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Recurrent PosteriorShoulder Instability
AbstractRecurrent posterior shoulder instability is an uncommoncondition. It is often unrecognized, leading to incorrect diagnoses,delays in diagnosis, and even missed diagnoses. Posteriorinstability encompasses a wide spectrum of pathology, rangingfrom unidirectional posterior subluxation to multidirectionalinstability to locked posterior dislocations. Nonsurgical treatmentof posterior shoulder instability is successful in most cases;however, surgical intervention is indicated when conservativetreatment fails. For optimal results, the surgeon must accuratelydefine the pattern of instability and address all soft-tissue and bonyinjuries present at the time of surgery. Arthroscopic treatment ofposterior shoulder instability has increased application, and avariety of techniques has been described to manage posteriorglenohumeral instability related to posterior capsulolabral injury.
Recurrent posterior shoulder in-stability is an uncommon condi-tion that is often unrecognized, lead-ing to incorrect diagnoses, delays indiagnosis, and even missed diag-noses.1 Posterior instability encom-passes a wide spectrum of pathoanat-omy that may affect the labrum,capsule, rotator interval, and bony ar-chitecture of the shoulder. Recurrentposterior subluxation is the mostcommon type of posterior instability.
Glenohumeral instability is com-mon, affecting approximately 2% ofthe general population.2 However,posterior instability occurs in only2% to 5% of those with shoulder in-stability.3 Trauma is thought to bethe underlying cause in approxi-mately half of patients with posteri-or instability.3 Although posterior
dislocation represents only 4% of alljoint dislocations,4 it is often easilymissed on clinical examination. Spe-cific imaging assessment is impor-tant. Recurrent posterior sublux-ation, which may present withinstability symptoms or simply aspain, is more common, particularlyin those who participate in high-riskathletic activities.
Relevant Anatomy andBiomechanics
The shoulder is the most mobile, butalso the least stable, joint in thebody because less than one third ofthe humeral head articulates withthe glenoid. Stability is conferred bya series of static and dynamic soft-tissue restraints that maintain thearticulation of the humeral headwith the glenoid while simulta-neously providing for a large range ofmotion.5
Peter J. Millett, MD, MSc
Philippe Clavert, MD
G. F. Rick Hatch III, MD
Jon J. P. Warner, MD
Dr. Millett is Co-Director, HarvardShoulder Service/Sports Medicine,Brigham & Womens Hospital,Massachusetts General Hospital,Boston, MA, and Assistant Professor,Department of Orthopaedic Surgery,Harvard Medical School. Dr. Clavert isAssociate Professor, Department ofOrthopaedics, CHRU Hautepierre,Strasbourg, France. Dr. Hatch isAssistant Professor, Sports Medicine/Shoulder & Elbow Services, Departmentof Orthopaedic Surgery, USC KeckSchool of Medicine, Los Angeles, CA.Dr. Warner is Professor, Department ofOrthopaedics, Harvard Medical School,Boston, MA, and Chief, HarvardShoulder Service, Department ofOrthopedics, Massachusetts GeneralHospital.
None of the following authors or thedepartments with which they areaffiliated has received anything of valuefrom or owns stock in a commercialcompany or institution related directly orindirectly to the subject of this article:Dr. Millett, Dr. Clavert, Dr. Hatch, andDr. Warner.
Reprint requests: Dr. Millett, SteadmanHawkins Clinic, 181 West MeadowDrive, Vail, CO 81657.
J Am Acad Orthop Surg 2006;14:464-476
Copyright 2006 by the AmericanAcademy of Orthopaedic Surgeons.
464 Journal of the American Academy of Orthopaedic Surgeons
Static RestraintsArticular factors such as joint
congruency, glenoid version, and hu-meral retrotorsion contribute tostatic joint stability. Bony abnormal-ities such as glenoid retroversion orposterior glenoid erosion can be pre-disposing causative factors for poste-rior shoulder instability.1
The glenoid labrum, a wedge-shaped fibrous structure consistingof densely packed collagen bundles,increases the depth and surface areaof the glenoid. It serves as an anchorpoint for the capsuloligamentousstructures, deepens the glenoid con-cavity, and reduces glenohumeraltranslation with arm motion.6 La-bral excision decreases the depth ofthe socket by 50% and reduces resis-tance to instability by 20%.6
The glenohumeral ligaments arethickened fibrous bands within thejoint capsule; these ligaments act atthe end ranges of motion and pro-vide static stability. Their functionis dependent on the position of thearm and the direction of the force ap-plied.7 For example, when the arm isadducted, the superior glenohu-meral ligament (SGHL) and coraco-humeral ligament (CHL) limit infe-rior translation and externalrotation of the humeral head. Addi-tionally, the SGHL and the CHL re-sist posterior translation of the hu-meral head when the shoulder is inflexion, adduction, and internal ro-tation. The inferior glenohumeralligament (IGHL) complex is com-posed of discrete anterior and poste-rior bands with an interposed axil-lary pouch that acts like ahammock, undergoing reciprocaltightening and loosening dependingon arm position. The posterior bandof the IGHL complex is the main re-straint to posterior translation of thehumeral head when the arm is ab-ducted.
The posterior capsule is definedas the area superior to the posteriorband of the IGHL complex. Theposterior capsule is the thinnest(1 mm) and perhaps weakest por-
tion of the shoulder capsule. It maylimit posterior translation when thearm is flexed, adducted, and inter-nally rotated.
The rotator interval plays a rolein static stability and is defined bythe borders of the supraspinatus su-periorly, the subscapularis inferi-orly, the coracoid process medially,and the biceps and humerus later-ally. The SGHL, medial glenohu-meral ligament, and CHL providevariable reinforcement to the rota-tor interval. The rotator interval andits constituents provide stabilityagainst inferior and posterior trans-lations, particularly when the arm isadducted and externally rotated.8
Evidence suggests that deficienciesin the rotator interval can contrib-ute to instability in patients withexcessive inferior or posterior trans-lation.9 In some individuals, the ro-tator interval may be composed ofloosely arranged collagen, whereasin others, it may be completelydevoid of tissue. This represents arotator interval capsular defectthat may need to be addressed inthe symptomatic shoulder, but itmay also be considered a normalanatomic variant in the stableshoulder.
Dynamic RestraintsDynamic stability is provided by
the rotator cuff, the deltoid, and thebiceps tendon through a concavity-compression effect on the humeralhead within the glenoid socket.10 Ofthe four muscles of the rotator cuff,the subscapularis provides the great-est resistance to posterior transla-tion.10,11 In addition, dynamic stabil-ity of the shoulder also is providedby the trapezius, serratus anterior,teres major, and latissimus dorsimuscles. Scapulothoracic motionmust be properly coordinated withglenohumeral motion so that theglenoid can be appropriately posi-tioned to provide a stable platformbeneath the humeral head.
Definitions: Laxity andInstability
The term instability is reserved forsymptomatic shouldersspecifical-ly, the sensation of the humeral headtranslating in the glenoid, which isfrequently associated with pain anddiscomfort.12 Instability is defined aspathologic joint translation thatcauses symptoms, or as the inabilityto keep the humeral head centeredwithin the glenoid cavity during ac-tive motion. Laxity is defined as aspecific translation for a particulardirection or rotation.13 Individualsmay have significant laxity and yetremain asymptomatic. Conversely,others with only minimal degrees oflaxity may have significant symp-toms of instability. The distinctionis important. Frequently, patientswith excessive shoulder laxity sus-tain a traumatic injury and subse-quently develop symptoms of insta-bility. Individuals with recurrentposterior subluxation generally havesymptomatic pain yet may or maynot have symptoms of instability.
Classification ofPosterior Instability
Posterior shoulder instability can beclassified by direction, degree, cause,and volition. Unidirectional posteri-or subluxation is the most frequentform of posterior instability. Posteri-or instability also can occur as bidi-rectional or multidirectional insta-bility.14
The degree of posterior instabili-ty can range from mild subluxationto frank dislocation. Recurrent pos-terior subluxation is the most com-mon form.
Posterior instability may be trau-matic (acquired) or atraumatic. Thetraumatic type is the more commonform.1 This can occur as a singletraumatic event with the shoulder inan at risk position (ie, flexion, ad-duction, and internal rotation) or asa culmination of multiple, smallertraumatic episodes. For example, an
Peter J. Millett, MD, MSc, et al
Volume 14, Number 8, August 2006 465
electrical shock producing posteriordislocation is a classic example of asingle traumatic event. An offensivelineman with the arms in the block-ing position would typify a predispo-sition to recurrent posterior sublux-ation because of the repetitiveloading. Posterior instability occur-ring secondary to overhead sportspresents more insidiously because ofthe gradual capsular failure from re-petitive microtrauma. Common pro-vocative activities include the back-hand stroke in racket sports, thepull-through phase of swimming,and the follow-through phases in athrowing activity or golf.
Posterior instability in the set-ting of an atraumatic history shouldalert the clinician to the possibilityof an underlying collage