25 Dealing with Glenoid Bone Loss in Reverse Shoulder Arthroplasty
This chapter is an overview of the treatment strategies for addressing glenoid deficiency while performing reverse shoulder arthroplasty. Primary and secondary glenoid bone losses each present unique challenges that must be recognized and addressed in order to maximize the chance of successful reverse shoulder arthroplasty.
Glenoid bone loss is a common complicating factor that requires special consideration in reverse shoulder arthroplasty (RSA). The proper management of glenoid bone loss is essential to a successful long-term patient outcome. An understanding of the etiology of bone loss and anatomy of the patient’s glenoid allows for surgical planning of optimal glenoid preparation and baseplate positioning. Glenoid bone loss may result primarily in the setting of arthritis or secondarily from previous arthroplasty. Primary glenoid bone loss can often be overcome through the recent advances of implant technology, eccentric reaming, or local autograft to restore the deficiency. Secondary bone loss is often more severe and therefore treatments may require a secondary incision to harvest autograft from a distant source, the use of allograft, and potential staging of the procedure. Identification and understanding of the etiology and severity of glenoid bone loss are important for successful RSA.
25.2 Primary Glenoid Bone Loss
Primary glenoid bone loss results from altered glenohumeral biomechanics in osteoarthritis. The altered biomechanics cause peripheral glenoid contact stresses and posterior glenoid erosion leading to glenoid retroversion and posterior humeral head subluxation. Walch et al1 utilized computed tomography (CT) to classify glenoid morphologies in patients with glenohumeral arthritis into four types based on glenohumeral alignment and glenoid erosion (► Fig. 25.1). Type A refers to central glenoid erosion, whether minor (A1) or major (A2). The next most common glenoid morphology, Type B, consists of asymmetric joint force distribution causing posterior subluxation of the humeral head. Type B1 is characterized by a narrowed posterior joint space and lack of posterior erosion, whereas Type B2 is characterized by eccentric posterior erosion and an associated biconcave appearance. Type B3 is characterized by more advanced posterior erosion with humeral head subluxation so that no paleoglenoid remains and acquired concentricity results. Type C glenoid is dysplastic with retroversion greater than 25 degrees. Type D wear pattern is for eccentric anterior glenoid wear. The classification describes different degrees of severity for each subset of glenoid morphology and its use has helped devise strategies for surgical treatment. For example, a patient with central erosion or minor posterior erosion may be successfully treated with eccentric reaming and the advances in implant technology without sacrifice of the critical subchondral glenoid bone for prosthetic support. On the other hand, more severe posterior erosion and posterior humeral head subluxation as in a Type B2 and B3 may require bone grafting. Standard reaming techniques with anatomic total shoulder arthroplasty (TSA) in the more severe types risk recurrent posterior subluxation and glenoid loosening or wear.2
25.2.1 Implant Advances
Inadequate bone stock for baseplate implantation is a major cause of failure in glenoid-deficient RSA. Despite advanced techniques in glenoid preparation, it is not uncommon for the surgeon to be left with the decision to implant the baseplate with some degree of incomplete coverage. It is therefore essential to maximize fixation in the remaining glenoid bone stock and/or grafted bone. Improvements in baseplate design over the past 15 years have equipped the surgeon with more options to overcome incomplete correction and to achieve successful fixation intraoperatively. Companies now offer short or long post baseplates capable of spanning bone graft with length remaining to adequately engage the native scapula (► Fig. 25.2a). Other implant design advances include coating for ingrowth/ongrowth, variable angle locking screws, and threaded posts for compression (► Fig. 25.2b). The threaded post features plasma spray coating for bony ingrowth. When using interposed bone graft for deformity correction, a long posted baseplate is preferred as it potentiates the possibility for ingrowth of the native scapula. In a biomechanical study, Formaini et al3 showed the impact of these advances. In their study, when a central screw–based baseplate with four peripheral locked screws is used, glenoid bone coverage of 50% or more results in stable fixation within the acceptable limits of micromotion for bone-implant ingrowth to occur. While the findings of the authors may be comforting to the surgeon, techniques for improved baseplate seating are still employed.
Superior and posterior augmented baseplates are another advancement that has been introduced as a potential solution to glenoid bone loss. Roche et al reported on a biomechanical comparison of two different techniques: (1) the fixation of superior augmented glenoid baseplates with the use of off-axis reaming and (2) an eccentric reaming technique with implantation of a standard glenoid baseplate in composite scapulae.4 No differences in baseplate displacement were observed either before or after cyclic loading between the groups. Despite this finding, long-term results of augmented baseplates in RSA are lacking.
25.2.2 Eccentric Reaming
Eccentric reaming is a method in which the surgeon preferentially reams the “High Side” of the glenoid surface to the level of the more worn portion. This technique is used in order to overcome a defect in version as in a B2 or B3 glenoid or in a defect in inclination often seen as sequelae of rotator cuff arthropathy. Eccentric reaming is utilized to provide a congruent surface for baseplate fixation. The technique is relatively simple and adds little time to the operation, but there are potential disadvantages to eccentric reaming.
The amount of eccentric reaming performed is limited by the available glenoid bone stock. With excessive reaming, bone stock decreases as the glenoid is medialized and therefore less bone is available for fixation. Furthermore, overmedialization may cause instability as soft-tissue tension is decreased and the deltoid vector is altered (► Fig. 25.3).5 Therefore, eccentric reaming is limited to mild deformities and often an incomplete correction may result in order to avoid overmedialization.6 More severe deformities have a high rate of failure with attempted correction by eccentric reaming.
25.2.3 Local Autograft
Bone grafting is a biological solution to more severe cases of bone loss that cannot be overcome by eccentric reaming or implant advances alone. Advantages to using bone graft include preservation of remaining glenoid bone stock and restoration of a more lateralized joint line and avoidance of instability caused by overmedialization. Additionally, as the bone graft incorporates, a permanent restorative solution is provided. Disadvantages to bone grafting include fixation failure, nonunion, and graft dissolution. Local sources of autograft include distal clavicle, portions of the humeral metaphysis, and the humeral head in primary cases.
In primary glenoid bone loss, the humeral head is available and serves as an ideal source of bone graft because there is no requirement of an additional incision and the utilized bone is harvested from osteotomized and otherwise discarded bone. Furthermore, the shape of the humeral head is inherently congruent to the glenoid defect assisting in graft preparation. Humeral head autograft with RSA is a successful treatment option of osteoarthritis with Types B2, B3, and C glenoid bone loss.7 The technique has been termed “bony increased-offset reversed shoulder arthroplasty or BIO-RSA” by Boileau et al.8 At our institution, we have found BIO-RSA to be very useful when dealing with glenoid bone loss. In a patient with a Type B2 or B3 glenoid, after preparation with a hole saw, the humeral head can be undercut on a bias to obtain bone graft sized for baseplate seating (► Fig. 25.4a, b). The bone graft can be reshaped to match the recipient glenoid on the back table and after glenoid preparation; the bone graft is impacted with the long-post baseplate insertion to restore depth and version (► Fig. 25.4c–f).