EQUINE VETERINARY EDUCATION / AE / july 2022
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the present study we could observe an increase in suture interdigitation between birth and adult age, suggesting an increased adaptation for shock absorption. However, what level of strain different suture lines in horses undergo during mastication is not known. In miniature swine, during normal mastication, the frontonasal suture has been shown to experience predominant compressive loading and exhibited a greater degree of interdigitation (Rafferty and Herring 1999). It can be hypothesised that in horses the frontonasal suture will also absorb the most compressive forces during mastication, which could explain why suture exostosis occurs most frequently at this location. However, we were not able to objectively identify different grades of interdigitation for each location in the present study. This study has a number of limitations including a small
1000 µm
Fig 8: Sutura maxillolacrimalis, 9-year-old horse, MMA sample, Giemsa stain. Note the suture with its interdigitation in dark blue (box) surrounded by osteons (arrows). The interdigitation is illustrated by white lineswithin the sutureandhasbeen classifiedas moderate to tightas the directional changes in the suture are greater than 90° in the visible sample. The nasolacrimalduct (NLD) canbe seen in the upper left-hand quadrant.
different rates depending on their location within/along the suture. In all three suture locations (internasalis, nasofrontalis and
maxillolacrimalis), there was clear interdigitation visible and the interdigitation seemed to increase with the age of the horse. Moreover, the serpent-like suture lines appear in all sutures not only in the outer, more horizontal plane but also in the vertical plane that connects the adjacent bones with one suture, the Sutura maxillolacrimalis in the 2-year-old horse, even appearing to be transected tangentially. However, the interpretation of the interdigitation is biased to the angle that the tissue was sectioned and therefore limited in its value. For this reason, the authors did not take a quantitative approach to describing the interdigitation of the sutures. Mature sutures unite bone surfaces through various
degrees of overlapping or interdigitation corresponding to the strain applied to it and are characterised by fibrous tissue that absorbs the load at a much higher level than the rigid skull bones are able to do (Jaslow 1990; Herring et al. 2001; Herring 2008). Research has shown that higher strain energy, and therefore a higher degree of interdigitation, in the suture corresponded to a greater ability to absorb shock (Jaslow 1990; Maloul et al. 2014). Three types of strain have been identified to influence the mechanical properties of sutures: impact loading, for example from falling or contact with a foreign object or intentional forces such as fighting; cyclic loading, for example during mastication; and quasi- static strain from the presence of adjacent tissues such as changes in intracranial pressure or direct strain to the facial sutures through growth of adjacent structures (Herring 2008; Moazen et al. 2016). Studies examining the skulls of mice and children reported an increase in suture interdigitation after birth as well as new bone growth, a dense network of collagen fibres and a high level of vascularisation in the suture borders (Rice 2008; Khonsari et al. 2012). Similarly, in
sample size that does not include weanlings, yearlings or horses over the age of nine. In addition, the lack of samples where the sutures were grossly fused does not allow conclusions to be made as to how gross fusion corresponds to histological fusion. This could be improved in further research by optimising consistency of sample location among horses as well as removing a larger area of suture line for comparison. In hindsight, it would have been beneficial to evaluate gross appearance before removing bone sections from the anatomical site where the suture was expected to be located. This would also have allowed bone to be removed for analysis in horses that no longer had grossly visible sutures. However, Klein et al. (2019) showed that the sutures are not always located in the same anatomical location – therefore, it would be necessary to take bone samples over a much larger area in cases where the suture is not grossly visible and examine a much larger number of histological samples. A number of methods have been suggested to evaluate
suture patency, including a qualitative method first suggested by Broca in 1875 that uses a scale of 0-4 based on the macroscopically visible degree of synostosis or a more quantitative analysis of suture closure such as that used by Hershkovitz et al. (1997) which measures the percentage of visible suture obliteration in order to classify a suture as totally closed, partially closed, partially open, totally open or prematurely closed (Broca 1875). Recent advancements in technology have allowed more sophisticated quantitative methods of evaluating suture morphology and patency such as 3D micro-computed tomography (lCT) (Maloul et al. 2010; Moazen et al. 2016). In conclusion, this study demonstrates that the examined
internasal, nasofrontal and maxillolacrimal sutures in horses up to 9 years of age are comprised of connective tissue and therefore histologically patent. Further studies would need to be done with horses of various ages, possibly horses up to 30 years of age, in order to determine whether these results are consistent among a larger sample group and at what age the facial sutures in horses fuse.
Authors’ declaration of interests No conflicts of interest have been declared.
Ethical animal research Not applicable.
© 2020 EVJ Ltd
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