Appears to me to be the most natural way of saying it, and I think it fits nicely into the sentence.

It's maybe a classic case of having to completely disconnect from the French, and rethink the situation in English.

That's my tuppence, or two cents across the wee pond!

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Note added at 6 hrs (2022-07-22 01:09:01 GMT)
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Often "encombrement" refers to floor space, in French (France), anyway, but here I don't think that's the case.

The word "volume" is an alternative, but I'm not too sure I prefer it over "space":

https://www.wordreference.com/fren/encombrement

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Note added at 6 hrs (2022-07-22 01:17:10 GMT)
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With the usual disclaimer that I'm not a specialist.

But in orthopaedics, it surely isn't surface area (referred to above as "floor space" for some crazy reason!), you need to fill in the gap, so to speak, for the patient's comfort. So it's volume or space, a three-dimensional thing.

... may be an option if a clou or broche - as mentioned in the client's explanation - was used for the procedure. If a copolymer was used for stabilizing, "filling" would be more appropriate (but the use of copolymer would be unusual, IMO, and I don't see a reason why a widened medullary canal shouldn't be amenable to complete filling with a paste-type material)