In vivo intervertebral disc mechanical deformation following a treadmill walking "stress test" is inversely related to T1rho relaxation time.

OBJECTIVE: To assess the in vivo relationship between the mechanical response of intervertebral discs (IVDs) to dynamic activity and IVD biochemical composition assessed via T1rho relaxation imaging. DESIGN: Eighteen asymptomatic participants with no history of low back pain (LBP), injury, or surgery underwent magnetic resonance (MR) imaging of their lumbar spine prior to and immediately following a treadmill walking "stress test." Anatomic (SPACE, FLASH) MR images were obtained pre- and post-exercise and utilized to measure IVD mechanical deformation. Quantitative (T1rho) imaging was performed pre-exercise to reflect IVD composition. Pre-exercise anatomic images were also utilized to assess IVD degenerative status based on the modified Pfirrmann scale. To quantify mechanical response, 3D surface models of the L1-L2-L5-S1 IVDs were created from manual segmentations of pre- and post-exercise anatomic images and utilized to assess changes in IVD height. IVD strain (%) was defined as change in IVD height normalized to pre-activity height. Linear mixed models were used to assess the relationships between IVD mechanical deformation (strain), composition (T1rho relaxation time), and degenerative status (Pfirrmann grade). RESULTS: Increased compressive IVD strain was associated with lower T1rho relaxation times in the nucleus pulposus (NP) of the disc (ßT1rho=5.07,CI:[1.52,7.77],Rmarg2=0.52,p=0.005). Thus, an inverse relationship between IVD strain and NP T1rho relaxation time was observed. CONCLUSION: The in vivo mechanical response of the IVD to the "stress test" was sensitive to differences in NP composition. The results of this study suggest that quantification of in vivo IVD mechanical function and composition may provide insight into IVD health.

Obesity impacts the mechanical response and biochemical composition of patellofemoral cartilage: An in vivo, MRI-based investigation.

Obesity is a primary risk factor for osteoarthritis. While previous work has addressed relationships between in vivo cartilage mechanics, composition, and obesity in the tibiofemoral joint, there is limited information on these relationships in the patellofemoral joint. The purpose of this study was to compare the patellofemoral cartilage mechanical response to walking in participants with normal and obese body mass indices (BMIs). Additionally, patellar cartilage T1rho relaxation times were measured before exercise to characterize the biochemical composition of the tissue. Fifteen participants (eight with normal BMI and seven with obese BMI) underwent baseline magnetic resonance imaging (MRI) of their right knee. They then walked on a treadmill for 20 min at a speed normalized to their leg length before a second MRI scan. Subsequently, three-dimensional models of the bones and articular surfaces of the patellofemoral joint were created via manual segmentation of the pre- and post-exercise MR images to compute cartilage thickness and strain. Strain was defined as the change in patellofemoral cartilage thickness normalized to the baseline thickness. Results showed that participants with an obese BMI exhibited significantly increased patellofemoral cartilage strain compared to those with a normal BMI (5.4 ± 4% vs. 1.7 ± 3%, respectively; p = 0.003). Furthermore, patellar cartilage T1rho values were significantly higher in participants with obese versus normal BMIs (95 ms vs. 83 ms, respectively; p = 0.049), indicative of decreased proteoglycan content in those with an obese BMI. In summary, the altered patellofemoral cartilage strain and composition observed in those with an obese BMI may be indicative of cartilage degeneration.

Osteoarthritis year in review 2018: mechanics.

OBJECTIVE: To review recent biomechanics literature focused on the interactions between biomechanics and articular cartilage health, particularly focused on macro-scale and human studies. DESIGN: A literature search was conducted in PubMed using the search terms (biomechanics AND osteoarthritis) OR (biomechanics AND cartilage) OR (mechanics AND osteoarthritis) OR (mechanics AND cartilage) for publications from April 2017 to April 2018. RESULTS: Abstracts from the 559 articles generated from the literature search were reviewed. Due to the wide range of topics, 62 full texts with a focus on in vivo biomechanical studies were included for further discussion. Several overarching themes in the recent literature were identified and are summarized, including 1) new methods to detect early osteoarthritis (OA) development, 2) studies describing healthy and OA cartilage and biomechanics, 3) ACL injury and OA development, 4) meniscus injury and OA development, and 5) OA prevention, treatment, and management. CONCLUSIONS: Mechanical loading is a critical factor in the maintenance of joint health. Abnormal mechanical loading can lead to the onset and progression of OA. Thus, recent studies have utilized various biomechanical models to better describe the etiology of OA development and the subsequent effects of OA on the mechanics of joint tissues and whole body biomechanics.

In vivo measurement of ACL length and relative strain during walking.

Although numerous studies have addressed the effects of ACL injury and reconstruction on knee joint motion, there is currently little data available describing in vivo ACL strain during activities of daily living. Data describing in vivo ACL strain during activities such as gait is critical to understanding the biomechanical function of the ligament, and ultimately, to improving the surgical treatment of patients with ACL rupture. Thus, our objective was to characterize the relative strain in the ACL during both the stance and swing phases of normal level walking. Eight normal subjects were recruited for this study. Through a combination of magnetic resonance imaging, biplanar fluoroscopy, and motion capture, we created in vivo models of each subject's normal walking movements to measure knee flexion, ACL length, and relative ACL strain during gait. Regression analysis demonstrated an inverse relationship between knee flexion and ACL length (R(2)=0.61, p<0.001). Furthermore, relative strain in the ACL peaked at 13±2% (mean±95%CI) during mid-stance when the knee was near full extension. Additionally, there was a second local maximum of 10±7% near the end of swing phase, just prior to heel strike. These data are a vital step in further comprehending the normal in vivo biomechanics experienced by the ACL. In the future, this information could prove critical to improving ACL reconstruction and provide useful validation to future computational models investigating ACL function.

Acute joint pathology and synovial inflammation is associated with increased intra-articular fracture severity in the mouse knee.

OBJECTIVE: Post-traumatic arthritis is a frequent cause of disability and occurs most commonly and predictably after articular fracture. The objective of this investigation was to examine the effect of fracture severity on acute joint pathology in a novel murine model of intra-articular fracture. DESIGN: Low and high energy articular fractures (n=25 per group) of the tibial plateau were created in adult male C57BL/6 mice. The acute effect of articular fracture severity on synovial inflammation, bone morphology, liberated fracture area, cartilage pathology, chondrocyte viability, and systemic cytokines and biomarkers levels was assessed at 0, 1, 3, 5, and 7 days post-fracture. RESULTS: Increasing intra-articular fracture severity was associated with greater acute pathology in the synovium and bone compared to control limbs, including increased global synovitis and reduced periarticular bone density and thickness. Applied fracture energy was significantly correlated with degree of liberated cortical bone surface area, indicating greater comminution. Serum concentrations of hyaluronic acid (HA) were significantly increased 1 day post-fracture. While articular fracture significantly reduced chondrocyte viability, there was no relationship between fracture severity and chondrocyte viability, cartilage degeneration, or systemic levels of cytokines and biomarkers. CONCLUSIONS: This study demonstrates that articular fracture is associated with a loss of chondrocyte viability and increased levels of systemic biomarkers, and that increased intra-articular fracture severity is associated with increased acute joint pathology in a variety of joint tissues, including synovial inflammation, cortical comminution, and bone morphology. Further characterization of the early events following articular fracture could aid in the treatment of post-traumatic arthritis.

The effects of femoral graft placement on in vivo knee kinematics after anterior cruciate ligament reconstruction.

Achieving anatomical graft placement remains a concern in Anterior Cruciate Ligament (ACL) reconstruction. The purpose of this study was to quantify the effect of femoral graft placement on the ability of ACL reconstruction to restore normal knee kinematics under in vivo loading conditions. Two different groups of patients were studied: one in which the femoral tunnel was placed near the anterior and proximal border of the ACL (anteroproximal group, n=12) and another where the femoral tunnel was placed near the center of the ACL (anatomic group, n=10) MR imaging and biplanar fluoroscopy were used to measure in vivo kinematics in these patients during a quasi-static lunge. Patients with anteroproximal graft placement had up to 3.4mm more anterior tibial translation, 1.1mm more medial tibial translation and 3.7° more internal tibial rotation compared to the contralateral side. Patients with anatomic graft placement had motion that more closely replicated that of the intact knee, with anterior tibial translation within 0.8mm, medial tibial translation within 0.5mm, and internal tibial rotation within 1°. Grafts placed anteroproximally on the femur likely provide insufficient restraint to these motions due to a more vertical orientation. Anatomical femoral placement of the graft is more likely to reproduce normal ACL orientation, resulting in a more stable knee. Therefore, achieving anatomical graft placement on the femur is crucial to restoring normal knee function and may decrease the rates of joint degeneration after ACL reconstruction.

Measurement of in vivo anterior cruciate ligament strain during dynamic jump landing.

Despite recent attention in the literature, anterior cruciate ligament (ACL) injury mechanisms are controversial and incidence rates remain high. One explanation is limited data on in vivo ACL strain during high-risk, dynamic movements. The objective of this study was to quantify ACL strain during jump landing. Marker-based motion analysis techniques were integrated with fluoroscopic and magnetic resonance (MR) imaging techniques to measure dynamic ACL strain non-invasively. First, eight subjects' knees were imaged using MR. From these images, the cortical bone and ACL attachment sites of the tibia and femur were outlined to create 3D models. Subjects underwent motion analysis while jump landing using reflective markers placed directly on the skin around the knee. Next, biplanar fluoroscopic images were taken with the markers in place so that the relative positions of each marker to the underlying bone could be quantified. Numerical optimization allowed jumping kinematics to be superimposed on the knee model, thus reproducing the dynamic in vivo joint motion. ACL length, knee flexion, and ground reaction force were measured. During jump landing, average ACL strain peaked 55±14 ms (mean and 95% confidence interval) prior to ground impact, when knee flexion angles were lowest. The peak ACL strain, measured relative to its length during MR imaging, was 12±7%. The observed trends were consistent with previously described neuromuscular patterns. Unrestricted by field of view or low sampling rate, this novel approach provides a means to measure kinematic patterns that elevate ACL strains and that provide new insights into ACL injury mechanisms.

The prediction of stress-relaxation of ligaments and tendons using the quasi-linear viscoelastic model.

Recent studies have questioned the ability of the quasi-linear viscoelastic (QLV) model to predict stresses and strains in response to loading conditions other than those used to fit the model. The objective of this study was to evaluate the ability of several models in the literature to predict the elastic stress response of ligament and tendon at strain levels higher than the levels used to fit the model. The constitutive models were then used to evaluate the ability of the QLV model to predict the overall stress response during stress relaxation. The models expressing stress as an exponential function of strain significantly overestimated stress when used at higher strain levels. The polynomial formulation of the Mooney-Rivlin model more accurately predicted the stress-strain behavior of ligament and tendon. The results demonstrate that the ability of the QLV model to accurately predict the stress-relaxation response is dependent in part on the accuracy of the function used to model the elastic response of the soft tissue.

The effect of tibiofemoral joint kinematics on patellofemoral contact pressures under simulated muscle loads.

Altered patellofemoral joint contact pressures are thought to contribute to patellofemoral joint symptoms. However, little is known about the relationship between tibiofemoral joint kinematics and patellofemoral joint contact pressures. The objective of this paper was to investigate the effect of tibiofemoral joint kinematics on patellofemoral joint pressures using an established in vitro robotic testing experimental setup. Eight cadaveric knee specimens were tested at 0 degrees, 30 degrees, 60 degrees, 90 degrees, and 120 degrees of flexion under an isolated quadriceps load of 400 N and a combined quadriceps/hamstrings load of 400 N/200 N. Tibiofemoral joint kinematics were measured by the robot and contact pressures by a TekScan pressure sensor. The isolated quadriceps loading caused anterior translation and internal rotation of the tibia up to 60 degrees of flexion and posterior translation and external rotation of the tibia beyond 60 degrees. The co-contraction of the hamstring muscles caused a posterior translation and external rotation of the tibia relative to the motion of the tibia under the quadriceps load. Correspondingly, the contact pressures were elevated significantly at all flexion angles. For example, at 60 degrees of flexion, the hamstrings co-contraction increased the posterior tibial translation by approximately 2.8 mm and external tibial rotation by approximately 3.6 degrees. The peak contact pressure increased from 1.4+/-0.8 to 1.7+/-1.0 MPa, a 15% increase. The elevated contact pressures after hamstrings co-contraction indicates an intrinsic relation between the tibiofemoral joint kinematics and the patellofemoral joint biomechanics. An increase in posterior tibial translation and external rotation is accompanied by an increase in contact pressure in the patellofemoral joint. These results imply that excessive strength conditioning with the hamstring muscles might not be beneficial to the patellofemoral joint. Knee pathology that causes an increase in tibial posterior translation and external rotation might contribute to degeneration of the patellofemoral joint. These results suggest that conservative treatment of posterior cruciate ligament injury should be reconsidered.

Effect of the posterior cruciate ligament on posterior stability of the knee in high flexion.

Most biomechanical studies of the knee have focused on knee flexion angles between 0 degrees and 120 degrees. The posterior cruciate ligament (PCL) has been shown to constrain posterior laxity of the knee in this range of flexion. However, little is known about PCL function in higher flexion angles (greater than 120 degrees ). This in vitro study examined knee kinematics before and after cutting the PCL at high flexion under a posterior tibial load and various muscle loads. The results demonstrated that although the PCL plays an important role in constraining posterior tibial translation at low flexion angles, the PCL had little effect in constraining tibial translation at 150 degrees of flexion under the applied loads.

A minimally invasive method for the determination of force in the interosseous ligament.

OBJECTIVE: The objective was to develop and utilize a minimally invasive testing system to determine the force in the interosseous ligament under axial compressive loads across the range of motion of the human forearm. DESIGN: Eleven fresh frozen human cadaveric forearms were used (51-72 years). BACKGROUND: Current studies investigating interosseous ligament forces altered the structure of the forearm by implanting load cells into the radius and ulna. This may affect load transfer through the forearm. Little information was available on interosseous ligament function over the entire flexion range of the elbow. METHODS: A robotic joint testing system was used to apply a 100 N compressive load to the forearm and measure the resulting displacement. Each forearm was tested with no disruption of the bones and soft tissues of the forearm. The principle of superposition was used to calculate the forces in the interosseous ligament and was indirectly validated using fluoroscopy. RESULTS: The force in the interosseous ligament ranged from a minimum of 8 N in neutral forearm rotation at full extension to a maximum of 43 N in supination at 30 degrees of flexion. The largest force was found in supination at all flexion angles. CONCLUSIONS: The interosseous ligament is an important structure in the stability of the forearm. The force in the interosseous ligament depends on the elbow flexion angle and forearm rotation. RELEVANCE: This study suggests that radial head fractures are best treated with the forearm in supination, since the interosseous ligament takes the largest load in this position. Complex injuries which have a poor prognosis, may require interosseous ligament reconstruction to improve clinical outcomes.