@jackiet Read online, but my understanding is that the diclofenac gel only penetrates a limited depth. So it is not considered useful for hip arthritis pain for example. I do not know about back pain, but here is an example article discussing how deep it has an effect.

Skin penetration and tissue permeation after topical administration of diclofenac
https://www.tandfonline.com/doi/full/10.1080/03007995.2017.1352497
EXCERPT: "... diclofenac has been shown to penetrate to a depth of around 3–4 mm ..."