The Biomechanical Role of Scaffolds in Augmented Rotator Cuff Tendon Repairs

Background Scaffolds continue to be developed and used for rotator cuff repair augmentation; however, the appropriate scaffold material properties and/or surgical application techniques for achieving optimal biomechanical performance remains unknown. The objectives of the study were to simulate a previously validated spring-network model for clinically relevant scenarios to predict: (1) the manner in which changes to components of the repair influence the biomechanical performance of the repair and (2) the percent load carried by the scaffold augmentation component. Materials and methods The models were parametrically varied to simulate clinically relevant scenarios, namely, changes in tendon quality, altered surgical technique(s), and different scaffold designs. The biomechanical performance of the repair constructs and the percent load carried by the scaffold component were evaluated for each of the simulated scenarios. Results The model predicts that the biomechanical performance of a rotator cuff repair can be modestly increased by augmenting the repair with a scaffold that has tendon-like properties. However, engineering a scaffold with supraphysiologic stiffness may not translate into yet stiffer or stronger repairs. Importantly, the mechanical properties of a repair construct appear to be most influenced by the properties of the tendon-to-bone repair. The model suggests that in the clinical setting of a weak tendon-to-bone repair, scaffold augmentation may significantly off-load the repair and largely mitigate the poor construct properties. Conclusions The model suggests that future efforts in the field of rotator cuff repair augmentation may be directed toward strategies that strengthen the tendon-to-bone repair and/or toward engineering scaffolds with tendon-like mechanical properties

Rotator Cuff Tendinosis in an Animal Model: Role of Extrinsic and Overuse Factors

The rat shoulder animal model has been used previously to study the role of intrinsic injury (modeled as an acute insult to the tendon), extrinsic injury (modeled as external subacromial impingement), and overuse factors on rotator cuff tendinosis. These studies demonstrated that it is possible to produce rotator cuff tendinosis with any one of these factors in isolation. The current study uses the rat shoulder model to study the roles of extrinsic compression, overuse, and overuse in combination with extrinsic compression, on the development of rotator cuff tendinosis. The results of this study demonstrate that the injury created by overuse plus extrinsic compression is greater than the injuries created by overuse or extrinsic compression alone, particularly when important biomechanical variables are considered. While ineffective in causing a change in supraspinatus tendon properties in animals with normal cage activity, extrinsic compression had a significant and dramatic effect when it was combined with overuse activity. Without an additional factor, such as overhead activity, the extrinsic compression alone may be insufficient to cause tendinosis. The results of the present study support the role of multiple factors in the etiology of some rotator cuff injuries. © 2002 Biomedical Engineering Society.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/44001/1/10439_2004_Article_482753.pd

Predicting normal glenoid version from the pathologic scapula: a comparison of 4 methods in 2- and 3-dimensional models

Correction of pathologic glenoid retroversion improves gleonhumeral mechanics and reduces glenoid component wear after total shoulder arthroplasty. Determining the amount of correction necessary can be difficult because of the wide range of normal glenoid version. We hypothesize that normal glenoid version can be predicted in a pathologic shoulder based on conserved relationships between the anterior glenoid wall, Resch angle, and the internal structures of the glenoid vault. Three-dimensional (3-D) computer tomography (CT) scan-based measurements of the anterior glenoid wall angle (AGWA), Resch angle (RA), and glenoid version were made in 58 scapulae from the Haeman-Todd Osteological Collection (Museum of Natural History in Cleveland, OH) and 19 paired scapulae from patients with unilateral osteoarthritis. Linear regression equations derived from the AGWA and RA and from a computer-generated vault model were used to predict native (nonpathologic) glenoid version as defined by the 19 nonpathologic scapula. Linear regression equations based on the measured AGWA or RA, as well as the glenoid vault model in the 19 pathologic scapulae, were able to accurately predict native glenoid version in the contralateral nonpathologic shoulder. This study demonstrates the ability to take 3-D CT scan-based measurements in a scapula with pathologic glenoid retroversion and predict the native (nonpathologic) glenoid version in the contralateral shoulder by using linear regression equations or a computer generated vault model. Such tools might assist in preoperative planning and intraoperative decision making to allow correction of pathologic glenoid retroversion

Mechanism for Superior Subluxation of the Glenohumeral Joint in Patients Who Have Fibrodysplasia Ossificans Progressiva

Progressive heterotopic ossification leads to ankylosis of the major joints in patients who have fibrodysplasia ossificans progressiva. Joint subluxation has not been recognized widely in patients with this disease. The clinical records and radiographs of 79 patients with fibrodysplasia ossificans progressiva were reviewed and, it was found that humeral to chest wall synostosis and subluxation of the glenohumeral joint had occurred in 21 % of skeletally immature patients and in 74% of skeletally mature patients. In fibrodysplasia ossificans progressiva, synostosis of the humeral shaft to the chest wall commonly occurs by 7 years of age, well before the age of proximal physeal closure. The continued growth of the proximal humeral physis in the presence of a humeral to chest wall synostosis causes the humeral head to migrate superiorly, thus promoting growth related subluxation. The clinical significance of this finding for patients who have fibrodysplasia ossificans progressiva is unknown, but this unique model will be useful in the study of shoulder biomechanics and growth plate physiology

Glenohumeral joint translations before and after total shoulder arthroplasty. A study in Cadavera

The purpose of the present study was to examine the motions of natural and prosthetically reconstructed glenohumeral joints without capsular contracture in the laboratory to obtain a better understanding of joint motion as it may relate to failure of the implant. Seven joints from fresh- frozen human cadavera were instrumented with a six-degrees-of-freedom magnetic tracking device to study patterns of translation and rotation with and without the application of simulated muscle forces (active and passive models). The specimens were tested before and after reconstruction of the joint with use of modified operative techniques and implants that had from zero to five millimeters of radial mismatch between the humeral head and glenoid components. The natural and reconstructed joints had similar patterns of translation, with larger rotations and translations observed in the passive model. On the average, the active translations of the natural joints were best reproduced by the reconstructed joints with less conforming articulations. The mean active translation in the natural joints was 1.5 millimeters along the anterior-posterior axis and 1.1 millimeters along the superior-inferior axis. The active translations in the reconstructed joints were observed to increase consistently as the conformity of the components decreased: the mean active anterior-posterior translations ranged from 0.3 millimeter for conforming components to 1.7 millimeters for components with a five-millimeter radial mismatch, and the mean active superior-inferior translations ranged from 0.4 to 1.1 millimeters. The patterns of translation during passive motions were less consistent, presumably because of the influence of capsular ligaments and because the translations were large enough for the articular surface of the humeral head to lose contact with the articular surface of the glenoid component. CLINICAL RELEVANCE: Glenohumeral translations during active motions were found to depend on articular conformity, which indicates that the choice of implants may have important consequences for normal motions. Totally conforming designs may impose a degree of restraint to translations that is higher than that in the natural joint. The patterns of translation may influence the longevity of the joint replacement with respect to stability, loosening of the glenoid component, and wear of the components

Effect of component conformity in total shoulder arthroplasty: theoretical and experimental observations

At present, there are several total shoulder arthroplasty systems available to the orthopaedic surgeon. The present work examines the relationships between transverse forces and translations for components with different conformities. A theoretical model on the knee joint is adopted and validated through an experiment. Theoretical predictions are compared with experimental results. Experimental results qualitatively confirm the validity of the theory. However, discrepancies are apparent between the theoretical and experimental results due to either deformation or rocking of the glenoid component. Results obtained imply that the maximal allowable force experienced by a glenoid component is not related to the amount of component conformity