COCCYX FRACTURE

“Ida” (Darwinius masillae), fossilized early primate, Eocene epoch, 47 million years old, Messel, Germany. I don’t know why we apes lost our tail. It is a subject that biologists discuss surprisingly little. Zoologists faced with this kind of conundrum often think comparatively. Look around the mammals, note where taillessness, (or a very short tail) has independently cropped up, and try to make sense of it. I don’t think anyone has done this systematically, and it would be a nice thing to undertake. Apart from apes, tail loss is found in moles, hedgehogs, the tailless tenrec (Tenrec ecaudatus), guinea pigs, hamsters, bears, bats, koalas, sloths, agoutis and several others. Perhaps most interesting for our purposes, there are tailless monkeys with a tail so short it might as well not be there., as in a Manx cat. The Barbary macaque Macaca sylvanus is a tailless monkey and, perhaps in consequence, is often miscalled the Barbary ape. The “Celebes ape” Macaca nigra is another tailless monkey. Jonathon Kingdon tells me it looks and walks just like a miniature chimpanzee. Madagascar has some tailless lemurs, such as the indri, and several extinct species including “koala lemurs”, (Megaladapis) and “sloth lemurs”, some of which were gorilla-sized.

Any organ which is not used will, other things being equal, shrink for reasons of economy if nothing else. Tails are used for a surprisingly wide variety of purposes amoung mammals. (Sheep keep a fat reserve in the tail. Beavers use it as a paddle. The spider monkey tail has a horny gripping pad and is used as a “fifth limb” in the treetops of South America. The massive tail of a kangaroo is spring loaded to assist bounding. Hoofed animals use the tail as a fly whisk. Wolves and many other mammals use it for singling, but this is likely to be secondary “opportunism” on natural selection’s part). But here we must be especially concerned with animals that live up trees. Squirrel tails catch the air, so a “leap”, is almost like flying. Tree-dwellers often have long tails as counterweights, or as rudders for leaping. Lorises and pottos...creep about the trees, slowly stalking their prey, and they have extremely short tails. Their relatives, the bushbabies, on the other hand, are energetic leapers, and they have long feathery tails. Tree sloths are tailless, like the marsupial koalas who might be regarded as their Australian equivalent, and both move slowly in the trees like lorises.

In Borneo and Sumatra, the long-tailed macaque lives up trees, while the closely related pig- tailed macaque lives on the ground and has a short tail. Monkeys that are active in trees usually have long tails. They run along the branches on all fours, using the tail for balance. They leap from branch to branch with the body in a horizontal position and the tail held out as a balancing rudder behind. Why, then, do gibbons, who are as active in trees as any monkey, have no tail? Maybe the answer lies in the very different way in which they move. All apes, as we have seen, are occasionally bipedal, and gibbons, when not brachiating, run along branches on their hind legs, using their long arms to steady themselves. It is easy to imagine a tail being a nuisance for a bipedal walker. My colleague Desmond Morris tells me that spider monkeys sometimes walk bipedally, and the long tail is obviously a major encumbrance. And when a gibbon projects itself to a distant branch it does so from a vertically hanging position, unlike the monkey’s horizontal leaping posture. Far from being a steadying rudder streaming out behind, a tail would be a positive drag for a vertical brachiator like a gibbon....

That is the best I can do. I think zoologists need to give more attention to the puzzle of why we apes lost our tail. The a posteriori counterfactual engenders pleasing speculations. How would the tail have sat with our habit of wearing trousers? It gives a different urgency to the classic tailor’s question, “Does Sir hang to the left or to the right?”

Richard Dawkins, “The Ancestor’s Tale”, Weidenfeld & Nicolson, 2004

One of the most stunning fossil discoveries of recent times is the enigmatic “Darwinius masillae” - affectionately known as “Ida”. It was discovered in 1983 at the Messel pit, a disused quarry near the village of Messel in Germany. The genus Darwinius was named in honor of Charles Darwin, and the species name masillae in honor of the town of Messel where the specimen was found. Ida represents one of the very earliest examples we have of a primate, and has been dated to the mid-Eocene epoch, 47 million years ago. It is a brilliant specimen being about 95 % intact, and so well preserved that you can see almost every bone in the skeleton, as well as outlines of skin and fur. Astonishingly preserved stomach contents from the young animal’s last meal of fruits and leaves have also been discerned.

Understandably the announcement to the world in 2009 of the discovery of this fossil created a sensation both in professional paleontological circles as well as within the wider media in general - the major reason being that the creature (a young female) was initially thought to be a possible direct ancestor of the apes - and so to humans. Its exact place in cladisitcs however is uncertain, and currently remains controversial. It is defiantly a primate of some sort, having opposable big toes for example, like nearly all primates, but it also has many other indeterminate archaic features, not easily classifiable. It somewhat resembles modern day lemurs, an early branch of the primate evolutionary tree. Yet Ida also has many important differences which may link it to another branch of the primate tree that leads to tarsiers, monkeys and apes, including humans. Intensive study is continuing. In any case Ida remains the most stunning example of early primate evolution, even if her exact place in the primate tree is uncertain at this time.

A striking feature of the fossil, which is immediately obvious to even the lay observer, is the length of its tail. Ida is about 2 feet long (58 cm), but over half of her length is provided by her tail. The tail was clearly an impressive and important part of early primate evolution. Which begs an interesting question. Why have some animals retained a tail, while others have lost it? The apes, from which humans are descended belong to the group that have lost their tails over the eons of time since the Eocene. Humans, and apes as a group in general do however have the vestigial evolutionary remains of the primal tail in the form of the fused terminal four segments of the vertebral column - collectively known as the “coccyx”. The magisterial evolutionary biologist, Richard Dawkins, has given this loss much thought. There is speculation amoung experts that when humans began to take on an exclusively bipedal gait, the tail was for one reason or another no longer required for its former purpose - whatever that was. This perhaps together with that fact that anatomical structures that are no longer used or useful to an animal will shrink and eventually be lost may help to explain things.

Ultimately we do not have a satisfactory understanding or even a generally accepted theory of how or why humans lost their tails - but as Richard Dawkins points out in his beautiful book, “The Ancestor’s Tale”, its loss, for whatever reason has no doubt been of immense benefit to all tailors!

COCCYX FRACTURE

Introduction

The coccyx, (commonly referred to as the “tailbone” in lay terminology) is infrequently fractured.

Coccyx fracture has traditionally been considered a “trivial” injury, not even worth the effort of a radiological diagnosis and that no treatment can be offered in any case.

This is untrue on both counts.

Coccyx fracture (and/ or dislocation) can result in long term, chronic pain and disability, which makes radiological diagnosis important when symptoms do not quickly settle.

CT or MRI are the imaging modalities of choice.

History

The word coccyx is derived from the Greek “kokkux” for the cuckoo bird, as its shape is said to mimic that of the curved arc of the cuckoo’s bill.

Anatomy

Coccyx anatomy, Gray’s Anatomy, 1918

The coccyx consists of the last 4 vertebrae of the spinal column fused together, to form a small triangular bone that articulates at its base with the distal end of the sacrum.

The coccygeal vertebrae consist of bodies only, but the first segment has a rudimentary transverse process and cornua.

The cornua are the remains of the pedicles and superior articular processes and project upward to articulate with the sacral cornua.

Mechanism

● Most commonly the mechanism of injury will be a fall directly onto the bone.

● Occasionally in may be seen in association with difficult childbirth

Complications

● Chronic coccyx pain - a condition termed coccydynia

♥ This can be the result of traumatic injury, but there also exists an idiopathic group of coccydynia cases, unrelated to (recognized) trauma.

● Pain on defecation which may lead to chronic constipation.

Clinical features

Important points of history

Coccygeal pain typically becomes worse with:

● Sitting

● Rising from a seated to a standing position

● Prolonged standing

Important points of examination

● Bruising may be present.

● Pain and point tenderness is maximal directly over the coccyx.

♥ The inferior tip of the coccyx can be palpated within the lower natal cleft, about 2.5 cm (1 inch) above the anus.

● Point tenderness can be also confirmed rectally, but this is not usually necessary.

Investigations

Plain radiography:

Plain radiography may diagnose a coccygeal fracture, but also may not, as the coccyx is often not well visualized on routine A-P and lateral radiographs, due to its oblique position relative to the film, and to the presence of gas and faecal material within the rectum and sigmoid colon.

These difficulties may be partially overcome by bowel preps and specialized tilted views.

The Intercoccygeal angle is the angle between the first coccygeal segment and the last coccygeal segment, which is the objective measurement of forward angulation of the coccyx.

CT Scan:

This is a god imaging modality, however has the drawback of irradiation to the region of the reproductive organs.

MRI

This is the best imaging modality in children, pregnant women or women of child bearing age, where it is desirable to avoid ionizing radiation.

Management

1. Analgesia:

● Pain can be severe, and titrated IV opioids may be required initially.

2. A doughnut type cushion can help alleviate direct pressure when in the sitting position.

3. Stool softeners may be required if constipation from painful defecation and/ or opioid analgesics becomes a significant problem.

4. Injections of local anesthetics and/or corticosteroids into the coccyx area may provide some additional temporizing relief in cases of protracted and severe symptoms.

5. Surgery:

● Occasionally surgical excision of the coccyx may be required for cases of chronic debilitating and severe symptoms, not controlled by more conservative means.

Appendix 1

Left: Anatomy of and muscles attachments of the coccyx (Gray’s Anatomy 1918). Right: Its articulation with the sacrum. 1

Australian Black Eared Cuckoo, “The Birds of Australia”, 1840-1848, lithographic print, John Gould. The coccyx, is so named from the Greek word “kokkux” for cuckoo. This is in reference to its curved shape that resembles that of the curved bill of a cuckoo bird.

References

1. Snell R.S. Clinical Anatomy for Medical Students 5th ed, 1995.

Dr J. Hayes September 2013