Optimal baseplate rotational alignment for locking-screw fixation in reverse total shoulder arthroplasty: a three-dimensional computer-aided design study.

BACKGROUND: Baseplate loosening in reverse total shoulder arthroplasty (RTSA) remains a concern. Placing peripheral screws into the 3 pillars of the densest scapular bone is believed to optimize baseplate fixation. Using a 3-dimensional computer-aided design (3D CAD) program, we investigated the optimal rotational baseplate alignment to maximize peripheral locking-screw purchase. METHODS: Seventy-three arthritic scapulae were reconstructed from computed tomography images and imported into a 3D CAD software program along with representations of an RTSA baseplate that uses 4 fixed-angle peripheral locking screws. The baseplate position was standardized, and the baseplate was rotated to maximize individual and combined peripheral locking-screw purchase in each of the 3 scapular pillars. RESULTS: The mean ± standard error of the mean positions for optimal individual peripheral locking-screw placement (referenced in internal rotation) were 6° ± 2° for the coracoid pillar, 198° ± 2° for the inferior pillar, and 295° ± 3° for the scapular spine pillar. Of note, 78% (57 of 73) of the screws attempting to obtain purchase in the scapular spine pillar could not be placed without an in-out-in configuration. In contrast, 100% of coracoid and 99% of inferior pillar screws achieved full purchase. The position of combined maximal fixation was 11° ± 1°. CONCLUSIONS: These results suggest that approximately 11° of internal rotation is the ideal baseplate position for maximal peripheral locking-screw fixation in RTSA. In addition, these results highlight the difficulty in obtaining optimal purchase in the scapular spine.

Biomechanical validation of medial pie-crusting for soft-tissue balancing in knee arthroplasty.

Balancing a varus knee is traditionally accomplished by releasing the medial soft-tissue sleeve off the tibia. Recently, "pie-crusting" (PC) medial structures has been described. In a biomechanical cadaver study we compared PC to traditional release (TR) to determine their effects on flexion and extension gaps. PC was done in five specimens along the anterior half of the medial soft-tissue sleeve and five along the posterior half, followed by a traditional release. In 90° flexion, valgus laxity after TR was significantly greater than after PC alone. PC of the anterior or posterior aspect of the medial soft-tissue sleeve can effect changes more in flexion than in extension, respectively. Complete TR did not provide more gap opening than PC in extension, but produced more effect in flexion.