Abstract
Introduction
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.
Materials and Methods
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.
Results
The lowest and highest torques to displace the PSG by 5° were around the lateral (−0.08±0.02 N⋅m) and superior axes (0.87±0.23 N⋅m), respectively. The lowest and highest forces to displace the PSG by 2mm were along the inferior (31.77± 6.30N) and posterior axes (64.80±0.79N), respectively. Although it yielded at a higher torque than about the lateral axis, CCW rotation about the posterior axis produced the earliest PSG dislodgement at 3.76° while the PSG dislodged after only 1.05mm in the anterior direction.
Discussion
The above results demonstrate that the tested PSG design produces similar docking rigidity for all tested rotations except rotations about the lateral axis, which provided 4 times less stability than the next lowest result. This indicates that the PSG may not provide sufficient resistance in this direction to prevent inadvertent mal-rotation. The relatively low rigidity in anterior and superior translation indicate that this PSG design may be prone to mal-positioning errors in these directions. With these data in mind, PSG docking rigidity is not equal in all loading directions which could play a role in the clinical accuracy. Furthermore, this indicates that a systematic, objective method for PSG design optimization may be warranted.
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