Medialized versus Lateralized COR in RTSA
At ICJR’s Pan Pacific Orthopaedic Congress, Dr. Joo Han Oh explored whether lateralization of the center of rotation in reverse shoulder arthroplasty implants avoids the problems created by medialization of the center of rotation. Below is the abstract of his presentation.
By Joo Han Oh, MD, PhD
Reverse total shoulder arthroplasty (RTSA) is a treatment option for patients with an irreparable massive rotator cuff tear or cuff tear arthropathy. [1,2]
The first RTSA implant designs maintained center of rotation (COR) near that of anatomic shoulder, which caused a relatively large torque on glenoid fixation and glenoid loosening. The Grammont reverse prosthesis medialized the COR, which minimized torque for glenoid fixation and increased deltoid efficacy by growing its abduction moment arm. Medialization, however, cause several problems, including scapular notching, [3,4] and instability. 
Medialization of the COR can be classified by either glenoid or humerus. Glenoid medialization is mainly associated with scapular notching and instability. Humerus medialization causes reduction of deltoid wrapping angle and rotator cuff tension, which results in decreased stability and active range of motion.
To avoid these problems, the concept of lateralization of the COR has recently been explored.
Medialized versus Lateralized
The challenge for current RTSA implant designs is to find the best balance between medialization of the glenosphere COR and lateralization of the humerus COR. Outcomes of concern include scapular notching, active rotational motion, and stability.
Lateralization of the Glenosphere
Lateralization of glenosphere can be achieved by increasing glenosphere thickness. A 1-mm increase in thickness results in a 5° increase in range of motion by and reduced scapular notching. However, this can also increase the shear forces on glenoid fixation.
The BIO (Bony Increased Offset) RTSA implant designed by Boileau et al is one of these concepts. By creating a long-necked scapula, this design can maintain the prosthetic COR with minimized torque to the glenoid rim, allowing for lateralization of glenoid and a 19% reduction in glenoid notching (Figure 1). 
Figure 1. Bony increased-offset RTSA implant.
Although similar in concept, prosthetic lateralization (metallic COR lateralization) causes elevated torque at the baseplate/glenoid interface, resulting in increased failure rates (Figure 2).
Figure 2 Medialized (Grammont) RTSA implant (A, left) and metallic lateralized RTSA implant (B, right). 
Glenoid loosening is theoretical concern for higher loads at the glenoid baseplate/bone interface in lateralized implant designs.
However, in a biomechanical study by Terrier et al,  no significant difference was found between standard RTSA and lateralized RTSA with regard to pull-out forces on the baseplate fixation screw. Only the location of the load differed, mostly on the upper screw of the lateralized implant or the lower screw of the BIO-RSA. 
In a computer simulation model, Virani et al  found no difference in movement of the baseplate between implants with or without 10 mm of lateralization.
Instability has been observed in 10% of cases as a dislocation within 6 weeks of surgery. Factors related to instability include proximal bone loss, subscapularis, and mechanical impingement. In lateralized-design RTSA implants, proximal pull directed to the glenosphere by the deltoid can enhance stability by increasing compression forces.
Lateralization of the Humerus
Lateralization of the humerus improves restoration of tension in the rotator cuff and increases deltoid wrapping. It depends on the humeral neck shaft angle (NSA) and humeral linear position.
A Grammont 155° humeral NSA applies tension on the deltoid by lowering the humerus, which helps to prevent dislocation. A medialized humerus shortens rotator cuff muscles, reduces the deltoid wrapping angle, and increases scapular notching. Reducing the humeral NSA will reduce scapular notching and allow the humerus to be lateralized. This does not change overall range of motion, but shifts where motion occurs.
A larger-diameter glenosphere will lateralize the humerus without lateralizing the COR and increasing joint stability. An extramedullary platform stem design with an anatomicl head osteotomy can also control the linear position. Conversey, intramedullary design such as the Grammont RSA results in humeral medicalization.
RTSA Design Classification
Various RTSA implant designs offer various glenosphere thicknesses for COR medialization and various humeral NSAs for humerus lateralization. Therefore, RTSA implant designs can be classified according to COR medialization relative to native glenoid and humerus lateralization relative to COR.
It is important to understand the advantages and disadvantages for each design (Table 1).
Table 1. RTSA Design Comparison
Medial Glenoid/Medial Humerus (MGMH) Design
With this design, the glenosphere COR is positioned medially near the native glenoid and the humeral component is positioned medially below the glenosphere. The result is a low glenoid loosing rate but a high scapular notching rate and poor improvements in active movement of internal and external rotation.
Lateral Glenoid/Medial Humerus (LGMH) Design
In this design, the glenosphere COR is positioned lateral to the native glenoid, while the humeral component is positioned medially below the glenosphere. The benefit is significantly reduced scapular notching and better improvement in active movement of internal and external rotation due to better tensioning of the rotator cuff muscles. However, there is a greater possibility of glenoid loosening.
Medial Glenoid/Lateral Humerus (MGLH) Design
Glenosphere COR in this design is positioned medially near native glenoid and the humeral component is positioned laterally on the top of humeral neck. This results in low glenoid loosening and reduced scapular notching. Better active abduction motion may occur due to a larger deltoid lever arm. Better tensioning of the rotator cuff muscles may be possible as a result of better active motion of internal and external rotation.
In RTSA, scapular notching may be prevented by reducing the implant’s NSA and increasing glenosphere thickness, while strength may be obtained from a longer lever arm and humeral lateralization. Stability may be increased by humeral lateralization a larger NSA.
Less medialization of the COR is recommended through use of a thicker and bigger glenosphere or bone graft behind the baseplate. A less-horizontal NSA will make the humerus more lateral and reduce scapular notching. A less-medialized humerus position will restore better active mobility by improved deltoid wrapping and rotator cuff tension.
Medialization or lateralization can be selected according to the patient’s individual needs. The surgeon can consider medialization as a primary option in cases of fixed elevation of the humeral head or pseudoparalysis. Lateralization can be more helpful in other cases, including a high risk of notching, younger age, massive rotator cuff tear with subscapularis tear, and possible active elevation with limited active external rotation.
Joo Han Oh, MD, PhD, is from the Department of Orthopaedic Surgery, Seoul National University College of Medicine, Seoul National University Bundang Hospital, Korea.
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