Mechanics of Glenohumeral Instability.
Last updated Friday, February 04, 2005
Superior stability: The same plus a unique additio
Superior stability benefits from all the same mechanisms as anterior,
posterior and inferior stability: glenoid orientation, muscle balance,
glenoid shape, ligamentous effects, adhesion/cohesion, the suction cup
and limited joint volume.
Centering and stabilization Compression of the humeral head into the glenoid concavity is an
important mechanism by which the head of the humerus is centered and
stabilized in the glenoid fossa to resist superiorly directed loads
(see figure 40). Even when a substantial supraspinatus defect is
present, compression from the subscapularis and infraspinatus can hold
the humeral head centered in the glenoid (see figure 41). More severe
cases of chronic rotator cuff deficiency, however, may be associated
with superior subluxation of the head of the humerus and wear on the
superior lip of the glenoid fossa (see figure 42). This erosive wear
flattens the superior glenoid concavity and thereby reduces the
effective glenoid depth in that direction. Once the effective superior
glenoid depth is lost, repair of the rotator cuff tendons or complex
capsular reconstructions cannot completely restore the glenohumeral
stability previously provided by concavity compression (see figure 43).
In addition to the mechanisms which stabilize the shoulder in other
directions, there is a unique aspect of superior stability: the ceiling
effect provided by the superior cuff tendon interposed between the
humeral head and the coracoacromial arch. As every shoulder surgeon has
observed, in the normal shoulder in a resting position there is no gap
between the humeral head, the superior cuff tendon and the
coracoacromial arch. As a result, the slightest amount of superior
translation compresses the cuff tendon between the humeral and the
arch. Thus when the humeral head is pressed upwards (for example when
pushing up from an arm chair or with isometric contraction of the
deltoid), further superior displacement is opposed by a downward force
exerted by the coracoacromial arch through the cuff tendon to the
humeral head. Ziegler et al (Ziegler et al, 1996) demonstrated this
stabilizing effect in cadavers by demonstrating acromial deformation
when the neutrally positioned humerus was loaded in a superior
direction. By attaching strain gauges percutaneously to the acromion
they were able to measure its deformation under load. The acromion thus
became an in situ load transducer. By applying known loads to the
acromion, they were able to derive calibration load-deformation curves
which were essentially linear. Superiorly directed loads applied to the
humerus were then correlated with resulting acromial loads and with
superior humeral displacement. In ten fresh cadaver specimens with the
superior cuff tendon intact but not under tension, superiorly directed
loads of 80N produced only 1.7 mm of superior displacement of the
humeral head relative to the acromion. When the cuff tendon was
excised, a similar load produced a superior displacement of 5.4 mm. (p
< .0001). In specimens where the cuff tendon was intact, an upward
load of 20 N gave rise to an estimated acromial load of 8 N. Greater
humeral loads up to 80 N were associated with a linear increase in
acromial load up to 55 N when an upward load of 80 N was applied (see
figure 43). In a single in vivo experiment where the acromion was
instrumented and calibrated as in the cadavers, very similar
relationships between upward humeral load and acromial load were noted
(see figure 43). These acromial loads must have been transmitted
through the intact cuff tendon. When the tendon was excised, the
humeral head rose until it contacted and again loaded the acromial
undersurface (see figure 44).
Flatow et al (AAOS 1996) (Flatow et al, 1996) used a cadaver model
to explore the active and passive restraints to superior humeral
translation. Whereas Ziegler's study was conducted with the arm in a
neutral position with axial loads, Flatow's involved abducting the
humerus with simulated deltoid and cuff muscle forces. Both groups
noted that the presence of the supraspinatus tendon limited superior
translation of the humeral head, even if there was no tension from
simulated muscle action.
Both Ziegler and Flatow cautioned that the effectiveness of the cuff
tendon as a superior stabilizing mechanism is dependent on an intact
coracoacromial arch. Sacrifice of the ceiling of the joint, the
coracoacromial ligament or the undersurface of the acromion, can be
expected to compromise the resistance to superior displacement of the
humeral head. Disclaimer
This resource has been provided by the University of Washington Department of Orthopaedics and Sports Medicine as general information only. This information may not apply to a specific patient. Additional information may be found at http://www.orthop.washington.edu or by contacting the UW Department of Orthopaedics and Sports Medicine.
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