Abstract
The purpose of the study was to evaluate the force and torque required to dissociate a humeral head from the unimplanted modular total shoulder replacement system from different manufacturers and to determine if load and torque to dissociation are reduced in the presence of bodily fluids. Impingement, taper contamination, lack of compressive forces, and interference of taper fixation by the proximal humerus have all been reported as possible causes for dissociation. Experimental values determined in this research were compared with literature estimates of dissociation force of the humeral head under various conditions to gain more understanding of the causes of recurrent dissociations of the humeral head. This study examined biomechanical properties under dry and wet conditions under clinically practiced methods. Mean load to dissociation (1513 N ± 508 N) was found to be greater than that exerted by the activities of daily living (578 N) for all implants studied. The mean torque to dissociation was (49.77 N·m ± 19.07 N·m). Analysis of R2 correlation coefficients and p-values (α = 0.05) did not show any significant correlation between dry/bovine, dry/wet, or wet/bovine for load, displacement, or torsional stiffness in the majority of tests performed. Wetting the taper with water or bovine serum did not reduce the dissociation force to a statistically significant degree. Torque and lack of compressive forces at the rotator cuff may be the cause of dissociation at values less than those of activities of daily living. Torque data are provided by this study, but further research is needed to fully appreciate the role of torque in recurrent dissociations.
Highlights
Most humeral head dissociations [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22] occur during activities of daily living (ADLs), which is of interest since the average force to dissociate a humeral head from components is much higher than would be expected during ADLs
The objective of the present study was to evaluate the load and torque to dissociation of humerus heads obtained from revision surgeries as retrieved implants, and to determine if load and torque to dissociation are reduced by the presence of bodily fluids
The mean distractive forces required to dissociate the well-seated prosthesis in this study are well above the predicted glenohumeral joint forces during ADLs (578 N)
Summary
Torque and lack of compressive forces at the rotator cuff may be the cause of dissociation at values less than those of activities of daily living. The present study uses this value to represent a best available value for forces at the glenohumeral joint since the data reported therein were from the human cadavers and not computationally determined. Accurate in vivo load parameters are not available; the present study assumes values obtained from cadavers.
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