In an explosive development for the city’s crippling pension crisis, doctors have determined that a firefighter who retired on a $75,000-a-year disability pension in 2003 — and went on to compete in brutal kickboxing competitions — is no longer disabled.

It’s the first time anyone can recall a disability retirement’s being disproved in the Fire Department, leaving officials scrambling to figure what to do with ex-firefighter John Giuffrida, 43.

“The Law Department, the pension board and the Fire Department are working toward some resolution,” said FDNY spokesman Jim Long.

Giuffrida has been retired on a three-quarters, tax-free disability pension for eight years, ever since FDNY doctors ruled he was suffering from asthma, other lung ailments and post-traumatic stress disorder after working in the Ground Zero rescue effort.

When The Post discovered him grappling with opponents in Thai kickboxing matches last year, Giuffrida insisted he had nothing to hide.

“The conditions that precluded me from being a firefighter in no way preclude me from living an active life in order to preserve my health,” the martial artist said at the time.

He didn’t return calls last week.

With the city’s pension bills exploding, critics have focused on the FDNY, where, The Post revealed last year, an overwhelming majority of firefighters and chiefs retire on lucrative disability pensions.

The Post analysis found:

* Nearly nine out of 10 of the FDNY’s 963 retirees since 2008 collect a disability pension.

* The average annual pension for new retirees is nearing $100,000 — up from about $50,000 20 years ago — and typically pays more than $2 million over a retiree’s remaining life span.

* Close to half of the 2,093 disability pensions granted from 2004 to 2008 were approved under “presumptive bills” — pension laws that automatically presume any city firefighter’s cancer or heart or lung problems are job related, ignoring other possible causes, such as smoking and obesity.

* Scores of firefighters who previously retired with regular service pensions are now applying and being approved for even costlier World Trade Center disability pensions, which cover family members even after the retiree’s death.

* Some questionable applications are emerging, including an FDNY doctor who never battled a blaze in line for a $95,000-a- year “heart bill” pension.

Fire Commissioner Sal Cassano committed last year to conducting medical checkups of disabled pensioners who had seemingly cured their disabilities.

The FDNY has always had that power — but has almost never used it.

The last instance was in 1975, when ex-firefighter Gary Muhrke was brought back to explain how he could possibly win a race to the top of the Empire State Building after going out with a bad back.

The medical board accepted his explanation that the strength needed to jog up 102 flights was different from that required to battle blazes.

But in the Giuffrida case, doctors — working for an “outside consultant” — for the first time concluded that a disabled retiree was able bodied.

Long said confidentiality rules prevented him from providing details of Giuffrida’s medical status.

The next step is for a three-doctor medical board to review the consultant’s findings before sending its recommendation to the FDNY pension board.

It’s anyone’s guess what happens after that.

“There are a lot of open questions,” conceded one official.

The city could order Giuffrida back to work, assuming he was able to pass the firefighters’ exam again and was willing to start over as a probie. He could also be asked to work at another city agency.

It is unclear what the precedent-setting case will mean to uniformed city workers retiring on disability, since most cops and firefighters retire after 20 years, when they are immune to medical recalls under the law.

Giuffrida was on the job for 12 years.

A second ex-firefighter who went out on medical disability and was retested, James Kadnar, was determined to still have a disability, according to Long.

Kadnar has been collecting $65,000 a year since 2006 based on a chronic sinus condition. But that didn’t stop him for applying to become a cop in North Carolina and undertaking what was described as a “vigorous four-month training program.”

david.seifman@nypost.com

