Fig. 10.1
CT arthrogram demonstrating eccentric posterior glenoid erosion with biconcavity consistent with Walch B2
Scapular dyskinesis is associated with abnormal scapular protraction [10]. Scapular protraction causes a decrease in scapular posterior tilt [10]. An abnormally protracted scapula in glenohumeral joint osteoarthritis may exacerbate posterior subluxation and contribute to the dreaded B2 glenoid. The challenge is to understand whether the protracted scapular position is the early culprit or whether the protracted scapular position is secondary to shoulder pain from osteoarthritis. Additional work will be needed to better understand the natural history.
Static posterior subluxation with early osteoarthritis in younger patients has been recognized as a challenging entity to surgically treat [13]. Primary shoulder osteoarthritis with static posterior subluxation was proposed as the first stage of primary glenohumeral osteoarthritis that predates posterior glenoid erosion [13]. Posterior humeral head subluxation is thought to be the cause of eccentric, posterior glenoid erosion [14]. Attempts at surgical correction with various approaches included posterior bone grafting with posterior capsule imbrications, posterior capsulorrhaphy, and others were completed. Follow-up demonstrated progression of osteoarthritis and persistent or recurrent posterior subluxation in all patients [13].
There is a growing body of literature assessing the challenges of the B2 glenoid. However, our current body of literature does not consider the dynamic role of the scapula related to glenoid wear. Static 3D studies are very important for our foundation of understanding glenoid erosion; however, a better understanding of the dynamic role of the scapula will be critical moving forward. The scapula is clearly not a static structure and undergoes tremendous range of motion. McClure et al. measured 3D in vivo scapular kinematics during dynamic movements to help understand normal scapular motion [15]. 3D motions sensors were attached to scapular bone pins placed in healthy volunteers. The average ratio of glenohumeral to scapulothoracic motion in this healthy population was 1.7:1 [15]. Measurements during active scapular plane elevation included an average of 50° upward rotation, 30° posterior tilt, and 24° external rotation [15]. So although small changes in glenoid version are important, the wide variations in scapular motion throughout shoulder range of motion must be considered.
A recent matched cohort study found that B2 osteoarthritic glenoids have significantly greater premorbid glenoid retroversion compared with non-arthritic, normal glenoids using 3D computed tomography reconstruction [16]. The authors concluded that greater premorbid glenoid retroversion is associated with posterior instability and may be a causative factor in eccentric glenoid wear [16]. This study did not consider the dynamic role of the scapula related to glenoid wear.
A separate three-dimensional (3D) comparative study assessed scapulohumeral relationship in osteoarthritic and non-arthritic shoulders [17]. The study sought to better understand eccentric loading in osteoarthritic shoulders. Eccentric, posterior loading has been associated with worse clinical outcomes and correlated with glenoid component failure in anatomic total shoulder arthroplasty [17–21]. Contrary, to the prior cited study, this group concluded that osteoarthritic shoulders “do not have increased native glenoid retroversion predisposing to the development of the pathologic change” [17]. The group did not incorporate dynamic scapular motion, but they recognized that “scapulohumeral evaluation is necessary to understand the biomechanical relationship of the shoulder” [17]. Scapular stabilization exercises prior to surgical intervention may help to improve scapular dyskinesis and the protracted scapular position in this population.
Implications for Surgical Treatment in Early Posterior Subluxation and Posterior Glenoid Erosion and Primary Glenohumeral Joint Osteoarthritis
The patient population with mild posterior subluxation, posterior labral tear, and early glenohumeral joint osteoarthritis remains challenging (Figs. 10.2 and 10.3). In this patient population that is not quite ready for a total shoulder arthroplasty, it is challenging to know the optimal treatment plan. As noted, Walch et al. were unable to determine a successful treatment algorithm in the challenging patient population with static posterior subluxation [13].
Fig. 10.2
Grashey radiograph demonstrating early osteoarthritis with joint space narrowing and an anterior inferior humeral osteophyte
Fig. 10.3
Axial MRI demonstrating mild posterior subluxation with a large posterior labral tear
Our treatment approach for patients with mild posterior subluxation and early glenohumeral osteoarthritis has been modified based on our understanding of the scapula, but follow-up data to substantiate results is currently lacking. We believe that this type of patient may ultimately develop the dreaded B2 glenoid. Preoperatively, we assess the scapula and determine the presence or absence of scapular dyskinesis along with corrective physical examination maneuvers including scapular assistance test and scapular retraction testing. We find that this population has a positive dynamic labral shear test with posterior joint line pain upon testing. We obtain plain radiographs to assess for osteoarthritis and for posterior subluxation. Magnetic resonance imaging (MRI) is obtained to further assess the extent of glenohumeral joint osteoarthritis, labral pathology, and for posterior subluxation. CT scan is often completed for additional assessment of glenoid morphology. Patients with scapular dyskinesis will undergo 6 weeks of scapular stabilization exercises. We have a comprehensive program that is described in the rehabilitation chapter. Some of our favorite exercises include sternal lift, step out, low row, inferior glide, robbery, and lawn mower exercises. These exercises are described in detail in the rehabilitation chapter.
Surgical intervention typically involves arthroscopic labral repair of the superior and posterior labrum (typically a 4-anchor repair), glenohumeral joint debridement including humeral osteophyte removal when indicated, and biceps tenodesis when indicated. Arthroscopic anterior capsular release is also completed if the patient has limited external rotation. Postoperative scapular rehabilitation is key and range of motion to prevent glenohumeral joint stiffness is critical.
Scapular Dyskinesis and Rotator Cuff Tear Arthropathy: Scapulohumeral Rhythm and the Role of Glenohumeral Joint Versus Scapulothoracic Motion
The prevalence of scapular dyskinesis and rotator cuff arthropathy has not been established. New research has assessed postoperative scapulohumeral and scapulothoracic motion in this patient population following reverse shoulder arthroplasty (RSA), but preoperative information is lacking. Similar to primary glenohumeral joint osteoarthritis, it is unclear if scapular dyskinesis resolves following reverse shoulder arthroplasty. It appears that the constraint in the reverse shoulder arthroplasty construct leads to less glenohumeral motion and places more demands on the scapulothoracic joint. Furthermore, it is unlikely that even an anatomic total shoulder arthroplasty restores the normal glenohumeral joint motion.
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