Paper 55: Kinematic Abnormalities in the Shoulder Girdle after Conservative or Surgical Treatment of the High-grade Acromioclavicular Joint Disruption

OBJECTIVES: Acromioclavicular joint (ACJ) injury is one of the most prevalent shoulder girdle injuries among young contact athletes, disrupting shoulder mechanics, which can adversely affect long-term joint health. However, there is considerable controversy regarding treatment selection and its role in the health of the shoulder complex over time. While there is some consensus that surgical treatment is needed when both AC and CC (coracoclavicular) ligaments are disrupted (Rockwood grades III-V), in practice distinguishing between high-grade injuries is challenging, and grading is not highly consistent among surgeons. Clinical studies comparing surgical and conservative treatment have reported comparable patient outcomes. However, they often have included outdated surgical approaches with known high complication rates (e.g., k-wire, hook-plate), undermining their current validity. Overall, data on the efficacy of conservative vs. surgical treatment for restoring ACJ stability and the impact of ACJ instability on shoulder girdle biomechanics is lacking, impairing optimal decision-making for ACJ injury treatment. This cross-sectional biomotion study aims to characterize the extent of biomechanical abnormalities of the shoulder girdle after surgical or conservative treatments in individuals treated for high-grade ACJ injury. Hypothesis: The kinematics of the shoulder girdle will remain abnormal regardless of the treatment type, but the abnormality will be more pronounced in the conservatively treated group. METHODS: Subjects: Individuals with a unilateral ACJ injury (Rockwood Grade ³ III; 2-5 years post- treatment) consented and participated in this IRB-approved cross-sectional study. Thus far, in this ongoing study, six subjects (one female, Height: 175 (12) cm, Weight: 81 (18) kg), BMI 26.1(3.1) kg/m(2)) who were treated surgically using suspensory fixation, and three subjects (179 (10.4) cm, 99.4 (13.6) kg, 30.9 (2.4) kg/m(2)) who were treated non-operatively were consented and tested. Measurements: A high-resolution 3D shoulder CT scan was acquired bilaterally for each subject. The patient-specific 3D bone models of the humerus and clavicle were created. The 3D in-vivo shoulder kinematics was measured bilaterally (with the contralateral shoulder serving as an internal control) using a Dynamic Stereo X-ray (DSX) system during three repetitions of the scapular plane arm elevation while subjects followed a metronome beat for consistent moving rhythm. Post-processing: The 3D poses of the scapula and clavicle were estimated according to validated methods in DSX-Suite (C-Motion, USA) by simultaneous aligning the bone-model contours acquired through CT to DSX images in both views in each frame. In addition, an anatomical coordinate system was determined for each bone, and joint rotations/translations were calculated according to the methods described elsewhere, which considers the abilities of DSX and CT-based bone models while adhering to ISB standards when relevant. Outcomes: The horizontal and vertical ACJ displacements were calculated in Visual 3D (C- Motion, USA). Also, ACJ separation was defined as an instantaneous Euclidean distance between acromion and clavicle centers, characterizing the ACJ subluxation during movements. The rotational stability of the ACJ was measured by calculating the instantaneous axial rotation of the scapula relative to the clavicle using the helical axis method. These outcomes can substantiate how the mechanical constraint of the shoulder girdle is maintained in surgically and conservatively ACJ-treated individuals during in-vivo muscle-driven activities. RESULTS: Figure (A)-(E) demonstrates the ACJ kinematic results normalized to the movement percent (mean±SE). No difference was noted in the axial rotation of the scapula relative to the clavicle between the study groups (Fig . B). The nonoperative group demonstrated a larger magnitude and range of ACJ separation (Fig. C), while the surgical cohort demonstrated smaller but comparable ranges and variations to controls. Fig. D shows that both treatment groups show increased posterior clavicular translations , with the conservative group showing poorer ACJ instability in the posterior direction. Also, an increase in superior clavicle translation (Fig. E) was detected in both groups but more significantly in the conservative group, indicating an inferior reduction of the CC ligaments. CONCLUSIONS: To our knowledge, this is the first DSX study reporting the ACJ instabilities 2-5 years post ACJ reconstruction surgery during in-vivo muscle-driven activity. Our preliminary results indicated that the operated side demonstrates increased horizontal and vertical instabilities compared to the uninjured contralateral. Also, ACJ separation remains abnormal, suggesting that the surgery may not fully restore the normative ACJ kinematics mid-to-long term. Furthermore, the ACJ kinematics of the few conservative participants in this study demonstrate extensive ACJ instabilities, potentially implying inferior kinematic outcomes in the conservative group compared to the surgical group. Especially the increased posterior clavicular instability in the conservative group is concerning since it was related to inferior long-term outcomes. While no definitive conclusion can be drawn at this stage due to the sub- optimal sample size, the current findings demonstrate the feasibility of addressing the study aim using the current method, supporting the study hypothesis and providing ample motivation to continue this line of research.