Optimal screw placement for base plate fixation in reverse total shoulder arthroplasty.

HYPOTHESIS: Scapular cortical thickness has not been fully characterized from the perspective of determining optimal screw placement for securing the glenoid base plate in reverse shoulder arthroplasty. MATERIALS AND METHODS: Twelve fresh frozen cadaveric scapulae underwent high resolution CT scans with 3-dimensional reconstructions and wall thickness analysis. Digital base plates were positioned and virtual screws were placed according to 2 scenarios: A - intraosseous through the entire course and exits a "safe region" with no known neurovascular structures; B - may leave and re-enter the bone and penetrates the thickest cortical region accessible regardless of adjacent structures. RESULTS: For scenario A, the optimal screw configurations were: (superior screw) length = 35 mm, 9° superior, 2° posterior; (inferior screw-A) length = 34 mm, 16° inferior, 5° anterior; (inferior screw-B) length = 31 mm, 31 inferior, 4 posterior; (posterior screw) length 19 mm, 29° inferior, 3° anterior. For scenario B: (superior screw) length = 36 mm, 28° superior, 10° anterior; (inferior screw) length = 35 mm, 19° inferior, 4° anterior; (posterior screw) length 37 mm, 23° superior, 3° anterior. The anterior screw was consistent between scenarios A and B, averaged 29 mm in length and was directed 16° inferior and 14° posterior. CONCLUSION: Thicker cortical regions were present in the lateral aspect of the suprascapular notch, scapular spine base, anterior/superior aspect of inferior pillar and junction of glenoid neck and scapular spine. Regions with high cortical thickness were accessible for both scenarios except for the posterior screw in scenario A.

Effects of mixing techniques on vancomycin-impregnated polymethylmethacrylate.

The use of antibiotic-impregnated polymethylmethacrylate in joint arthroplasty is widespread. The Food and Drug Administration has approved commercially prepared antibiotic bone cement, but in a climate of increasingly drug-resistant bacteria, orthopedic surgeons often hand-mix their own. A recent study reported the effects on drug elution of different mixing methods designed to decrease antibiotic particle size and distribute those particles more uniformly. Theoretically, these mixing techniques could also improve antibiotic cement strength; however, the actual effects of these techniques on cement strength are undefined. In the present study, 3 different methods of mixing vancomycin with bone cement were compared. We conclude that the addition of vancomycin to polymethylmethacrylate at commonly accepted concentrations does substantially decrease cement strength and that more complex mixing techniques do not improve cement strength significantly.