In 2024, South Dakota replaced knees on about six times the share of fee-for-service Medicare patients as Hawaii

The numbers first. South Dakota records about 34 knee replacements per 1,000 people in traditional Medicare — the fee-for-service side, the part that bills Medicare directly. Hawaii records about 5, and the country as a whole about 18. The high rates cluster in the Plains.

The obvious objection is the denominator, and it is the right one. Divide by nothing and the largest states sit on top of every map, whatever their surgeons do. Each state is divided by its own fee-for-service population, which removes that — and rules out adjusting further: Medicare Advantage covers a different share of people in every state, and a per-1,000 fee-for-service rate is the one version of this figure that needs no assumption about them.

The thing being divided matters too, and here it is people. Each patient counts once on the knee-replacement claim in a year, so someone who has both knees replaced appears once. Count operations and you get a bigger number; count billed lines and bigger again. Holding that unit still in every state is what lets the spread be read side by side.

So the 6.2 times is real arithmetic. What it is not is an explanation. How old and how heavy each state's Medicare population is, how common arthritis is locally, how many orthopedic surgeons practice there, and where local thresholds for operating fall could each plausibly matter. The rate measures the variation, not its cause.

One boundary rides on the whole of it: this measures where the surgery was billed, not where the patient lives. For an operation people generally have close to home the two are near enough — but they are not the same thing, and this map is the first.