Mr. Fabian told investigators he lay groaning on the floor of his prison cubicle for almost an hour before hobbling to lunch. A sergeant sent him to the medical unit, and he was driven 80 miles to a hospital in Albany, where doctors removed part of his right testicle in emergency surgery.

State corrections officials moved to fire the officer. But the politically powerful union exercised a contract provision that puts disciplinary cases like this one before an arbitrator who is jointly chosen by the union and the corrections department. Arbitrators often end up “splitting the baby” to keep both sides satisfied and ensure they are chosen for future cases. Even when compelling evidence warrants dismissal, officers often get off with less.

For example, since 2010, the state has tried to fire 30 prison guards — and prevailed only eight times. In the same period, an additional 80 cases brought against corrections officers, sergeants and lieutenants were settled with their unions for penalties other than dismissal.

The arbitrator in the Bukowski case found that the officer had used excessive force and lied to investigators. But the arbitrator lowered the penalty from dismissal to a 120-day suspension without pay, making a mockery of the process.

Arbitrators’ decisions are considered final and binding; in the past courts have ruled that those decisions may not be overturned even when they commit errors of law or fact. In this case, however, state officials refused to return the officer to the job, and the union, the New York State Correctional Officers and Police Benevolent Association, sued to reinstate him. A state court ruled that suspension for this kind of assault “shocks the judicial conscience” and ordered the case reheard.