Evaluation of Recurrent Instability.
Last updated Friday, November 16, 2007
Recurrent traumatic instability
Traumatic instability is instability that arises from an injury of
sufficient magnitude to tear the glenohumeral capsule, ligaments,
labrum or rotator cuff or to produce a fracture of the humerus or
glenoid.
TUBS A typical patient is a 17-year-old skier whose recurrent anterior
instability began with a fall on an abducted, externally rotated arm
(although the condition has been reported in individuals as young as
three years old. (Endo et al, 1993) In order to injure these strong
structures, a substantial force must be applied to them. The most
common pathology associated with traumatic instability is the avulsion
of the anteroinferior capsule and ligaments from the glenoid rim.
Substantial force is required to produce this avulsion in a healthy
shoulder. While this load may be applied directly (for example, by
having the proximal humerus hit from behind), an indirect loading
mechanism is more common. Indirect loading is most easily understood in
terms of a simple model of the torques involved. When the upper
extremity is abducted and externally rotated by a force applied to the
hand, the following equation for torque equilibrium is a useful
approximation, if we attribute the major stabilizing role to the
ligament (see figure 6): T = B * E/R where "T" is the tension in the
inferior glenohumeral ligament, "R" is the radius of the humeral head,
"B" is the abduction external rotation load applied to the hand, and
"E" is the distance from the center of the humeral head to the hand. If
the radius of the humeral head is 2.5 cm and the distance from the head
center to the hand is one meter, this formula suggests that the
inferior glenohumeral ligament would experience a load 40 times greater
than that applied to the hand. From this example we can see that a
relatively small load is required to produce the characteristic lesion
of traumatic instability if this load is applied indirectly through the
lever arm of the upper extremity.
Avulsion of the anterior glenohumeral ligament mechanism (see figure
7) deprives the joint of stability in positions where this structure is
a check rein, such a in maximal external rotation and extension of the
arm elevated near the coronal plane. Thus, it is evident that in
recurrent traumatic instability, problems are most likely to occur when
the arm is placed in a position approximating that in which the
original injury occurred. Midrange instability may also result from a
traumatic injury because the glenoid concavity may be compromised by
avulsion of the labrum or fracture of the bony lip of the glenoid (see
figure 8). Lessening of the effective glenoid arc compromises the
effectiveness of concavity compression, reduces the balance stability
angles, reduces thesurface available for adhesion-cohesion, and
compromises the ability of the glenoid suction cup to conform to the
head of the humerus.
The corner of the glenoid abuts against the insertion of the cuff to
the tuberosity when the humerus is extended, abducted, and externally
rotated (see figure 9). (Liu and Boynton, 1993; Matsen, Lippitt, Sidles
et al, 1994; Montgomery and Jobe, 1994; Rossi et al, 1994; Walch et al,
1991; Walch et al, 1993) Thus, the same forces which challenge the
inferior glenohumeral ligament are also applied to the greater
tuberosity-cuff insertion area. It is not surprising, therefore, that
posterolateral humeral head defects, tuberosity fractures and cuff
injuries may be a part of the clinical picture of traumatic
instability. The exact location and type of traumatic injury depends on
the age of the patient and the magnitude, rate, and direction of force
applied. Avulsions of the glenoid labrum, glenoid rim fractures and
posterolateral humeral head defects are more commonly seen in young
individuals. In patients over the age of 35, traumatic instability
tends to be associated with fractures of the greater tuberosity and
rotator cuff tears. This tendency increases with increasing age at the
time of the initial traumatic dislocation. Thus, as a rule, younger
patients require management of anterior lesions and older patients
require management of posterior lesions.
The posterior lateral humeral head defect is a common feature of
traumatic instability. These lesions are often noted after the first
traumatic dislocation and tend to increase in size with recurrent
episodes. This impaction injury usually occurs when the anterior corner
of the glenoid is driven into the posterior lateral humeral articular
surface. It is evident that this injury is close to the cuff insertion.
Large head defects compromise stability by diminishing the articular
congruity of the humerus.
To help recall the common aspects of traumatic instability, we use
the acronym TUBS. The instability arises from a significant episode of
Trauma, characteristically from abduction and extension of the arm
elevated in the coronal plane. The resulting instability is usually
Unidirectional in the anteroinferior direction. The pathology is
usually an avulsion of the labrum and capsuloligamentous complex from
the anterior inferior lip of the glenoid, commonly referred to as a
Bankart lesion. With functionally significant recurrent traumatic
instability, a Surgical reconstruction of this labral and ligament
avulsion is frequently required to restore stability.
The reader is referred to an review of the pathology and
pathogenesis of traumatic instability by Wirth and Rockwood. (Wirth and
Rockwood, 1993)
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