Daniel Choi

Effects of glenosphere positioning on impingement-free internal and external rotation after reverse total shoulder arthroplasty

INTRODUCTION Patients may experience a loss of internal rotation (IR) and external rotation (ER) after reverse total shoulder arthroplasty (RTSA). We hypothesized that alterations in the glenosphere position will affect the amount of impingement-free IR and ER. MATERIALS AND METHODS Computed tomography (CT) scans of the scapula and humerus were obtained from 7 cadaveric specimens, and 3-dimensional reconstructions were created. RTSA models were virtually implanted into each specimen. The glenosphere position was determined in relation to the neutral position in 7 settings: medialization (5 mm), lateralization (10 mm), superior translation (6 mm), inferior translation (6 mm), superior tilt (20°), and inferior tilt (15° and 30°). The humerus in each virtual model was allowed to freely rotate at a fixed scaption angle (0°, 20°, 40°, and 60°) until encountering bone-to-bone or bone-to-implant impingement (180° of limitation). Measurements were recorded for each scaption angulation. RESULTS At 0° scaption, only inferior translation, lateralization, and inferior tilt (30°) allowed any impingement-free motion in IR and ER. At the midranges of scaption (20° and 40°), increased lateralization and inferior translation resulted in improved rotation. Supraphysiologic motion (>90° rotation) was seen consistently at 60° of scaption in IR. Superior translation (6 mm) resulted in no rotation at 0° and 20° of scaption for IR and ER. CONCLUSIONS Glenosphere position significantly affected humeral IR and ER after RTSA. Superior translation resulted in significant restrictions on IR and ER. Optimal glenosphere positioning was achieved with inferior translation, inferior tilt, and lateralization in all degrees of scaption.

Humeral version in reverse shoulder arthroplasty affects impingement in activities of daily living.

BACKGROUND Impingement after reverse shoulder arthroplasty (RSA) has been correlated with implant design and surgical techniques. Previous studies suggested that humeral retroversion can reduce impingement and increase external rotation range of motion (ROM). The purpose of this study was to determine how humeral version affects impingement in activities of daily living (ADLs). MATERIALS AND METHODS A single surgeon performed virtual RSA on 30 arthritic shoulders that were reconstructed from preoperative computed tomography scans. For each subject, the humeral component was placed into 5 versions: -40°,-20°, 0°, +20°, and +40° (- indicates retroversion, + indicates anteversion). Intra-articular and extra-articular impingement was calculated for 10 ADLs. Impingement-free ROM was also calculated for abduction, forward flexion, scapula plane elevation, and internal/external rotation (standardized tests). Risk of impingement for ADLs was assessed as the collective duration and frequency of impingement across all motions. Frequent impingement sites were identified. RESULTS For the ADLs, 0° version showed the least amount of impingement. In contrast, 40° retroversion resulted in the largest ROM for the standardized tests (118° ± 19° abduction, 109° ± 16° forward flexion, 111° ± 10° scapula plane elevation, 140° ± 15° internal/external rotation). The site of impingement changed with version: retroversion increased the extra-articular impingement, and anteversion increased the contact between the inferior glenoid and the humeral cup. CONCLUSIONS Humeral version can significantly affect impingement in RSA. Maximizing ROM in standardized tests may not reduce the risk of impingement during ADLs. Our results showed that an average 0° of version should be preferred, but the large variability among subjects suggested that optimum version may vary among individuals.

Trabecular bone microarchitecture and characteristics in different regions of the glenoid.

Success of shoulder surgery depends on implant fixation to the glenoid trabecular bone. The purpose of this study was to evaluate the anatomic characteristics of the normal glenoid trabecular bone microarchitecture to help assist in implant design and provide data for finite element analyses. Eight cadavers without evidence of osteoarthritis were used. Glenoids were scanned with micro-computed tomography and then divided into lateral and medial, then superior, inferior, anterior, and posterior quadrants (8 total segments). Each segment was analyzed for total mineral density, bone volume fraction, structure model index, and trabecular thickness (Tb.Th), number (Tb.N), and separation. Bone volume fraction was significantly higher (P<.05) in the posterolateral (20.8%±4.5%) and posteromedial (18.6%±2.5%) regions. Both Tb.N and Tb.Th were also highest in the posterolateral (Tb.N, 1.74±0.374 mm; Tb.Th, 0.148±0.017 mm) and posteromedial (Tb.N, 1.49±0.401 mm; Tb.Th, 0.165±0.016 mm) regions. Trabecular separation was greatest in the superomedial segment (1.00±0.181 mm) and lowest in the posterolateral region (0.663±0.121 mm). For structural model index, both the posterolateral (0.314) and posteromedial (0.312) regions had lower values than the other regions. The posterior segment of the normal glenoid in both the lateral and medial regions has the highest density, which is attributed to the increased trabecular number and thickness with decreased separation. This increased density may be attributed to the posterior directed loading of the glenohumeral joint. The trabecular microarchitecture in the glenoid is plate-like, as indicated by the low structural model index.

Analyse fonctionnelle comparative du positionnement de l’implant glénoïdien d’une prothèse d’épaule inversée entre une méthode 3D interactive et une méthode traditionnelle

Introduction A previous study by our group suggested that surgeons were not able to achieve the same accuracy in computer surgeries without full vision of scapulae. However, it is unknown whether planned prosthetic position for correcting alignment would result in better functional outcomes. The aim of this study is to analyze the effect of glenoid component positioning on the postoperative impingement free range of motion during standard glenohumeral kinematics. Materials and methods Thirty 3D-CT scapulas with highly eroded glenoid were involved. Two surgeons (A and B) performed blind RSA in Mimics software, when only the glenoid was visible. Surgeon A performed full 3D structure assisted virtual surgery on the same group of scapulae, and achieved high accuracy in correcting tilt and version. For each subject, the center of the frontal surface of glenoid metaglene from 3D assisted surgeries was defined as the planned position for baseplate. Translation of baseplate was measured in sagittal plane (anterior-posterior and superior-inferior directions) as the distance from the planned position to the center of baseplate from blind surgeries. For three postoperative situations, four standard glenohumeral kinematics (abduction, forward flexion, and internal external rotation) were simulated. Impingements between the humeral prosthesis, humerus and scapula during motion were recorded. Results Relative to the planned position from 3D assisted surgeries, no statistical difference was found on positioning by different surgeons for the anterior-posterior and superior-inferior translations of the baseplate. No statistical difference was found in impingement free range of motion among three operative conditions for each activity. Compared to the planned positions, more inferiorly translated baseplates resulted in increased impingement free range of motion in forward flexion ( P P P P P Discussion Without the guidance of the full 3D structure, the location deviation of baseplate positioning for both surgeons were similar, but showed significant variability. However, surgical plans focusing only on alignment accuracy in RSA may not result in better positioning of glenoid component, therefore may not improve impingement free range of motion for patients. A more inferior and posterior position may be advantageous for a better functional outcome. Future studies should integrate better designs for both alignment and positioning accuracy.