*4.1. Convertible Implants*

Humeral revision can be as complicated as glenoid revision in revision shoulder arthroplasty. Humeral revision is complicated by humeral stems that are challenging to remove and at the same time complicated by bone loss leading to the inability to place new implants with adequate fixation. Solutions on the humeral side include stemmed implants that are convertible, so removal does not need to be performed. Convertible systems have

the benefits of an easier revision but can be complicated by improper positioning of the original stem or failure of ability to reduce the implant at the time of revision. Component removal, if required, can be challenging depending on fixation methods and adequacy of fixation. Options include breaking up the fixation from above, osteotomy or windows. In the setting of cement, removal systems can significantly improve the ease of revision as well as eliminate the need for windows to remove cement and plugs. This is especially true in the setting of infection. In the setting of bone loss proximally, options for humeral revision include long-stem cemented or uncemented stems with or without proximal replacement using bone or metal. Various options have their own advantages and disadvantages.

#### *4.2. Humeral Bone Loss*

Humeral bone loss is usually seen in the revision setting after failed ORIF, hemiarthroplasty for fracture and failed anatomic TSA, as well as sequalae from a fracture malunion or nonunion or oncologic resection. Like glenoid bone loss, humeral bone loss is a difficult problem associated with several complications post-operatively after an RSA. One commonly cited complication is the loss of rotator cuff function, particularly ER, due to the loss of tuberosities. Loss of the tuberosity is also associated with decreased contour that decreases the deltoid wrap and subsequently alters the deltoid vector. Lastly, aseptic humeral loosening is seen due to the lack of metaphyseal osseous support which increases the torsional forces in the diaphysis.

Treatment options for this include humeral allograft prosthetic composite implants or proximal humeral replacement systems. For the replacement systems, metallic augmentation is used to restore the absent proximal humerus bone to restore the deltoid wrap. These systems also have built in modularity to allow for adjusted length and offset as needed. They rely on diaphyseal fixation and can be either cemented or cementless depending on the quantity and quality of bone available distally. These require bilateral full length humerus films to quantify the amount of bone loss requiring restoration.

#### **5. Innovations in Arthroplasty Technologies**

#### *5.1. Patient-Specific Instrumentation and Pre-Operative Planning*

Patient-Specific Instrumentation (PSI) systems have been developed to help surgeons more accurately implant the glenoid prosthesis. A patient's preoperative 3D CT scan is used to create a 3D virtual surgery tool that enhances the surgeon's ability to prepare the glenoid surface as well as fix the implant and screws. A meta-analysis of 12 studies comprising 227 participants found that PSI, compared to standard instrumentation methods, significantly improved glenoid positioning and decreased the number of malpositioned components from 68.6% to 15.3% [99]. These systems can be particularly helpful in cases of altered glenoid morphology. Hendel et al. found that in patients with preoperative retroversion >16◦, surgeons utilizing PSI were able to place the glenoid component within 1.2◦ of the ideal position [100]. Though implantation accuracy may be improved with PSI, the long-term clinical outcomes remain to be seen. In knee arthroplasty, for instance, PSI and robotic-assisted surgery have failed to demonstrate improvements in long-term clinical outcomes [101,102]. Robotic-assisted total shoulder arthroplasty is on the horizon, but prior to its widespread implementation, there must be careful consideration of its costs, benefits, and long-term outcomes.

Patient-specific computer modeling and surgeon-controlled 3D planning software have emerged as valuable tools for preoperative planning in shoulder arthroplasty. Statistical shape modeling technology can help quantify glenoid bone defects and virtually reconstruct the glenoid, thus assisting the surgeon to choose a suitable glenoid implant. 3D technologies can predict impingement-free ROM, which could help prevent notching or possible instability secondary to impingement. They also allow the surgeon to virtually plan implant size, implant seating and positioning, appropriate reaming depth, and compare different implant designs before even entering the OR. Patient specific guides can also be created based on these virtual models for use in the OR. The senior author's current

preference is to use Blueprint TM (Stryker, Kalamazoo, MI, USA) 3D planning software for a vast majority of cases.

#### *5.2. Augmented and Mixed Reality Applications in Total Shoulder Arthroplasty*

Augmented reality (AR) that is a "digital display overlay on real-world surfaces, allowing for depth perception" can be used in preoperative planning and intraoperative guidance during shoulder arthroplasty [103]. AR has been used in multiple orthopedic procedures and its applications are broadening. Ponce et al. utilized an AR device to enable a surgeon to interact remotely with another surgeon during a TSA via livestreamed video, allowing remote mentoring and guidance [104].

Mixed reality (MR), which consists of a "digital display overlay combined with interactive projected holograms", allows the surgeon to view the real world while manipulating the digital content generated by the device [103]. Gregory et al., in their proof-of-concept study, successfully utilized the HoloLens MR system (Microsoft) to perform a standard RSA, with an operative time of 90 min and a post-op CT confirming proper prosthetic positioning [105].

The ability to visualize data in real time and improve the accuracy of surgical intervention make these reality technologies promising tools for the shoulder arthroplasty surgeon. However, the prohibitive costs of these tools, for now at least, limit their widespread application.
