Capsule function following anterior dislocation: implications for diagnosis of shoulder instability.

During shoulder dislocation, the glenohumeral capsule undergoes non-recoverable strain, leading to joint instability. Clinicians use physical exams to diagnose injury and direct repair procedures; however, they are subjective and do not provide quantitative information. Our objectives were to: (1) determine the relationship between capsule function following anterior dislocation and non-recoverable strain; and (2) identify joint positions at which physical exams can be used to detect non-recoverable strain in specific capsule regions. Physical exams were simulated at three joint positions including external rotation (ER) using robotic technology before and after anterior dislocation. The resulting joint kinematics, strain distribution in the capsule, and non-recoverable strain were determined. Following dislocation, anterior translation increased by as much as 48% (0° ER: p = 0.03; 30° ER: p = 0.03; 60° ER: p < 0.01). Capsule sub-regions with less non-recoverable strain required more ER to detect differences in the strain ratios between the intact and injured joint. Strain ratio changes on the humeral side of the posterior axillary pouch (0.31 ± 0.32) were significant at all joint positions (0° ER: p = 0.03; 30° ER: p = 0.048; 60° ER: p = 0.04), whereas strain ratio differences on the humeral side of the anterior axillary pouch (0.18 ± 0.21) were significant only at 60° of ER (p = 0.03). Therefore, standardizing physical exams for joint position could help surgeons identify specific locations of non-recoverable strain that may have been ignored.

Injury to the anteroinferior glenohumeral capsule during anterior dislocation.

BACKGROUND: Glenohumeral dislocation commonly results in permanent deformation of the glenohumeral capsule. Knowing the location and extent of tissue damage may aid in improving diagnostic and repair procedures for shoulder dislocations. Therefore, the objectives of this study were to determine: (1) the strain in the anteroinferior capsule at dislocation and (2) the location and extent of injury to the anteroinferior capsule due to dislocation by quantifying the resulting non-recoverable strain. METHODS: A robotic/universal force-moment sensor testing system was used to anteriorly dislocate six cadaveric shoulders. The magnitude of the maximum principle strain at dislocation and the resulting non-recoverable strain due to dislocation in the anteroinferior capsule were measured by tracking the change in the location of a grid of strain markers from a reference position. FINDINGS: The glenoid side of the capsule experienced higher strains at dislocation than the humeral side. The greatest strains at dislocation were found on the glenoid side of the anterior band (strain ratio of 0.60), but the greatest non-recoverable strains were found in the posterior axillary pouch (strain ratio of 0.34 on the glenoid side and 0.31 on the humeral side). INTERPRETATION: These findings suggest that even though the glenoid side of the anterior band undergoes more deformation during anterior dislocation, the most permanent deformation occurs in the posterior axillary pouch, and surgeons should consider also plicating the posterior axillary pouch when performing repair procedures following anterior dislocation. In the future, the mechanical properties of the normal and injured glenohumeral capsules will be compared.

Effects of simulated injury on the anteroinferior glenohumeral capsule.

Glenohumeral dislocation results in permanent deformation (nonrecoverable strain) of the glenohumeral capsule which leads to increased range of motion and recurrent instability. Minimal research has examined the effects of injury on the biomechanical properties of the capsule which may contribute to poor patient outcome following repair procedures. The objective of this study was to determine the effect of simulated injury on the stiffness and material properties of the AB-IGHL during tensile deformation. Using a combined experimental and computational methodology, the stiffness and material properties of six AB-IGHL samples during tensile elongation were determined before and after simulated injury. The AB-IGHL was subjected to 12.7 ± 3.2 % maximum principal strain which resulted in 2.5 ± 0.9 % nonrecoverable strain. The linear region stiffness and modulus of stress-stretch curves between the normal (52.4 ± 30.0 N/mm, 39.1 ± 26.6 MPa) and injured (64.7 ± 21.3 N/mm, 73.5 ± 53.8 MPa) AB-IGHL increased significantly (p = 0.03, p = 0.04). These increases suggest that changes in the tissue microstructure exist following simulated injury. The injured tissue could contain more aligned collagen fibers and may not be able to support a normal range of joint motion. Collagen fiber kinematics during simulated injury will be examined in the future.