Reverse shoulder glenoid loosening: an evaluation of the initial fixation associated with six different reverse shoulder designs.

INTRODUCTION: This study quantified glenoid fixation before and after cyclic loading of six reverse shoulder prosthesis designs when secured to low and high density bone substitute blocks.

METHODS: A displacement test quantified fixation of six reverse shoulder designs: 38 mm Equinoxe standard offset (EQ), 38 mm Equinoxe lateral offset (EQL), 36 mm Depuy Delta III (DRS), 36 mm Zimmer (ZRS), 32 mm DJO RSP (DJO), and a 36 mm Tornier BIO-RSA (BIO), secured to 0.24 and 0.48 g/cm 3 polyurethane blocks as a shear (357 N) and compressive (50 N) load were applied before and after cyclic loading. Displacement was measured with a dial indicator in the directions of the applied loads along the superior/inferior axis. A cyclic test rotated each glenosphere (N= 7) at 0.5 Hz for 10 k cycles as 750 N was constantly applied. A two-tailed Student's unpaired t-test compared mean displacements.

RESULTS: The average displacement of the EQ, EQL, ZRS, DJO, DRS, and BIO-RSA devices in the low density substrate was 182, 137, 431, 321, 190, and 256 microns, respectively. The average displacement of the EQ, EQL, ZRS, DRS, and BIO-RSA devices in the high density substrate was 102, 95, 244, 138, and 173 microns, respectively. Pre- and post-cyclic displacement was significantly less in the high density bone substitutes than in the low density bone substitutes for the majority of implant comparisons. During the cyclic test, six of seven ZRS devices failed at an average of 2,603 cycles, one of seven 32 mm DJO failed at 7,342 cycles, and four of seven BIO devices failed at an average of 2,926 cycles. All seven of the EQ, EQL, and DRS devices remained well fixed throughout cyclic loading.

DISCUSSION AND CONCLUSIONS: This study quantified glenoid fixation of six reverse shoulder designs; significant differences in fixation were observed between nearly every implant design tested.