasive treatment method for primary sinusitis [27]. After widening the caudal sinonasal channel, the flow-rate of fluids from the SMC into the middle nasal meatus was significantly higher than before, whereas dilatation of the rostral sinonasal channel was not possible. This reflects that detailed anatomical knowledge and a spatial sense of these structures are essential for diagnostical and surgical intervention. By viewing the 3D models of the sinonasal channel system and knowing the relations to other anatomical structures, the results of the balloon dilatation can be interpreted. The caudal sinonasal channel originates at the common sinonasal channel, turns caudolaterally and ends in the SMC. This channel is ventrally limited by the thin dorsal part of the maxillary septum, which is the caudal, ‘bulla-like’ protrusion of the ventral conchal sinus (Bulla septi sinuum maxillariuma). Depression of this part was visible in every horse after dilatation [27]. The entrance into the rostral sinus system is much more complicated, due to angulation of the rostral sinonasal channel and the elasticity of the ventral limitation of this channel. The ventral limitation is formed by the spiral lamella of the ventral concha. The fusion of this lamella with the maxillary bone resulted in returning of this lamella to its basic position after removing plastic probes from the rostral sinonasal channel, which was visible during macroscopic preparation. This lamella seemed to be fixed in position. Even if there is transnasal access to this part of the sinonasal channel system, which was rarely possible due to the 90° angulation [27], balloon sinuplasty of this part of the sinonasal channel system may be of limited value. If instruments are optimized for transnasal assessment of the rostral sinonasal channel, the insertion of drug eluting biodegradable stents as described in human medicine [26] could be a novel, minimal-invasive, successful technique.
In one study the procedure of equine paranasal sinusography is described [28]. By placing 130 ml contrast medium into the conchofrontal sinus, small amount of contrast agent was visible in the SMR. Fluids from the caudal sinus system can flow through the caudal sinonasal channel into the common sinonasal channel. It is the authors’ assumption that from here fluids can drain either via the nasomaxillary aperture into the nasal cavity or via the rostral sinonasal channel into the rostral sinus system. This fluid flow would be a gravitational dependent flow in relation to head position. Clinicians should be aware of this fluid flow, because pathological conditions of the caudal sinus system can be due to fluid fillings of the rostral paranasal sinus system without a primary pathological cause within the rostral sinus compartments.