Fred Starr recollects: The metallurgy of in-situ repairs
The day after I got the cheque for £7,000 from the bus company, ‘in full and final settlement for your broken ankle’, with the usual disclaimers about admitting liability, my leg swelled up like a balloon and the pain was intense.
Two years earlier, coming out of the RAF Club on Piccadilly on a drizzly day and boarding a double decker, I slipped over on a floor swimming with rain water. My right leg went underneath me and I knew that something had gone. When I forced myself up onto the good leg (never try to stand on a broken leg – you risk a compound fracture). I could sense that both the lower leg bones were broken from the way the right leg dangled.
The bus halted. Police arrived, breathalysing the driver. Then came an inspector and two ambulances. By now, there was a traffic jam stretching back from Piccadilly Circus to Hyde Park Corner. I had at last made my mark on London.
I was in St Thomas’ Hospital for just over a week. My lower leg was cut open and a number of titanium splints screwed into place around the tibia and fibula. The splints were only to position the bones while they healed, not to take loads. I struck up a good relationship with Mr George, the surgeon, who said that if the bones did not heal properly, some part of the titanium splint would fail by fatigue after about six months. Given that the material is an alpha-beta, Ti-6Al-4V alloy, with a fatigue strength of around 600MPa, the possibility of such a failure suggests how much load we put on our skeletons.
The early X-rays of the leg were not for the fainthearted. When this part of the leg breaks, it is not like snapping a carrot, but more like stepping on a Bic ballpoint. The bones were still joined to some extent, but there were menacing-looking splinters protruding into the surrounding flesh. The principal features of the X-ray were the 20 round-headed screws, of the chipboard type, with sharp, prominent threads designed to bite into the bone, holding the titanium plates in place.
Gradually, over the next few months, the splinters disappeared, and the weak area, initially almost transparent on the X-rays, gradually restored itself to full solidity. I was told that the pieces of titanium could stay and should give no trouble, although I might look forward to reminders of their presence whenever I went through airport security.
All was well, until, as recounted, my leg swelled up the day after the cheque arrived. The swelling was diagnosed as an infection, and although antibiotics helped, every so often I would have a flare up. Eventually I was back in the operating theatre, this time at Guy’s Hospital, as after Mr Jones and Mr Latif, colleagues of Mr George, had seen me, it was decided that the titanium should come out, pronto. Since then I have been okay.
But no one was really able to tell why I had begun to have difficulties. Titanium is compatible with the human body – its one drawback being that new bone material can grow over the metal. If splints have to be removed, this over-growth has to be scraped off to reveal the location of the screws.
I have my own pet theory. The Youngs modulus of titanium is at least 10 times that of bone, so when the two are subject to a bending stress, the bone will tend to bend away from the titanium. The load on the chipboard screws will be high, with the threads being ripped out of bone material. A gap would then form between the titanium splint and bone, with the blood vessels being torn in this area. Resting the leg would allow temporary rehealing.
I still have the occasional twinge, but nothing more. And although I always thought fracture mechanics a subject for engineers rather than metallurgists, it has been my privilege, over the past months, for my body to have been serviced by some of the best Fracture Mechanics in our National Health Service.