The Unstable Anatomic Total Shoulder Arthroplasty



The Unstable Anatomic Total Shoulder Arthroplasty


Sri Pinnamaneni, MD

Lawrence Gulotta, MD



INTRODUCTION

Stability in the normal native glenohumeral joint is maintained by the interaction of multiple static and dynamic stabilizers. The bony articulation plays a minor role in providing stability. Dynamic stabilizers, including the deltoid and rotator cuff, contribute significantly to the stability of the glenohumeral joint by providing active compression of the humeral head against the glenoid and within the coracoacromial arch during shoulder range of motion (ROM).

When performing an anatomic total shoulder arthroplasty (ATSA), many of the static and dynamic stabilizers of the native shoulder are altered. These include extensive capsular releases and often resections. In addition, the subscapularis tendon is generally taken down and then repaired at the end of the operation. With these alterations, successfully assessing and maintaining adequate stability of the implant can be challenging. In an ATSA, minimal stability is provided by the implanted metal on polyethylene bearing surface; therefore, implant size, position, and soft-tissue balancing are of paramount importance.

The number of ATSAs has increased significantly over the last decade and is projected to continue to increase. While ATSA shows excellent long-term outcomes and 10-year survivorship, the complication rates can be as high as 14.7%.1,2 With increasing numbers of ATSAs being performed around the world, surgeons performing this procedure should be well versed in the diagnosis and management of instability following ATSA. The purpose of this chapter is to provide an overview of the diagnosis and management of instability after ATSA.


EPIDEMIOLOGY AND CLASSIFICATION

Instability following ATSA is a common cause of persistent pain and dysfunction and represents a relatively frequent indication for revision surgery. The rate of shoulder instability in the literature after ATSA has ranged from 1.0% to 31%.1,2,3,4 While the reported rate of instability after shoulder arthroplasty has decreased over the last decade, instability after ATSA still represents the third most common complication (after glenoid loosening and glenoid wear).2 Instability after shoulder arthroplasty is a challenging problem to treat and can result from a combination of soft-tissue insufficiency and implant malpositioning.5,6 Despite revision surgery for instability, over two-thirds of patients can have recurrent symptoms.5

Instability following ATSA is generally classified by the direction of displacement—anterior, posterior, superior, or inferior. It is further characterized based on chronicity (early vs late) and the cause of instability. When assessing and addressing instability following ATSA, it is essential to understand the direction, chronicity, and cause of the instability to determine an appropriate treatment plan.


DIAGNOSTIC EVALUATION



Physical Examination

A careful physical examination is important to establish the diagnosis and direction of instability. Inspection for deltoid and rotator cuff atrophy is important. Location of previous incisions and status of the surrounding skin should be evaluated, especially if there is a suggestion of infection. Any concern for infection should prompt the clinician to proceed with further diagnostic workup as described in Chapter 32.

Passive and active ROM should be evaluated, with particular attention to deltoid and rotator cuff function. Deficits of passive and active ROM should be noted. In the early postoperative setting, increased passive external rotation can point toward possible subscapularis failure.12 The operative shoulder should also be assessed for anterosuperior escape with active forward elevation.

The positions of apprehension should be assessed. A comprehensive motor and sensory examination should be performed with specific attention to axillary nerve and brachial plexus. Postoperative axillary nerve palsy is a known complication of ATSA. Focused special testing should also be completed. The lift-off and belly-press test should be performed to assess the subscapularis. However, these tests are often difficult to perform in the setting of an ATSA. The sulcus test can reveal inferior laxity, and the load and shift test can assess for increased anterior and posterior translation.5,8


Standard Radiographs

Standard radiographs are an essential component of the initial evaluation. Radiographic studies should include a true anteroposterior image (Grashey) of the glenohumeral joint, a scapular Y view, and an axillary view. The true anteroposterior radiograph shows implant position, fixation, and the relationship between the humeral and glenoid components. The scapular Y lateral view can show the anterior to posterior position of the implant in relation to the glenoid. Additionally, an axillary view is essential to evaluate the glenoid component position, fixation, glenoid bone stock, version, and the relative position of the humeral component in relation to the glenoid. Comparison to previous radiographs can identify progressive changes, including osteolysis, implant loosening, eccentric glenoid wear, and superior migration. Anterior translation can indicate subscapularis insufficiency. Superior translation is consistent with superior rotator cuff insufficiency.


Advanced Imaging

While standard radiographs can provide useful information, the utility of the radiographs can be impacted by the patient’s positioning and by the imaging technique.13,14 In this context, a computed tomography (CT) scan can help evaluate the glenoid component position, fixation, glenoid bone stock, glenoid version, and the position of the humeral component in relation to the glenoid.13,14 It is essential to utilize metal artifact reduction techniques to enhance visualization. An ultrasound can evaluate the rotator cuff without metallic artifact. However, an ultrasound cannot evaluate for implant loosening and position.15 Magnetic resonance imaging (MRI) is oftentimes not helpful because of the associated artifact. An MRI with multiacquisition with variable-resonance image combination (MAVRIC) sequencing may be useful to identify the soft-tissue integrity, including fatty infiltration of the muscle bellies.16 Alternatively, a CT arthrogram can evaluate the rotator cuff and assess glenoid bone stock, version, implant position, and fixation. If a CT arthrogram is planned, it can be used as an opportunity for aspiration if there is concern about infection.


Anterior Instability


Etiology

The incidence of anterior instability is approximately 0.9%1,2 and represents 16% of all instability after ATSA.1,2 The most common cause of anterior instability is disruption or insufficiency of the subscapularis tendon.1,2,12 After ATSA, the subscapularis tendon acts as a primary anterior compressor and serves to balance the posterior rotator cuff. Additionally, it also acts as a static soft-tissue restraint to anterior subluxation. Subscapularis function can be variable in patients following ATSA.17 In a retrospective study of 41 patients, Miller et al showed that the lift-off test and belly-press test was abnormal in the majority of patients.17

Subscapularis tendon failure after ATSA is a potentially devastating complication that can result in significant pain, disability, and even implant failure.6,8,12,17 Most cases of subscapularis failures are related to soft-tissue integrity and implant positioning, but can be multifactorial (TABLE 34.1). Soft-tissue integrity risk factors for subscapularis failures include a history of prior anterior shoulder surgeries that violate the subscapularis, including prior lengthening procedures for internal rotation contractures and previous open anterior stabilization procedures.12 Aggressive physical therapy,
early return to increased activity, or acute trauma in the early postoperative period can place excessive stress on the subscapularis repair and result in a subscapularis compromise.








Authors have proposed different techniques of subscapularis management during ATSA to optimize postoperative healing, including tendon-to-tendon repair, tendon-to-bone repair, and lesser tuberosity osteotomy.18,19,20,21,22 The reported failure rate of primary subscapularis repairs and reattachments has been reported to be as high as 44%.19 A compilation of the current literature is inconclusive whether one technique is clearly superior to another to prevent postoperative subscapularis failure.18,19,20,21,22 Any technique that optimizes subscapularis healing will help improve clinical outcomes after ATSA.

Implant malpositioning in the form of excessive combined humeral or glenoid component anteversion can cause unexpected stress on the anterior structures and the subscapularis repair resulting in anterior instability.7,23 Considering there is a wide variation in the normal humeral head retroversion and glenoid version, an anatomic humeral head osteotomy to re-create the anatomic retroversion and treating anterior glenoid bone loss is essential during the index ATSA.24 Excessive lateralization by overstuffing the joint with a large humeral head, large glenoid component, or placing the humeral component in varus can place undue stress on the subscapularis repair and result in failure.6,8,12,19

The diagnosis of chronic anterior instability can be subtle. Subscapularis failure can present with progressive development of internal rotation weakness or a sense of apprehension with the arm in abduction and external rotation. Symptoms may present more acutely after a traumatic event. Increased external rotation on the operative side compared to the contralateral side or a significant change from earlier examinations can point to a subscapularis tear. Special testing, including belly-press and the lift-off test for subscapularis testing, is not highly reliable in the setting of ATSA.17 In cases of apparent anterior instability or subluxation, standard radiographs can be diagnostic (FIGURE 34.1). If the subscapularis was managed with a lesser tuberosity osteotomy during the index surgery, it is very important to evaluate radiographs for displacement of the reattached lesser tuberosity. In subtle cases of anterior instability, MRI, CT arthrograms, and ultrasound may be helpful in the diagnosis of rotator cuff tears, tendon quality, retraction, and associated muscle atrophy (FIGURES 34.2 and 34.3). CT scan can also help identify the version and positioning of the humeral and glenoid components.






Jun 23, 2022 | Posted by in ORTHOPEDIC | Comments Off on The Unstable Anatomic Total Shoulder Arthroplasty

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