ISTA_Effect_of_Loading_Scenario
Evaluating the Efficacy of Measuring Implant-bone Fixation Methods in the Evaluation of Orthopaedic Implants
Presenting Author: David E. Cunningham
Contributing Authors: Cole Fleet, Dr. D. Ferguson, Dr. G. Athwal, Dr. J. Johnson
Introduction
Fixation is an important outcome measure in the evaluation of shoulder implants. The ASTM F2028 standard for the dynamic evaluation of glenoid loosening and disassociation outlines criteria for the quantification of the initial fixation of glenoid implants. To date, studies evaluating bone-implant fixation have employed loading protocols mostly isolated to simulating forces in the local coronal plane as a proxy to evaluate bone-implant relative motion when the maximum shear load in a normal shoulder is naturally sustained. This experimental study was conducted to determine if additional loading protocols are also warranted during the evaluation of glenoid baseplate primary stability. The objective of this study was to evaluate the effects of loading magnitude and load direction that not only characterize the objective ASTM-suggested inferior-superior shear load (0.42BW in shear, 0.51BW in compression), but also those that represent a clinically relevant range of activities of daily living (Figure 1).
Figure 1: 3D Load Vectors on Glenoid During Anatomical Motions
Methods
Five human cadaveric scapulae (male, 83±11 years) were implanted with keeled polyethylene glenoid baseplates (Olympia™ Total Shoulder System, Wright Medical Group) using bone cement. An optical machine vision USB3 camera (acA4096-30uc, Basler AG, Ahrensburg, SH, Germany) outfitted with a c-mount premium lens (FL-BC3518-9M, Ricoh Canada, ON, Canada) was used in conjunction with ProAnalyst ® to measure relative bone-implant motion at the inferior edge of the glenoid component. Five separate loading scenarios were employed. These included 4 loading conditions intended to approximate those experienced during daily living, as well as the ASTM-suggested standard loading protocols for the S-I direction. A custom designed synchronously loaded 3-dimensional loading apparatus was employed.
Results
The mean relative bone-implant displacements ranged from [0.113 mm – 0.031 mm] (Figure 1) over all loading cases, indicating a limited variability in the response to loading states developed at the glenoid (Figure 2). It was noted that the no-weight forward-flexion motion caused similar bone-implant displacements to the ASTM inferior-superior profile. An ANOVA test resulted in only slight statistically significant differences in bone-implant relative motion at the glenoid baseplate-bone interface (p = 0.049), but no relative bone-implant displacements were larger than those experienced by the ASTM-suggested worst-case scenario values. These results suggest that the existing method of evaluating fixation of polyethylene glenoid baseplate designs is ultimately sufficient, but may benefit from replicating anatomically relevant loading scenarios, as the varied loading magnitudes and directions ultimately result in slightly different levels of liftoff at the inferior edge of the glenoid baseplate; dependent on the load profiles utilized.
Figure 2: Clinically Relevant Loading Scenario vs Relative Bone-Implant Displacement Magnitude.
Conclusion
The precedent methods of evaluating the efficacy of (polyethylene) glenoid components are sufficient in representing the worst-case scenarios in an adequately wide range of activities that might be experienced by a patient post-surgery. The results of this investigation validate the existing suggested loading protocols in the evaluation of the fixation of polyethylene glenoid baseplates. Further work is needed to determine if the testing of implants with metal baseplates and reversed shoulder arthroplasty components follows similar patterns.