Biomedical Engineering Reference
In-Depth Information
posterior band of the inferior glenohumeral ligament in thrower shoulders. When
a posterior capsular contracture develops, the center of rotation of the humerus, or
the contact point of the humerus on the glenoid, is shifted postero-superiorly. This
shift functionally increases the length of the anterior aspect of the capsule, which
provides more clearance for the greater tuberosity, diminishing the glenohumeral
contact point of the anterior-inferior aspect of the capsule with proximal part of
the humerus. As a result, the biceps anchor is peeled back under tension, causing
injury to the postero-superior structures, especially the postero-superior aspect of
the labrum (SLAP lesion). The so-called peel-back progression mechanism permits
further laxity of the anterior aspect of the capsule [
19
,
35
]. With the glenohumeral
internal rotation deficit model, one attempts to identify throwers at risk for shoulder
injury by quantifying the internal rotation deficit individuals are considered to have
a clinically relevant glenohumeral internal rotation deficit when there is a loss
of internal rotation of the throwing shoulder as compared with the non-throwing
side. Such deficits are commonly found in overhead throwers, when compared with
measurements on the contralateral side, as well as concomitant increases in external
rotation.
Some studies suggested an osseous adaptation as a possible explanation for
the increased external rotation observed on the throwing arm, namely an increase
on the angle of the humeral head retroversion, or humeral torsion [
33
]. More
external rotation range in the dominant arm could be seen as a strategy to improve
performance, allowing increased cocking of the throwing arm which leads to higher
ability to generate power and speed or release [
36
]. Other authors though do not
look at these adaptations as single benefits but as abnormal stresses at the joints and
the surrounding tissues which may cause shoulder pain, decreased performance or
some unspecific shoulder disorders [
37
,
38
]. Pieper [
8
] found an augmented angle
of retroversion (up to 15
ı
) in the dominant shoulder of 51 handball players, when
compared with the non-dominant shoulder. This retroversion seems to increase the
available external rotation range-of-motion (ROM) but at the same time reduced the
ability of the rotator cuff to control high forces or velocities through the extremes
to shoulder ROM which could lead to excessive humeral head translation and
culminate in shoulder pain [
31
,
33
]. Thus, it remains unclear whether there are
benefits or disadvantages associated to changes in humeral torsion.
Humeral torsion may not be the only mechanism that explains the external rota-
tion gain in throwers. It seems that the looseness of the connective that surrounds and
stabilizes the glenohumeral joint may also play a role. The inferior glenohumeral
ligament complex (IGHLC) is considered to be the most restraining structure at the
late cocking position [
39
,
40
] followed by the coracohumeral ligament [
39
]. It is
likely that with the continuous excessive external rotation in throwing mechanics,
the anterior capsule and the anterior band of the IGHLC may become looser than
normal subjects [
41
,
42
]. The link between looseness of the anterior band of the
IGHLC, increased anterior and inferior humerus head translations and humeral
external rotation was demonstrated in cadaveric models [
42
].
