Bipolar lateralization in reverse shoulder arthroplasty for avoidance of scapular notching

Reverse shoulder arthroplasty (RSA) has become a well-established treatment option for multiple disorders of the shoulder joint. In recent years, implant designs and configurations have been modified in order to improve function and avoid complications. Lateralization on the glenoid and the humeral side has been described to improve function and decrease radiographic scapular notching. Data on the clinical and radiographic results of bipolar lateralization in RSA are lacking. In 38 cases, RSA was performed using an uncemented humeral short-stem component with a 145° neck–shaft angle in combination with bone lateralization on the glenoid side (Bony Increased-Offset Reversed Shoulder Arthroplasty, BIO-RSA; Wright Medical Inc., Memphis, TN, USA). Mean follow-up was 19 months (range 12–34 months). Patients were followed clinically using the Constant score as well as range of motion for shoulder flexion and external rotation. Radiographs in two different plains were analyzed for implant seating and the occurrence of scapular notching. There was a significant increase in all measured clinical parameters. Forward flexion increased from a mean of 75° preoperatively to 151° postoperatively, and mean Constant score increased from 21 to 71 points postoperatively (p < 0.001). Glenoid notching of grade 1 according to Sirveuax was observed in 3 out of 35 cases (9%); no grade 2, 3, or 4 notching was present. Revision surgery was necessary in one case (3%). RSA with bipolar lateralization leads to excellent clinical outcomes, low complication rates, and low rates of radiographic scapular notching. Longer follow-up and prospective randomized trials are needed. Level IV.

Reverse shoulder arthroplasty (RSA) has become a well-established treatment option for multiple disorders of the shoulder joint [1][2][3][4]. Significant pain relief and improvement of function have been published [2][3][4][5]. The implant concept introduced by Paul Grammont with a medialized center of rotation on the glenoid side and a 155°neck-shaft angle on the humeral side has shown good long-term results [1,6,7]. Initially, complications such as dislocations and infections were frequent, but decreased substantially with surgeons' growing experience [1]. The radiographic phenomenon of scapular notching describes impingement of the polyethylene liner against the scapular neck, leading to wear debris and potential loosening of components in the long term [8]. Therefore, in recent years implant designs and configurations have been modified. In order to avoid scapular notching, inferior baseplate positioning as well as lateralization on the glenoid side with bone grafting from the humeral head or with metallic augmentation became popular [9,10]. Alongside lateralization, bone defects on the glenoid as well as glenoid version and inclination can be addressed by augmentation [9,10]. On the humeral side the neck-shaft angle was modified by some manufactures to 145-or 135-degree inclination. First results for lateralization on the glenoid or humeral side are promising; however, results for bipolar lateralization in RSA are limited [2,11].
An excellent option for virtual testing aimed at an "impingement-free" range of motion is represented by CT-based three-dimensional (3D) planning tools [12]. The purpose of this study was to analyze the short-term results of a consecutive cohort of patients treated with RSA and bipolar lateralization who underwent preoperative CT-based 3D planning. The hypothesis was that clinical results improve over time and that scapular notching can be effectively avoided.

General information
Between March 2017 and January 2019, a consecutive series of 38 RSAs with bipolar lateralization were performed in one institution by the first author (PR). In all cases the same RSA system was used (Wright Medical Inc., Memphis, TN, USA). Patients were included in a prospectively recorded database; however, data analysis was performed in a retrospective fashion. All patients were seen in routine clinical controls. Inclusion criteria were the following: 1. preoperative CT-based 3D planning, 2. treatment with the same reverse shoulder implant, 3. complete clinical and radiographic data, and 4. a minimum follow-up of 12 months.
The 38 replacements were performed in 37 patients. Mean age at the time of surgery was 75 years (range 48-86 years). There were 28 women and 9 men. The right shoulder was treated 24 times and the left 14 times. Indications for surgery were cuff-deficient shoulders in 22 cases, primary osteoarthritis with severe glenoid erosion in 11, rheumatoid arthritis in 3, and posttraumatic arthritis and instability arthritis in one case each. Mean follow-up was 19 months (range 12-34 months).

Radiographic protocol
Preoperatively, immediately postoperatively, and at the most recent follow-up, radiographs in two plains (anteroposterior and outlet view) were performed in a standardized fashion. Additionally, CT scans for 3D planning were acquired before surgery according to the protocol. Three-dimensional planning was performed with Blueprint software (Wright Medical Inc.). The goal of preoperative planning was to test different implant configurations as well as implant positions in order to maximize the range of motion and minimize scapular notching. Glenoid morphology was classified according to Bercik et al. [31]. In all cases, the best solution to reach the goal was bipolar lateralization with either bone grafting on the glenoid side (Bony Increased-Offset Reversed Shoulder Arthroplasty, BIO-RSA; Wright Medical Inc.) or metallic augmentation in combination with an uncemented humeral implant that offers a neck-shaft angle of 145°. Whenever possible, the senior author and surgeon of this study (PR) favored lateralization with bone augmentation (. Figs. 1 and 2).
All radiographs were analyzed for implant loosening, incorporation of the

Clinical evaluation
All patients were examined in a standardized and comparable fashion before surgery and at each follow-up examination. Range of motion was measured in degrees with a standard goniometer for forward flexion and external rotation with the arm hanging at the side and an elbow flexion of 90°. Internal rotation was measured according to the region of patient's back that was reached with the thumb. The Constant and Murley score (CS) was used as a measurement tool [13,14]. Additionally, the Subjective Shoulder Value (SSV) was used.

Surgical technique
All patients were treated under general anesthesia and were placed in the beach chair position. A deltopectoral approach was used and the cephalic vein was prepared laterally. A 1-cm incision of the upper pectoralis major tendon was made in order to visualize the subscapularis tendon correctly, and was repaired after the procedure with nonabsorbable sutures. Tenotomy of the subscapularis tendon was performed and periglenoidal release was undertaken after placement of a humeral head retractor. Tenodesis of the long head of the biceps was performed systematically. Thereafter, the humeral head was dislocated. A bone graft for lateralization on the glenoid side was prepared with the manufacturer's instruments and resection of the humeral head was performed anatomically in a free-hand technique. Multiple broaches were used for preparation of the proximal humerus according to preoperative planning. After achieving adequate rotational stability of the final broach, the glenoid was exposed. According to the 3D planning, the central guide pin was placed and slight reaming was performed at the inferior part of the glenoid. Remaining cartilage was removed with a curette. Drill holes were placed with a 2.0-mm drill for better ingrowth of the bone augmentation. The baseplate with a 25-mm post was augmented with the bone graft that was shaped by using a rongeur. According to planning, the angled shaped bone graft was measured with a ruler to have the same lateralization as compared to the planning. The baseplate was impacted and fixed with two compression and two self-locking screws. The diameter of the baseplate was 25 mm in 28 cases and 29 mm in 10 cases. In all but one case bone augmentation was performed. In one case a baseplate with 3 mm of concentric lateralization (Perform Reverse, Wright Medical Inc.) was used. A 36-mm glenosphere was used in 30 cases and a 42-mm glenosphere in 8. The humeral head was then exposed, and the humeral tray was positioned. A highoffset (3.5-mm) tray was used in 20 cases and a low-offset (1.5-mm) tray was used in 18 cases. Tray position 6 was chosen in 30 cases and position 12 in 8. Care was taken that the tray did not overlap the highest point of the greater tuberosity, to avoid overlengthening of the arm. A + 6mm liner was used in all cases after trial reduction. The final implant was placed thereafter. An uncemented short stem was used in all cases, with a neck-shaft angle of 145°. The subscapularis tendon was repaired with 6-8 nonabsorbable sutures. The wound was closed in layers and patients were immobilized in a sling for 2 days. After 2 days patients were allowed to remove the sling and to use the arm according to their pain level. Free range of motion was allowed 2 days after surgery.

Statistics
The paired t-test was used to compare preoperative with most recent follow-up outcome (flexion, external and internal rotation, and CS). The level ofsignificance was set at p < 0.05.
All clinical results are shown in .

Radiographic results
Glenoid morphologyshowed anA1 glenoid in 16 cases, an A2 in 8, a B1 in 3, and a B2 in 11. No signs of implant loosening were detected on standard radiographs, neither on the humeral nor on the glenoid side. Full incorporation of the bone augmentation on the glenoid was seen in 33 out of 34 cases. In one case partial osteolysis of the bone graft was detected 25 months after surgery in a rheumatoid patient (. Fig. 3). However, this patient was very satisfied with the clinical result and no signs of radiographic implant loosening were found. Glenoid notching was observed in 3 out of 35 cases (9%), with grade 1 according to Sirveuax; no grade 2, 3, or 4 notching was present. No case of humeral loosening was detected.

Revisions
In one case revision surgery was necessary (3%). This patient had a massive cuff-tear arthropathy with a huge inferior bone spur on the glenoid neck (. Fig. 4). Three months after RSA (. Fig. 5) this patient suffered a non-traumatic dislocation in combination with a fatigue fracture of the acromion (. Fig. 6). Closed reduction was performed and the patient was placed Internal rotation was graded according to the Constant score by the level on the back that can be reached with the thumb in points: thigh = 0 points; buttock = 2 points; sacrum = 4 points; sacroiliac joint = 6 points; waist = 8 points; between shoulder blades = 10 points in a sling for 4 weeks; however, re-dislocation occurred. Revision was performed 6 months after the initial surgery with a liner 3 mm higher on the humeral side and a lateralizing glenosphere (. Fig. 7).
No re-dislocation has occurred to date (12 months after revision).

Discussion
RSA has become a very frequent surgical procedure for multiple degenerative disorders of the shoulder joint. In former days, this implant concept was used exclusively in patients with cuff-tear arthropathy [7]. However, as the initially high complication rates decreased over time, indications were expanded to other diagnoses including massive rotator cuff tears [15], rheumatoid arthritis [16,17], posttraumatic arthritis [5,[18][19][20][21], and primary osteoarthritis [22]. Compared to anatomic shoulder replacement, RSA also relieves pain and improves shoulder function, although rotation (especially internal rotation) may be compromised. A frequent complication in anatomic replacement is radiographic loosening of the glenoid component, particularly in the long term [23,24]. Loosening of the glenoid component in RSA is a rare complication in primary conditions with good bone stock on the scapula [25]. However, the radiographic phenomenon of scapular notching was frequently detected in several studies [1,8,26]. The contact between the humeral liner and the neck of the scapula frequently leads to  [27]. Different implant modifications have been developed in recent years in order to avoid scapular notching. The first option to achieve a greater notching-free range of motion is reduction of the neck-shaft angle on the humeral side. Historically, the neck-shaft angle in RSA was 155°.
Modern implants usually offer a lower and/or modular neck-shaft angle of 135°o r 145°. Due to the lower angle, notching of the liner becomes less likely and the component is less distalized and more lateralized. The second option to reduce notching is lateralization on the glenoid side. This can be achieved by an augmented baseplate (with metal or bone), a lateralized glenosphere, or both [2,10].
Several studies have been published to showthatlateralizedhumeralandglenoid components have lower rates of radiographic scapular notching compared to medialized implants [3,11]. However, data on bipolar lateralization are limited.
Werthel et al. published a descriptive analysis of different frequently used RSA systems and found 28 different configurations in 22 different implants and a high variability in global lateralization with between 13.1-35.8mm [28].
Recently, Boileau et al. published their 5-10-yearresults ofthe BIO-RSA concept in a cohort of 143 consecutive patients [2]. All patients received biologic augmentation on the glenoid side with a bone graft from the humeral head in order to lateralize the component and lengthen the scapular neck. They found graft incorporation in 96% of cases and grade 3 and 4 notching according to Sirveaux was present in 18%. Low body mass index, superior inclination of the baseplate, and a flush glenosphere without inferior overhang were risk factors for scapular notching. They hypothesized that a humeral component with a neck--shaft angle <155°in combination with a lateralized baseplate should lead to less notching. The results of the current study seem to support this statement, as we found an overall low frequency of scapular notching in only 9% of cases, all of which were grade 1. However, it must be mentioned that Boileau et al. [2] published mid-to long-term results, whereas the current investigation presents short-term results only.
One strength of the current study is that all patients underwent preoperative CT-based 3D planning and multiple implant configurations were tested for each patient virtually in order to achieve the best scenario for an impingement-free range of motion. In all cases bipolar lateralization was the most effective solution to avoid notching and achieve the best possible range of motion in all planes. Patients were treated according to preoperative planning and concordance between planning and implantation was high, as previously shown by Raiss et al.
The effectiveness of the philosophy of bipolar lateralization with a short-stem humeral component with a 145°neck--shaft angle in combination with bone augmentation on the glenoid side is underlined by the study of Lädermann et al. [29]. These authors recently published their results on the effect of humeral and glenosphere design on range of motion based on 3D planning software. They compared 30 different implant configura-tions and found that only 5 out of 30 combinations were able to restore >50% of the anatomic range of motion in all directions. Only a lateralized neck-shaft angle (145°) in combination with eccentric, large, or lateralized spheres was able to restore the range of motion accordingly.
Another aspect of lateralization in addition to reduction of scapular notching and improvement of range of motion is implant stability. In a biomechanical study Pastor et al. showed that particularly lateralization on the glenoid side significantly increased anterior stability of the artificial joint in different abduction angles [30]. Only a minor effect on stability was found for a lower humeral neck-shaft angle.
One patient in the current series suffered a dislocation which was related to underestimation of the pathology by the surgeon, as the severe cuff-tear arthropathy led to a huge inferior spur on the scapula neck that was not adequately removed during the first operation and which probably levered out the humeral component.
The limitations of the current study are its monocentric and retrospective design as well as the short follow-up and relatively low number of patients. Longer follow-up and a higher number of patients are necessary to adequately compare the findings to the abovementioned studies. CT scans were not acquired postoperatively to analyze incorporation of the bone grafts. However, the current study comprises a cohort of patients who were treated consistently with preoperative 3D planning and the same implant by one surgeon.

Conclusion
Reverse shoulder arthroplasty with bipolar lateralization leads to good and satisfying clinical outcomes, low complication rates, and low rates of radiographic scapular notching.