A DOCTOR slaps an X-ray onto a screen. “There,” she says, pointing with her pencil. “That’s the source of the problem.” What she is studying is not a nail embedded within half an inch of an eye, nor an arm snapped in two like a Kit-Kat. In its own way, however, this particular if unthrilling image is far more interesting.

Alongside the cluster of bones that form the knee-cap sits a tiny bone, a humble-looking nodule that, like the remote islands of St Kilda or Rockall, lies far out to sea from the mainland. This diminutive dot, floating some distance from the thigh bone and shin, is known as the fabella, or “little bean”. Those in possession of one doubtless have a less fond name for it because, harmless as it may look, it could be related to osteoarthritis. Certainly, those suffering from this wretched ailment are twice as likely to have a fabella tucked in the tendon behind their knee. I suppose it’s a distinction, of a sort.

For a start, not all of us have a fabella. For a long time, indeed, it appeared to be going the way of the dodo, declining into extinction as humankind evolved into a sleeker sort of machine. Now, however, it appears to be making something of a comeback. A study by Imperial College London shows that, in the past century, the presence of the fabella worldwide has risen threefold, from 11.2 per cent to 39 per cent. The likeliest cause for its re-emergence on this scale is thought to be better nutrition. The problem with that equation, of course, is that as we grow taller and heavier, more pressure is put upon the knee and its archipelago of hard-working bones.

Lest you fear you are accidentally reading the Lancet instead of The Herald, this is not such an arcane topic as it first seems. Trust me, since publication of this report in the Journal of Anatomy, the word fabella will already have entered the mainstream. In some quarters it will probably be the only subject of discussion.

The seven ages of man are distinguished by generational preoccupations. For most people the first 40-odd years are absorbed by exams, sex and rental accommodation, settling down and having children, DIY, the cost of property and further education. Thereafter comes the care of elderly parents. After which, with the Indian summer of our lives beckoning, the musculoskeletal system takes centre stage.

As the years gather pace, at any gathering of those of a similar vintage conversation inevitably, and usually inexorably, comes around to this. Brexit and Independence, environmental calamity and luxury cruises are of fleeting interest in comparison with such an evergreen complaint. Whatever the occasion, be it a birthday or a wake, those of a certain age will form a huddle, during which all joints are allowed their moment in the sun. Slipped discs, tennis elbows and sore hips are given their due, but pride of place – the champion marrow of the show – is the knee and its manifold malfunctions.

Jim McColl announced his retirement from presenting the Beechgrove Garden recently, in part, he says, because age is taking its toll: “in the sense that if I get down on my knees, I’m not sure I can get back up again”. He is over 80, yet there are folk half his age with the same problem. If my experience is in any way representative, then those who are martyrs to the region near the popliteal ligament account for about half the population (it might explain why you so often find coins lying on the pavement, since they cannot risk bending to retrieve them). That estimation is likely to soar as we all live longer, creaking ever louder like so many unoiled doors.

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I encountered the word popliteal in a novel by John Banville, renowned for his extensive vocabulary. The only time I met him, I asked if he remembered using it. Not only did he have no memory of it, but he could not recall what it meant. Presumably his knee caps are still in good order.

At a drinks party the other evening, the state of our knees was a failsafe ice-breaker. One by one, as if we were dealing out cards, we disclosed the condition of this lower hinge, and whether it was one knee or both that had failed us. From accidents and sporting injuries to arthritis, we must have sounded like a convention of orthopaedists. In fact, in my part of the world, heavily populated with former rugby players and farmers, fell-runners and horse riders, there are so many experts in the vagaries of knee replacement surgery and post-operative complications we could be as valuable for medical scientific study as a remote tribe in Borneo.

No matter where in the world, any group of over-50s will have its fair share of the afflicted. I only realised I had joined this unenviable club when the old joke about crouching to pick up something and wondering what else you could do while down there began to sound like an opportunity too good to be missed.