We present 2 year results of a prospective natural history study of Type III Acromioclaviclular joint dislocation (ACJD) treated non-operatively. Previous natural history studies are compromised by inconsistent definitions of the grade of injury and non-validated scoring tools; they do not identify which patients will have ongoing symptoms. This trial documents the strength and subjective recovery over time, and identifies risk factors for poor outcome and need for surgery. Patients with Rockwood Type III ACJD received a standardised rehabilitation protocol (6 sessions of physiotherapy). Clinical assessment was performed at presentation, 3, 6, 12 and 24 months after injury, including isometric strength testing, pain (VAS/10) and subjective scores – Subjective percentage of normal (SPON), American Shoulder and Elbow (ASES), Oxford Shoulder Score (OSS) and Quick Disabilities of Arm Shoulder and Hand (qDASH). 28 male patients were recruited, 26 reached 12, and 9 to 24 months follow up. Two required surgery and one emigrated. The mean age was 39 (15 to 67). Initial mean pain was 3/10, SPON 51% (6–95) and strength was 76% of the other side. By 3 months mean subjective recovery was 70% and strength 90%. Strength recovered to 99% of normal by 12 months but subjective scores remained at mean 90%; by 24 months subjective scores were mean 94%. 2 patients had subjective scores <80%. There was a wide range of initial subjective scores and weakness after Type III ACJD. 95% of strength had recovered by 6 months and subjective scores recovered to 94% of normal by 24 months. 4 patients (14%) did poorly with 2 requiring surgery. Low initial subjective score and inability to abduct the arm at presentation were risk factors for a poor outcome

Introduction. Alignment of the acetabular cup and femoral components directly affects hip joint loading and potential for impingement and dislocation following total hip arthroplasty (THA) [1]. Changes to the lines of action and moment generating capabilities of the muscles as a result of component position may influence overall patient function. The objectives of this study were to assess the effect of component placement on hip joint contact forces (JCFs) and muscle forces during a high demand step down task and to identify important alignment parameters using a probabilistic approach. Methods. Three patients following THA (2 M: 28.3±2.8 BMI; 1 F: 25.7 BMI) performed lower extremity maximum isometric strength tests and a step down task as part of a larger IRB-approved study. Patient-specific musculoskeletal models were created by scaling a model with detailed hip musculature [2] to patient segment dimensions and mass. For each model, muscle maximum isometric strengths were optimized to minimize differences between model-predicted and measured preoperative maximum isometric joint torques at the hip and knee. Baseline simulations used patient-specific models with corresponding measured kinematics and ground reaction forces to predict hip JCFs and muscle forces using static optimization. To assess the combined effects of stem and cup position and orientation, a 1000 trial Monte Carlo simulation was performed with input variability in each degree of freedom based on the ±1 SD range in component placement relative to native geometry reported by Tsai et al. [3] (Figure 1). Maximum confidence bounds (1–99%) were predicted for the hip JCF magnitude and muscle forces for three prime muscles involved in the task (gluteus medius, gluteus minimus and psoas). HJC confidence bounds were compared to Orthoload measurements from telemetric implants from 6 patients performing the step down task. Sensitivity of hip JCF and muscle force outputs was quantified by Pearson Product-Moment correlation between the input parameter and the value of each output averaged across four points in the cycle. Results. Variation in the placement of the stem and cup produced an average maximum confidence bound (1–99%) in hip JCF of 277.7±91.1N and forces of 259.4±58.3N in the gluteus medius for all three patients (Figure 2). Sensitivity to cup and stem placement varied among the three patients; however, in general, hip JCFs were more sensitive to the position of the stem than the cup (Figure 3). Hip JCF was most sensitive to stem anteversion (0.64±0.10) and the superior/inferior stem position (0.42±0.19). Discussion. Variation in stem anteversion and medial/lateral cup position contributed the largest amount of variability in hip JCF and muscle forces during a step down task. The probabilistic analysis characterized bounds for output parameters, considering interactions between alignment parameters. Alignments that avoid increases in JCF and muscle loading during high demand tasks may lead to earlier recovery of function, by reducing muscle fatigue and the need to develop compensatory movement patterns. Acknowledgements. This research was supported in part by DePuy-Synthes