Anyone who has placed a golf ball onto a tee knows how easy it is to dislodge the ball from the shallow surface of the tee and see the ball drop to the ground (often followed by laughing members of the foursome reminding you – “That’s One”).
Concavity Compression
The hip joint is a deep ball and socket joint, where stability is largely attributed to the constraint created by the depth of the socket that wraps around the femoral head. The knee joint is roughly a hinge, where one or more ligaments are under tension in all positions. Stability in the knee is created by this ligamentous tension through it’s range of motion. The shoulder joint does not benefit from either a deep socket or tension of ligaments through its motion. Another mechanism is needed to maintain the ball in its location on the relatively flat surface of the socket.
Matsen, et al. Practical Evaluation and Management of the Shoulder, 1994.
“Convcavity compression”, a concept described by University of Washington surgeons Matsen and Harryman, describes the necessary conditions for shoulder stability: a convex humeral head that sees a compressive force into the convex glenoid concavity will achieve stability if the applied direction of the force remains within the limits set by the margins of the surface of the glenoid.
Matsen, et al. Practical Evaluation and Management of the Shoulder, 1994.
What applies the force? It is largely from contraction of the rotator cuff muscles. One will notice that in a relaxed patient (for example, someone under anesthesia or a patient in a relaxed state on an exam table), it is relatively easy to translate and even dislocate the humeral head off of the glenoid. However, if the awake patient gently recruits the muscle with a little abduction, the stabilization effect of concavity compression comes into play and stability is achieved. (On a related note – this is why relying on an “exam under anesthesia” is much less helpful in determining stability issues than appreciating physical exam findings in an awake patient where testing of this mechanism is more reliably achieved).
The importance of the labrum in stability of the shoulder relates to its role in creation of depth and width of the concavity. In fact, it is accepted that it provides 50% of the glenoid depth.
Matsen, et al. Practical Evaluation and Management of the Shoulder, 1994.
The loss of the labrum compromises the “stability ratio”, especially in directions below the equator of the joint (as shown by Matsen and Harryman in their 1994 text). This can be considered the essential lesion of instability, as will be discussed further in future posts.
Matsen, et al. Practical Evaluation and Management of the Shoulder, 1994.
Other mechanisms of stability of the shoulder (especially in mid ranges of motion) include “adhesion-cohesion”, “limited joint volume” effect, and the “suction cup” effect. All are described in Matsen’s 1994 text. Concavity compression should be considered the primary mechanism for stability, as this is the mechanism that surgeons are effectively able to influence with operative procedures.
Capsular Constraint
I discussed the structure of the capsule and its role in stiffness in my last post (link). The capsule develops tension at the extremes of motion, where it acts as a check rein to further motion. Tension before the extremes of motion are achieved would limit mobility and create stiffness. Thus, stiffness and instability could be considered to lie on opposite ends of a spectrum. Obviously, it is also quite unusual to see shoulders with problems related to both stiffness and instability.
Defined areas of thickening of parts of capsule form the superior (SGHL), middle (MGHL), and inferior glenohumeral ligaments (IGHL). The SHGL limits external rotation (ER) with arm at the side (ERS), the MGHL ER in midranges of abduction, and the anterior band of the IGHL limits ER with the arm abducted.
The region on the anteroinferior quadrant has an essential role related to anter0inferior stability of the shoulder. While it can limit motion in ERA, loss of it’s integrity will lead to instability of the shoulder at this extreme position. This is the essential lesion of instability (“Bankart lesion”): the labrum and capsule tear off the anteroinferior quadrant of the shoulder.
Matsen, et al. Practical Evaluation and Management of the Shoulder, 1994.
Recurrent instability is common without surgical repair of the labral complex below the equator of the shoulder. Moving above the equator risks overtensioning the shoulder by overtensioning the MGHL – which plays little role in stability of classic anterior dislocators. Surgeons should avoid placing anchors between the 12 oclock position and the anterior equator of the glenoid, to avoid iatrogenic injury to the MGHL. This would cause restricion of ER of the shoulder. You may remember the table in the last post detailing how these parts of the capsule limit motion in predictable movements (link) . The anterior band of the IGHL can limit motion in ERA when tight, and instability in this same direction when incompetent or loose. And while tightness of the posterior capsule limits XBA (cross body adduction), incompetence results in instability of the shoulder with the arm in this position.
Capsular Laxity
A key concept to understand is the difference between laxity and instability. Matsen and colleagues measured laxity on anesthetized patients. They found that shoulders undergoing stabilization surgery for INSTABILITY had similar capsular LAXITY to other shoulders. Intuitively, one would think that lax shoulder would dislocate more than others. But this doesn’t seem to be the case. Laxity of the capsule is quite normal, and ranges in laxity and flexibility are expected to be seen. Thus caution is indicated about relying on the “exam under anesthesia” to determine an instability problem. Drawer testing and sulcus testing shoulder not be relied upon to direct surgical care. Instability is a functional issue where a patient is unable to maintain the humeral head concentrically reduced on the glenoid. The direction of instability is best determined by a careful history and physical exam of an awake patient. Also, restoring stability to a shoulder is not simply about removing laxity from the capsule. Indeed – overtightening the capsule can lead to functional stiffness and lack of satisfaction with the surgical procedure.
Summary
Stability of the shoulder is largely attributed to concavity compression, which is the functional compression of the humeral head into a concave socket by the pull of the rotator cuff. It should not be confused with laxity of the shoulder, which is needed to allow normal and functional movement of the shoulder. Further posts will discuss the nature of different types of instability – including direction (anterior/posterior/multidirectional), origin (atrauamtic vs traumatic), and even pathology (soft tissue vs bone loss).
Matsen, Lippitt, Sidles, Harryman. Practical Evaluation and Management of the Shoulder, 1994.
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