https://www.oarsijournal.com/article/S1063-4584(15)00877-8/pdf

Mechanics and biology in intervertebral disc degeneration: a vicious circle

As a step towards developing a finite element model of the knee that can be used to study how the variables associated with a meniscal replacement affect tibio-femoral contact, the goals of this study were 1) to develop a geometrically accurate three-dimensional solid model of the knee joint with special attention given to the menisci and articular cartilage, 2) to determine to what extent bony deformations affect contact behavior, and 3) to determine whether constraining rotations other than flexion/extension affects the contact behavior of the joint during compressive loading. The model included both the cortical and trabecular bone of the femur and tibia, articular cartilage of the femoral condyles and tibial plateau, both the medial and lateral menisci with their horn attachments, the transverse ligament, the anterior cruciate ligament, and the medial collateral ligament. The solid models for the menisci and articular cartilage were created from surface scans provided by a noncontacting, laser-based, three-dimensional coordinate digitizing system with an root mean squared error (RMSE) of less than 8 microns. Solid models of both the tibia and femur were created from CT images, except for the most proximal surface of the tibia and most distal surface of the femur which were created with the three-dimensional coordinate digitizing system. The constitutive relation of the menisci treated the tissue as transversely isotropic and linearly elastic. Under the application of an 800 N compressive load at 0 degrees of flexion, six contact variables in each compartment (ie., medial and lateral) were computed including maximum pressure, mean pressure, contact area, total contact force, and coordinates of the center of pressure. Convergence of the finite element solution was studied using three mesh sizes ranging from an average element size of 5 mm by 5 mm to 1 mm by 1 mm. The solution was considered converged for an average element size of 2 mm by 2 mm. Using this mesh size, finite element solutions for rigid versus deformable bones indicated that none of the contact variables changed by more than 2% when the femur and tibia were treated as rigid. However, differences in contact variables as large as 19% occurred when rotations other than flexion/extension were constrained. The largest difference was in the maximum pressure. Among the principal conclusions of the study are that accurate finite element solutions of tibio-femoral contact behavior can be obtained by treating the bones as rigid. However, unrealistic constraints on rotations other than flexion/extension can result in relatively large errors in contact variables.

A Finite Element Model of the Human Knee Joint for the Study of Tibio-Femoral Contact

The menisci and their insertions into bone (entheses) represent a functional unit. Thanks to their firm entheses, the menisci are able to distribute loads and therefore reduce the stresses on the tibia, a function which is regarded essential for cartilage protection and prevention of osteoarthrosis. The tissue of the hypocellular meniscal body consists mainly of water and a dense elaborate type I collagen network with a predominantly circumferential alignment. The content of different collagens, proteoglycans and nonproteoglycan proteins shows significant regional variations probably reflecting functional adaptation. The meniscal horns are attached via meniscal insertional ligaments mainly to tibial bone. At the enthesis, the fibres of the insertional ligaments attach to bone via uncalcified and calcified fibrocartilages. This anatomical configuration of gradual transition from soft to hard tissue, which is identical to other ligament entheses, is certainly essential for normal mechanical function and probably protects this vulnerable transition between 2 biomechanically different tissues from failure. Clinical treatment of meniscal tears needs to be based on these special anatomical and functional characteristics. Partial meniscectomy will preserve some of the load distribution function of the meniscus only when the meniscal body enthesis entity is preserved. Repair of peripheral longitudinal tears will heal and probably preserve the load distribution function of the meniscus, whereas radial tears through the whole meniscal periphery or more central and complex tears may be induced to heal, but probably do not preserve the load distribution function. There is no proof that replacement of the meniscus with an allograft can reestablish some of the important meniscal functions, and thereby prevent or reduce the development of osteoarthrosis which is common after meniscectomy. After implantation, major problems are the remodelling of the graft to inferior structural, biochemical and mechanical properties and its insufficient fixation to bone which fails to duplicate a normal anatomical configuration and therefore a functional meniscal enthesis.

https://onlinelibrary.wiley.com/doi/pdf/10.1046/j.1469-7580.1998.19320161.x

The menisci of the knee joint. Anatomical and functional characteristics, and a rationale for clinical treatment

Study design An in vivo study of the biologic and biomechanical consequences of static compressive loading on the mouse tail intervertebral disc. Objectives To determine whether static compression in vivo alters the biologic activity of the disc and leads to diminished biomechanical performance. Summary of background data Static compressive stress that exceeds the disc's swelling pressure is known to change hydration and the intradiscal stress distribution. Alterations in hydration and stress have been associated with changes in disc cell activity in vitro and in other collagenous tissues in vivo. Methods Mouse tail discs were loaded in vivo with an external compression device. After 1 week at one of three different stress levels, the discs were analyzed for their biomechanical performance, morphology, cell activity, and cell viability. A second group of mice were allowed to recuperate for 1 month after the 1-week loading protocol to assess the disc's ability to recover. As an aid to interpreting the histologic and biologic data, finite-element analysis was used to predict region-specific changes in tissue stress caused by the static loading regimen. Results With increasing compressive stress, the inner and middle anulus became progressively more disorganized, and the percentage of cells undergoing apoptosis increased. The expression of Type II collagen was suppressed at all levels of stress, whereas the expression of aggrecan decreased at the highest stress levels in apparent proportion to the decreased nuclear cellularity. Compression for 1 week did not affect the disc bending stiffness or strength but did increase the neutral zone by 33%. As suggested by the finite-element model, during sustained compression, tension is maintained in the outer anulus and lost in the inner and middle regions where the hydrostatic stress was predicted to increased nearly 10-fold. Discs loaded at the lowest stress recovered anular architecture but not cellularity after 1 month of recuperation. Discs loaded at the highest stress did not recover anular architecture, displaying islands of cartilage cells in the middle anulus at sites previously populated by fibroblasts. Conclusions The results of the current project demonstrate that static compressive loading initiates a number of harmful responses in a dose-dependent way: disorganization of the anulus fibrosus; an increase in apoptosis and associated loss of cellularity; and down regulation of collagen II and aggrecan gene expression. The finite element model used in this study predicts loss of collagen fiber tension and increased matrix hydrostatic stress in those anular regions observed to undergo programmed cell death after 1 week of loading and ultimately become populated by chondrocytes after one month of recuperation. This correspondence conforms with the suggestions of others that the cellular phenotype in collagenous tissues is sensitive to the dominant type of tissue stress. Although the specific mechanisms by which alterations in tissue stress lead to apoptosis and variation in cell phenotype remain to be identified, our results suggest that maintenance of appropriate stress within the disc may be an important basis for strategies to mitigate disc degeneration and initiate disc repair.

Compression-induced degeneration of the intervertebral disc: an in vivo mouse model and finite-element study.

The mechanical behavior of the entire anulus fibrosus is determined essentially by the tensile properties of its lamellae, their fiber orientations, and the regional variation of these quantities. Corresponding data are rare in the literature. The paper deals with an in vitro study of single lamellar anulus lamellae and aims to determine (i) their tensile response and regional variation, and (ii) the orientation of lamellar collagen fibers and their regional variation. Fresh human body-disc-body units (L1–L2, n=11) from cadavers were cut midsagittally producing two hemidisc units. One hemidisc was used for the preparation of single lamellar anulus specimens for tensile testing, while the other one was used for the investigation of the lamellar fiber orientation. Single lamellar anulus specimens with adjacent bone fragments were isolated from four anatomical regions: superficial and deep lamellae (3.9±0.21 mm, mean ± SD, apart from the outer boundary surface of the anulus fibrosus) at ventro-lateral and dorsal positions. The specimens underwent cyclic uniaxial tensile tests at three different strain rates in 0.15 mol/l NaCl solution at 37°C, whereby the lamellar fiber direction was aligned with the load axis. For the characterization of the tensile behavior three moduli were calculated: Elow (0–0.1 MPa), Emedium (0.1–0.5 MPa) and Ehigh (0.5–1 MPa). Additionally, specimens were tested with the load axis transverse to the fiber direction. From the second hemidisc fiber angles with respect to the horizontal plane were determined photogrammetrically from images taken at six circumferential positions from ventral to dorsal and at three depth levels. Tensile moduli along the fiber direction were in the range of 28–78 MPa (regional mean values). Superficial lamellae have larger Emedium (p=0.017) and Ehigh (p=0.012) than internal lamellae, and the mean value of superficial lamellae is about three times higher than that of deep lamellae. Tensile moduli of ventro-lateral lamellae do not differ significantly from the tensile moduli of dorsal lamellae, and Elow is generally indifferent with respect to the anatomical region. Tensile moduli transverse to the fiber direction were about two orders of magnitude smaller (0.22±0.2 MPa, mean ± SD, n=5). Tensile properties are not correlated significantly with donor age. Only small viscoelastic effects were observed. The regional variation of lamellar fiber angle ϕ is described appropriately by a regression line |ϕ|=23.2+0.130×α (r2=0.55, p<0.001), where α is the polar angle associated with the circumferential position. The single anulus lamella may be seen as the elementary structural unit of the anulus fibrosus, and exhibits marked anisotropy and distinct regional variation of tensile properties and fiber angles. These features must be considered for appropriate physical and numerical modeling of the anulus fibrosus.

Single lamellar mechanics of the human lumbar anulus fibrosus

Study DesignThe structure-function relationship of anulus fibrosus of nondegenerate lumba intervertebral discs was investigated.ObjectivesThe tensile properties and biochemical composition of single lamella specimens from human anulus librosus and their variations with anatomic region were determine

D. L. Skaggs

Papers

Regional variation in tensile properties and biochemical composition of the human lumbar anulus fibrosus

Radial tie fibers influence the tensile properties of the bovine medial meniscus.