An orthopaedic surgeon has identified the plate found in the corpse of 4.7-metre crocodile 'MJ' as a bone plate used to heal fractures in humans.

An orthopaedic plate and six screws were found in the crocodile's stomach. ( Supplied: Koorana Crocodile Farm )

Central Queensland crocodile farmer John Lever routinely examines crocodiles that die at the Koorana Crocodile Farm.

He was surprised to find what is now believed to be a human orthopaedic plate inside MJ, who passed away about a month ago.

Mr Lever said UK-based medical experts had confirmed the device was fitted onto a human bone after thoroughly examining photographs.

"A company in England called Which Medical Device, and also an orthopaedic surgeon in England, they teamed up to come back to us and say it's definitely a human orthopaedic plate," he said.

"We've found out the manufacturing company of these plates [is] called Synthes."

In an email to Mr Lever, UK-based radiologist Phil Haslam and orthopaedic surgeon Craig Gerrand said they believed the plate was attached to a human bone.

"My orthopaedic colleague is pretty sure this is a 3.5 millimetre DCP [dynamic compression plate] from AO [an international community of medical specialists in the field of musculoskeletal trauma]," the email said.

"The symbol is mostly rubbed off. It was almost certainly used in a human.

"We at Which Medical Device would be very keen to be kept updated if this leads to identification of anyone please."

The crocodile farmer said finding the age of the device would help the investigation.

"We've got a contact in that company who's prepared to receive photographs and try and identify the period in which it was made," he said.

"It'll give us some idea at least and then we can link that up with people that've gone missing with no explanation.

"We've already had two people contact us with relatives that have gone missing in north Queensland and feel there may be some connection there.

When MJ died, John Lever opened up his stomach to find this inside. ( Supplied: Koorana Crocodile Farm )

"The police haven't been in touch with me at all and I don't think there's much they could do at present.

"Once we can get the plate identified properly and the period in which it was made, maybe [we can] find out if any of these missing people did have some fractures that needed some reparation."

Mr Lever purchased MJ from the Johnstone River Crocodile Farm in Innisfail six years ago, but prior to that he was wild.

4.7-metre crocodile MJ was found with an orthopaedic plate in his stomach around seven months after he stopped eating. ( Supplied: Koorana Crocodile Farm )

The Innisfail crocodile farm has since changed ownership, and when the ABC contacted them the new owners said they had nothing to do with MJ.

Crocodile farmer John Lever said he had tried to contact the original farm owners multiple times with no luck yet.

Human or animal?

Australian Orthopaedic Association president David Martin said the surgical plate found inside MJ is used to stabalise broken bones.

"For a lot of medium-sized bones the most effective way of stabalising is to hold the bone in proper position until it heals — to put a metal plate along the bone and hold it there with screws," Dr Martin said.

"There's no actual difference between a human plate and an animal plate, other than the shape and thickness.

"Smaller, lighter bones need smaller, thinner, shorter plates. Bigger, heavier bones need obviously bigger, heavier plates.

"A plate that could be used on a human forearm, they would be the same sort of thickness and length as, say, a small to medium-sized dog shinbone."

John Lever says they tried to feed MJ mince in the months before he died, but he was sick and would spit his food out. ( Supplied: Koorana Crocodile Farm )

The senior orthopaedic surgeon said he had called a veterinary friend to ask about the plate.

"[I said] 'are there plates that can be used in humans that can also be used in animals?' and he said 'absolutely'," Dr Martin said.

"He suggested that the plate is a Synthes plate and that's the most common type of plate that's used in veterinary medicine.

"It's a plate that would commonly be used in humans, but vets have access to those plates as well."

Identifying the plate's age

The surgeon said he had never heard of a plate showing up in a crocodile, but orthopaedic implants were often recovered post mortem from crematoriums.

"There was a case when the Bali Bombings were on, a patient of mine who I'd done a knee replacement [on], they were obviously having trouble identifying him," he said.

"I could let them know the size and type of knee replacement that I'd popped into this fellow.

"They were able to then identify him because they had a body that had a knee replacement inside of it."

The surgeon said the implants are designed with longevity in mind.

"There are people who have had orthopaedic implants in for 40, 50, 60, 70 years now. They are incredibly durable, they don't rust, which is one of the reasons they're so expensive," he said.

"I've been doing orthopaedics since the early 1980s and the form of most of these stainless steel plates and the screw design really hasn't changed significantly since then.

"The implant companies have been very responsible in keeping good designs in constant manufacture for many, many years.

"In fact most plates, whether they're made by Synthes or any of the other companies, the screw heads will be exactly the same."

'Eternal mystery'

Dr Martin said without thoroughly examining the plate, in his opinion it was more likely to be from an animal than a human.

"It's probably more likely than not that there was a bone with a plate attached that the crocodile has eaten," he said.

John Lever says he often finds rocks, onion bags, broken bottles, hooks and plastic inside crocodiles. ( Supplied: Koorana Crocodile Farm )

"Then it's been there for a while, and the bone has been dissolved away by time and the stomach acid.

"My best guess? I think it's probably been used on a dog at some stage and the dog has gone too close to the water. Unless they find a missing human.

"But they're never going to be able to tell what that was on."

The surgeon said the plates are common and would be extremely difficult to track.

"We'd use 10 or 20 of those plates a week, and if you think about the length of time that's gone on and the size of Australia, there'd be a lot of those plates that've been sold over the years," he said.

"I think it's going to be a curiosity, and there's going to be lots of 'where was the croc? What was he up to?'

"But I think it's going to be one of those lovely, eternal mysteries."