I doubt that being the author on both papers will, in itself, have any effect. If you remove yourself from either, in other words, I'd expect the decision of the publisher to be unchanged.
But the issue is that they seem awfully similar, though it isn't my field. Unless you have some reason why the effect should be different for one joint rather than another, I'd worry that the publisher won't want both of them.
However, if the methodologies (surgical and/or research) of the papers are sufficiently different, which you haven't suggested, then a publisher might be interested in both.
Is is possible that you combine the two papers in to one? As an outsider in the field, this would seem like a more interesting paper to a publisher, even if there are some differences in outcomes. It surprises me that you haven't suggested this already, since you know of the similarities.
FWIW, there are no ethical issues in submitting both papers, whether at the same time or not and no matter the authorship.