Abstract
Introduction
Good outcomes in reverse shoulder arthroplasty (RSA) rely in part on stability of the humeral component. Traditionally humeral components have been cemented, however there has been recent interest in press-fit fixation of humeral components in RSA. Lateralization of the head center in RSA can impart larger moments on the humeral component than for anatomic reconstructions, increasing the importance of distal humeral canal preparation for implant stability. To date, the primary stability of any type of press-fit humeral prosthesis has been largely unexplored. The goal of this study is to evaluate the effect of over-reaming the distal humeral canal in a press-fit humeral component in RSA.
Methods
Computed tomography (CT) data of the shoulder were obtained from 55 shoulders. Images were segmented to produce digital models of the humerus. Humeral components for RSA (2mm diameter size increments) were sized and placed per the surgical technique, including preparation of the humerus with the appropriate reamers (1mm increments). Finite element models for each specimen were created with heterogeneous bone properties derived from the CT scan. Pressfit between the bone and stem was resolved to quantify the initial contact pressure on the stem; each stem was then loaded at 566N oriented 20° lateral and 45° anterior. Overall motion of the stem was measured, as well as interfacial micromotion in the porous coating region (Fig. 1). The effect of line-to-line (L2L) reaming and over-reaming by 1 mm was evaluated using an unpaired Student's t-test, with significance defined at p<0.05.
Results
Across all specimens, stem sizes 8 (n=3), 10 (n=25), 12 (n=20), 14 (n=2), and 16 (n=1) were used. Stem motion ranged from approximately 250–750μm; micromotion remained under 300μm (Fig. 2). Stem motion was significantly less for L2L reaming as compared to over-reaming for both size 10 (p=.008) and size 12 (p=.002) stems; micromotion was significantly less for size 12 (p=.002) stems. L2L reaming to a larger diameter stem resulted in significantly reduced stem motion (average 390μm versus 530μm, p<.001) and micromotion (average 53μm versus 135μm, p=.001) than over-reaming and using a smaller diameter stem. Stem rotation following L2L reaming was generally below 0.5°, and exceeded 0.75° when over-reaming.
Discussion and Conclusion
Reaming of the humeral canal directly impacts the stability of humeral stems in RSA. Even with satisfactory proximal press-fit, over-reaming enables increased rotation of the stem under functional loading prior to cortical engagement, and results in increased micromotion. In cases in which the reamer and stem offerings result in over-reaming, L2L reaming to the next larger stem significantly reduces stem motion and micromotion. However, reaming up also removes distal cortical bone, and thus the strength of the prepared humerus must be considered. In conclusion, line-to-line reaming significantly reduces the micromotion of humeral stems as compared to over-reaming.
