Mechanical Properties and Microstructural Collagen Alignment of the Ulnar Collateral Ligament During Dynamic Loading

The alteration of the structure and macroscopic mechanical response of porcine patellar tendon by elastase digestion

Background: The treatment of patellar tendon injury has always been an unsolved problem, and mechanical characterization is very important for its repair and reconstruction. Elastin is a contributor to mechanics, but it is not clear how it affects the elasticity, viscoelastic properties, and structure of patellar tendon. Methods: The patellar tendons from six fresh adult experimental pigs were used in this study and they were made into 77 samples. The patellar tendon was specifically degraded by elastase, and the regional mechanical response and structural changes were investigated by: (1) Based on the previous study of elastase treatment conditions, the biochemical quantification of collagen, glycosaminoglycan and total protein was carried out; (2) The patellar tendon was divided into the proximal, central, and distal regions, and then the axial tensile test and stress relaxation test were performed before and after phosphate-buffered saline (PBS) or elastase treatment; (3) The dynamic constitutive model was established by the obtained mechanical data; (4) The structural relationship between elastin and collagen fibers was analyzed by two-photon microscopy and histology. Results: There was no statistical difference in mechanics between patellar tendon regions. Compared with those before elastase treatment, the low tensile modulus decreased by 75%-80%, the high tensile modulus decreased by 38%-47%, and the transition strain was prolonged after treatment. For viscoelastic behavior, the stress relaxation increased, the initial slope increased by 55%, the saturation slope increased by 44%, and the transition time increased by 25% after enzyme treatment. Elastin degradation made the collagen fibers of patellar tendon become disordered and looser, and the fiber wavelength increased significantly. Conclusion: The results of this study show that elastin plays an important role in the mechanical properties and fiber structure stability of patellar tendon, which supplements the structure-function relationship information of patellar tendon. The established constitutive model is of great significance to the prediction, repair and replacement of patellar tendon injury. In addition, human patellar tendon has a higher elastin content, so the results of this study can provide supporting information on the natural properties of tendon elastin degradation and guide the development of artificial patellar tendon biomaterials.

Elbow Ulnar Collateral Ligament Repair With Suture Augmentation Is Biomechanically Equivalent to Reconstruction and Clinically Demonstrates Excellent Outcomes: A Systematic Review

PURPOSE

To systematically review (1) biomechanical properties of augmented elbow ulnar collateral ligament (UCL) repair compared with reconstruction and (2) clinical efficacy and complication rates of UCL repair with and without augmentation.

METHODS

A systematic review was completed August 15, 2023, identifying articles that (1) biomechanically compared suture augmented UCL repair and reconstruction and (2) clinically evaluated medial elbow UCL repairs. Search terms included: "UCL repair" OR "internal brace" OR "suture augmentation" AND "UCL reconstruction." For inclusion, biomechanical studies compared augmented repair with reconstruction; clinical studies required clinical outcomes with minimum 6-month follow-up. Biomechanical data included torsional stiffness, gap formation, peak torque, and failure torque. Clinical data included return to previous level of play, time to return, functional outcomes, and complications.

RESULTS

In total, 8 biomechanical and 9 clinical studies were included (5 with and 4 without augmentation). In most biomechanical studies, augmented repairs demonstrated less gap formation, with equivalent torsional stiffness, failure load, and peak torque compared with reconstruction. Clinical outcomes in 104 patients without augmentation demonstrated return to previous level of 50% to 94% for nonprofessional athletes and 29% for professional baseball pitchers. Suture augmented repairs in 554 patients demonstrated return to previous level from 92% to 96%, at 3.8 to 7.4 months, with Kerlan Jobe Orthopaedic Clinic scores of 86 to 95. The overall complication rate for augmented UCL repair was 8.7%; most commonly ulnar neuropraxia (6%).

CONCLUSIONS

Biomechanically, UCL repair with augmentation provided less gapping with equivalent torsional stiffness and failure compared with reconstruction. Clinically, augmented UCL repair demonstrated excellent return to previous level of play and Kerlan Jobe Orthopaedic Clinic scores with modest complications and time to return. Augmented UCL repair is biomechanically equivalent to reconstruction and may be a viable alternative to reconstruction in indicated athletes.

CLINICAL RELEVANCE

UCL repair with suture augmentation is biomechanically equivalent to reconstruction and clinically demonstrates excellent outcomes.

Multi-organ phenotypes in mice lacking latent TGFβ binding protein 2 (LTBP2)

BACKGROUND

Latent TGFβ binding protein-2 (LTBP2) is a fibrillin 1 binding component of the microfibril. LTBP2 is the only LTBP protein that does not bind any isoforms of TGFβ, although it may interfere with the function of other LTBPs or interact with other signaling pathways.

RESULTS

Here, we investigate mice lacking Ltbp2 (Ltbp2-/- ) and identify multiple phenotypes that impact bodyweight and fat mass, and affect bone and skin development. The alterations in skin and bone development are particularly noteworthy since the strength of these tissues is differentially affected by loss of Ltbp2. Interestingly, some tissues that express high levels of Ltbp2, such as the aorta and lung, do not have a developmental or homeostatic phenotype.

CONCLUSIONS

Analysis of these mice show that LTBP2 has complex effects on development through direct effects on the extracellular matrix (ECM) or on signaling pathways that are known to regulate the ECM.

Effect of matrix properties on transmission and reflectance mode division-of-focal-plane Stokes polarimetry

Significance

Division-of-focal-plane Stokes polarimetry is emerging as a powerful tool for the microstructural characterization of soft tissues. How individual extracellular matrix (ECM) properties influence polarimetric signals in reflectance or transmission modes of quantitative polarized light imaging (QPLI) is not well understood.

Aim

We aimed to investigate how ECM properties affect outcomes obtained from division-of-focal-plane polarimetric imaging in reflectance or transmission modes.

Approach

Tunable collagen gel phantoms were used to modulate ECM properties of anisotropy, collagen density, crosslinking, and absorber density; the effects of degree of linear polarization (DoLP) and angle of polarization (AoP) on polarimetry outcomes were assessed. A model biological tissue (i.e., bovine tendon) was similarly imaged and evaluated using both reflectance and transmission modes.

Results

Reflectance QPLI resulted in decreased DoLP compared with transmission mode. A 90 deg shift in AoP was observed between modes but yielded similar spatial patterns. Collagen density had the largest effect on outcomes besides anisotropy in both imaging modes.

Conclusions

Both imaging modes were sufficiently sensitive to detect structural anisotropy differences in gels of varying fiber alignment. Conclusions drawn from phantom experiments should carry over when interpreting data from more complex tissues and can help provide context for interpretation of other Stokes polarimetry data.

Biomechanical Properties of Knee Medial Collateral Ligament Compared to Palmaris Longus for Ulnar Collateral Ligament Reconstruction

Ulnar collateral ligament reconstruction (UCLR) is frequently performed among injured overhead-throwing athletes. One of the most common graft choices when performing a UCLR is the ipsilateral palmaris longus tendon (PL). The purpose of this study was to investigate the material properties of aseptically processed cadaveric knee collateral ligaments (kMCL) as a potential graft source for UCLR and compare them to the gold standard PL autograft. Each PL and kMCL cadaveric sample was subjected to cyclic preconditioning, stress relaxation, and load-to-failure testing, and the mechanical properties were recorded. PL samples exhibited a greater average decrease in stress compared to the kMCL samples during the stress-relaxation test (p < 0.0001). PL samples also demonstrated a greater average Young's modulus in the linear region of the stress-strain curve compared to the kMCL samples (p < 0.01). The average yield strain and maximum strain of kMCL samples were significantly greater than the PL, p = 0.03 and 0.02, respectively. Both graft materials had comparable maximum toughness and demonstrated a similar ability to deform plastically without rupture. The clinical significance of our result is that prepared knee medial collateral ligament allografts may provide a viable graft material for use in the reconstruction of elbow ligaments.

Multiparity and Aging Impact Chondrogenic and Osteogenic Potential at Symphyseal Enthesis: New Insights into Interpubic Joint Remodeling

Pregnancy and childbirth cause adaptations to the birth canal to allow for delivery and fast recovery. To accommodate delivery through the birth canal, the pubic symphysis undergoes changes that lead to the interpubic ligament (IpL) and enthesis formation in primiparous mice. However, successive deliveries influence joint recovery. We aimed to understand tissue morphology and chondrogenic and osteogenic potential at symphyseal enthesis during pregnancy and postpartum in primiparous and multiparous senescent female mice. Morphological and molecular differences were found at the symphyseal enthesis among the study groups. Despite the apparent incapacity to restore cartilage in multiparous senescent animals, the symphyseal enthesis cells are active. However, these cells have reduced expression of chondrogenic and osteogenic markers and are immersed in densely packed collagen fibers contiguous to the persistent IpL. These findings may indicate alterations of key molecules in the progenitor cell population maintenance of the chondrocytic and osteogenic lineages at the symphyseal enthesis in multiparous senescent animals, possibly compromising the mouse joint histoarchitecture recovery. This sheds light on the distention of the birth canal and the pelvic floor that may play a role in pubic symphysis diastasis (PSD) and pelvic organ prolapse (POP), both in orthopedic and urogynecological practice in women.

Biomechanical Effect of Differential Tensioning on Suture-Augmented Ulnar Collateral Ligament Reconstruction of the Elbow

BACKGROUND

Medial ulnar collateral ligament (mUCL) reconstructions are becoming increasingly prevalent among the overhand throwing population. Suture tape augmentation has the potential to provide biomechanical advantages over standard docking reconstruction. However, the optimal tensioning of the suture augmentation technique has not yet been evaluated.

PURPOSE

To compare the subfailure biomechanical performance and graft strain of a standard docking mUCL reconstruction to an mUCL reconstruction using suture tape augmentation tensioned with 1 mm or 3 mm of laxity.

STUDY DESIGN

Controlled laboratory study.

METHODS

A total of 18 cadaveric elbows were dissected to the mUCL anterior band and biomechanically assessed via a valgus torque protocol to failure. Elbows were randomly assigned to be reconstructed via (1) a standard docking technique, (2) a suture-augmented reconstruction with 1-mm laxity, or (3) a suture-augmented reconstruction with 3-mm laxity. Reconstructed elbows were then subjected to the same loading protocol. Subfailure mechanical properties, failure mode, and mUCL/palmaris strain were assessed.

RESULTS

All reconstruction groups had decreased rotational stiffness, torque at 5° of angular rotation, and resilience compared with matched native controls. There were no differences in transition torque between groups. The failure mode of suture-augmented specimens was most often due to bone tunnel failure or reaching the maximum allowable angular displacement. In native controls or docking reconstructions, the primary failure mechanism was in the ligament or graft midsubstance. There were no significant differences in strain on the reconstructed or suture-augmented groups at any laxity compared with native controls.

CONCLUSION

Suture augmentation results in similar subfailure joint biomechanical properties as the standard docking reconstruction procedure at both laxity levels in a cadaveric model. There are improvements in the failure mode of suture-augmented specimens compared with standard docking. Graft strain may be modestly reduced in the 1-mm laxity group compared with other reconstruction groups.

CLINICAL RELEVANCE

Suture augmentation at both 1-mm and 3-mm laxity appears to offer similar advantages in subfailure biomechanics to standard docking reconstruction of the mUCL, with some improvements associated with failure mode. Strain data suggest a potential avoidance of graft stress shielding when tensioning the suture augmentation to 3-mm laxity, which is not as apparent with 1-mm laxity.

Single shot quantitative polarized light imaging system for rapid planar biaxial testing of soft tissues

Quantitative Polarized Light Imaging (QPLI) is an established technique used to compute the orientation of collagen fibers based on their birefringence. QPLI systems typically require rotating linear polarizers to obtain sufficient data to estimate orientation, which limits acquisition speeds and is not ideal for its application to mechanical testing. In this paper, we present a QPLI system designed with no moving parts; a single shot technique which is ideal to characterize collagen fiber orientation and kinematics during mechanical testing. Our single shot QPLI system (ssQPLI) sorts polarized light into four linear polarization states that are collected simultaneously by four cameras. The ssQPLI system was validated using samples with known orientation and retardation, and we demonstrate its use with planar biaxial testing of mouse skin. The ssQPLI system was accurate with a mean orientation error of 1.35° ± 1.58°. Skin samples were tested with multiple loading protocols and in each case the mean orientation of the collagen network reoriented to align in the direction of primary loading as expected. In summary, the ssQPLI system is effective at quantifying collagen fiber organization, and, when combined with mechanical testing, can rapidly provide pixel-wise measures of fiber orientation during biaxial loading.

Mechanical and Microstructural Properties of Meniscus Roots Vary by Location

BACKGROUND

Despite the growing awareness of the clinical significance of meniscus root tears, there are relatively limited biomechanical and microstructural data available on native meniscus roots that could improve our understanding of why they are injured and how to best treat them.

PURPOSE/HYPOTHESIS

The purpose of the study was to measure the material and microstructural properties of meniscus roots using mechanical testing and quantitative polarized light imaging. The hypothesis was that these properties vary by location (medial vs lateral, anterior vs posterior) and by specific root (anteromedial vs anterolateral, posteromedial vs posterolateral).

STUDY DESIGN

Descriptive laboratory study.

METHODS

Anterior and posterior meniscus roots of the medial and lateral meniscus were isolated from 22 cadavers (10 female, 12 male; mean ± SD age, 47.1 ± 5.1 years) and loaded in uniaxial tension. Quantitative polarized light imaging was used to measure collagen fiber organization and realignment under load. Samples were subjected to preconditioning, stress-relaxation, and a ramp to failure. Time-dependent relaxation behavior was quantified. Modulus values were computed in the toe and linear regions of the stress-strain curves. The degree of linear polarization (DoLP) and angle of polarization-measures of the strength and direction of collagen alignment, respectively-were calculated during the stress-relaxation test and at specific strain values throughout the ramp to failure (zero, transition, and linear strain).

RESULTS

Anterior roots had larger moduli than posterior roots in the toe (P = .007) and linear (P < .0001) regions and larger average DoLP values at all points of the ramp to failure (zero, P = .016; transition, P = .004; linear, P = .002). Posterior roots had larger values across all regions in terms of standard deviation angle of polarization (P < .001). Lateral roots had greater modulus values versus medial roots in the toe (P = .027) and linear (P = .014) regions. Across all strain points, posterolateral roots had smaller mean DoLP values than posteromedial roots.

CONCLUSION

Posterior meniscus roots have smaller modulus values and more disorganized collagen alignment at all strain levels when compared with anterior roots. Posterolateral roots have lower strength of collagen alignment versus posteromedial roots.

CLINICAL RELEVANCE

These data findings may explain at least in part the relative paucity of anterior meniscus root tears and the predominance of traumatic posterolateral roots tears as compared with degenerative posteromedial root tears.

Mechanical properties and microstructural organization of common ulnar collateral ligament grafts: Palmaris longus and gracilis tendons

Ulnar collateral ligament (UCL) injuries are becoming increasingly common. The palmaris longus (PL) and gracilis (GR) tendons are the most common grafts used in UCL reconstructions. While clinical studies have demonstrated relatively similar outcomes for either graft, there is little quantitative data describing these grafts from a material perspective, specifically the mechanical and microstructural properties of these tissues and how they respond under dynamic loading. The purpose of this descriptive laboratory study was to quantify and compare the mechanical and microstructural properties of PL and GR tendons. A total of 13 PL and 11 GR cadaveric human tendons were obtained. Each specimen was divided into three subregions and subjected to preconditioning, ramp-and-hold stress-relaxation and ramp-to-failure testing. Mechanical parameters were computed for each sample, and a polarized light imaging technique was used to simultaneously evaluate dynamic microstructural properties during testing. The PL had larger toe- and linear-region modulus values than the GR. Within the GR, the distal subregion had stronger collagen alignment than the proximal subregion at the zero, transition and linear portions of the stress-strain curve. The PL and GR, have similar mechanical properties and similar microstructural alignment under load. The PL graft has similar properties throughout its length whereas the GR properties exhibited slight differences in strength of alignment along its length. The PL and GR exhibit larger moduli values and more strongly/uniformly aligned collagenous microstructure when qualitatively compared to data previously published on the native UCL.

Donor age and sex have limited effects on the mechanical and microstructural properties of human connective tissues

Connective tissues, such as tendons, ligaments, and capsules, play a large role in locomotion and joint stability and are often subjected to traumatic injuries and degeneration. The purpose of this study was to evaluate if the mechanical and microstructural properties of connective tissues correlate with the age and sex of the human donor. Dissected samples were prepared for mechanical testing, consisting of 10 cycles of preconditioning, a stress-relaxation ramp and hold, and a quasi-static ramp to failure. During the testing protocol, the microstructural organization of tissues was analyzed using quantitative polarized light imaging. A linear mixed model was used to assess whether tissue type, donor age, or donor sex were significantly associated with mechanical and microstructural tissue properties. Tissue type had a significant effect on all parameters, while donor age and sex did not. Groupings by tissue type (i.e., tendon vs. ligament vs. capsule) were evident for microstructural data, with tendons having a tighter grouping and ligaments having a larger spread of values. The interaction of tissue type and age yielded a significant effect for linear modulus only (p = 0.007), with the palmaris tendon appearing to have the largest contribution to this effect. There were no significant interaction effects between sex and tissue type or donor age. Donor age appears to affect linear modulus in some, but not all, tissue types. Otherwise, age and sex do not have significant effects on the mechanical and microstructural properties of the range of connective tissues that were analyzed in this study.

The Effects of Differences in the Morphologies of the Ulnar Collateral Ligament and Common Tendon of the Flexor-Pronator Muscles on Elbow Valgus Braking Function: A Simulation Study

The anterior bundle (AB) and posterior bundle (PB) of the ulnar collateral ligament and the anterior common tendon (ACT) and posterior common tendon (PCT) of the flexor-pronator muscles have an independent form and an unclear form. The purpose of this study was to clarify the effect of differences in the morphologies of the AB, PB, ACT, and PCT on the elbow valgus braking function. This investigation examined three elbows. In the classification method, the AB, PB, ACT, and PCT with independent forms constituted Group I; the AB, ACT, and PCT with independent forms and the PB with an unclear form constituted Group II; the AB, PB, ACT, and PCT with unclear forms constituted Group III. The strains were calculated by simulation during elbow flexion at valgus at 0° and 10°. At 0° valgus, Group I and Group II showed similar AB and PCT strain patterns, but Group III was different. At 10° valgus, most ligaments and tendons were taut with increasing valgus angle. The average strain patterns of all ligaments and tendons were similar for the groups. The AB, PB, ACT, and PCT may cooperate with each other to contribute to valgus braking.

Instant polarized light microscopy for imaging collagen microarchitecture and dynamics

Collagen fibers are a primary load-bearing component of connective tissues and are therefore central to tissue biomechanics and pathophysiology. Understanding collagen architecture and behavior under dynamic loading requires a quantitative imaging technique with simultaneously high spatial and temporal resolutions. Suitable techniques are thus rare and often inaccessible. In this study, we present instant polarized light microscopy (IPOL), in which a single snapshot image encodes information on fiber orientation and retardance, thus fulfilling the requirement. We utilized both simulation and experimental data from collagenous tissues of chicken tendon, sheep eye, and porcine heart to evaluate the effectiveness of IPOL as a quantitative imaging technique. We demonstrate that IPOL allows quantitative characterization of micron-scale collagen fiber architecture at full camera frame rates (156 frames/second herein).

Posterior Inferior Comminution Significantly Influences Torque to Failure in Vertically Oriented Femoral Neck Fractures: A Biomechanical Study

OBJECTIVES

To evaluate axial fracture obliquity and posterior inferior comminution in vertically oriented femoral neck fractures (FNFs) in the physiologically young patient. A biomechanical investigation was designed to evaluate the impact of these fracture elements on torque to failure using cannulated screw (CS) and sliding hip screw fixation.

METHODS

Four Pauwels III FNF models were established in synthetic femurs: (1) vertically oriented in the coronal plane (COR), (2) coronal plane with axial obliquity (AX), (3) coronal plane with posterior inferior comminution (CCOM), and (4) coronal plane with axial obliquity and posterior inferior comminution (ACOM). In each group (n = 10), specimens were fixed using either 3 CSs or a sliding hip screw with supplemental antirotation screw (SHS). Quasistatic cyclic ramp-loading to failure was performed using a custom testing jig combining axial preloading and torsional ramp-loading. The primary outcome was torque to failure, defined as angular displacement ≥5 degrees.

RESULTS

In the CS group, torque to failure was 40.2 ± 2.6 Nm, 35.0 ± 1.4 Nm, 29.8 ± 1.5 Nm, and 31.8 ± 2.2 Nm for the COR, AX, CCOM, and ACOM fracture groups, respectively (P < 0.05). In the SHS group, torque to failure was 28.6 ± 1.3 Nm, 24.2 ± 1.4 Nm, 21.4 ± 1.2 Nm, and 21.0 ± 0.9 Nm for the COR, AX, CCOM, and ACOM fracture groups, respectively (P < 0.05). In both constructs, groups with posterior inferior comminution demonstrated significantly lower torque to failure compared to the COR group (P < 0.05). The CS construct demonstrated higher torque to failure in all groups when compared to the SHS construct (P < 0.01).

CONCLUSIONS

Posterior inferior comminution significantly affects torque to failure in vertically oriented FNFs. Three peripherally placed CSs may resist combined axial and torsional loading better than a sliding hip screw construct.

Mechanical Roles in Formation of Oriented Collagen Fibers

Collagen is a structural protein that is widely present in vertebrates, being usually distributed in tissues in the form of fibers. In living organisms, fibers are organized in different orientations in various tissues. As the structural base in connective tissue and load-bearing tissue, the orientation of collagen fibers plays an extremely important role in the mechanical properties and physiological and biochemical functions. The study on mechanics role in formation of oriented collagen fibers enables us to understand how discrete cells use limited molecular materials to create tissues with different structures, thereby promoting our understanding of the mechanism of tissue formation from scratch, from invisible to tangible. However, the current understanding of the mechanism of fiber orientation is still insufficient. In addition, existing fabrication methods of oriented fibers are varied and involve interdisciplinary study, and the achievements of each experiment are favorable to the construction and improvement of the fiber orientation theory. To this end, this review focuses on the preparation methods of oriented fibers and proposes a model explaining the formation process of oriented fibers in tendons based on the existing fiber theory. Impact statement As the structural base in connective tissue and load-bearing tissue, the orientation of collagen fibers plays an extremely important role in the mechanical properties and physiological and biochemical functions. However, the current understanding of the mechanism of fiber orientation is still insufficient, which is greatly responsible for the challenge of functional tissue repair and regeneration. Understanding the mechanism of fiber orientation can promote the successful application of fiber orientation scaffolds in tissue repair and regeneration, as well as providing an insight for the mechanism of tissue histomorphology.

Experimental mechanical strain measurement of tissues

Strain, an important biomechanical factor, occurs at different scales from molecules and cells to tissues and organs in physiological conditions. Under mechanical strain, the strength of tissues and their micro- and nanocomponents, the structure, proliferation, differentiation and apoptosis of cells and even the cytokines expressed by cells probably shift. Thus, the measurement of mechanical strain (i.e., relative displacement or deformation) is critical to understand functional changes in tissues, and to elucidate basic relationships between mechanical loading and tissue response. In the last decades, a great number of methods have been developed and applied to measure the deformations and mechanical strains in tissues comprising bone, tendon, ligament, muscle and brain as well as blood vessels. In this article, we have reviewed the mechanical strain measurement from six aspects: electro-based, light-based, ultrasound-based, magnetic resonance-based and computed tomography-based techniques, and the texture correlation-based image processing method. The review may help solving the problems of experimental and mechanical strain measurement of tissues under different measurement environments.