My employer had Cigna last year when I had my implant, as well as an ankle replacement.
They payed all but my out of pocket limit on all claims.
I just looked at my record, and I had claims totaling over $429k last year submitted. Of course, the amounts paid were a lot less, due to insurance agreements with providers.
I suspect it's limitations mandated by your employer, and not necessarily the insurance company.
Definitely keep fighting.
