The instrumented sheep knee to elucidate insights into osteoarthritis development and progression: A sensitive and reproducible platform for integrated research efforts

BACKGROUND

Osteoarthritis of the knee is a very common condition that has been difficult to treat. The majority of cases are considered idiopathic. Much research effort remains focused on biology rather than the biomechanics of such joints. Some new methods were developed and validated to better appreciate the subtleties of the biomechanical integrity of joints, and how changes in biomechanics can contribute to osteoarthritis.

METHODS

Over the past 15 years our lab has enhanced the sensitivity of the assessment of knee biomechanics of an instrumented, trained large animal model (sheep) of osteoarthritis and integrated the findings with biological and histological assessments. These new methods include gait analysis before and after injury followed by robotic validation post-sacrifice, and more recently using Fibre Bragg Grating sensors to detect alterations in cartilage stresses.

RESULTS

A review of the findings obtained with this model are presented. The findings indicate that sheep, like humans, exhibit individual characteristics. They also indicate that joint kinetics, rather than kinematics may better define the alterations induced by injury. With the addition of Fibre Bragg Grating sensors, it has been possible to measure with good accuracy, alterations to cartilage stresses following a controlled knee injury.

INTERPRETATION

Using this model as Proof of Concept, this sheep system can now be viewed as a sensitive platform to address many questions related to risk for development of idiopathic osteoarthritis of the human knee, the efficacy of potential interventions to correct biomechanical disruptions, and how joint biomechanics and biology are integrated during aging.

Influence of peracetic acid-ethanol sterilisation on the biomechanical properties of human meniscus transplants

Purpose

Meniscus allograft transplantation (MAT) is a possible treatment for patients suffering with pain after meniscectomy. Here, peracetic acid (PAA) sterilised meniscus transplants were investigated on whether they would provide an adequate alternative to fresh-frozen transplants in their viscoelastic and mechanical properties.

Methods

In this analysis, 31 menisci donors (26 male and 5 female) were included. The average donor age was 49.87 years, ranging from 32 to 65 years. Menisci of matched pairs of knees underwent chemical sterilisation while counterparts were left fresh-frozen. Stiffness and load to failure were determined via suture retention. Further menisci were analysed while attached to the tibial bone block using a novel test device to mimic physiological load distribution. Meniscus relaxation, stiffness and failure loads were determined. Histology and biphasic properties of the menisci were examined and results were analysed using paired t-tests.

Results

A novel custom built test device allowed the application of physiological loads for suture retention testing and revealed no significant differences between PAA sterilised (14.85 ± 4.46 N/mm, 50.49 ± 17.01 N) and fresh-frozen (18.26 ± 4.46 N/mm, 59.49 ± 21.07 N) regarding stiffness and failure load, respectively. Furthermore, initial 200 N loading showed significantly higher strain in sterilised menisci (18.87 ± 1.56) compared to fresh frozen (13.81 ± 1.04). Load relaxation experiments demonstrated significantly lower relaxation for sterilised menisci (77.71 ± 1.62) compared to fresh-frozen (89.11 ± 1.00, p-value < 0.0001).

Conclusion

Peracetic acid sterilised human menisci performed equally to fresh-frozen counterparts in a suture retention test and in physiological failure testing providing an adequate alternative. However, meniscus relaxation, biphasic properties and strain were shown to be significantly different between the groups. A common problem of MAT is graft extrusion or shrinkage, therefore the parameters measured here should be considered and may influence meniscus extrusion after transplantation.

Level of evidence

n/a (experimental study)

Correlation of damage score in PTOA with changes in stress on cartilage in an ovine model

Objective

There is a high risk of developing osteoarthritis (OA) following traumatic injury to the knee. Severe ligament injuries can disrupt the integrity of the multicomponent knee at both biological and biomechanical levels. We hypothesize changes in cartilages stresses could lead to tissue damage and development of OA.

Design

The in-vivo gait kinematics of the stifle (knee) joint of four adult female ovine subjects were recorded prior to and at ten-and-twenty weeks following partial ACL-MCL transection. The subjects were sacrificed and the experimental joint from each subject was mounted on a parallel robotic system programmed with the kinematic findings. Ten custom-built Fibre Bragg Grating optic sensors were arranged to measure contact stresses on the surface of the tibial plateau articular cartilage. These sensors provide the first accurate stress measurements in a joint during gait replication using the previously recorded in-vivo kinematics. The relationship between the results obtained and observed focal damage was assessed.

Results

The locations on the tibial plateaus that experienced the greatest change in contact stresses corresponded with the locations of focal damage development. No direct link was detected between individual animal differences in kinematics and variations in stress magnitudes or the development of focal cartilage damage.

Conclusions

The findings highlight the importance of mechanical stress determinants in the integrated set point for the knee (with individual variation), and how injury-related stress changes correlate with development of PTOA.

Mapping Stresses on the Tibial Plateau Cartilage in an Ovine Model Using In-Vivo Gait Kinematics

Understanding stresses within the knee joint is central to understanding knee function, and the etiology and progression of degenerative joint diseases such as post-traumatic osteoarthritis. In this study, in vivo gait kinematics of four ovine subjects were recorded using a highly accurate Instrumented Spatial Linkage (ISL) as each subject walked on a standard treadmill. The subjects were then sacrificed, and the right hind limbs removed. Ten purpose-built Fibre Bragg Grating (FBG) sensors were positioned within each stifle joint and used to measured contact stresses on the articulating surface of the tibial plateau as the recorded gait was replicated using a 6-degrees-of-freedom parallel robotic system. This study provides the first accurate, direct measurement of stress in a joint during in vivo gait replication. It was hypothesized that the results would indicate a direct link between gait kinematics and measured stress values. Contrary to this expectation no direct link was found between individualistic differences in kinematics and differences in stress magnitudes. This finding highlights the complex multifactorial nature of stress magnitudes and distribution patterns across articular joints. The results also indicate that stress magnitudes within the knee joint are highly position dependent with magnitudes varying substantially between points only a few mm apart.

Measuring the Internal Stress in Ovine Meniscus During Simulated In Vivo Gait Kinematics: A Novel Method Using Fibre Optic Technology

Changes in stress transferred across articular joints have been described as a substantial factor in the initiation and progression of joint disease such as post-traumatic osteoarthritis and have thus been of interest to biomechanical researchers. However, to date, stress magnitudes within the menisci have not been successfully measured. In this study, a novel method for measuring stress within the menisci is presented. Small Fibre Bragg Grating (FBG) sensors were inserted inside menisci and used to measure mechanical stress during replicated gait cycles. In-vitro stress measurements within the menisci were preformed for healthy gait and gait following surgical damage to the joints. Together with our capability to reproduce in vivo motions accurately, the improvements in fibre optic technology have allowed for the first direct measurement of mechanical stress in menisci.

Stress Measurements on the Articular Cartilage Surface Using Fiber Optic Technology and In-Vivo Gait Kinematics

It has been hypothesized a change in stress on the cartilage of a joint is a significant factor in the initiation and progression of post-traumatic osteoarthritis. Without a reliable method for measuring stress, this hypothesis has largely gone untested. In this study, a novel, repeatable, and reliable method for measuring stress on the surface of articular cartilage in articular joints is presented. Small Fiber Bragg Grating (FBG) sensors capable of measuring normal stress between contact surfaces in diarthrodial joints were developed and validated. The small size of these sensors (diameter of 125-300 μm and sensing length of 1 mm) allows them to be inserted into the joint space without the removal of biomechanically relevant structures. In-vitro stresses on the surface of the cartilage for both healthy and surgically damaged joints were measured after implantation of the FBG sensors using in vivo generated gait kinematic data and a 6-degrees of freedom parallel robot. Along with our capability to reproduce in vivo motions accurately and the improvements in fiber optic technology, this study describes the first direct measurement of stress in a joint using in vivo gait kinematics.

Robotically Simulated Pivot Shift That Represents the Clinical Exam

A thorough understanding of anterior cruciate ligament (ACL) function and the effects of surgical interventions on knee biomechanics requires robust technologies and simulation paradigms that align with clinical insight. In vitro orthopedic biomechanical testing for the elucidation of ACL integrity doesn't have an established testing paradigm to simulate the clinical pivot shift exam on cadaveric specimens. The study aim was to develop a robotically simulated pivot shift that represents the clinical exam. An orthopedic surgeon performed a pivot shift on an instrumented ACL-deficient cadaver leg to capture 6 degree-of-freedom motion/loads. The same knee was mounted to the robot and the sensitivity of the motion/loading profiles quantified. Three loading profile candidates that generated positive pivot shifts on the instrumented knee were selected and applied to 7 ACL-intact/deficient specimens and resulted in the identification of a profile that was able to induce a positive pivot shift in all ACL-deficient specimens ( p < 0.001). The simulated shifts began at 22 ± 8° and ended at 33 ± 6° of flexion with the average magnitude of the shifts being 12.8 ± 3.2 mm in anterior tibial translation and 17.6 ± 4.3° in external tibial rotation. The establishment and replication of a robotically simulated clinical pivot shift across multiple specimens show the robustness of the loading profile to accommodate anatomical and experimental variability. Further evaluation and refinement should be undertaken to create a useful tool in evaluating ACL function and reconstruction techniques. Statement of clinical significance: Creation and successful demonstration of the simulated clinical pivot shift validates a profile for robotic musculoskeletal simulators to analyze ACL related clinical questions. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 37:2601-2608, 2019.

Use of pre-clinical surgically induced models to understand biomechanical and biological consequences of PTOA development

Post-traumatic osteoarthritis (PTOA) development is often observed following traumatic knee injuries involving key stabilising structures such as the cruciate ligaments or the menisci. Both biomechanical and biological alterations that follow knee injuries have been implicated in PTOA development, although it has not been possible to differentiate clearly between the two causal factors. This review critically examines the outcomes from pre-clinical lapine and ovine injury models arising in the authors' laboratories and differing in severity of PTOA development and progression. Specifically, we focus on how varying severity of knee injuries influence the subsequent alterations in kinematics, kinetics, and biological outcomes. The immediate impact of injury on the lubrication capacity of the joint is examined in the context of its influence on biomechanical alterations, thus linking the biological changes to abnormal kinematics, leading to a focus on the potential areas for interventions to inhibit or prevent development of the disease. We believe that PTOA results from altered cartilage surface interactions where biological and biomechanical factors intersect, and mitigating acute joint inflammation may be critical to prolonging PTOA development. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:454-465, 2017.

Recent advances in computational mechanics of the human knee joint

Computational mechanics has been advanced in every area of orthopedic biomechanics. The objective of this paper is to provide a general review of the computational models used in the analysis of the mechanical function of the knee joint in different loading and pathological conditions. Major review articles published in related areas are summarized first. The constitutive models for soft tissues of the knee are briefly discussed to facilitate understanding the joint modeling. A detailed review of the tibiofemoral joint models is presented thereafter. The geometry reconstruction procedures as well as some critical issues in finite element modeling are also discussed. Computational modeling can be a reliable and effective method for the study of mechanical behavior of the knee joint, if the model is constructed correctly. Single-phase material models have been used to predict the instantaneous load response for the healthy knees and repaired joints, such as total and partial meniscectomies, ACL and PCL reconstructions, and joint replacements. Recently, poromechanical models accounting for fluid pressurization in soft tissues have been proposed to study the viscoelastic response of the healthy and impaired knee joints. While the constitutive modeling has been considerably advanced at the tissue level, many challenges still exist in applying a good material model to three-dimensional joint simulations. A complete model validation at the joint level seems impossible presently, because only simple data can be obtained experimentally. Therefore, model validation may be concentrated on the constitutive laws using multiple mechanical tests of the tissues. Extensive model verifications at the joint level are still crucial for the accuracy of the modeling.