Am J Sports Med 1996 May;24(3):286-292
Department of Orthopaedic Surgery, University of Pittsburgh, Pennsylvania, USA.
We conducted this cadaveric study to define a biomechanical rationale for rotator cuff function in several deficiency states. A dynamic shoulder testing apparatus was used to examine change in middle deltoid muscle force and humeral translation associated with simulated rotator cuff tendon paralyses and various sizes of rotator cuff tears. Supraspinatus paralysis resulted in a significant increase (101%) in the middle deltoid force required to initiate abduction. This increase diminished to only 12% for full glenohumeral abduction. The glenohumeral joint maintained ball-and-socket kinematics during glenohumeral abduction in the scapular plane with an intact rotator cuff. No significant alterations in humeral translation occurred with a simulated supraspinatus paralysis, nor with 1-, 3-, and 5-cm rotator cuff tears, provided the infraspinatus tendon was functional. Global tears resulted in an inability to elevate b eyond 25 degrees of glenohumeral abduction despite a threefold increase in middle deltoid force. These results validated the importance of the supraspinatus tendon during the initiation of abduction. Glenohumeral joint motion was not affected when the "transverse force couple" (subscapularis, infraspinatus, and teres minor tendons) remained intact. Significant changes in glenohumeral joint motion occurred only if paralysis or anatomic deficiency violated this force couple. Finally, this model confirmed that rotator cuff disease treatment must address function in addition to anatomy.
J Biomech 1995 May;28(5):489-499
Orthopaedic Department, Hannover Medical School, Germany.
This study introduces a dynamic shoulder model, where forces were applied to individual muscles in ten cadaveric specimens. The model provided reproducible glenohumeral joint motion and thereby allowed the investigation of active, glenohumeral joint mechanics. Forces were created by servo-actuated hydrodynamic cylinders and applied to the deltoid muscle and to the rotator cuff through wire cables. Computerized regulation initiated precise, time controlled cycles of glenohumeral joint motion. The position of the glenohumeral joint in all spatial orientations was measured and recorded using an ultrasonic sensor device. Reproducibility of glenohumeral joint motion was demonstrated on the basis of five cycles of glenohumeral joint elevation. Repeatability variance of position measurements for five cycles of elevation averaged 0.80 degrees for abduction, 0.75 degrees for anteflexion and 1.36 degrees for internal rotation. Arm weight and f orce distribution at the shoulder musculature were estimated according to the literature. In comparison to estimated physiologic conditions, a one third increase of arm weight led to a significant (p < 0.05) decrease of elevation of 20%, a one third decrease of arm weight to an average increase of elevation of 18% (p < 0.05). Exclusion of the supraspinatus muscle caused a significant (p < 0.05) 6% decrease of elevation of the glenohumeral joint. Without force applied to the subscapularis and infraspinatus/teres minor muscles, elevation decreased 16% (p < 0.05). A decrease of glenohumeral joint elevation of 25% resulted when force was applied to the deltoid muscle alone (p < 0.05).
J Bone Joint Surg Am 1997 Mar;79(3):433-440
Department of Surgery, University of Michigan, Ann Arbor 48109-0486, USA.
We examined the role of the glenohumeral and coracohumeral ligaments as well as the forces provided by the rotator cuff muscles, the long head of the biceps, the anterior and middle deltoids, and the pectoralis major in the stabilization of the glenohumeral joint in the posterior direction. Simulated muscle forces were mechanically applied to eight shoulder specimens. The humeroscapular position for testing simulated the 90-degree forward-flexion (humerothoracic) position used clinically for the so-called jerk test, which is the most clinically important position with regard to posterior instability of the shoulder. Experiments were performed with a variety of configurations of ligamentous and capsular cuts, humeral rotation, and levels of muscle force. Stability was investigated by measuring the force required to subluxate the humeral head a specified amount from its reduced position. Of the muscles and ligaments tested, t he subscapularis muscle contributed the most to this subluxation force. The coracohumeral ligament was an effective contributor in neutral humeral rotation, and the inferior glenohumeral ligament was an effective contributor in internal humeral rotation. The long head of the biceps was found to reduce the subluxation force in certain positions. CLINICAL RELEVANCE: It is widely agreed that a complex interaction of passive and active stabilizing structures and forces is necessary for clinical stability of the shoulder. The present study identified the contributions of ligaments and muscles to posterior stability of the shoulder in the position of greatest clinical importance--posterior subluxation with the shoulder in forward flexion.
Am J Sports Med 1995 May;23(3):270-275
Orthopaedic Research Laboratories, University of California, School of Medicine, Sacramento 95817, USA.
To determine the influence of rotator cuff muscle activity on humeral head migration relative to the glenoid during active arm elevation we studied five fresh cadaveric shoulders. The shoulder girdles were mounted in an apparatus that simulated contraction of the deltoid and rotator cuff muscles while maintaining the normal scapulothoracic relationship. The arms were abducted using four different configurations of simulated muscle activity: deltoid alone; deltoid and supraspinatus; deltoid, infraspinatus, teres minor, and subscapularis; and deltoid, supraspinatus, infraspinatus, teres minor, and subscapularis. For each simulated muscle configuration the vertical position of the humeral head in relation to the glenoid was determined at 30 degrees, 60 degrees, 90 degrees, and 120 degrees of abduction using digitized anteroposterior radiographs. Both muscle activity and abduction angle significantly influenced the glenohu meral relationship. With simulated activity of the entire rotator cuff, the geometric center of the humeral head was centered in the glenoid at 30 degrees but had moved 1.5 mm superiorly by 120 degrees. Abduction without the subscapularis, infraspinatus, and teres minor muscles caused significant superiorly directed shifts in humeral head position as did abduction using only the deltoid muscle. These results support the possible use of selective strengthening exercises for the infraspinatus, teres minor, and subscapularis muscles in treatment of the impingement syndrome.
J Shoulder Elbow Surg 1997 Jul;6(4):371-379
Department of Surgery, University of Michigan, Ann Arbor, USA.
This study examines the stabilizing factors of the glenohumeral joint against inferior translation over a range of subluxations. Factors examined included the glenohumeral capsular ligaments, the coracohumeral ligament, the rotator cuff forces, and the long head of the biceps force. Simulated muscle forces were applied to eight shoulder specimens with the arm near 0 degrees abduction. Stability was defined as the force required to inferiorly sublux the joint to a specified translation from the centered position and was evaluated under varying configurations of capsule cuts, humeral rotation, and muscle loads. The supraspinatus and biceps muscle forces were found to be important active stabilizers. Thus, tension in the long head of the biceps did not tend to depress the humeral head. The inferior glenohumeral ligament was an important passive stabilizer in external rotation. Understanding the effects of these factors adds insight into the under lying biomechanics of clinical shoulder instability.
Clin Orthop 1996 Sep;330:13-30
Orthopaedic Research Laboratory, Columbia University, New York, NY, USA.
The shoulder is characterized foremost by its mobility and large range of motion. The glenohumeral joint is notable for its relative lack of bony constraint, relying heavily on the congruent articulating surfaces and surrounding soft tissue envelope for static and dynamic stability. Effective function in the articulation is achieved by a complex interaction between the various articular and soft tissue restraints. The rotator cuff muscles center the humeral head in the congruent glenoid fossa through the midrange of motion, when the capsuloligamentous structures are lax. However, incongruent joints, especially in positions of loading asymmetry (in external rotation), have larger translations that occur at the extremes of motion. Excessive translations are then effectively restricted by the mechanical properties of the inferior glenohumeral ligament. When the capsule is tightened anteriorly it results in an anterior tether and causes an associa ted posterior shift in contact on the glenoid. The posterior migration of the humeral head center and glenohumeral contact are again more pronounced in shoulders with reduced congruence. Additional studies of normal motion in different planes, the effects of rotator cuff pathology and dysfunction on the kinematics of the joint, proprioception of the capsule, and biomechanical tests of the inferior glenohumeral ligament and other components of the joint capsule at strain rates associated with injury, need to be conducted to understand the specifics of normal shoulder function and the pathophysiologic processes that occur during shoulder degeneration.
J Bone Joint Surg Br 1993 Jan;75(1):137-140
Royal Infirmary of Edinburgh, Scotland.
We studied five cadaver shoulders to determine the strength relationship of the four rotator cuff muscles. The mean fibre length and volume of each muscle were measured, from which the physiological cross-sectional area was calculated. This value was used to estimate the force which each muscle was capable of generating. The lever arm of each muscle about the humeral head was then measured and the moment exerted was calculated. The strength ratios between the muscles were more or less constant in the five specimens. Subscapularis was the most powerful muscle and contributed 53% of the cuff moment; supraspinatus contributed 14%, infraspinatus 22% and teres minor 10%. The force-generating capacity of the subscapularis was equal to that of the other three muscles combined.
Clin Orthop 1990 May;254:29-34
Department of Orthopaedics, University of Washington, Seattle 98105.
The glenohumeral joint capsules of 23 shoulders in which the rotator cuff was not torn were studied by gross dissection and histologic methods. The cuff tendons were resected, leaving the intact capsule attached to the bones. This dissection method provided a unique overview of the capsule in situ and allowed the areas of cuff tendon and muscle attachment to be mapped. The capsule was found to be a continuous cylinder between humerus and glenoid. On approximately one-third of the capsule (the portion adjacent to the humeral tuberosities), tight insertions of cuff tendons were noted. The superior segment between subscapularis and supraspinatus contained the coracohumeral ligament. This segment appeared to reinforce the cuff through a transversely oriented band similar to the glenohumeral ligaments.
Clin Orthop 1980 Jan;146:37-41
Man's shoulder girdle is of the general pattern of his tree-swinging ancestors. With assumption of the upright posture, man's thorax has flattened from anterior-posterior. This results in a rotation of the scapula to a position of 45 degrees with the sagittal plane. In addition to this, man's forearm is habitually used in a position of approximately 45 degrees of internal rotation. This places the biceps tendon "off its trolley" and leads to biceps tinosynovitis. Motions of the glenohumeral joints result from the force couple of the deltoid muscle plus the rotator cuff muscles. The rotator cuff alone can abduct the arm with 50% force throughout the full range of its motion. In the absence of the supraspinatus muscle, however, the force couple is disrupted. Although initiation of abduction is with full force, the force rapidly falls off to 90 degrees. Above 90 degrees the arm can barely support its o wn weight. Although the shoulder undergoes progressive degenerative changes with age, the necessity for a joint implant, either partial or total--although successful--is rare.
J Biomech 1990;23(5):405-415
Department of Orthopedics, Mayo Clinic/Mayo Foundation, Rochester, MN 55905.
The three-dimensional orientation of the shoulder girdle musculature was studied in five cadaver shoulders in the position of function at 90 degrees of abduction and 90 degrees of external rotation using a method of computer assisted gross muscle cross-section analysis. The muscle volume, muscle fiber length, and physiological cross-sectional area were obtained by dissecting two specimens. The line of action, the magnitude and orientation of the moment were calculated for each muscle crossing the shoulder joint. The quantitative description of the moment potential of muscle forces influencing shoulder function was thus obtained. The most effective flexors of the shoulder which also appear to resist anterior dislocation in the position studied are the pectoral, the short head of the biceps, coracobrachialis, anterior deltoid, and the subscapularis. Most of the rotator cuff muscles and the posterior deltoid acted as adductors, while the anterior deltoid, long a
Clin Orthop 1990 Aug;257:76-85
Department of Orthopaedic Surgery, Karolinska Hospital, Stockholm, Sweden.
Muscle activity and coordination in ten shoulders were studied in five healthy subjects using electromyography (EMG) recorded during standardized loaded movements, i.e., flexion, extension, abduction, external rotation, and internal rotation at 0 degrees, 45 degrees, and 90 degrees of abduction. Bipolar surface and intramuscular fine-wire electrodes were used, and the EMG signal was low-pass filtered, full-wave rectified, and time-averaged. Activity from the subscapularis, supraspinatus, infraspinatus, pectoralis major (sternoclavicular part), the anterior, middle, and posterior parts of the deltoid, and the latissimus dorsi was recorded in parallel. In order to allow a comparison of the activity in a subject's different muscles and the activity in specific muscles between different individuals, the EMG was normalized. Muscle activity occur red simultaneously in muscles producing the movement and in antagonistic muscles. Coordination due to muscle contractions plays a significant role in stabilizing the shoulder joint. The infraspinatus, subscapularis, and latissimus dorsi acted as stabilizers during flexion; the subscapularis acted as a stabilizer during external rotation and with the supraspinatus during extension.
J Shoulder Elbow Surg 1995 May;4(3):209-218
Department of Orthopaedic Surgery, School of Medicine, University of Occupational and Environmental Health, Kitakyushu, Japan.
Dynamic analysis of intraarticular pressure was performed in 180 glenohumeral joints. The intraarticular pressure demonstrated characteristic changes during shoulder movement. In the healthy group minimum pressure was measured at 40 degrees of elevation in the scapular plane. In patients with contractures the pressure increased in the early phase of elevation and persisted. Pressure changes of the group with incomplete tears of the rotator cuff resembled those of the healthy group, whereas those in the massive tear group showed only slight pressure changes. When patients with rotator cuff tears had no limitation of arm elevation, pressure changes close to the normal pattern were found. However, when active elevation was markedly limited, no significant pressure changes were noted in some cases. In unstable shoulders the rate of pressure increase was slower, and the range of pressure changes was more limited than that in the healthy group. It i s speculated that changes in intraarticular pressure reflect the intraarticular volume. Measurement of the intraarticular pressure may be clinically useful in functional assessment.
Orthop Rev 1994 Jan;23(1):45-50
Orthopaedic Research Laboratories, University of Michigan, Ann Arbor.
Recently, considerable importance has been ascribed to that portion of shoulder stability which may be provided by active muscle forces. Joint proprioception likely has a considerable role in muscular stabilization of the shoulder by providing information to the central nervous system for the management of muscular activity. Normal human shoulder proprioception has not yet been thoroughly characterized. We have measured shoulder joint proprioception in a population of subjects without known shoulder abnormalities by quantifying the subjects' ability to correctly detect passive shoulder rotation in the abducted shoulder. We have found absolute angular proprioception to range from an average "best" of 0.78 degrees to a "worst" of 1.08 degrees. Individuals who have clinically determined generalized joint laxity are significantly less sensitive in proprioception (P < .002). Detection of external rotation is significantly m ore sensitive than detection of internal rotation (P < .001). Detection of external rotation becomes significantly more sensitive as the limit of external rotation is approached. We have concluded that these findings suggest capsular tightening as one possible mechanism for shoulder proprioception.
Am J Sports Med 1995 May;23(3):301-306
Department of Orthopaedic Surgery, Louisiana State University Medical Center, New Orleans, USA.
The existence of a reflex arc from the glenohumeral capsule to several muscles crossing the shoulder joint was determined in the feline model. Three branches of the axillary nerve terminating in the glenohumeral capsule were identified and electrically stimulated with supramaximal, 100-microseconds pulses using bipolar hook electrodes. Stimulation of the anterior and the inferior axillary articular nerves elicited electromyographic activity in the biceps, subscapularis, supraspinatus, and infraspinatus muscles. Stimulation of the posterior axillary articular nerve elicited electromyographic activity in the acromiodeltoid muscle. Transection of the three articular nerves just distal to their emergence from the main axillary nerve resulted in the absence of any electromyographic activity in the muscles on stimulation, confirming the afferent nature of the articular branches. The time from application of the stimulus to the appearance of a response in the muscles varied from 2.7 msec in the biceps to 3.1 msec in the supraspinatus. The existence of a reflex arc from mechanoreceptors within the glenohumeral capsule to muscles crossing the joint confirms and extends the concept of synergism between the passive (ligaments) and active (muscles) restraints of the glenohumeral joint. This provides new information in orthopaedic sciences that has direct application in modification of surgical repairs and therapeutic modalities of shoulder injuries.
Am J Sports Med 1986 Jan;14(1):12-17
Measurement of glenohumeral joint motion has, for the most part, been performed with the simple goniometer. The purpose of this paper is to describe a method for measuring and recording static and dynamic external rotation of the glenohumeral joint using the Cybex II Isokinetic Dynamometer and an MFE model 815 X-Y plotter. Static range of motion was assessed at 60 deg/sec using a slow arm action externally from a 90 degree abduction position. Dynamic motion was recorded at 300 deg/sec using a rapid rotational whipping action from a 90 degree abducted position. Measurements were recorded on three groups of athletes, and the influences of sex, hand dominance,
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