Background: The current treatment of coronoid process fractures of the ulna
is based on the classification system of Regan and Morrey. We found no bio
mechanical studies that specifically addressed the role of the coronoid pro
cess in elbow stability. In the present investigation, the elbows of cadave
ra were tested before and after fracture of the coronoid process to assess
the stabilizing contribution of the coronoid process under axial loading.
Methods: Six fresh-frozen cadaveric elbows were tested mechanically. All so
ft tissue surrounding the elbow including the skin, was left intact. An axi
al load compressing the elbow joint was applied along the shaft of the fore
arm in the sagittal plane. A displacement of fifteen millimeters per minute
was applied until a load of 100 newtons was attained. Each elbow was teste
d in 15, 30, 45, 60, 75, 90, 105, and 120 degrees of flexion. Next, less th
an 25 percent, 25 to 50 percent, or more than 50 percent of the coronoid pr
ocess was fractured with an osteotome under radiographic guidance, and the
testing was repeated. Each elbow served as its own control, and one elbow w
as used for two tests; therefore, a total of seven situations were investig
ated. The difference in displacements between the intact and osteotomized e
lbows was measured.
Results: There was no significant difference, at any flexion position, in p
osterior axial displacement between the intact elbows and the elbows in whi
ch 50 percent or less of the coronoid process was fractured (type I and typ
e II) (p = 0.43). There were significant differences, across all flexion po
sitions, in posterior axial displacement between the intact elbows and the
elbows in which more than 50 percent of the coronoid process was fractured
(type III) (p = 0.006). Specimens with a type-III fracture also showed a si
gnificant increase in displacement compared with specimens with a type-I or
type-II fracture (p = 0.012). Specifically, from 60 to 105 degrees of flex
ion, a significant increase in posterior translation of up to 2.4 millimete
rs was found (p < 0.05).
Conclusions: In response to axial load, elbows with a fracture involving mo
re than 50 percent of the coronoid process displace more readily than elbow
s with a fracture involving 50 percent or less of the coronoid process, esp
ecially when the elbow is flexed 60 degrees and beyond.
Clinical Relevance: These data provide biomechanical support for the curren
t treatment of coronoid process fractures and may help to explain why type-
III coronoid process fractures often have a poor prognosis.