The coronoid process provides anterior buttress against
posterior subluxation or displacement. The radial head prevents valgus
instability, and the coronoid process prevents varus instability. The coronoid
process also provides attachment for the anterior bundle of the MCL and
attachment to the anterior capsule. The anterior capsule attaches 6mm distal to
the tip of the coronoid process. The anterior bundle of the medial collateral
ligament attaches to the sublime tubercle 18mm distal to the tip of the
coronoid process. You need to know the difference between the insertion of the
MCL and the insertion of the brachialis as seen here. If the fracture of the
coronoid process tip is small, the brachialis should insert distal to the tip
of the coronoid process. There are two types for the mechanism of injury: posterolateral
rotatory displacement and varus and posteromedial rotatory displacement.
Posterolateral rotatory displacement is a fracture of the radial head, fracture
of the coronoid process tip, and dislocation of the elbow. Varus and
posteromedial rotatory displacement are associated with fracture of the
anteromedial coronoid process. The LCL tears from the humerus, and the MCL may
not be ruptured. In posterior elbow dislocation and posterolateral instability,
the lateral side fails first with the medial side failing last. This valgus and
supination can result in the terrible triad. Patient with instability after
elbow fracture dislocation always has a coronoid fracture, and it can
redislocate in a cast or after surgery. Elbow dislocation with Type II coronoid
process fracture and non-reconstructable comminuted radial head fracture. Treated
by repair of the lateral collateral ligament, do radial head arthroplasty, and
do ORIF of the coronoid process. This is an example of the terrible triad
(dislocation of the elbow, coronoid fracture, and radial head fracture) and you
need to fix all these injuries. Address each injury to restore elbow stability.
If you have an elbow dislocation with fracture of the olecranon tip fracture
and a radial head fracture, the likely pattern of instability is valgus
posterolateral rotatory instability. There will be rupture of the LCL from the
humerus and varus force will cause medial facet fracture, and this is the
malignant fracture pattern. To recognize the posteromedial facet injury, look
at the AP view x-ray in addition to the lateral view x-ray (in the lateral view
you may miss it). In large medial coronoid fracture and elbow dislocation,
there probably will be varus posteromedial rotatory instability, and it will
affect the anteromedial facet of the coronoid. In fracture of the coronoid
process, the x-ray is difficult to interpret. The fracture may be mistaken for
a radial head fracture. The structures overlap, and we may miss the fracture.
In the lateral view radiograph, you find a chip a bone. AP view radiograph will
find a nondislocated elbow with an anteromedial coronoid process fracture. if
you miss the anteromedial coronoid process fracture, you will get progressive
narrowing of the joint space from lateral to medial between the medial trochlea
and the coronoid process. This entity (anteromedial facet fracture) that gives
posteromedial instability, occurs in conjunction with lateral collateral
ligament injury. When you see this fracture, suspect anteromedial coronoid
fracture, especially when you cannot find a radial head fracture. You may also
find narrowing of the joint space between the medial trochlea and the coronoid
process. CT scan is usually very helpful. There are two known classification
systems: Regan & Morrey Classification and O’Driscoll Classification. Regan
& Morrey Classification is based on viewing the lateral x-ray. In Regan
& Morrey Classification, there are three fracture types based on viewing
the lateral x-ray. Type I is a shear fracture of the tip of the coronoid
process. Type II involves up to 50% of the coronoid process. Type III involves
more than 50% of the coronoid process. This is a very simple classification
system, but the problem is that it does not show the malignant fracture
pattern. The O’Driscoll classification is very helpful, and it will show the
anteromedial facet fracture that will create posteromedial instability. The
O’Driscoll classification can be the tip, anteromedial facet, or basal. The
O’Driscoll classification recognized the anteromedial facet fracture caused by
varus posteromedial rotatory force. This fracture could be missed on the x-ray
and can cause degenerative joint disease.
