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Objectives: This study sought to examine fatigue characteristics of 2 polyaxial locking screw designs: locking cap (LC) and cross-threaded (CT). The goal was to compare LC and CT implants at 0, 10, and 15 degrees of angulation to determine the effect of locking mechanism on screw-plate interface failure. The hypothesis was that LC implants would have superior fatigue properties in comparison to CT designs and that increased angulation of the screw would have a negative impact on the fatigue life of CT implants, but would not have any effect on LC implants., Methods: A total of 72 screws were tested in 4 upper extremity implants. Implants were subjected to cyclic shear loads and subsequent ramp to failure. Performance characteristics were statistically compared using nonparametric statistical methods., Results: Fatigue testing demonstrated that LC designs were consistently able to sustain a significantly higher number of cyclic loads than CT designs. There were no significant differences in the number of cycles sustained by LC designs because of changes in screw angle, but CT implants exhibited decreases in screw stability with increasing angulation., Conclusions: Likely because of the spherical screw head geometry, LC fatigue characteristics are not influenced by the orientation of the screw relative to the plate. Application of an LC in the operating room requires additional time, but provides significantly more robust fixation of the screw, especially at oblique angles to the plate and provides a more predictable and consistent biomechanical result.

Achilles tendon ruptures are common and devastating injuries; however, an optimized treatment and rehabilitation protocol has yet to be defined. Therefore, the objective of this study was to investigate the effects of surgical repair and return to activity on joint function and Achilles tendon properties after 3 weeks of healing. Sprague-Dawley rats (N = 100) received unilateral blunt transection of their Achilles tendon. Animals were then randomized into repaired or non-repaired treatments, and further randomized into groups that returned to activity after 1 week (RTA1) or after 3 weeks (RTA3) of limb casting in plantarflexion. Limb function, passive joint mechanics, and tendon properties (mechanical, organizational using high frequency ultrasound, histological, and compositional) were evaluated. Results showed that both treatment and return to activity collectively affected limb function, passive joint mechanics, and tendon properties. Functionally, RTA1 animals had increased dorsiflexion ROM and weight bearing of the injured limb compared to RTA3 animals 3-weeks post-injury. Such functional improvements in RTA1 tendons were evidenced in their mechanical fatigue properties and increased cross sectional area compared to RTA3 tendons. When RTA1 was coupled with nonsurgical treatment, superior fatigue properties were achieved compared to repaired tendons. No differences in cell shape, cellularity, GAG, collagen type I, or TGF-β staining were identified between groups, but collagen type III was elevated in RTA3 repaired tendons. The larger tissue area and increased fatigue resistance created in RTA1 tendons may prove critical for optimized outcomes in early Achilles tendon healing following complete rupture. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 34:2172-2180, 2016., (© 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc.)

Introduction: Lower patient satisfaction and high rates of plate prominence has led to the use of lower profile, smaller plates in the treatment of midshaft clavicle fractures. Specifically regarding the use of 2.7mm reconstruction plates, there lacks biomechanical comparison to its more robust 3.5mm counterpart. This study was designed to compare the mechanical properties of anteroinferior plate fixation on a clavicle fracture model using either 2.7mm or 3.5mm reconstruction plates., Methods: Forty-eight synthetic left clavicles were divided into two groups based on the type of fixation: 3.5mm or 2.7mm pelvic reconstruction plate fixed in the anteroinferior position. Fixation was tested on AO/OTA 15B1.3 transverse midshaft fractures. Each specimen underwent the following three mechanical tests: axial compression, torsion, and four-point bending., Results: Significant differences were observed in axial (p=0.016) and torsional (p=0.00097) stiffness between the two groups. The average bending rigidity (EI) was found to be significantly lower for the 2.7-mm plates as compared to the 3.5-mm plates (p=0.03). The loading scenarios performed in the mechanical tests did not lead to failure of any implants., Conclusion: While our results show clear mechanical superiority of 3.5-mm reconstruction plates over 2.7-mm plates, superior results in the clinical setting may not necessarily translate. With exceptional mechanical strength also noted for the 2.7mm plate, well above the biomechanical properties of an intact clavicle, these results may obviate the need for robust plates in general., (Copyright © 2016 Elsevier Ltd. All rights reserved.)

Ectopic tendon mineralization can develop following tendon rupture or trauma surgery. The pathogenesis of ectopic tendon mineralization and its clinical impact have not been fully elucidated yet. In this study, we utilized a mouse Achilles tendon injury model to determine whether ectopic tendon mineralization alters the biomechanical properties of the tendon and whether BMP signaling is involved in this condition. A complete transverse incision was made at the midpoint of the right Achilles tendon in 8-week-old CD1 mice and the gap was left open. Ectopic cartilaginous mass formation was found in the injured tendon by 4weeks post-surgery and ectopic mineralization was detected at 8 to 10weeks post-surgery. Ectopic mineralization grew over time and volume of the mineralized materials of 25-weeks samples was about 2.5 fold bigger than that of 10-weeks samples, indicating that injury-induced ectopic tendon mineralization is progressive. In vitro mechanical testing showed that max force, max stress and mid-substance modulus in the 25-weeks samples were significantly lower than the 10-weeks samples. We observed substantial increases in expression of bone morphogenetic protein family genes in injured tendons 1week post-surgery. Immunohistochemical analysis showed that phosphorylation of both Smad1 and Smad3 was highly increased in injured tendons as early as 1week post-injury and remained high in ectopic chondrogenic lesions 4-weeks post-injury. Treatment with the BMP receptor kinase inhibitor (LDN193189) significantly inhibited injury-induced tendon mineralization. These findings indicate that injury-induced ectopic tendon mineralization is progressive, involves BMP signaling and associated with deterioration of tendon biomechanical properties., (Copyright © 2016 International Society of Matrix Biology. Published by Elsevier B.V. All rights reserved.)

Background: a rat model of supraspinatus overuse has suggested mechanisms governing tendon degeneration; however, delineating which changes are pathologic or simply physiologic adaptations to increased loading remains a question. The objective of this study was to develop and characterize a rat exercise model that induces systemic and local shoulder adaptations without mechanical injury to the supraspinatus tendon., Methods: exercise rats completed a treadmill training protocol for 12 weeks. Body, fat pad, and heart weights were determined. Supraspinatus tendon collagen content, cross-sectional area, and mechanical properties were measured. Supraspinatus muscle cross-sectional area, weight, and the expression of mitochondrial oxidative phosphorylation (OXPHOS) proteins were measured. Humeri were analyzed with μCT and mechanically tested., Results: exercise decreased fat pad mass. Supraspinatus muscle hypertrophied and had increased OXPHOS proteins. Humerus trabecular bone had increased anisotropic orientation, and cortical bone showed increased bone and tissue mineral density. Importantly, the supraspinatus tendon did not have diminished mechanical properties, indicating that this protocol was not injurious to the tendon., Conclusion: this study establishes the first rat exercise protocol that induces adaptations in the shoulder. Future research can use this as a comparison model to study how the supraspinatus tendon adapts to loading and undergoes degeneration with overuse.

High cholesterol remains a significant healthcare problem, as more than 13% of adults in the U.S. are affected by hypercholesterolemia. The detrimental effects the disease has on cardiovascular health are well-documented, but the effects on the musculoskeletal system, and more specifically on tendons, have not been thoroughly examined. This paper provides an overview of work performed in our lab with various animal models to elucidate the relationship between high cholesterol and tendon biomechanical integrity and ability to heal. These studies highlight the complexity of relationships between multiple factors that influence tendon biomechanics, and it has offered a better understanding of the implications of high cholesterol on healthy and healing tendons.

Tendinopathy is a debilitating musculoskeletal condition which can cause significant pain and lead to complete rupture of the tendon, which often requires surgical repair. Due in part to the large spectrum of tendon pathologies, these disorders continue to be a clinical challenge. Animal models are often used in this field of research as they offer an attractive framework to examine the cascade of processes that occur throughout both tendon pathology and repair. This review discusses the structural, mechanical, and biological changes that occur throughout tendon pathology in animal models, as well as strategies for the improvement of tendon healing. Cite this article: Bone Joint Res 2014;3:193-202., (©2014 The British Editorial Society of Bone & Joint Surgery.)

Model-based estimation of in vivo contact forces arising between components of a total knee replacement is challenging because such forces depend upon accurate modeling of muscles, tendons, ligaments, contact, and multibody dynamics. Here we describe an approach to solving this problem with results that are tested by comparison to knee loads measured in vivo for a single subject and made available through the Grand Challenge Competition to Predict in vivo Tibiofemoral Loads. The approach makes use of a "dual-joint" paradigm in which the knee joint is alternately represented by (1) a ball-joint knee for inverse dynamic computation of required muscle controls and (2) a 12 degree-of-freedom (DOF) knee with elastic foundation contact at the tibiofemoral and patellofemoral articulations for forward dynamic integration. Measured external forces and kinematics were applied as a feedback controller and static optimization attempted to track measured knee flexion angles and electromyographic (EMG) activity. The resulting simulations showed excellent tracking of knee flexion (average RMS error of 2.53 deg) and EMG (muscle activations within ±10% envelopes of normalized measured EMG signals). Simulated tibiofemoral contact forces agreed qualitatively with measured contact forces, but their RMS errors were approximately 25% of the peak measured values. These results demonstrate the potential of a dual-joint modeling approach to predict joint contact forces from kinesiological data measured in the motion laboratory. It is anticipated that errors in the estimation of contact force will be reduced as more accurate subject-specific models of muscles and other soft tissues are developed.

Modern total knee arthroplasty (TKA) is an effective procedure to treat pain and disability due to osteoarthritis, but some patients experience quadriceps weakness after surgery and have difficulty performing important activities of daily living. The success of TKA depends on many factors, but malalignment of the prosthetic components is a major cause of postoperative complications. Significant variability is associated with femoral and tibial component rotational alignment, but how this variability translates into functional outcome remains unknown. We used a forward-dynamic computer model of a simulated squatting motion to perform a parametric study of the effects of variations in component rotational alignment in TKA. A cruciate-retaining and posterior-stabilized version of the same TKA implant were compared. We found that femoral rotation had a greater effect on quadriceps forces, collateral ligament forces, and varus/valgus kinematics, while tibial rotation had a greater effect on anteroposterior translations. Our findings support the tendency for orthopedic surgeons to bias the femoral component into external rotation and avoid malrotation of the tibial component., (Copyright © 2011 Orthopaedic Research Society.)