Anterior glenohumeral instability is a topic in which the exact etiology still is not well understood in terms of biomechanics. Static (capsule and ligaments) and dynamic factors (muscle contraction) are responsible for glenohumeral joint stability. 3,4,6,11,13–20,22–24,26 There has been an increasing interest in the dynamic stability, as has been reported in clinical and research studies on the anterior glenohumeral instability. 3,4,13,15–20,29 It may be a shift in the philosophy how to treat this difficult problem.
Dynamic Glenohumeral Stability
During the late cocking phase of throwing, the humerus maintains its level of abduction and moves into the scapular plane while externally rotating from 46° to 170°. In this position, the head of the humerus is angled so that it can stretch the anterior structures. This creates the potential for anterior instability. Jobe et al 12 reported that the pitcher with an unstable shoulder might begin with some compensatory mechanics, such as moving the humerus into the coronal plane. When the humerus moves into the coronal plane, the head of humerus angles even more anteriorly. The anterior capsule and ligaments should be protected in a dynamic way to avoid additional damage.
Numerous studies have shown that contraction of muscles across a joint may lead to increased stability. 3,6,17,18,28,29 Although dynamic stability mechanisms potentially could operate throughout the entire range of motion (ROM), its importance may vary according to the position of the glenohumeral joint. Recently, Lee et al 13 developed a new method that could quantify dynamic glenohumeral stability provided by any muscle (individual rotator cuff muscle) in a given shoulder position. Dynamic stability index, a new biomechanical parameter, was calculated in a specific glenohumeral position comprising the effects attributable to the concavity-compression mechanism and the shear force generated by a muscle. They showed quantitatively that the rotator cuff muscles primarily are dynamic stabilizers. The rotator cuff provided substantial stability even in the extreme ROM such as the late cocking phase.
The deltoid is the largest and maybe the most important muscle of the shoulder girdle. It consists of three major parts, with the anterior deltoid taking origin from the anterior and superior surfaces of the outer thirds of the clavicle and anterior acromion, the middle deltoid from the lateral margin of the acromion, and the posterior deltoid from almost the entire scapular spine. The most important function of the deltoid is forward elevation in the scapular plane. However, differences in activity of the three portions of the deltoid relative to arm position have been observed through electromyographic analysis. 25 The function of the deltoid might be highly differentiated and not be restricted only to abducting moment of the arm. Although its integrity is critical to shoulder function, little has been studied about stabilizing function of the deltoid.
The purpose of the current study was to quantify dynamic glenohumeral stability provided by the deltoid muscle using the dynamic stability index that was described previously. 13 The current authors also estimated the contribution of the three heads of the deltoid muscle in providing joint constraint during the late cocking phase of throwing, and compared that with rotator cuff muscles.
MATERIALS AND METHODS
Ten fresh-frozen shoulders from human cadavers (age range, 48–74 years) were prepared. The glenohumeral joint was disarticulated by resecting the glenohumeral joint capsule after rotator cuff muscles were released from the scapula origin, whereas three heads of the deltoid muscle was preserved intact. The glenoid labrum was preserved. The glenoid neck was osteotomized 2.5 cm medial to its articular surface. A specially-designed acrylic frame was constructed to permit placement of the humerus in the desired glenohumeral position (Fig 1). The distal shaft of the humerus was fixed to the frame on which the load-cell (AMTI Model FS 160A-600; Barry Wright, Watertown, MA) was mounted. An anatomic neutral position was the position in which the humerus is unelevated (parallel to the medial border of the scapula) and in 0° rotation (the elbow was flexed and the forearm was perpendicular to the coronal plane). The osteotomized glenoid was replaced back to the neck of the scapula and fixed temporarily for exact positioning of the humerus on the scapula. The scapula was fixed rigidly to the mounting device, which enabled the scapula and its glenoid to be anatomically aligned to the humeral head in any glenohumeral position. The glenoid piece then was taken out of the place before data collection. Then, the position of the humeral head was maintained by secure fixation of the entire frame. Muscle contraction was simulated by application of a constant force of 20 N to each muscle individually by means of a hanging weight system. A six-degree-of-freedom electromagnetic tracking device (Fastrak, Polhemus Navigational Sciences Division, Colchester, VT) was used to measure position and orientation of the glenohumeral joint. The load-cell permitted accurate resolution of the forces that were applied to the humeral head by the deltoid muscle force across the joint. Anatomic axes for the measurement of force vectors were defined to be in line with the anteroposterior (AP) and superoinferior axes of the glenoid. The mediolateral axis was defined as perpendicular to both axes.
To simulate the late cocking phase of throwing, testing was done with the humerus in the 45° extension (coronal plane) and in the scapular plane, while glenohumeral joint was maintained in 60° abduction and externally rotated to 90°. The glenohumeral joint in 60° abduction corresponded to the shoulder in 90° abduction. The force components in the mediolateral (compression), AP (shear), and superoinferior (shear) directions generated by three heads of the deltoid muscle/tendon unit were measured as described previously. 13 The following calculations and derivations were made on the force vector data:EQUATION
where Fcomp, FA–P shear and FS–I shear denoted measured compressive, anterior shear, and superior shear forces, respectively. The denominator of the above equations represented a vector sum of the measured force vector.
The dynamic stability index (DSI) in the anterior direction was calculated assuming the stability ratio in the anterior direction was 0.35 as determined previously 13;EQUATION
The higher the dynamic stability index, the greater the dynamic glenohumeral stability in the corresponding direction.
Deltoid Muscle Force Vectors
The line of action of each head of the deltoid, representing the direction of its force vector, was grossly observed by noting the relative positions of the origins, insertions and centroids of each muscle belly (Fig 1A). All the muscles appeared to have compressive and shear force in the AP direction. The deltoid made an additional contribution to stability by circumscribing the protruding portion of humeral spheroid and creating compression to increase stability (Fig 1B).
Figure 2 included compressive forces generated by the three heads of the deltoid muscle and the rotator cuff muscles in the abducted and extended shoulder (shown as a percentage of the force applied to a muscle) to compare the difference. Magnitude of the compressive force component generated by three heads of the deltoid muscle was significantly lower than that by the rotator cuff (p < .05). With external rotation simulating the position of anterior shoulder instability, the compressive force components by three heads decreased significantly (p < .05). The posterior head generated greater compressive force than the other heads in these positions (p < .005).
Figure 3 shows AP shear force generated by three heads of the deltoid muscle and the rotator cuff in the abducted and extended shoulder. The shear force changed its direction and magnitude significantly as the humerus was rotated to the end ROM. Three heads of the deltoid muscle revealed substantial anterior shear force component, which destabilize the joint, regardless of the degree of external rotation when the arm was abducted and extended. The anterior shear force by the anterior head was significantly greater than those by the other heads in these positions (p < .05).
Dynamic Stability Index of the Deltoid in Anterior Direction
Dynamic glenohumeral stability in the anterior direction provided by the deltoid muscle was quantified by the dynamic stability index. Figure 4 shows the dynamic stability index in the anterior direction (DSIanterior) of four muscles of the rotator cuff and three heads of the deltoid for the glenohumeral joint, which has the stability ratio of 0.35 in the anterior direction. Dynamic stability provided by the individual muscle varied substantially with different glenohumeral position.
The dynamic stability index of three heads revealed negative values in the neutral rotation and external rotation of 90°, with the arm abducted in the extended position. This result implies quantitatively that contraction of the deltoid muscle in these positions does not have stabilizing action and would dislocate the glenohumeral joint anteriorly when there is not a net opposing force exerted by the other muscles. The anterior head of the deltoid was the strongest dislocator among three heads in the abducted and extended shoulder simulating the anterior shoulder instability.
With the glenohumeral joint abducted 60° and externally rotated 90° in the scapular plane, all the heads of the deltoid revealed positive dynamic stability index values (Fig 5). This means the deltoid muscle could stabilize the shoulder in this particular position. Mid and posterior heads were found to stabilize the glenohumeral joint more efficiently than the anterior head (p < .05).
Practical Implication of Quantitative Dynamic Stability in Anterior Glenohumeral Instability
The deltoid is a large bulky muscle, comprising approximately 20% of the shoulder muscles. Therefore, the function of the deltoid as a stabilizer is thought to be significant. The current study shows the dynamic glenohumeral stability provided by the deltoid can be quantified by the dynamic stability index defined previously. 13 Quantitative determination of dynamic stability in the position of the anterior glenohumeral instability implies several important points in the treatment of the anterior shoulder instability.
First, the deltoid generated significant anterior shear force in the position of anterior shoulder instability, but still was providing the dynamic stability except for the extended glenohumeral position. This is in contrast to the rotator cuff that provides substantial stability in the end ROM and in the midrange of motion. Numerous reports 1,2,5,7–10,21,24,26,27,30 suggested that failure after instability surgery may be associated with excessive capsular laxity. However, the current results on quantitative analysis of the dynamic stability suggests that higher rate of recurrence after Bankart repair, especially with arthroscopic procedure, might not be from residual excessive capsular laxity. At least, asymptomatic laxity does not need to be sacrificed in fear that the capsular laxity possibly may lead to recurrent instability after the operation.
Second, selective strengthening of muscles that is efficient in providing dynamic stability in the specific position of instability is necessary. For anterior glenohumeral instability, the anterior cuff (subscapularis) and the posterior cuff (infraspinatus and teres minor) were more efficient in providing stability. Mid and posterior parts of the deltoid were less destabilizing to the joint than the anterior head in the position of anterior instability. In addition, the dynamic stability provided by the deltoid was less efficient when the arm was extended into the coronal plane. Strengthening of the scapular muscle is strongly recommended to avoid this vulnerable position.
The resulting stability from concavity compression is related to the depth of the concavity and the magnitude of the compressive force. 18 The depth of the glenoid fossa is provided by the shape of the glenoid bone, by the increased thickness of the articular cartilage at the periphery of the glenoid fossa, and by the glenoid labrum. Concavity of the glenoid should be restored fully in patients with instability by anatomic reattachment of a detached labrum and glenohumeral ligament back to the glenoid rim to restore effective dynamic stability. The strong relationship between depth and stability from concavity suggests that dynamic stabilization mechanism is compromised when the glenoid is developmentally small or flat or when the effective concavity of the glenoid has been lessened by Bankart lesion or glenoid rim fracture.
The deltoid generated significant shear force and compressive force in the position of anterior shoulder instability. The deltoid provided dynamic stability with the arm in the scapular plane and only was decreasing the stability of the shoulder with the arm in the coronal plane. The mid and posterior heads should be strengthened vigorously in anterior shoulder instability because they provide more stability, generating higher compressive force and lower shear force than the anterior head. Scapular muscles should be balanced to avoid the vulnerable glenohumeral position where the arm is extended beyond the scapular plane. Dynamic glenohumeral stability provided by the three heads of the deltoid could be quantified by the dynamic stability index. 13
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Patrick J. McMahon, MD; and Thay Q. Lee, PhD—Guest Editors