There is nothing in this sentence that requires the use of the subjunctive.
Kommt auch eine Osteonekrose in Betracht?
is a correct sentence with the same meaning.
However, since the question implicitly suggests that whoever is asked has overlooked or wrongly excluded the possibility of an osteonecrosis, I would often use a Konjunktiv II of politeness here. It may help understanding this to regard this as an irrealis which maximally attenuates the factuality of my suggestion beforehand, i.e., I state that an osteonecrosis actually does not need to be considered – but that’s really only an approach to understand the subjunctive of politeness.
In particular, I would not use the subjunctive here, if I am not talking to somebody who is responsible for making diagnosis. For example, if I were talking with somebody about a medical check he received, I could ask:
Halten die Ärzte eine Osteonekrose für möglich?
But in this context, it would be grammatically wrong to ask:
Hielten die Ärzte eine Osteonekrose für möglich?
(Do the doctors consider an osteonecrosis?)
Compare to the following sentences which also differ only in this politeness aspect (and are both correct):
Kannst Du mir das Salz reichen?
Könntest Du mir das Salz reichen?
(Could you pass me the salt?)
Haben sie gerade eine Minute Zeit für mich?
Hätten sie gerade eine Minute Zeit für mich?
(Do you have a minute for me? – Can I take one minute of your time?)
Ich brauche einen Laib Brot.
Ich bräuchte einen Laib Brot.
(I need a loaf of bread.)
Note that though this is often claimed, the Konjunktiv does not express possibilities in modern German (except for some fossilised expressions) but only happens to be often used for something that is possible. In the above examples for the subjunctive of politeness, the variant without the Konjunktiv does not imply any different factuality of what is said. This is implied by the context (with or without Konjunktiv).
If this sentence weren’t a queston, i.e., if a doctor reports to another that an osteonecrosis is considered, it would be:
Eine Osteonekrose kommt auch in Betracht.
Using the Konjunktiv II here would imply that it does not actually need to be considered for some reason.
Eine Osteonekrose käme auch in Betracht. Aber die Blutwerte sprechen dagegen.
(An osteonecrosis would need to be considered, if it weren’t for the result of the blood test.)