Abstract

In mammalians, there is a considerable amount of variation in the development of ossification centers in the vertebrae.⁶ Rommel⁷ reported the uniqueness of superficial location of the vertebral non-epiphyseal neurocentral sutures in cetaceans, which normally should be found within the centrum in other mammalian species. Disarticulated pedicles or laminae in neonatal cetaceans were observed; however, formal literature is devoid of any reference to the fusion pattern of vertebral non-epiphyseal sutures, and its relationship with age during the growth and developmental process in cetaceans. The incidence of vertebral epiphyseal fusion is clearly reported; however, structural variability in the basic appearance of the ossifying vertebral non-epiphyseal sutures of cetaceans has rarely been described. They may be mistaken for fracture lines and, therefore, become clinically important when evaluating carcasses after trauma, with congenital skeletal anomalies or metabolic bone disease.

In human forensic investigation, multidetector computed tomography (MDCT) has been used successfully to evaluate the ossification stage of postcranial skeletal sutures for chronological and skeletal age estimation,^2,4 Kot and his team⁵ reported "fractured vertebral processes" in stranded cetacean using postmortem MDCT, suggestive of normative age-related vertebral morphological variability. This preliminary study aimed to investigate the applicability of MDCT on the assessment fusion pattern of vertebral non-epiphyseal sutures in cetaceans, and its possible implication on age estimation.

The progression of vertebral non-epiphyseal sutures fusion was assessed in 20 Neophocaena phocaenoides carcasses (6 males, 8 females and 6 unknown sex; body length 67–170 cm; condition code ranged from 2–4), stranded in the Hong Kong waters. Chronological age estimation of the carcass was determined by the record of total body length. Carcasses were categorized into 4 age groups:⁸ a) < 1 year (body length < 100 cm), b) 1–5 years (body length between 100–140 cm), c) 6–9 years (body length between 140–150 cm), d) > 10 years (body length > 150 cm). All carcasses underwent whole body MDCT scan using Toshiba 16-row multislice CT scanner Alexion^TM (Toshiba Medical Systems, Tochigi, Japan), with slice thickness of 1 mm. Volumetric data were reconstructed and reformed for multiplanar reconstruction using an open medical image viewer OS X (Horos Project).

The stage of vertebral non-epiphyseal sutures fusion was evaluated using the classification of stages modified from Kellinghaus and his team³ (Table 1). The degree of vertebral non-epiphyseal suture fusion of 10 carcasses was compared with gross necropsy.

Table 1. Definition of the stages of vertebral non-epiphyseal sutures fusion

Preliminary results indicated that there were discrepancies in the fusion pattern of vertebral non-epiphyseal sutures at 4 different vertebral levels, among the 4 age groups as shown in Table 2.

Table 2. Score of the non-epiphyseal sutures in vertebral column of different age groups

All vertebral non-epiphyseal sutures were fused in carcass with the age above 6 years. In carcass with age 1–5 years, fusion pattern varied at the cervical and thoracic lamina, while other vertebral non-epiphyseal sutures were fused. Minimal fusion in all vertebral non-epiphyseal sutures was noted in carcass with age below 1 year. These fusion patterns of vertebral non-epiphyseal sutures suggested their age-related ossification. Fusion of the non-epiphyseal sutures enhances the mechanical limits of the vertebral processes and articular facets, together with the vertebral intervertebral size and shape, allows the range of dynamic postures and muscle activity. With these, cetaceans may also stabilize the vertebral column and increase the mechanical advantages of muscle system¹ to facilitate locomotion in aquatic habitat during the process of growth and development.

To conclude, MDCT provides a convenient, noninvasive and holistic means to assess vertebral non-epiphyseal sutures fusion, and corresponding fusion patterns of vertebral non-epiphyseal sutures among the 4 age groups were recorded and suggested to be age-related.

Acknowledgements

This project was financially supported by the Hong Kong Research Grants Council (Grant number: UGC/FDS17/M07/14). The authors would like to thank the Agriculture, Fisheries and Conservation Department of the Hong Kong SAR Government for the continuous support in this project. Sincere appreciation is also extended to veterinarians, staff and volunteers from Ocean Park Hong Kong, Ocean Park Conservation Foundation Hong Kong, and Tung Wah College for paying great effort on the stranding response and necropsy in this project. Special gratitude is owed to technicians from Hong Kong Veterinary Imaging Center for CT and MRI for this research.

* Presenting author
+ Student presenter

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