In April 2003, an American mountaineer and mechanical engineer called Aron Lee Ralston dislodged a boulder as he made a solo descent of Bluejohn Canyon in south-eastern Utah. The rock trapped his arm and pinned his right wrist to the side of the canyon wall. After five days without food or water, dehydrated and hallucinating, he had to break his forearm and amputate it with a dull pocketknife to break free. He then made his way through the rest of the canyon, rappelled down a twenty-metre drop, and hiked eleven miles to safety.

As a surgeon, I read his autobiography and watched James Franco play him in the film 127 Hours, and wondered how desperate you’d have to be to find the strength to do such a thing.

Now I know.

I’m cool and collected as I pick up the rusty Stanley knife. Resilience isn’t just about exercising your strengths: it’s about finding the strength in your weaknesses. The inability to tune in to my emotions has sometimes made it hard for me to move through the world, but now it’s my saving grace. I’m a rational person: an analytical, problem-solving scientist. Self-pity isn’t an option. I can’t use brute force to break the handcuff: I’ve tried. My son doesn’t have time to wait for an unlikely rescue. I’ve known since the moment Stacey told me my son was trapped here with me I only ever had one choice – which means it wasn’t a choice at all.

I tug off one of my running shoes, and unthread the shoelace, tying it as tightly as I can around my wrist, using my teeth to pull the knot firm. I need a tourniquet, or I’ll bleed to death.

I have one advantage over Aron Ralston: my anatomical knowledge.

As any surgeon – or butcher – will tell you, severing a limb at the joint is the most efficient method: the bones are held together by sinew and tendon and muscle and cartilage, all far easier to part than bone. I know exactly where to cut to remove the last two fingers and knuckles of my left hand.

I’ll never operate again, of course. My surgical skills may be about to save my life but using them now will ensure I never set foot in an operating room again.

For most people, losing two fingers would be upsetting, distressing, shocking even: but in the end it wouldn’t impact their lives to any significant degree. You can still do up your buttons with six fingers and two thumbs. You can still ski downhill and send a text and roll pastry and bathe your child. But for a pianist, or a gymnast, or a world-class heart surgeon, it’s life-changing.

I’m a surgeon, but I’m a mother first.