Patient Specific Guides (PSGs) are used to increase the accuracy of arthroplasty. PSGs achieve this by incorporating geometry that fits in one unique position and orientation on a patient's bone. Sufficient docking rigidity ensures PSGs do not shift before being fixed by pins. Despite the importance of PSG docking rigidity, minimal research has been conducted on this issue. This study aims to determine whether commercially available PSGs, in their equilibrium position, provide sufficient stability for reliable surgical use. A commercially available PSG (Glenoid PSG, BLUEPRINT™, Wright Medical) was analyzed and tested in this study; the mechanical performance of this guide was assessed using a custom testing apparatus mounted to a universal testing machine (UTM) (MTI-10k, Materials Testing Inc), assembled with a high-precision load cell (MiniDyn Type 9256C, Kistler). The apparatus accepts an additively manufactured glenoid surrogate and was designed to transform vertical crosshead forces from the UTM into PSG-applied forces transverse to the glenoid plane along anterior-posterior and superior-inferior axes and PSG-applied torques about lateral, anterior, and superior axes. Three trials were recorded for each force and torque application. Prior to each test, the glenoid surrogate and PSG were articulated together with a constant 27N compressive force — equivalent to the normal force exerted by a surgeon using the guide — applied using springs. Forces were recorded when the guide was displaced 2mm by transverse loads or 5° by torque application; if the guide visibly dislodged from the glenoid surrogate before either criterion was met, force was recorded at the time of dislodgement. If no PSG movement occurred, testing ceased at 75N or 1.19N⋅m, depending on the test type.Introduction
Materials and Methods
Reverse shoulder arthroplasty (RSA) provides an effective alternative to anatomic shoulder replacements for individuals with cuff tear arthropathy, but certain osteoarthritic glenoid deformities make it challenging to achieve sufficient long term fixation. To compensate for bone loss, increase available bone stock, and lateralize the glenohumeral joint center of rotation, bony increased offset RSA (BIO-RSA) uses a cancellous autograft for baseplate augmentation that is harvested prior to humeral head resection. The motivations for this computational study are twofold: finite element (FE) studies of BIO-RSA are absent from the literature, and guidance in the literature on screw orientations that achieve optimal fixation varies. This study computationally evaluates how screw configuration affects BIO-RSA graft micromotion relative to the implant baseplate and glenoid. A senior shoulder specialist (GSA) selected a scapula with a Walch Type B2 deformity from patient CT scans. DICOM images were converted to a 3D model, which underwent simulated BIO-RSA with three screw configurations: 2 divergent superior & inferior locking screws with 2 convergent anterior & posterior compression screws (SILS); 2 convergent anterior & posterior locking screws and 2 superior & inferior compression screws parallel to the baseplate central peg (APLS); and 2 divergent superior & inferior locking screws and 2 divergent anterior & posterior compression screws (AD). The scapula was assigned heterogeneous bone material properties based on the DICOM images’ Hounsfield unit (HU) values, and other components were assigned homogenous properties. Models were then imported into an FE program for analysis. Anterior-posterior and superior-inferior point loads and a lateral-medial distributed load simulated physiologic loading. Micromotion data between the RSA baseplate and bone graft as well as between the bone graft and glenoid were sub-divided into four quadrants.INTRODUCTION
METHODS
