Last Updated: Monday, February 4, 2013
In addition to the motion at the glenohumeral joint the upper end of the humerus biceps tendon and rotator cuff must glide smoothly inside a sleeve consisting of the acromion deltoid coracoid coracoacromial ligament and the muscles originating from the coracoid. This motion is described as motion at the humeroscapular motion interface.
Excursions of up to 4 cm take place at this nonarticular interface.
Loss of smooth motion
Smooth motion at this interface is required for normal shoulder function. Loss of this smooth motion can result from alterations in the normal postural relationships of the humerus and scapula. Tightness of the posterior capsule can produce obligate superior translation of the humerus on elevation forcing abrasive contact between the cuff and the undersurface of the coracoacromial arch. Cuff deficiency may also allow superior translation of the humerus in relation to the scapula creating localized abrasion of the proximal humerus beneath the unyielding acromion. Repeated forced contact of the cuff or proximal humerus with the undersurface of the anterior acromion may grind up the remaining interposed rotator cuff tissue and humeral articular cartilage. Destruction of the joint surface associated with the excessively superior position of the humeral head in massive cuff deficiency is known as cuff tear arthropathy.
Roughness of the superficial aspect of the motion interface (the undersurface of the coracoacromial arch) may also result from developmental or acquired abnormalities in the shape of the acromion. Roughness of the deep aspect of the interface may result from complete or partial thickness cuff tears involving the upper surface of the tendon by sutures or lumpy tendon attachments following cuff repair by prominent tendon calcifications or by abnormal prominence of the tuberosities. Finally roughness at the humeroscapular interface can result from a thickened subacromial bursa or from posttraumatic or postsurgical scarring.
Most important problems of shoulder stiffness concern the movement between the humerus (arm bone) and the scapula (shoulder blade). Thus specific evaluation of this aspect of motion (rather than overall shoulder motion) is important in determining the problem and optimal management. This is accomplished by examining the movement of the arm with one hand while the shoulder blade is held still with the other.
A number of different anatomic factors limit normal humeroscapular motion including capsuloligamentous check reins abutment of the cuff and capsular insertions against the margin of the glenoid and humeroscapular bony contact.
Capsule and ligaments
Tension in the glenohumeral capsule and ligaments limits rotation of the humeral head. Tension in the inferior capsule for example restricts elevation. Tension in the anterior and posterior portions of the capsule restricts external and internal rotation respectively.
In eight cadaver shoulders we investigated the kinematic effects of the rotator interval capsule/coracohumeral ligament a particularly important aspect of the glenohumeral capsular complex which lies between the coracoid process the bicipital groove the subscapularis tendon and the supraspinatus tendon.
We found that this area of the capsule limited humeroscapular elevation in the plus 90 degree and minus 90 degree scapular planes but not in the zero degree scapular plane. Tightness of this specialized portion of the capsule also restricted adduction and external rotation but not internal rotation of the humerus.
Insertional abutment against glenoid
At the extremes of humeroscapular rotation the margins of the articular surfaces of the humeral head and glenoid come into contact. The humeral attachments of the capsule and rotator cuff border the articular surface of the humeral head. The labrum borders the articular surface of the glenoid. When these two groups of structures come into contact motion is limited unless the cuff insertions slide past the labrum and into the joint.
Bony factors can limit the range of humeroscapular motion. In abduction the proximal humeral shaft can contact the acromion. In cross-body movement the humerus can contact the coracoid. In internal rotation the lesser tuberosity can contact the glenoid.
Soft tissue causes of limited humeroscapular motion
Shoulder stiffness resulting from abnormalities of the glenohumeral joint surface is discussed in a later section. Here we consider stiffness in the presence of normal glenohumeral joint surfaces that is stiffness resulting from problems of the humeroscapular soft tissues. Two variations of soft tissue restriction of humeroscapular motion are recognized. The term frozen shoulder refers to an idiopathic limitation of humeroscapular motion from contracture and loss of compliance of the glenohumeral joint capsule. By contrast in a post-traumatic or post-surgical stiff shoulder adhesions scarring and capsular contracture result from previous injuries or surgery to the soft tissues around the glenohumeral joint and non-articular humeroscapular motion interface.
Contracture of the glenohumeral capsule may be generalized or localized. Localized capsular contractures produce predictable limitations of shoulder motion:
- Posterior-inferior tightness limits anterior elevation internal rotation of the elevated arm and cross body adduction.
- Posterior-superior tightness limits reach up the back.
- Anterior-superior tightness limits external rotation at the side.
- Anterior-inferior tightness limits external rotation of the elevated arm.
Capsular tightness not only limits motion but causes obligate translation. When rotational torque is applied to the humerus in a direction that tightens one aspect of the capsule the head of the humerus may be forced in the opposite direction. Therefore we would expect that when the capsule is tight anteriorly and an external rotation torque is applied the humeral head is forced posteriorly. This phenomenon may relate anterior capsular tightness and posterior humeral subluxation to the posterior glenoid wear seen commonly in glenohumeral osteoarthritis. It is also consistent with the posterior glenohumeral subluxation and posterior glenoid erosion in shoulders with excessively tight anterior capsular repairs--a condition we refer to as capsulorrhaphy arthropathy.
Similarly tightness of the posterior capsule may produce obligate anterior-superior translation with shoulder flexion.
In a series of experiments using cadaver shoulders we found that humeral elevation in the plus 90 degree scapular plane with a torque of three Newton-meters produced anterior translation of 5 mm and superior translation of 0.5 mm. When the posterior capsule was shortened surgically the anterior translation on forward elevation increased to over 7 mm and the superior translation to over 2 mm. These translations are sufficient to press the humeral head and cuff against the coracoacromial arch producing "subacromial impingement." These data suggest that "impingement signs" (either in maximal flexion or in abduction internal rotation) are likely to be positive in the presence of a tight posterior capsule.
The phenomenon of obligate translation suggests that caution should be exercised in applying large rotational torques to shoulders with tight capsules because of the risk of forcing obligate translation and increasing joint contact pressures.