Knee-jerk reaction: it’s a patellar thing.
Obviously the ideal situation in life, as far as the knee is concerned, is to keep the same old knee functioning as well as it did when it was young and springy. But we can’t stay young and springy for ever and some wear and tear is inevitable. Some of us wear and tear more than others, for a variety of reasons. Some of us can’t hold onto our cartilage for one thing, or damage it.
My friend Art realized he needed some newness of knee when he neared seventy. He began to feel a little hampered in mobility, so he got a partial replacement: some nice fresh titanium parts. Art shared his x-rays with me and I fooled around with the idea of extra material — stuff that’s not actually bone. It’s interesting to compare the durable, living bone and its textures with the durable, nonliving titanium. The texture of the bone and the almost translucent transmittal of light contrasts so starkly with the stuff that is not bone. It’s a little unsettling, although the pieces obviously fit together very well. The whole structure works together — the living and the not-living.
 And Art is back to traveling all over the world and, I would bet, that by now he can’t tell where the real knee ends and the new knee begins. I would think he feels lucky that partial replacement sufficed. I’ll ask him when my husband and I are meet up with him tomorrow in Uruguay. We’ll be doing the stillness stuff we’re both so good at while Art is off exploring.
(The red is my idea — it’s there simply because it’s my favorite color.)