Mechanics of Glenohumeral Instability.
Last updated Friday, February 04, 2005
LigamentsProperties of ligaments Each glenohumeral ligament has clinically important properties which
can be characterized by the relationship of the distance between its
origin and insertion and its tension. (Frank, 1996) These properties
include:
- Its resting length (how far can its origin and insertion be separated with minimal force),
- Its elastic deformability (how much additional separation of the
origin and insertion can be achieved by the application of larger
forces without permanently changing the ligament's properties), and
- Its plastic deformability (beyond the ligament's elastic limit, how
much additional separation between the origin and insertion can be
achieved by the application of larger forces which permanently deform
the ligament up to the point where the ligament fails).
These properties can be demonstrated as a plot of the ligament's
tension versus the distance between the ligament's origin and
insertion. The same relationship pertains whether the ligament's origin
and insertion are separated by translation of the humeral head or by
rotation (see figure 27).
At point "A", the origin and insertion of the ligament are closely
approximated. At point "B", the origin and insertion have been
separated enough to initiate tension in the ligament. Thus, the resting
length of the ligament is shown as A-B. Stefko et al (Stefko et al,
1995) measured the length of the anterior band of the IGHL to be 37
millimeters.
Additional separation of the origin and insertion causes increasing
ligament tension. Up to point "C", this separation is elastic (i.e. it
does not result in permanent change in the ligament). Further
separation of the origin and insertion plastically deform the ligament
up to the point "D" where the ligament fails at a tension of "S". The
midsubstance strain to failure has been measured from 7 per cent - 11
per cent. (Stefko et al, 1995)
Such graphs are helpful in describing the properties of ligaments:
- The strength of a ligament is the amount of tension it can take before failure (S).
- The laxity of a ligament is the amount of translation (see figure
28) or rotation (see figure 29) it allows from a specified starting
position when a small load is applied. Ligaments with long A-B
distances demonstrate substantial laxity if the starting point for
laxity testing is close to "A." Laxity is diminished when the joint is
positioned near the extremes of motion; that is when the starting point
for the axity measurement is close to "B" (see figure 27).
- Ligaments with small A-B distances are short or contracted.
- Translational and rotational laxity are equivalent: they both
reflect the ability to separate the attachment points of the ligament.
- A typical relationship between humeroscapular position and torque
(capsular tension X humeral head radius) is shown in figure 30.
(Matsen, Lippitt, Sidles et al, 1994) Note that the greatest part of
glenohumeral motion and function takes place in the area where there is
no tension in the capsule (corresponds to zone A-B in figure 27). Also
note that at the limits of motion (corresponds to zone B-C), the torque
increases rapidly with changes in position as suggested by the rapid
increase in tension shown in figure 27.
These diagrams help distinguish laxity from instability. Normally
stable shoulders may demonstrate substantial laxity; consider the very
lax but very stable glenohumeral joints of gymnasts. In a most
important study, Emery and Mullaji (Emery and Mullaji, 1991) found that
of 150 asymptomatic shoulders in school children, 50 per cent
demonstrated positive signs of "increased laxity".
Some investigators have measured increased laxity in patients with
glenohumeral instability. (Cofield et al, 1993; Jerosch et al, 1991;
Jerosch et al, 1991; Jorgensen and Bak, 1995; Marquardt and Jerosch,
1991) However, recent evidence indicates that these differences are not
always significant. (Harryman et al, 1992; Lippitt et al, 1994; Matsen,
Lippitt, Sidles et al, 1994) Starting in a neutral position, the
translational laxities of eight normal living subjects were found to be
8 ± 4, 8 ± 6, and 11 ± 4 mm, in the anterior,
posterior and inferior directions, respectively. Interestingly,
virtually identical laxities were measured in sixteen patients
requiring surgery because of symptomatic recurrent instability (see
figure 31), indicating that in these subjects, the measured laxity was
not the determinant of glenohumeral stability. (Lippitt, Harris,
Harryman et al, 1994; Matsen, Lippitt, Sidles et al, 1994) Sperber and
Wredmark (Sperber and Wredmark, 1994) found no differences in joint
volume or capsular elasticity between healthy and unstable shoulders.
These results indicate the amount of laxity cannot be used to
distinguish clinically stable shoulders from those which are unstable.
The stretchiness of a ligament is its elasticity. Ligaments with
long B-C distances (see figure 27) are stretchy and have "soft" end
points on clinical laxity tests. Ligaments with short B-C distances are
stiff and have "firm" endpoints on clinical laxity tests.
Biochemical composition (as in Ehlers Danlos), anatomical variation
(anomalies of attachment), use (or disuse), age, disease (e.g.
diabetes, frozen shoulder) injury, and surgery (e.g. capsulorrhaphy)
can affect the strength, laxity and stretchiness of glenohumeral
ligaments.
Ligamentous stabilization The glenohumeral ligaments exert two stabilizing effects:
- They serve as check reins, restricting the range of joint positions
to those which can be stabilized by muscle balance. This is important
because at extreme glenohumeral positions, the net humeral joint
reaction force becomes increasingly difficult to balance within the
glenoid (see figure 32). For example, excessive abduction, extension
and external rotation of the shoulder may allow the net humeral joint
reaction force to exceed the anterior-inferior balance stability angle.
Similarly, excessive posterior capsular laxity allows the net humeral
joint reaction force to achieve large angles with the glenoid center
line, angles which may exceed the posterior balance stabilityangle.
Furthermore, at the extremes of motion, the muscles tend to be near
their maximal extension, a position in which their force-generating
capacity is diminished. (Lieber, 1992)
The patient can modify the check rein function by altering the
position of the scapula (see figure 33). Surgeons can modify the check
rein function: capsular tightening moves points B, C and D closer to
point A, reducing laxity (see figure 27). The check rein function is
inoperant when the ligament is not under tension (i.e. when the
humeroscapular position is within the tension free zone (A-B in figure
27, see also figure 30). - When torque is applied to the humerus so that a ligament
come under tension, this ligament applies a force to the proximal
humerus. Because of the attachments of the ligament this countervailing
force both compresses the humeral head into the glenoid fossa and also
resists displacement in the direction of the tight ligament (see figure
34).
An analysis of ligament function (footnote 1) demonstrates the limits
of the stability provided by ligaments acting alone. For example, it
suggests that if the torque resulting from a force of a modest 10
pounds applied to the arm at a distance of 40 inches from the center of
a humeral head with a one inch radius was resisted only by the tension
in the inferior glenohumeral ligament (IGHL), the IGHL would need to be
able to withstand a tension of 400 lbs (see figure 34).
If tension in the ligament exceeds the strength of the ligament (S),
the ligament breaks. A few investigations have attempted to measure the
strength of the glenohumeral capsular ligaments. Kaltsas (Kaltsas,
1983) has studied some of the material properties of the shoulder
capsule and found it to be more elastic and stronger than the capsule
of the elbow. He noted that the entire glenohumeral capsule ruptured at
2000 Newtons of distraction (450 lbs). Stefko et al (Stefko et al,
1995) found the average load to failure of the entire IGHL to be 713 N
or 160 lbs. Bigliani et al (Bigliani et al, 1992) noted in sixteen
cadaver shoulders that the IGHL could be divided into three anatomical
regions: a superior band, an anterior axillary pouch, and a posterior
axillary pouch, of which the thickest was the superior band (2.8 mm).
With relatively low strain rates, the stress at failure was found to be
nearly identical for the three regions of the ligament, averaging 5.5
MPa, which is 5.5 Newtons (1.2 lbs) per square millimeter. Thus to
function as the primary stabilizer for a load of 300 lbs as in the
example above, the IGHL's of these cadavers would need to be 250 square
millimeters in cross section. Thus no experimental measurements have
demonstrated that the IGHL alone is sufficiently strong to balance the
torque resulting from a load of 10 pounds applied to the arm at a
distance of 30 inches from the center of a humeral head.
Excessive ligament tension can produce obligate translation of the
humeral head. Harryman et al (Harryman, Sidles, Clark et al, 1990)
demonstrated that certain passive motions of the glenohumeral joint
forced translation of the humeral head away from the center of the
joint. This obligate translation occurs when the displacing force
generated by ligament tension (quantity "P" in figure 34), overwhelms
the concavity compression stability mechanism (see figure 36).
In Harryman's study anterior humeral translation occurred at the
extremes of flexion and cross body adduction while posterior humeral
translation occurred at the extremes of extension and external
rotation. Operative tightening of the posterior portion of the capsule
increased the anterior translation on flexion and cross-body adduction
and caused it to occur earlier in the arc of motion compared with the
intact joint. Operative tightening of the posterior part of the capsule
also resulted in significant superior translation with flexion of the
glenohumeral joint. These data indicate that glenohumeral translation
may occur in sports when the joint is forced to the extremes of its
motion, such as at the transition between late cocking and early
acceleration. Such obligate translations may account for the posterior
labral tears and calcifications seen at the posterior glenoid in
throwers. In addition, these results point to the hazard of
overtightening the glenohumeral capsule, which may result in a form of
secondary osteoarthritis known as capsulorrhaphy arthropathy. Hawkins
and Angelo (Hawkins and Angelo, 1990b) pointed to these complications
of obligate translation in overtightened capsular repairs.
Footnotes Footnote 1: The magnitude of the countervailing force is determined
by the applied torque and limited by the strength of the ligament. The
direction of this force is tangent to the humeral head at the point of
its contact with the glenoid rim.
The countervailing force mechanism operates in the arc B-C where the
ligament is elastically deformed. If the ligament behaves perfectly
elastically, the tension in the ligament provides a stabilizing force
(T) where:
T = (Angular position - angle B) diameter of humeral head Pi/360° * spring constant of the ligament.
This relationship predicts that until angle B is reached, no force
is generated by the ligament, the larger the angle past position B, the
more force is generated (up to the elastic limit), stiffer ligaments
generate more force for a given angular displacement, and larger
humeral heads generate more force for each degree of angular
displacement.
Ligament tension results from applied torque. When an externally
applied force B acts at a distance E from the center of the humeral
head it creates a torque (Q) which is the product of B and E (see
figure 9). If this torque is resisted by a ligament closely applied to
the humeral head (i.e. the effective moment arm equals the head
radius(R)), the tension in the ligament (T) is
T = Q/ R = B * E/R
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