The middle ear is the part we're concerned with. The tympanic membrane ('eardrum') conducts sound to small bones (the malleus, incus, and stapes, or the 'ossicles') which then do complicated things with nerves. Nobody has asked me about sensorineural hearing loss and anyways, it's generally irreversible.**
There are a number of more-or-less standardized tests used to measure and characterize hearing loss. You can read about them here. (For a good time, read a bunch of papers wherein ENTs argue about the relative subjectivity of the audiogram, a boring-looking line graph.) In this case, since the hearing loss has a mechanical (conductive) cause, the audiogram is in fact the test in question- the faintest decibel level a person can hear at a certain frequency.
Surgery in general
I have read a lot of very convincing papers about surgical success (see also a book called "The Checklist Manifesto"); they suggest that the two biggest predictors of a patient's outcome, aside from the particulars of the patient's condition, are how many of that surgery the medical center performs per year, and how many of that procedure the surgeon has personally performed. (The use of checklists is, naturally, strongly recommended.)
Surgical studies in general frequently suffer from low N - that is, they don't have that many patients, so they lump everyone who got a certain surgery into one big group regardless of why they got the surgery specifically. This makes it challenging to interpret the results. This is also probably part of why surgeons, even more than other doctors, often rely on personal experience more than randomized controlled trials.
How To Fix Mechanical Hearing Loss?
You rebuild the part that had problems.
The tympanic membrane, it turns out, can be reconstructed. There are two main methods for major damage: using a piece of fascia (fibrous muscle covering) generally taken from behind the ear.
Here is a somewhat disturbing video of an endoscopic fascia tympanoplasty (seriously, don't watch it; you don't want to see what goes on inside people). Basically they take a little piece of fibrous gook, rough up the edges of the tympanic membrane, fill the ossicular space with gel gook, stuff the fibrous gook down into the ear, pack it in with more gel gook, and then it magically grows together and the gel slowly gets hydrolyzed or resorbed or vanishes in some way.
The other way is to take a little piece of cartilage and do more or less the same thing. Many studies in reputable journals suggest that cartilage tympanoplasty has a higher graft rate and a lower rate of surgical failure requiring a repeat of the surgery.
My understanding is that, because the entire ear canal ends up packed, one will be functionally deaf in that ear for quite some time, until everything heals/ dissolves.
* Complications in Tympanoplasty
Like any surgery, things can go wrong. There can be nerve damage if the surgeon isn't very, very careful. They can damage the little bones inside the ear if they aren't very, very careful. Extremely rarely, people react to stuff used in surgery, like surgical ointments. How common are these? It's hard to say. But, without a doubt, the most common bad outcome is that it simply doesn't work. Depending on the method and the surgeon, the tympanoplasty failure rate varies from 5% to 40%. It's also nearly impossible to know which group one will fit into. Obviously, methods that, in general, work more of the time are to be preferred, and surgeons with a high success rate are also to be preferred.
This can be loosely summarized as 'repairing or replacing some of the tiny bones in the inner ear - malleus, incus, stapes.' There are many variations. Also, a large percentage of trials are sponsored by the manufacturer, and are therefore
Essentially, you can fix the bones with other bone from somewhere (the patient, or a cadaver, one guesses); you can replace some of the bones with a polymer, with titanium, or with a hydroxyapatite/titanium substance; you can replace all of the bones; you can use a "titanium malleus replacement prosthesis (MRP) ... inserted into the external auditory canal". You can do something with a footplate which is completely beyond me (MRP-to-footplate; TM-to-footplate; other-stuff-to-footplate). You can do vein grafts (what these have to do with anything is also beyond me).
The main problem is, it's apparently quite hard to figure out what's going on with itty bitty tiny bones inside the ear without opening it up. So I ended up saying to my friend that I have no idea how to predict which procedure or procedures might be used, and there's an awful lot of them, and therefore I can supply very little information.
*I paraphrase, but this was the general idea.
** Irrelevant digression: my grad thesis project was on a set of proteins involved in hereditary progressive sensorineural hearing loss. I've read about a lot of hard-of-hearing mice.