The effect of screw position on the initial fixation of a reverse total shoulder prosthesis in a glenoid with a...

Patients with rotator cuff tear arthropathy can be treated successfully with a reverse total shoulder prosthesis. In patients with significant glenoid bone loss, achieving stable bone fixation can be challenging, as the surgeon must know when bone grafting is necessary and when the plan to implant the reverse total shoulder prosthesis should be abandoned because of the likelihood of early implant loosening. The purposes of this study were (1) to determine the initial stability of a metal glenoid implant fixed in a glenoid with a central cavitary defect and (2) to determine whether an altered screw configuration would sufficiently resist implant micromotion and, thereby, allow bone ingrowth to occur. The Delta III reverse total shoulder glenoid implant was fixed into foam scapulae with a uniform density similar to normal glenoid bone density. The control group implants were fixed into foam scapulae without a glenoid defect, by use of the standard surgical technique for screw placement. The second group was fixed into foam scapulae containing a central cavitary glenoid defect, by use of the standard surgical technique for screw placement. The central cavitary defect was meant to simulate the bone loss typically found after the removal of a loose pegged glenoid implant, and it was created with a 4-pegged glenoid drill guide. A third group was fixed into foam scapula with a central cavitary glenoid defect, with an experimental screw configuration by use of a posterior screw directed toward the spine of the scapula and an anterior screw directed inferior to the central peg. All specimens were loaded with 500 cycles of 1 body weight (70 kg) to simulate the forces generated during arm elevation that occur during the first 3 months after surgery. Micromotion between the implant and the foam bone was measured with a digital video motion analysis system (accuracy, +/- 2.6 microm). After loading of the implant with 70 kg for 500 cycles in the superior direction, the mean micromotion was 54 microm (SD, 22) in the control group, 159 microm (SD, 70) in the second group, and 86 microm (SD, 32) in the third group (P = 0.003). Fixing the posterior screw into the spine of the scapula and directing the anterior screw below the central peg decreased the micromotion of a metal glenoid implant fixed in a glenoid with a cavitary defect by 46% and, more importantly, reduced the micromotion below the critical threshold of 150 microm, which is necessary for bone ingrowth and long-term survival of the implant.