The Four Causes of Nonunion
Only 31% of nonunions have a single cause, so fixing the obvious problem and stopping is how a nonunion comes back. About 1 in 6 that look completely clean on every preoperative test still grow an organism once the tissue reaches microbiology.
Takeaways
- A nonunion has stopped healing and a delayed union is still moving. The FDA line is 9 months from injury with no radiographic progress across 3 consecutive months. Bones differ and the clock is arbitrary, so read serial films rather than the calendar (StatPearls, Bone Nonunion, 2023).
- Sort every nonunion into 4 causes - mechanics, infection, dead bone with a gap, and host. In 100 consecutive long bone nonunions the frequencies ran mechanical 59%, dead bone with a gap 47%, host 43% and infection 38% (Bone Joint Res 2016;5:512-19, PMID 27784669).
- Fixing one cause and stopping is how a nonunion comes back. Only 31% of that cohort had a single attributable cause - 55% had two, 14% had three, and 1% had all four - so the workup is an audit of all 4 causes (Bone Joint Res 2016;5:512-19, PMID 27784669).
- Abundant callus with a persistent fracture line is hypertrophic - the bone is trying and the construct is moving, so the fix is stability. Absent callus with tapered sclerotic ends is atrophic and reads as failed biology instead. Mechanics was the most common single cause at 59% (Bone Joint Res 2016;5:512-19, PMID 27784669).
- Atrophic nonunions are not just dead bone. Biopsies from healing fractures, hypertrophic nonunions and atrophic nonunions showed no significant difference in median vessel count across the 3 groups (J Orthop Res 2002;20:593-9, PMID 12038636). The failure looks osteogenic rather than vascular.
- Send tissue for culture on every nonunion, including the clean-looking ones. Pooled across 21 studies and 2397 patients, 16% of presumed aseptic long bone nonunions grew an organism and 10% were true infection, with coagulase-negative staphylococci in 59% of isolates (Arch Orthop Trauma Surg 2024;144:701-21, PMID 38006438).
- A surprise positive culture predicts more operations and the same final result. Secondary surgery ran 22% after a surprise positive culture against 6% with negative cultures, and final healing reached 98% to 100% whatever grew (Arch Orthop Trauma Surg 2024;144:701-21, PMID 38006438).
- Ask about smoking and diabetes, and treat a history of infection as the loudest flag. Across 38,465 patients, nonunion was associated with infection (OR 7.0), open fracture (OR 2.6), smoking (OR 1.7) and diabetes (OR 1.6) (EFORT Open Rev 2022;7:516-25, PMID 35900220).