The dynamic mechanical viscoelastic properties of the temporomandibular joint disc: The role of collagen and elastin fibers from a perspective of polymer dynamics.

The temporomandibular joint disc is a structure, characterized as heterogeneous fibrocartilage, and is composed of macromolecular biopolymers. Despite a large body of characterization studies, the contribution of matrix biopolymers on the dynamic viscoelastic behavior of the disc is poorly understood. Given the high permeability and low concentration of glycosaminoglycans in the disc, it has been suggested that poro-elastic behavior can be neglected and that the intrinsic viscoelastic nature of solid matrix plays a dominant role in governing its time-dependent behavior. This study attempts to quantify the contribution of collagen and elastin fibers to the viscoelastic properties of the disc. Using collagenase and elastase, we perturbed the collagen and elastin fibrillar network in porcine temporomandibular joint discs and investigated the changes of dynamic viscoelastic properties in five different regions of the disc. Following both treatments, the storage and loss moduli of these regions were reduced dramatically up to the point that the tissue was no longer mechanically heterogeneous. However, the proportion of changes in storage and loss moduli were different for each treatment, reflected in the decrease and increase of the loss tangent for collagenase and elastase treated discs, respectively. The reduction of storage and loss moduli of the disc correlated with a decrease of biopolymer length. The present study indicates that the compositional and structural changes of collagen and elastin fibers alter the viscoelastic properties of the disc consistent with polymer dynamics.

Restrained Differential Growth: The Initiating Event of Adolescent Idiopathic Scoliosis?

STUDY DESIGN: An experimental model study and a short review of literature. OBJECTIVE: The purpose of this study was to explore a new hypothesis suggesting that the curvatures seen in adolescent idiopathic scoliosis (AIS) originate from restrained differential growth between the vertebral column and the surrounding musculo-ligamentary structures. SUMMARY OF BACKGROUND DATA: Despite decades of research, there is no generally accepted theory on the physical origin of the severe spinal deformations seen in AIS. The prevailing theories tend to focus on left-right asymmetry, rotational instability, or the sagittal spinal profile in idiopathic scoliosis. METHODS: We test our hypothesis with a physical model of the spine that simulates growth, counteracted by ligaments and muscles, modeled by tethers and springs. Growth of the spine is further restrained by an anterior band representing the thorax, the linea alba, and abdominal musculature. We also explore literature in search of molecular mechanisms that may induce differential growth. RESULTS: Differential growth in the restrained spine model first induces hypokyphosis and mild lateral bending of the thoracic spine, but then suddenly escalates into a scoliotic deformity, consistent with clinical observations of AIS. The band simulating the ventral structures of the body had a pivotal effect on sagittal curvature and the initiation of lateral bending and rotation. In literature, several molecular mechanisms were found that may explain the occurrence of differential growth between the spine and the musculo-ligamentary structures. CONCLUSION: While AIS is a three-dimensional deformation of the spine, it appears that restrained differential growth in the sagittal plane can result in lateral bending and rotation without a pre-existing left-right asymmetry. This supports the concept that AIS may result from a growth imbalance rather than a local anatomical defect. LEVEL OF EVIDENCE: N/A.

Electrospun Matrices for Pelvic Floor Repair: Effect of Fiber Diameter on Mechanical Properties and Cell Behavior.

Electrospun matrices are proposed as an alternative for polypropylene meshes in reconstructive pelvic surgery. Here, we investigated the effect of fiber diameter on (1) the mechanical properties of electrospun poly (lactic-co-glycolic acid)-blended-poly(caprolactone) (PLGA/PCL) matrices; (2) cellular infiltration; and (3) the newly formed extracellular matrix (ECM) in vitro. We compared electrospun matrices with 1- and 8 µm fiber diameter and used nonporous PLGA/PCL films as controls. The 8-µm matrices were almost twice as stiff as the 1-µm matrices with 1.38 and 0.66 MPa, respectively. Matrices had the same ultimate tensile strength, but with 80% the 1-µm matrices were much more ductile than the 8-µm ones (18%). Cells infiltrated deeper into the matrices with larger pores, but cellular activity was comparable on both substrates. New ECM was deposited faster on the electrospun samples, but after 2 and 4 weeks the amount of collagen was comparable with that on nonporous films. The ECM deposited on the 1-µm matrices, and the nonporous film was about three times stiffer than the ECM found on the 8-µm matrices. Cell behavior in terms of myofibroblastic differentiation and remodeling was similar on the 1-µm matrices and nonporous films, in comparison to that on the 8-µm matrices. We conclude that electrospinning enhances the integration of host cells as compared with a nonporous film of the same material. The 1-µm matrices result in better mechanical behavior and qualitatively better matrix production than the 8-µm matrices, but with limited cellular infiltration. These data are useful for designing electrospun matrices for the pelvic floor.

Biomechanical evaluation of a novel nucleus pulposus prosthesis in canine cadaveric spines.

Partial disc replacement is a new surgical technique aimed at restoring functionality to degenerated intervertebral discs (IVDs). The aim of the present study was to assess biomechanically the behaviour of a novel nucleus pulposus prosthesis (NPP) in situ and its ability to restore functionality to the canine IVD after nuclectomy alone or after combined dorsal laminectomy and nuclectomy. Nine canine T13-L5 specimens (L2L3 group) and 10 L5-Cd1 specimens (LS group) were tested biomechanically in the native state, after nuclectomy (L2L3 group) or after combined dorsal laminectomy and nuclectomy (LS group), and after insertion of the NPP. Range of motion (ROM), neutral zone (NZ), and neutral zone stiffness (NZS) were determined in flexion/extension, lateral bending, and axial rotation. Nuclectomy alone and combined dorsal laminectomy and nuclectomy caused significant instability in all motion directions. Implantation of the NPP resulted in significant restoration of the parameters (ROM, NZ, and NZS) towards the native state; however, fragmentation/herniation of the NPP occurred in 47% of the cases. In conclusion, the NPP has the ability to improve functionality of the nuclectomized canine IVD. The high rate of NPP failure requires modifications directed at the integrity of the NPP and its confinement to the nuclear cavity.