So Sorry: No Terrible Consequences to Speak Of
Phineas Gage was a 19th century workman famous for having survived an injury far worse that the one you're describing. On the right image, you can see that a heavy rod violently ascended through his left cheek, above his molars, burst through his left eye socket and proceeded to pass through his brain before exiting the right side of his skull. The left image is of Mr Gage afterwards. Still a rakish rogue, with his missing eye and famous rod in hand! His injuries were almost all cognitive and behavioural.
You're describing a much less damaging injury, where a spike pierces the floor of the mouth, pierces or more likely violently pushes the tongue aside and lodges in his hard palate. Being from 1920s Boston, your character, especially since he's a policeman is undoubtedly Irish and probably got into into a bit of a tussle with some other, even drunker, Irishman, perhaps leaving him stabbed by a sailor with a marlinspike in his pocket. Very handy, your basic marlinspike!
A spike is essentially a longish, tapered tool with a round cross-section. The end may be sharp or blunt. This is ideal.
The likely extent of your character's injuries will be as follows:
- Skin and subcutaneous tissues will, of course, be punctured and stretched to accommodate the widening shaft of the spike.
- There is a suite of muscles in this area: the digastric attaches to the mandible and runs in a triangle shape back towards the hyoid bone near the top of the larynx; beneath this is the mylohyoid, a broad muscle that comprises much of the structure of the floor of the mouth. It is possible that one side or the other of the digastric will be damaged, but probably not severed. A spike, depending on the angle of attack, will most likely "slot" roughly to midline, which will push those muscles out of the way. A small enough spike might pierce the digastric but will probably not sever it: it will just divide the muscle belly and push the tissues aside. The mylohyoid will certainly be pierced, but again will not be severely damaged.
- Passing up through the sublingual salivary glands, the spike may injure a gland or damage the salivary duct.
- The tongue itself is very muscular and will almost certainly be injured. A spike might pierce the tissue, and as with the other soft tissues it's gone through, will do little more than divide the muscle fibres and push them aside. Most of the tongue muscles attack posteriorly and to the mandible.
- The hard palate is the thick bone that forms the roof of the mouth and floor of the nasal cavity. The spike will probably be checked by the bone. If the tip passes into the nasal cavity, a fistula might form communicating between the nose and mouth.
Even in the 1920s, this injury would be a cinch for a good, Army trained surgeon to take care of!
Inspect and explore the nasal cavity; tamp down any dislodged bone; suture or cauterise the mucosa.
In the oral cavity, explore the injury to the roof of the mouth caused by the spike. It would be relatively simple to close the soft tissue injury injury in the roof of the mouth. A few stitches ought to do the trick.
Same with the tongue. Explore the wound, clean it out, suture the top, suture the middle if needed and then the bottom.
Same with the floor of the mouth. Explore the wound, clean it out. The salivary duct injury repair was by this time more than a century old, so no real worries there. Tack together torn soft tissues.
Coming down to the structural muscles, explore, irrigate, repair.
Skin, same: explore, clean, repair.
Consequences: The tongue can take quite a beating. People pierce them. People slit them. People bite them. Early in recovery, of course his mouth and jaw will will be very sore. Eating and drinking may be difficult, so he may have to be fed. If his nurse is cute, he'll be happy to be fed! Happily, his caretakers will almost certainly have an invalid feeder in the kitchen. These are designed to help feed people liquid diets when they're sick.
Once the wounds heal, your character will be able to talk as well as he could before the injury.
Men came back from the trenches with real facial injuries. Your character...how can I put this? Twas a mere flesh wound.
- What medical care for this injury would be provided in this era? -- No antibiotics, so your character's greatest threat will be from infection. This is the Post-WWI era. Surgeons have learned a lot about reconstructing the face after mind-blowing injuries on the battlefield. Your guy does not suffer from a mind-blowing injury. Nurses also learned a lot about pre-, intra- and post-surgical patient care & wound management. Pain killers are available (morphine, opium, laudanum, etc); anesthesia was well known (ether, airway management, etc).
- How easy it will be for him to communicate with or without sign language? -- No need for sign language. In the immediate post-operative period, a notebook and pencil ought to suffice. Soon enough, he'll be able to talk quite normally. At worst, he might suffer a slight impediment.
- Whether his job as a police sergeant will be affected (and how so) -- Ultimately, there should be no reason why he can't get back to his normal duties.
- How easy it will be for him to get assistance in learning sign language, what with deaf schools in 1920 being primarily oral education based. -- No need.