Scoliosis is a deformity of the spine and trunk, often as a result of a traumatic injury, syndromic conditions, neuromuscular diseases or disorders. All mammals have the ability to develop scoliosis. Idiopathic scoliosis means when scoliosis is developed without any underlying cause, and is only observed in humans. Although the most common form of scoliosis is idiopathic, there is not a lot known about its origins, and the reasons as to why it develops in humans in the absence of trauma, but not in other species, remains a mystery.
What Is Scoliosis?
Scoliosis is an abnormal lateral curvature of the spine. It is a complex, three-dimensional problem, and affects about 2-3 percent of the population, or an estimated six to nine million people in the United States. In humans, scoliosis can develop in infancy or early childhood, however the primary onset age is 10-15 years old. Treatment for scoliosis is currently focused on limiting progression of the spinal curve until skeletal maturity. Very little is known about the nuances of the compensatory mechanism of the spine – which is the way the body makes up for the spine deformity.
Researchers from the Netherlands decided to investigate idiopathic scoliosis by investigating scoliosis in a minke whale, hoping to study whether the compensatory curves would be similar in the whale to that observed in human idiopathic scoliosis, despite different origins.
Common Minke Whales
Minke whales (Balaenoptera acutorostrata) are part of the baleen whale family, and are the smallest of the finback whales, growing up to approximately 10 meters in length. The scientific name for minke whales translates to “winged whale” (Balaenoptera) “sharp snout” (acutorostrata), however they received their common name from a whale spotter named Meincke, who mistook a minke whale for a blue whale. They tend to be dark gray, with a white underbelly. They have a worldwide distribution, appearing in all oceans and adjoining seas, however it does appear that they prefer cooler regions over the tropics. They migrate seasonally for feeding and breeding, covering long distances through various oceans, although some individuals have home ranges and are not highly migratory.
Studying Scoliosis in a Whale
A new study published in the journal Scientific Reports studied the rale occurrence of scoliosis in a stranded minke whale. A multidisciplinary team of biologists, pathologists, veterinarians, taxidermists, radiologists and orthopedic surgeons were all involved in the necropsy and imaging analysis of the whale. The researchers hypothesized that scoliosis is a universal compensatory mechanism of the spine that consists of vertebral rotation into the curve, accompanied by anterior lengthening of the intervertebral disks, which can be caused by different primary challenges to spinal equilibrium. They further hypothesized that the unique prevalence of idiopathic scoliosis in humans is due to the unique upright posture of humans, with the center of gravity positioned straight above the pelvis. This posture results in decreased rotational stability, making it more prone than other spines in nature to result in scoliosis.
Whales are not known to spontaneously develop scoliosis. Although several reports on cetaceans with scoliosis exist, all the cases have a clear cause, mostly due to a traumatic event such as a ship collision. The study examined a young common minke whale that was found stranded in July 2019 in the Netherlands. The stranded whale had obvious spine trauma, and severe local post-traumatic scoliosis. The scoliosis in the whale appeared to have initiated compensatory spinal curves in the area of the spine that were not affected by trauma, in an attempt of the animal to re-align its trunk. The researchers were interested in these compensatory curves, hoping they could provide insight into the intrinsic mechanisms that govern the alignment of the mammalian spine.
The researchers involved in the study proposed that scoliosis is a universal compensatory mechanism that can occur as a response to a (perceived) disturbance of spinal equilibrium. The biggest difference between the human spine and that of other mammals is not in its anatomy, but how it is biomechanically loaded. This is not because humans walk on two legs (since many other animals do), but rather that their center of gravity is located more posteriorly (i.e. further back) than other species. This makes the human spine unique when compared to other mammalian spines, as it is both quadrupedal and bipedal in nature, making it a rotationally less stable structure.
The objective of the study on the scoliosis of the minke whale spine was to investigate the mechanism through which scoliosis developed in the normal, unaffected area of the spine (the area that experienced no trauma), in an animal that does not develop scoliosis idiopathically. The acute primary scoliosis caused by the trauma in the whale, resulted in the head and tail of the whale being out of line, probably inhibiting proper movement and swimming. The whale most likely compensated for this restriction in movement by attempting to realign its head and tail, inducing compensatory curves in the spine. Looking at the 3D morphology, these compensatory curves strongly resemble human scoliosis.
Unfortunately the study had one limitation – the scoliosis occurring in the stranded minke whale could not be compared to non-scoliotic whales, due to the low frequency of stranded minke whales in the Netherlands, as well as their large size exceeding the capacity of most CT-scanning facilities.
In line with their hypotheses, the researchers observed that the compensatory curves that developed in the unaffected area of the spine of a whale that had suffered localized spinal trauma, shows strong similarities in 3D configuration with different types of human scoliosis. This implies a shared and uniform mechanical basis for scoliosis, suggesting that any perceived shifts in spinal equilibrium can lead to a uniform response. The unique biomechanics of the human spine, with decreased rotational stability, may explain why humans experience idiopathic scoliosis relatively easily, without any obvious trauma.
While more information is still needed to establish the connection of scoliosis across different mammals, the discovery of the rare minke whale’s spine trauma and compensatory spinal curves have certainly provided a head start.