Monteggia fracture-dislocations are defined by the presence of
In reality, the injury is one and the same - the injury plane runs from the ulna fracture site, through the proximal interosseous membrane, and then the radiohumeral joint.
An injury that might have resulted in a second fracture at the radial shaft, produces a dislocation of the radial head instead. The ulnohumeral joint is spared.
I would not.
In children, it is possible to successfully treat Monteggia fractures without surgery.
In adults, without prompt surgical care, this injury will predictably result in a permanently stiff elbow, with lots of pain. The risk-benefit analysis is clearly in favor of surgery for this injury. Surgical treatment is standard in virtually 100% of cases.
This is an example of a 44 year old male with a Monteggia injury sustained after a fall:
Zooming in at the above picture, I have marked in red the path of the injury.
Here is an xray of the above elbow 3 months after surgical treatment. At this time the patient had zero pain, with full motion both at the elbow and wrist. Healing is occurring by primary intention, with no callus formation.
Below is the x-ray at 1 year, showing a fully healed fracture.