Posterior Bone Block / Posterior Shoulder Instability

Posterior shoulder instability is far less common than anterior instability and is frequently missed — the operation you choose depends entirely on two things: the instability pattern (traumatic vs atraumatic, unidirectional vs multidirectional, voluntary vs involuntary) and the amount of posterior glenoid bone loss. Get the pattern wrong and you operate on the wrong patient; get the bone loss wrong and you do the wrong operation on the right patient. The decision that drives everything — the 25% bone-loss threshold. Quantify posterior glenoid bone loss on a CT with 3D reconstruction (Pico method or best-fit circle, expressed as a percentage of the inferior glenoid diameter). Plain films are insufficient.

Two red flags before you ever offer surgery. First, voluntary dislocation: a patient who can deliberately dislocate using muscular contraction needs a formal psychosocial or psychiatric evaluation before any surgical consideration — surgery in voluntary dislocators with underlying psychopathology has uniformly poor outcomes. Second, multidirectional instability: never operate before an adequate rehabilitation trial, and never perform a bone block where there is no bone loss. Consent realistically. Counsel the patient that the open posterior bone block is salvage surgery, not a high-reliability operation like an anterior Latarjet: long-term data show tested recurrent instability in around 30%, major graft resorption in around 62%, and degenerative progression in about two-thirds at median 9-year follow-up (Villefort/Gerber 2022, PMID 36911764). Specific risks to consent: axillary nerve injury (deltoid paralysis), loss of external rotation from overtightening, graft prominence or resorption, recurrent instability, and iliac-crest donor-site morbidity (pain, lateral femoral cutaneous nerve injury, haematoma, infection). Setup. Lateral decubitus is preferred for posterior access (beach chair is an alternative). General anaesthesia plus an interscalene block. Before draping, mark the posterolateral acromion corner and measure and mark the 5–6 cm axillary nerve safe zone inferiorly — this single landmark governs the whole posterior dissection.

The goal is to restore a posterior bony and/or soft-tissue restraint to the humeral head. Two operations sit under this page: the open posterior bone block (Eden-Hybinette) for the bony-deficient shoulder, and the arthroscopic posterior Bankart repair for the soft-tissue shoulder. Both share the same critical safety principle — protect the axillary nerve, which lies about 5–6 cm below the posterolateral acromion corner. The exposure is laid out in full below (and in depth on the posterior approach to the shoulder page).

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Rehabilitation — posterior Bankart repair vs posterior bone block

Expected outcomes. Arthroscopic posterior capsulolabral repair is a reliable first-line operation for unidirectional posterior instability without significant bone loss: Bradley et al. 2006 (PMID 16567458) reported mean ASES improving from 50 to 86 with 89% returning to sport at mean 27 months in 100 shoulders, though recurrence and failure to return to the same level are higher in contact athletes. Posterior bone block outcomes are mixed — arthroscopic iliac-crest bone block achieved Rowe 18 to 82 with union in all 19 shoulders at short follow-up (Schwartz/Lafosse 2013, PMID 23337111), but long-term open data are sobering: Villefort/Gerber 2022 (PMID 36911764) reported only modest score gains, around 30% tested recurrent instability, 62% major graft resorption, and degenerative progression in two-thirds at median 9 years. Frame the bone block as salvage for failed soft-tissue surgery or significant posterior glenoid deficiency, not a first-line high-reliability procedure. MDI after inferior capsular shift or arthroscopic plication does well in roughly 80–90% of well-selected patients who have failed rehabilitation, with worse outcomes in hyperlax and voluntary patients. Complications — recognition, prevention, management

Epidemiology. Posterior instability constitutes only 2–10% of all shoulder instability — far less common than anterior — and is underdiagnosed, with posterior dislocation missed in up to 60% of cases on initial presentation. Natural history. Traumatic posterior instability with a labral tear has a low spontaneous healing rate. Atraumatic instability with capsular laxity may respond to rehabilitation (up to 80–85% in MDI). Untreated engaging lesions progress to symptomatic instability. Classification framework. Four axes determine management:

• Traumatic vs atraumatic: a single traumatic event (axial load to a forward-flexed, internally rotated arm — a football tackle or a fall) produces a posterior labral tear (posterior Bankart) or Kim lesion; atraumatic instability is insidious, from repetitive microtrauma (bench pressing, swimming) or generalised ligamentous laxity, with capsular redundancy rather than a discrete labral tear.
• Unidirectional vs multidirectional: pure unidirectional posterior instability responds better to surgery; MDI requires rehabilitation first.
• Voluntary vs involuntary: voluntary dislocation (deliberate dislocation by muscular contraction) demands a psychosocial workup before any surgery. Posterior glenoid and soft-tissue anatomy. The normal glenoid is pear-shaped and slightly retroverted (average 2–7°); the posterior glenoid rim is the critical stabilising structure for posterior instability, so posterior bone loss must be quantified on CT. The posterior labrum is a fibrocartilaginous thickening deepening the socket — a posterior labral tear (posterior Bankart) is a detachment from the posterior rim, while the Kim lesion is a concealed incomplete tear within the labral substance without surface disruption. The posterior capsule reinforces the joint; a tight posterior capsule paradoxically causes anterosuperior humeral head migration (GIRD — glenohumeral internal rotation deficit) and is associated with superior labral pathology in overhead athletes. Infraspinatus originates in the infraspinous fossa, inserts on the middle facet of the greater tuberosity, provides external rotation and posterior dynamic stabilisation, and is supplied by the suprascapular nerve (C5–C6). Teres minor originates on the lateral border of the scapula, inserts on the inferior facet of the greater tuberosity, provides external rotation and posteroinferior dynamic stabilisation, and is supplied by an axillary nerve branch — placing it at risk inferiorly. Key evidence. Kim SH et al. 2004 (PMID 15346113) defined the Kim lesion (concealed incomplete posteroinferior labral avulsion) in 15 shoulders. Bradley et al. 2006 (PMID 16567458) reported arthroscopic posterior capsulolabral reconstruction in 100 shoulders with mean ASES improving from 50 to 86 and 89% returning to sport at mean 27 months. Schwartz/Lafosse et al. 2013 (PMID 23337111) reported arthroscopic posterior bone block in 19 shoulders with Rowe improving from 18 to 82 and radiographic union in all. Villefort/Gerber et al. 2022 (PMID 36911764) provided the cautionary long-term open data: at median 9-year follow-up, around 30% tested recurrent instability, 62% major graft resorption, and degenerative progression in 67%. Burkhead and Rockwood 1992 (PMID 1634579) showed an exercise programme gave 80% good or excellent results in atraumatic instability but only 16% in traumatic — selection matters. The glenoid track / on-track-off-track concept (Di Giacomo, Itoi and Burkhart 2014, PMID 24384275) is validated for anterior instability; its posterior application is by analogy only.

References 1. Kim SH, Ha KI, Yoo JC, Noh KC (2004). Kim's lesion: an incomplete and concealed avulsion of the posteroinferior labrum in posterior or multidirectional posteroinferior instability of the shoulder. Arthroscopy. 20(7):712–720. PMID: 15346113. DOI: 10.1016/j.arthro.2004.06.012 2. Bradley JP, Baker CL 3rd, Kline AJ, Armfield DR, Chhabra A (2006). Arthroscopic capsulolabral reconstruction for posterior instability of the shoulder: a prospective study of 100 shoulders. Am J Sports Med. 34(7):1061–1071. PMID: 16567458. DOI: 10.1177/0363546505285585 3. Schwartz DG, Goebel S, Piper K, Kordasiewicz B, Boyle S, Lafosse L (2013). Arthroscopic posterior bone block augmentation in posterior shoulder instability. J Shoulder Elbow Surg. 22(8):1092–1101. PMID: 23337111. DOI: 10.1016/j.jse.2012.09.011 4. Villefort C, Stern C, Gerber C, Wyss S, Ernstbrunner L, Wieser K (2022). Mid-term to long-term results of open posterior bone block grafting in recurrent posterior shoulder instability: a clinical and CT-based analysis. JSES Int. 7(2):211–217. PMID: 36911764. DOI: 10.1016/j.jseint.2022.12.008 5. Di Giacomo G, Itoi E, Burkhart SS (2014). Evolving concept of bipolar bone loss and the Hill-Sachs lesion: from "engaging/non-engaging" lesion to "on-track/off-track" lesion. Arthroscopy. 30(1):90–98. PMID: 24384275. DOI: 10.1016/j.arthro.2013.10.004 6. Burkhead WZ Jr, Rockwood CA Jr (1992). Treatment of instability of the shoulder with an exercise program. J Bone Joint Surg Am. 74(6):890–896. PMID: 1634579 7. Neer CS 2nd, Foster CR (1980). Inferior capsular shift for involuntary inferior and multidirectional instability of the shoulder. J Bone Joint Surg Am. 62(6):897–908.