gms | German Medical Science

Objectives: Glenoid loosening, still a main complication for shoulder arthroplasty, was suggested to be related not only to glenohumeral conformity, cementing techniques, orientation, or implant design, but also to bone quality. However the effect of bone quality has never been experimentally tested. In this study, a novel cyclic rocking horse test was conducted on paired cadaver specimens of a wide range of bone density implanted with either keeled or pegged fixation designs.

Methods: Six pairs of cadaveric scapulae were scanned with a QCT to calculate trabecular bone mineral density (BMD) and implanted with either pegged or keeled cemented anatomical glenoid components. The implanted samples were then mounted on a custom mechanical testing setup to simulate rocking horse test under a constant 350N glenohumeral load while a humeral head component was moved cyclically in the inferior and superior directions within 90% of the subluxation range according to typical testing standards (ASTM F2808-14). The samples were then cyclically tested for 23000 cycles. Compression and distraction were measured with LVDT after 1000, 4000 and 23000 rocking cycles. Inferior and superior implant displacements were then tested for statistical differences (Wilcoxon-Mann-Whitney) between implant design and with the reference initial displacements. Relationships were finally established between the measured edge displacements and BMD in the periprosthetic zone.

Results and Conclusion: Mean measured edge displacements showed no statistical difference between keeled and pegged designs (p>0.05) at all measurement points, but were statistically different from the initial reference measurement at even 1000 cycles for both implant designs (p>0.05). At 4000 cycles, the pegged design lifted by 160μm in both inferior and superior edges, while the keeled lifted by a mean 200μm. For both implant designs, superior and inferior distractions were highest in specimens where BMD below the lifting edge was lower, showing a trend of increased distraction with decreased BMD, with the lowest density specimen allowing more than 900μm distraction at the superior and inferior implant edges (Figure 1 [Fig. 1]).

This study of glenoid loosening on cadaver specimens has shown that significant amounts of implant distraction (>0.2mm) could occur after 4000 cycles, even with half the load of the current ASTM standard in artificial bones. Using such test model, no significant effect of implant design could be observed. However, results showed that distraction was greater for specimens of lower periprosthetic BMD. This suggests that fixation failure will most likely occur in bone of lower density, and that fixation design itself may play a secondary role.