44.1 Introduction

Accurate glenoid component placement is an important technical goal in shoulder arthroplasty. Multiple biomechanical studies have shown that glenoid component malposition in either version or inclination leads to compromised fixation and stability. ^{1 – 6} Glenoid component malposition also has clinically significant implications for implant longevity and function. ^{7 – 9} Standard methods to address glenoid wear patterns include eccentric reaming, bone grafting, and augmented components. In shoulders with glenoid deformity, these methods often are technically difficult and can lead to glenoid component malposition and early aseptic loosening. To prevent glenoid component malposition, CT scans have been used to develop patient-specific guides to improve the accuracy of glenoid component placement. Compared with traditional instrumentation, patient-specific guides have been shown to improve the accuracy of version and inclination and to reduce significant outliers in a cadaver study of 70 arthritic shoulders. ¹⁰ We describe our surgical technique with patient-specific instrumentation (PSI) in shoulder arthroplasty with specific tips and pearls.

44.2 Preoperative Planning

Standard preoperative planning begins with a radiographic examination of the shoulder. Our standard shoulder series includes anteroposterior (AP), true AP (Grashey), and axillary lateral views. The axillary lateral view provides information about the glenoid wear pattern and version of the glenoid. The true AP view provides an idea of the glenoid inclination. If rotator cuff function is in question, MRI, CT arthrography, or ultrasound can provide information to aid in deciding between an anatomic and a reverse shoulder arthroplasty.

Plain radiographs frequently are inadequate for characterization of glenoid wear patterns because of interference from the axillary soft tissues. Therefore, CT scans are often obtained for further evaluation of glenoid anatomy and for consideration of PSI. Multiple implant companies have 3D glenoid component planning guides with PSI ( Table 44.1 ). The CT protocol used at our institution is in accordance with the patient-specific guide protocol and uses soft-tissue algorithms and 0.625 × 0.625 mm slices at 120 kVp. Patients are placed supine in the CT scanner with the arm externally rotated (palm up). 2D DICOM (digital imaging and communications in medicine) images are segmented to create a 3D representation of the patient’s scapula. The Signature planning technique uses the 3D CT imaging to plan implantation in neutral version based on the method of Friedman et al. ¹¹ This method aligns the implant version by aiming toward the medial border of the scapula and is defined as an anatomic version. Glenoid inclination is planned preoperatively in neutral inclination based on the methods of Churchill et al ¹² and De Wilde et al ¹³ ; an average of the results from both groups (8 degrees inclined from the anatomic axis projecting perpendicular to the medial border of the scapula) was used. A web-based portal is used by the surgeon to review and plan the starting point for guide pin placement ( Fig. 44.1 ). Anatomic total shoulder implants are placed in the center of the glenoid based off the anteroposterior and superoinferior measurements of the glenoid. Placement of the baseplate components in reverse total shoulder arthroplasty is planned for the anteroposterior center of the glenoid, but positioned slightly inferior such that the inferior aspect of the baseplate is flush with the inferior glenoid rim. The position and orientation of the implants can be changed by the surgeon on the web-based portal and submitted to the implant company for review and creation of the guide. The patient-specific guide and a bone model of the scapula for reference are then produced ( Fig. 44.2 ).

The importance of careful preparation and planning by the surgeon cannot be overstated. Only by studying the CT scan and truly understanding the glenoid deformity can an effective plan be generated. Surgeons are specifically discouraged from simply applying default values without further evaluation.