It is one of those static, chill days you get in Edinburgh towards the end of winter. Ice sheets the pavements and roads; no wind stirs the blackened branches of the trees; the fallen foliage from the now distant autumn is frost-gilded and crisp underfoot.

I am swathed in multiple layers of merino wool, a scarf covering half my face, and I am holding myself stiffly upright on the very edge of a stool in a small and glaringly lit room. Despite the wool, despite my mittens and sheepskin-lined boots, I am unremittingly, unavoidably cold. Chronic pain, I am discovering, is tiring, draining, domineering: it absorbs all your energy and focus; it drives other thoughts from your head. My body seems unable to keep itself at a livable temperature, so preoccupied is it with the extreme discomfort of my back.

In the room with me is a doctor from Australia and I am wondering to myself: how can he be wearing just a shirt under that white coat? Doesn’t he feel the cold? How can he be unaffected by this temperature?

I have just explained to him that, three days ago, I leaned sideways to move a counter in a game I was playing with my children and I felt a crunch, followed by a rip, and then a horrible shifting sensation as something slid out of place in my lower back. Pain spread like a stain, outwards and upwards, and I have, ever since then, been unable to move, sit, walk or stand without unbelievable agony.

We are, he and I, gazing at an x-ray sheet on a lightbox. I’ve always had a deep fascination for x-rays: what a gift, what an unaccountable power, to be presented with the shaded, layered images of your inner workings, to be granted an oddly prescient glimpse of what you’ll look like in your grave.

Other x-rays have shown me my cranium, with its recognisably ridged nose, my chaos of teeth, pitted with the stark geometry of fillings; I’ve seen the spread bones of my hands, the linear metatarsals of my toes, the neat socket of my ankle. But never this, before now: the astonishing twinned halves of the sacroiliac region.

The antipodean doctor points out bones, joints, nerves, to help me get my bearings on this strange map of grey and white and black.

He says my left sacroiliac joint has slipped out of alignment, causing my current state.

“And when did you break your sacrum?” he says, leaning closer to peer at something.

“What?” I say, from behind my scarf.

He repeats the question, turning around.

I tilt my head to look up at him and an invisible, answering knife slices through my side. “I haven’t broken my sacrum,” I mutter, wincing, tightening my hold on myself. “Or at least … I don’t think I have. Have I?”

The doctor raises his eyebrows. “You don’t remember?” he says.

The lower back is a lesson in symmetry, with the curved wings of the pelvis flaring out from the sacrum, which is cupped like an open palm and pierced with a line of paired holes. The coccyx curves up, beneath, a vestigial reminder of our simian origins. On an x-ray, the area resembles a butterfly or silver moth, pinned to a dark velvet board. There are parts of the body which, taken in magnified isolation, look strange or spindly or peculiar or unidentifiable, but the sacral area is unmistakable and unusually beautiful. It is part angel, part lepidopteran, part Rorschach inkblot.

The sacrum, the triangular, pitted central bone, is a complex, multifaceted cog, performing numerous functions. It is crucial for load-bearing, supporting the entire spine above it, and for accommodating the spinal nerves; it articulates with the hip bones, connects with the final lumbar vertebrae, above, and the coccyx, or tailbone, below. Strong ligaments connect it to the ilium bones of the pelvis: these joints are L-shaped and capable of a small amount of movement.

In children, the sacrum is formed of five separate vertebrae, which start to fuse into a single bone at around the age of 18. Women’s sacrums tend to be shorter than men’s, with more breadth and curvature, to allow greater capacity in the pelvis.

‘Part angel, part lepidopteran, part Rorschach inkblot’ … the sacrum, or ‘holy bone’. Photograph: Sebastian Kaulitzki/Getty/Science Photo Library RF

The term “sacrum”, which was coined by 18th-century anatomists, comes from the Latin name os sacrum, which means “sacred bone”. Before the anatomists came along, the sacrum was also known in English as “the holy bone”.

All this anatomical and linguistic information is unknown to me as I perch gingerly in the doctor’s office, as I watch him point to a tiny grey line, like a river seen from a plane, on one side of the sacrum on the lightbox.

I can see what he means. There is evidence of fracture on the bone. I am able to see it. I also know that the x-ray is mine: I can see my name, reversed so that the surname precedes my initial, in the corner. But can I really have broken it and not known? How is that possible?

I look down at my hands, a minuscule movement that causes a shiver of discomfort from my shoulder blades, down to the base of my spine, exactly where the line of fracture must be.

“I fell,” I say to the doctor, “on a marble floor.”

What I don’t tell him is that we were on holiday, in Italy. That one of my children had been very sick. That it was early spring, Easter time, and my job that week, as the mum, was to get the family back on track, to show them that we were going to have a great time, despite illness and stress and dashes to hospital. I wasn’t exactly wearing a jester’s hat, but I might as well have been. I had an assignment from a newspaper to write about a garden of stone monsters, built by a grieving duke for his dead wife. Monsters or no monsters, we were going to enjoy ourselves: I would make sure of it.

One morning, I was with my children beside a pond outside the villa; my son was beside me and my two daughters were chasing emerald-backed lizards in and out of the rosemary bushes. I had drawn up my feet and I was sitting cross-legged in a rickety garden chair. My son said something – I forget what – and I laughed, throwing back my head.

Balance has never been my strong point. I am forever falling sideways, lurching into bookcases or banisters or doorjambs. Turning my head can cause me to topple over. I often trip over things that aren’t actually there.

So I laughed, cross-legged in my chair, and what happened next seemed to occur in slow motion. I saw the median line of the pond edge, the box hedge, the eaves of the villa tilt. The scene of the water, my children, the tiles dropped away from me and I saw instead a flash of treetops, the arrowing path of a bird. I heard the noise before I was aware of the impact: a crashing thud inside the border of my body, travelling upwards in a great sonic wave towards my ear canals.

The next moment, I was supine, in a different place entirely, as if I had dropped through a trapdoor. Here I was on a level with marble flagstones, with feet, with the lip of the pond. It seemed suddenly hard to breathe, to inflate my lungs. I tried to roll sideways, to right myself, but there was a pain so enormous, so severe, at the base of my back that I couldn’t move. It was a large presence, this pain, with tentacles and claws: it gripped me tightly in its clutches, it drove an iron fist into my spine.

My children were crowding round me. I could see their sandalled feet, the hems of their clothes. The youngest was patting me on the arm, saying, “Mama, Mama.” Tears were streaming from my eyes and I could hear strange, hoarse, gasping sounds.

“Get your dad,” I managed to say.

It was hard to get up, to move: this I remember, more than anything else. My husband tried to help me up, but each time he touched me, I screamed. The flagstones around the pond, the instruments of my destruction, suddenly seemed like the best place to be, to remain. I would just lie here, curled like a prawn, on my side for the rest of the week. That would work, wouldn’t it?

Somehow they got me into the house, my husband and my 11-year-old boy. I recall spending some hours lying on my front on a bed, allowing tears to leak, in a drivelly and directionless fashion, into some pillows. My children came in and out, awed and silent. My husband’s worried face hove into view, coming nearer and nearer. “Do we need to go to hospital?” he asked.

No, I muttered. The idea of going anywhere, of raising myself from this position, of – good grief – folding myself into some kind of vehicle, made me want to vomit. It hurt to move my leg, to curl my toes, to turn my head, to brush a hair off my forehead. It hurt to blink.

Maggie O’Farrell. Photograph: Kate Peters/The Guardian

What I learned that day, before I had linguistic confirmation that the bone I had injured was in some way “sacred” and “holy”, before I had pored over pictures of it, occupying the very middle of our bodies, holding up the spine, forming the basis for the nerves to our brains, the movement of our legs, was that the sacrum is central to us.

It lies at our middle. If we are wheels, the sacrum is our hub. All roads lead to it; everything flows from it. Without it, that small, hand-sized bone, we can’t move.

In Italy, however, I was moving the very next day. I had no choice.

I had three children to look after, one of whom was still unwell, a holiday to undertake, an article to research and write. So I did what parents have always done. I took painkillers, I got myself out of bed, I stood upright, I hobbled on.

I went to the garden, because I had no choice: the stone monsters had to be seen, the article had to be written. I recall a certain amount of difficulty getting myself out of the car. I clung for a moment to the hot metal of the roof, gulping back a sob, and then stuffed bags of ice down my clothes. I was past caring what the chic Italians around me might think.

I leaned on the buggy for support as we processed around the pathways, the seat of my trousers filled with a rubble of ice cubes. I peered up at the lichenous faces of the monsters who weren’t so very monstrous after all, but inscrutable, mired in weeds and soil, their gazes directed above the heads of those who had come to look at them. “Are you all right?” my husband kept asking me, and because I didn’t want to alarm the children, I said: “I’m fine.”

In the cold room in Edinburgh, I condense this story to its bare essentials. I tell the doctor that I fell, on holiday, several years ago, and that ever since then I am prone to sudden and acute injuries from strangely little cause.

“Any problem with my back,” I tell him, “seems to go straight there.”

The doctor nods. “Well, it would,” he says, tapping the x-ray.

“Injuries like that can be life-changing.”

In my mid-20s, I lived in London, where I worked for a newspaper. It was a job that required me to sit for long hours in front of a monitor, clicking again and again on a mouse, scrolling up, scrolling down, zooming in, zooming out, over and over again, five days a week, sometimes until midnight. It wasn’t long before my back seized up: I had pins and needles between my shoulder blades and a numbness down my right arm. My boss, nervous about RSI absenteeism, sent me two floors up to see the company physiotherapist.

She was the first in a long line of practitioners on whose couches I would lie, face down, while they prodded and measured and tested my back.

“My God,” was what she said as she pummelled me with an electric massager. “You have the back of an 80-year-old. Of course,” she shouted, over the noise of the hellish machine, “the problem’s all coming from down here.” I felt her hand descend on my lower back.

“Something’s really not right in your lumbar. Your SI joints move around too much – they’re hypermobile. And your tailbone seems kind of crooked.”

As with the Australian doctor who would x-ray me, years later, there was a lot I could have said in reply, but didn’t. At the time, I got up off the couch, my back feeling kneaded and almost bruised, and went back to work.

I made phone calls, I chased copy, I discussed layouts, as if it was a normal day, but all with the sensation that there was something behind me, something only I could see, nebulous and malevolent, something I thought I had outrun, a long time ago, but here it was, back to haunt me, placing its clammy hand on my shoulder and saying, you didn’t think you’d get away that easily, did you?

The uneven curvature of my coccyx was caused by spending upwards of a year lying on my back, in bed, as a child. My spine grew, of course, as would that of any eight-year-old, but it grew crooked, from the pressure of the mattress beneath. I’d contracted encephalitis and the virus was eating away at my brain, a maggot through an apple, making lacework of the neural pathways. Most of the lasting neurological damage was done to my cerebellum, that part of the brain involved in movement and coordination, but the virus also dismantled many of the neuromuscular junctions in my spine and pelvis. Muscles in my back and upper legs were left much weakened and foreshortened.

Don’t get me wrong: I consider myself to be an extremely lucky person. The doctors first said that I would die; when I didn’t, they said I wouldn’t walk again. To have recovered, to have found a loophole out of one of these destinies, let alone both, strikes me as the very best fortune a person could ever have.

As as twentysomething journalist, however, it came as a shock to realise that I hadn’t left – couldn’t leave – all this behind me. At that age, you believe yourself invincible. You think the world will always be like this, that a life of working long hours and staying out late and barely eating and flitting from one rental flat to another is a permanent state.

Your teens, your childhood, seem indistinct and distant, in contrast to the speedy Technicolor of the present. What happened to that child, that teenager, might well have happened to somebody else.

What I hadn’t realised then is that parts of life come in and out of focus as you get older. Events that might at the time have seemed to pass without consequence may return with great import; something you thought you might have shrugged off or come through can always rear its head again, very much the way a virus can lie dormant in your system.

The truth is that my sacroiliac region is a part of me that could tell a long story, should anyone wish to hear it. It is my synecdoche and also my equivalent of Achilles’ famed heel, which didn’t make it into the magic waters of the Styx. I had avoided an early death, I managed to find a way out of a life of incapacity and dependency. In my teens and 20s, I fled from this knowledge; I wanted to put as much distance as possible between myself and that ailing, immobile child in a hospital bed. I ran away from her, as fast and as far as I could, but my sacroiliac area, my lower back, was always there to remind me that I could do no such thing, that she is me and I am her. There will be no escape.

It is of course a very, very small price to pay. So what if my coccyx curves too much to the right? So what if my pelvic ligaments are too loose and the muscles connected to them too tight or too weak or atrophied or whatever it is they are? So what if I had to wear what the obstetricians called a “truss” throughout my pregnancies, to hold the bones of my pelvis together? So what if I broke my sacrum and didn’t realise? I can walk, I can grip a pen, I can lead an independent life, and that’s more than the neurologists expected for me.

As I sit staring at the healed fracture on my x-ray, as the doctor guides my hand to the corresponding place on my lower back and I feel, yes, a tiny calcified lump, a frozen pea beneath the skin, I am struck by the strangeness of it all. We think we know our bodies, these shells of blood and muscle and tissue and bone, but they lead lives of their own, they keep secrets from us. We inhabit them but they remain unknowable, elusive, brave, carrying on with the business of living, despite our accidents and choices and incursions and foolishnesses.

I leave the hospital walking slowly and carefully, with shortened, hesitant steps, the gait of a woman wearing leg irons. Crossing the car park, I am filled with an unfamiliar and absurd desire to apologise to my back. I had no idea, I want to say, I didn’t realise, I didn’t know. I edge my way over wet tarmac, navigating the banks of cleared snow, and think, for the first time: we can’t go on like this.

This is what I have learned about living with pain: you need to be careful that your baseline for what’s acceptable doesn’t sink too low. There were days, after my fall in Italy, when I found myself thinking, well, I can’t turn my head to the left but it’s fine because I can still turn it to the right. It’s too painful for me to sit down, but if I just balance my laptop on a desk, a cardboard box and three dictionaries, and if I stand in front of this wobbly ziggurat, then I can keep working. I can’t bend down to tie my laces but, hey, I’ll just find some slip-on shoes.

Some injuries are life-changing, the doctor said, and so I have duly changed my life. These days, a year or so on from my x-ray, my sacrum and I have reached an equilibrium of a very tentative and hesitant nature. Our relationship is unambiguously uneven: the sacrum is in charge and I am the willing, reverential supplicant.

I am filled at all times with a dutiful respect for the holy bone and pay regular obeisance to it. I do whatever I can to keep it happy. I rub scented ointment into it, I appease it with hot packs. I have special cushions to ease its comfort, all over the house; I take one with me when I travel, for hard and unforgiving surfaces in trains or planes or airports. In the manner of a cat meeting a dog for the first time, I will eye up a chair before I will commit to sitting in it. I avoid any that are too soft or too hard, or ones that recline too much or are shaped like a bucket. Anything that might tip backwards doesn’t even get a second glance.

I don’t cross my legs, I don’t lift anything heavier than a bag of flour, I don’t row boats, I have to tell my daughter that no, I can’t carry her, no matter how tired her legs are. I even forgo the pleasure of sledging.

Like a religious fanatic, a medieval mystic, I prostrate myself for my sacrum, on a mat, at least three times a day. I have a series of 12 exercises, “for sacral stability”, the sheet says. I move through them, always in the same order, with mute, pious regularity, once in the morning, once at midday and once in the evening. I never miss a single one of these physical novenas.

As I do them, moving my legs one way, my arms the other, bending and supplicating my spine, I can hear my sacral joints clicking, realigning, settling themselves, speaking to me. It is a wordless language, almost as old as I am, and I am happy to hear it.

All Hail the Holy Bone by Maggie O’Farrell was originally published in Granta 145: Ghosts. Visit granta.com/guardian for a special subscription offer with a 25% discount for Guardian readers

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