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% (08 ER: p ¼ 0.03; 308 ER: p ¼ 0.03; 608 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 AE 0.32) were significant at all joint positions (08 ER: p ¼ 0.03; 308 ER: p ¼ 0.048; 608 ER: p ¼ 0.04), whereas strain ratio differences on the humeral side of the anterior axillary pouch (0.18 AE 0.21) were significant only at 608 of ER (p ¼ 0.03). Therefore, standardizing physical exams for joint position could help surgeons identify specific locations of non-recoverable strain that may have been ignored. ß

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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±21.3N/mm, 73.5±53.8MPa) 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.

The anteroinferior glenohumeral capsule (anterior band of the inferior glenohumeral ligament (AB-IGHL), axillary pouch) limits anterior translation, particularly in positions of external rotation. [1, 2] Permanent tissue deformation that occurs as a result of dislocation contributes to anterior instability, but, the extent and effects of this injury are difficult to evaluate as the deformation cannot be seen using diagnostic imaging. Clinical exams are used to identify the appropriate location of tissue damage and current arthroscopic procedures allow for selective tightening of localized capsule regions; however, identifying the specific location for optimal treatment of each patient is challenging. Although the reliability of clinical exams has been shown to change with joint position [3] a standardized procedure has yet to be established. This lack of standardization is particularly problematic since capsule function is highly dependent upon joint position [4–7], and could be responsible for failed repairs attributed to plication of the wrong capsular region [8]. Understanding the relationship between the location of tissue damage and changes in capsule function following anterior dislocation could aid clinicians in diagnosing and treating anterior instability. Therefore, the objective of this work was to compare strain distributions in the anteroinferior capsule before and after anterior dislocation in order to identify joint positions at which clinical exams would be capable of detecting damage (nonrecoverable strain) in specific locations.

Knee osteoarthritis (OA) is one of the most prevalent chronic conditions affecting older adults and commonly leads to pain and functional limitations. Many individuals with knee OA also report episodes of knee instability, which has been shown to adversely affect their ability to perform weight-bearing functional tasks. [1] Recently it was reported that individuals with knee OA and reports of joint instability demonstrate significant reductions in their sagittal and transverse plane rotational knee joint motion. [2] It is conceivable that the decreased rotational joint motion may represent a compensatory attempt to avoid pain and/or to stabilize an unstable knee joint. However, this movement strategy may be problematic in the long term as it could lead to increased compressive loading and a reduction of shock absorption capabilities. Since abnormal rotation is associated with altered joint contact position [3], and the internal/external rotation axis in the normal knee is located on the medial side [4,5], this patient population may exhibit a decreased contact path length in the lateral compartment during the loading response phase of gait. The combination of a reduced contact path length and cartilage loss could in turn lead to a decreased dynamic joint space [6] and concentration of joint stresses responsible for disease progression. Therefore, the objectives of this work were to 1) compare the tibiofemoral joint contact path length and dynamic joint space in the medial and lateral compartments during the loading response phase of gait in individuals with knee OA and complaints of joint instability to a healthy control group and 2) investigate the relationship between these parameters.