Adam C. Abraham

Hyperelastic properties of human meniscal attachments

Meniscal attachments are ligamentous tissues anchoring the menisci to the underlying subchondral bone. Currently little is known about the behavior of meniscal attachments, with only a few studies quantitatively documenting their properties. The objective of this study was to quantify and compare the tensile mechanical properties of human meniscal attachments in the transverse direction, curve fit experimental Cauchy stress-stretch data to evaluate the hyperelastic behavior, and couple these results with previously obtained longitudinal data to generate a more complete constitutive model. Meniscal attachment specimens were tested using a uniaxial tension test with the collagen fibers oriented perpendicular to the loading axis. Tests were run until failure and load-optical displacement data was recorded for each test. The medial posterior attachment was shown to have a significantly greater elastic modulus (6.42±0.78 MPa) and ultimate stress (1.73±0.32 MPa) when compared to the other three attachments. The Mooney-Rivlin material model was selected as the best fit for the transverse data and used in conjunction with the longitudinal data. A novel computational approach to determining the transition point between the toe and linear regions is presented for the hyperelastic stress-stretch curves. Results from piece-wise non-linear longitudinal curve fitting correlate well with previous linear elastic and SEM findings. These data can be used to advance the design of meniscal replacements and improve knee joint finite element models.

NANOINDENTATION OF HUMAN MENISCAL SURFACES

Menisci are crescent shaped fibrocartilaginous structures which support load distribution of the knee. The menisci are specifically designed to fit the contour of the femoral condyles, aiding to disperse the stresses on the tibial plateau and in turn safeguarding the underlying articular cartilage. The importance of the meniscal superficial layer has not been fully revealed and it is suspected that this layer plays a pivotal role for meniscal function. In this study, both femoral (proximal) and tibial (distal) contacting meniscal surfaces were mechanically examined on the nano-level among three distinct regions (anterior, central and posterior) of the lateral and medial menisci. Nanoindentation testing showed no significant differences among regions, surfaces or anatomical locations, possibly elucidating on the homogeneity of the meniscal superficial zone structure (E(instantaneous): 3.17-4.12MPa, E(steady-state): 1.47-1.69MPa). Nanomechanical moduli values were approximately an order of magnitude greater than micro-scale testing derived moduli values. These findings validate the structural homogeneity of the meniscal superficial zone, showing that material properties are statistically similar regardless of meniscal surface and region. Understanding the mechanical behavior of meniscal surfaces is imperative to properly design an effective meniscal replacement.

From meniscus to bone: A quantitative evaluation of structure and function of the human meniscal attachments

Meniscus efficacy at promoting joint congruity and preventing osteoarthritis hinges on enthesis integrity. Gross-scale tensile testing, histomorphometry and magnetic resonance imaging reveal significant differences between the four attachments, implying that each must endure a unique mechanical environment, which dictates their structure. However, little data exists to elucidate how these interfaces have adapted to their complex loading environment, particularly on a relevant scale, as the enthesis transitions through several unique zones in less than a millimeter. In our study we leveraged nanoindentation to determine viscoelastic material properties through the transition zones. Additionally, we employed histological techniques to evaluate the enthesis structure, including collagen organization and interdigitation morphometry. Mechanical evaluation revealed the medial posterior insertion site to be significantly more compliant than others. Collagen fiber orientation and dispersion as well as interdigitation morphometry were significantly different between attachments sites. These findings are clinically relevant as a disproportionate amount of enthesis failure occurs in the medial posterior attachment. Also, meniscal enthesis structure and function will need to be considered in future reparative and replacement strategies in order to recreate native meniscus mechanics and prevent osteoarthritis propagation.

Deleterious effects of osteoarthritis on the structure and function of the meniscal enthesis

OBJECTIVE The ability of menisci to prevent osteoarthritis (OA) is dependent on the integrity of the complex meniscal entheses, the attachments of the menisci to the underlying subchondral bone (SB). The goal of this study was to determine mechanical and structural changes in meniscal entheses after the onset of OA. DESIGN Healthy and osteoarthritic meniscal entheses were evaluated for changes in histomorphological characteristics, mineralization, and mechanical properties. Glycosaminoglycans (GAG) and calcium in the insertion were evaluated with histological staining techniques. The extent of calcium deposition was assessed and tidemark (TM) integrity was quantified. Changes in the mineralized zone of the insertion were examined using micro-computed tomography (μCT) to determine bone mineral density, cortical zone thickness, and mineralization gradient. Mechanical properties of the entheses were measured using nano-indentation techniques to obtain material properties based on viscoelastic analysis. RESULTS GAG thickness in the calcified fibrocartilage (CFC) zone and calcium content were significantly greater in osteoarthritic anterior meniscal entheses. TM integrity was significantly decreased in OA tissue, particularly in the medial anterior (MA) enthesis. The mineralized zone of osteoarthritic meniscal entheses was significantly thicker than in healthy entheses and showed decreased bone mineral density. Fitting of mineralization data to a sigmoidal Gompertz function revealed a lower rate of increase in mineralization in osteoarthritic tissue. Analysis of viscoelastic mechanical properties revealed increased compliance in osteoarthritic tissue. CONCLUSIONS These data suggest that significant changes occur at meniscal enthesis sites with the onset of OA. Mechanical and structural changes in meniscal entheses may contribute to meniscal extrusion, which has been shown to increase the progression of OA.

Phenomenological consequences of sectioning and bathing on passive muscle mechanics of the New Zealand white rabbit tibialis anterior

Skeletal muscle tissue provides support and mobility of the musculoskeletal system. Numerical modeling of muscle tissue aids in understanding disease pathophysiology, however, the effectiveness is dependent on accurately accounting for various tissue phenomena. Muscle modeling is made difficult due to the multitude of constituents that contribute to elastic and viscous mechanisms. Often, deterministic single fiber or fiber bundle studies are undertaken to examine these contributions. However, examination of whole, intact and structurally altered tissue and comparison to findings at the myofibril scale can help elucidate tissue mechanics. Stress relaxation tests at 10% strain were performed on 28 New Zealand White rabbits tibialis anterior muscles for whole, intact muscle and sub-sectioned muscle samples. Additionally, to aid in examining viscous effects, sub groups were tested with and without a phosphate buffered saline bath. The steady-state elastic modulus was not significantly different between groups. Interestingly, sectioning did result in a negative Poissons ratio following tensile loading. Additionally, sectioning resulted in altering the viscous tissue response as the time to reach steady-state was significantly faster than whole muscle samples (p<0.05), as well as the linear relaxation rate from 0 to 0.1 (p<0.01), 1 to 10 (p<0.05), and 10 to 100 s (p<0.05). Bathing tissue resulted in a significantly greater amount of percent stress relaxation for whole muscle (p<0.01). These findings provide new insight into the differing mechanical characteristics of whole and sectioned muscle tissue.

Internal pressure of human meniscal root attachments during loading.

This study investigated the internal fluid pressure of human cadaver meniscal root attachments. A pressure micro‐sensor was implanted inside each attachment site. Tibiofemoral joints were compressed to 2× body weight at various flexion angles and pressure recorded for 20 min. The anterior cruciate ligament (ACL) was then transected and joints retested. Lastly, a longitudinal incision of the lateral posterior (LP) horn was made and the joint retested. Ramp pressure was defined as the pressure when 2× body weight was reached, and equilibrium pressure was recorded at the end of the hold period. The medial posterior (MP) attachment was subjected to greater ramp pressure than the medial anterior (p = 0.002) and greater equilibrium pressure than all other root attachment sites (p < 0.001). Flexion angle had a significant effect on pressure as full extension was greatest at ramp (p = 0.040). Transection of the ACL decreased ramp pressure in the LP attachment (p = 0.025) and increased equilibrium pressure (p = 0.031) in the MP attachment. The results suggest that repair strategies should be developed which reconstruct the MP attachments to be sufficient to withstand large pressures. Furthermore, since meniscal pressure is highest at full extension, this fact should be considered when prescribing rehabilitation following repair of an attachment.

Efficacy of P188 on lapine meniscus preservation following blunt trauma

Traumatic injury to the knee leads to the development of post-traumatic osteoarthritis. The objective of this study was to characterize the effects of a single intra-articular injection of a non-ionic surfactant, Poloxamer 188 (P188), in preservation of meniscal tissue following trauma through maintenance of meniscal glycosaminoglycan (GAG) content and mechanical properties. Flemish Giant rabbits were subjected to a closed knee joint, traumatic compressive impact with the joint constrained to prevent anterior tibial translation. The contralateral limb served as an un-impacted control. Six animals (treated) received an injection of P188 in phosphate buffered saline (PBS) post trauma, and another six animals (sham) received a single injection of PBS to the impacted limb. Histological analyses for GAG was determined 6 weeks post trauma, and functional outcomes were assessed using stress relaxation micro-indentation. The impacted limbs of the sham group demonstrated a significant decrease in meniscal GAG coverage compared to non-impacted limbs (p<0.05). GAG coverage of the impacted P188 treated limbs was not significantly different than contralateral non-impacted limbs in all regions except the medial anterior (p<0.05). No significant changes were documented in mechanics for either the sham or treated groups compared to their respective control limbs. This suggests that a single intra-articular injection of P188 shows promise in prevention of trauma induced GAG loss.

Regional Comparisons of Nano-Mechanical Properties of Deep Zone Menisci

Osteoarthritis (OA) is a dehabilitating condition that is highly prevalent in today’s society, frequently leading to a knee replacement. OA is characterized by loss of articular cartilage. Previous research has shown articular cartilage preservation is dependent on the structural integrity of the menisci [1, 2]. The human menisci are two crescent shaped fibrocartilaginous structures that provide fundamental load distribution within the knee joint, ultimately aiding to attenuate stresses at the tibio-femoral site [3–5].Copyright

Internal pressure of the human meniscal attachments during physiological and pathological loading

Knee menisci are semi-lunar, fibrocartilaginous structures that convert applied compressive loads to circumferential hoop stresses which are attenuated at the tibial plateau via the meniscal attachments [1]. These specialized interfaces, located at the lateral anterior (LA), lateral posterior (LP), medial anterior (MA), and medial posterior (MP) horns, are crucial to maintaining mechanical functionality of menisci, thereby preventing osteoarthritis [2]. Soft-tissues subjected to compression and shear loads are known to possess proteoglycans, which aid in resisting these applied stresses by retaining interstitial fluid [1]. Interestingly, proteoglycans are also known to be present at the meniscus to bone interface [3]. Despite the presence of these biphasic attachments and their important role in joint load transfer, there have been no quantitative investigations of the mechanical environment within these attachments under physiological and pathological loads. Recent development of a novel pressure microsensor now allows direct observation of fluid pressure, which will aid in understanding the internal mechanical environment within the attachment sites. Previous work indicates that the attachment sites are geometrically, mechanically, and histologically different [4], thus it is hypothesized that the fluid pressures would likewise differ.Copyright

Long Term Effect of P188 on Meniscus Preservation Following Blunt Trauma

Acute knee joint injury has been associated with the development and progression of secondary osteoarthritis (OA). Previous work implicates that acute damage to tissue matrix and cells of the meniscus and articular cartilage may play important roles in early-stage OA [1]. Additionally, it has been shown that articular cartilage matrix repair hinges on chondrocyte preservation [2]. Therefore, inhibition of cell death may halt tissue degeneration. Recently, the FDA-approved surfactant Poloxamer 188 (P-188) has been shown to decrease acute cell death by repair of its plasma membrane, as well as mediate p38 signaling and subsequent inflammatory and apoptotic signaling leading to a reduction in degeneration of impacted cartilage [3, 4]. Therefore, it was hypothesized that matrix glycosaminoglycans of the meniscus will be preserved in the long-term following traumatic impaction and subsequent treatment with P-188.Copyright