Effects of Anterior Cruciate Ligament Deficiency on Tibiofemoral Cartilage Thickness and Strains in Response to Hopping

Associations Between Body Mass Index, Gait Biomechanics, and In Vivo Cartilage Function After Exercise in Those With Anterior Cruciate Ligament Reconstruction

BACKGROUND

Both high body mass index (BMI) and anterior cruciate ligament reconstruction (ACLR) independently influence knee osteoarthritis risk. Preliminary evidence shows the combination of these risk factors leads to poorer recovery and altered biomechanical outcomes after ACLR, but few studies have directly evaluated early changes in cartilage health between normal-BMI and high-BMI groups in this population.

PURPOSE

To evaluate ultrasound-based measures of cartilage strain and compositional changes (via echo-intensity [EI]) in response to an incline walking stress test between normal-BMI and high-BMI individuals with ACLR. A secondary evaluation was conducted of associations between habitual walking biomechanics (ie, ground-reaction forces, sagittal knee kinetics and kinematics) and cartilage strain and EI outcomes.

STUDY DESIGN

Controlled laboratory study.

METHODS

Gait biomechanics and femoral trochlear ultrasound analyses were evaluated in 64 participants with ACLR who had normal BMI (BMI < 27.0; n = 40) and high BMI (BMI ≥ 27.0; n = 24). Ultrasound images were collected bilaterally before and after an incline treadmill walk, and medial and lateral trochlear strain and EI changes pre-post exercise were used to compare BMI groups and limbs. Gait outcomes included ground-reaction forces, peak sagittal plane knee moments, angles, and excursions and were used to determine associations with cartilage outcomes in the entire cohort.

RESULTS

High-BMI individuals with ACLR exhibited greater medial trochlear cartilage strain in the ACLR limb compared with normal-BMI individuals (approximately 6%; P < .01). In those with high BMI, the ACLR limb exhibited greater medial trochlear strain relative to non-ACLR limbs (approximately 4%; P < .05), but between-limb differences were not observed in the normal-BMI group (P > .05). Medial trochlear EI changes were greater bilaterally in those with high BMI compared with normal-BMI ACLR counterparts (approximately 10%; P < .01). Last, individuals who walked with greater peak knee flexion angles exhibited less medial cartilage strain (ΔR2 = 0.06; P = .025).

CONCLUSION

The data suggested that high BMI affects cartilage functional properties after ACLR, whereas smaller knee flexion angles were associated with larger medial cartilage strain.

CLINICAL RELEVANCE

High-BMI individuals with ACLR may represent a subset of patients exhibiting earlier declines in cartilage functional integrity in response to loading, necessitating additional or more targeted interventions to mitigate disease development.

Anterior cruciate ligament injury should not be considered a contraindication for medial unicompartmental knee arthroplasty: Finite element analysis

PURPOSE

The research objective of this study is to use finite element analysis to investigate the impact of anterior cruciate ligament (ACL) injury on medial unicompartmental knee arthroplasty (UKA) and explore whether patients with ACL injuries can undergo UKA.

METHODS

Based on the morphology of the ACL, models of ACL with diameters ranging from 1 to 10mm are created. Finite element models of UKA include ACL absence and ACLs with different diameters. After creating a complete finite element model and validating it, four different types of loads are applied to the knee joint. Statistical analysis is conducted to assess the stress variations in the knee joint structure.

RESULTS

A total of 11 finite element models of UKA were established. Regarding the stress on the ACL, as the diameter of the ACL increased, when a vertical load of 750N was applied to the femur, combined with an anterior tibial load of 105N, the stress on the ACL increased from 2.61 MPa to 4.62 MPa, representing a 77.05% increase. Regarding the equivalent stress on the polyethylene gasket, a notable high stress change was observed. The stress on the gasket remained between 12.68 MPa and 14.33 MPa in all models. the stress on the gasket demonstrated a decreasing trend. The equivalent stress in the lateral meniscus and lateral femoral cartilage decreases, reducing from the maximum stress of 4.71 MPa to 2.61 MPa, with a mean value of 3.73 MPa. This represents a reduction of 44.72%, and the statistical significance is (P < 0.05). However, under the other three loads, there was no significant statistical significance (P > 0.05).

CONCLUSION

This study suggests that the integrity of the ACL plays a protective role in performing medial UKA. However, this protective effect is limited when performing medial UKA. When the knee joint only has varying degrees of ACL injury, even ACL rupture, and the remaining structures of the knee joint are intact with anterior-posterior stability in the knee joint, it should not be considered a contraindication for medial UKA.

Immediate and Delayed Effects of Joint Loading Activities on Knee and Hip Cartilage: A Systematic Review and Meta-analysis

BACKGROUND

The impact of activity-related joint loading on cartilage is not clear. Abnormal loading is considered to be a mechanical driver of osteoarthritis (OA), yet moderate amounts of physical activity and rehabilitation exercise can have positive effects on articular cartilage. Our aim was to investigate the immediate effects of joint loading activities on knee and hip cartilage in healthy adults, as assessed using magnetic resonance imaging. We also investigated delayed effects of activities on healthy cartilage and the effects of activities on cartilage in adults with, or at risk of, OA. We explored the association of sex, age and loading duration with cartilage changes.

METHODS

A systematic review of six databases identified studies assessing change in adult hip and knee cartilage using MRI within 48 h before and after application of a joint loading intervention/activity. Studies included adults with healthy cartilage or those with, or at risk of, OA. Joint loading activities included walking, hopping, cycling, weightbearing knee bends and simulated standing within the scanner. Risk of bias was assessed using the Newcastle-Ottawa Scale. Random-effects meta-analysis estimated the percentage change in compartment-specific cartilage thickness or volume and composition (T2 relaxation time) outcomes. The Grading of Recommendations Assessment, Development and Evaluation (GRADE) system evaluated certainty of evidence.

RESULTS

Forty studies of 653 participants were included after screening 5159 retrieved studies. Knee cartilage thickness or volume decreased immediately following all loading activities investigating healthy adults; however, GRADE assessment indicated very low certainty evidence. Patellar cartilage thickness and volume reduced 5.0% (95% CI 3.5, 6.4, I² = 89.3%) after body weight knee bends, and tibial cartilage composition (T2 relaxation time) decreased 5.1% (95% CI 3.7, 6.5, I² = 0.0%) after simulated standing within the scanner. Hip cartilage data were insufficient for pooling. Secondary outcomes synthesised narratively suggest knee cartilage recovers within 30 min of walking and 90 min of 100 knee bends. We found contrasting effects of simulated standing and walking in adults with, or at risk of, OA. An increase of 10 knee bend repetitions was associated with 2% greater reduction in patellar thickness or volume.

CONCLUSION

There is very low certainty evidence that minimal knee cartilage thickness and volume and composition (T2 relaxation time) reductions (0-5%) occur after weightbearing knee bends, simulated standing, walking, hopping/jumping and cycling, and the impact of knee bends may be dose dependent. Our findings provide a framework of cartilage responses to loading in healthy adults which may have utility for clinicians when designing and prescribing rehabilitation programs and providing exercise advice.

No Difference Unicompartmental Knee Arthroplasty for Medial Knee Osteoarthritis With or Without Anterior Cruciate Ligament Deficiency: A Systematic Review and Meta-analysis

BACKGROUND

A functional intact anterior cruciate ligament (ACLI) is considered to be a prerequisite for unicompartmental knee arthroplasty (UKA). However, UKA has been shown to have good clinical efficacy in ACL-deficient (ACLD) knees at 3 to 10 years follow-up. Therefore, the role of ACLD in UKA remains controversial, and more evidence is needed to clarify the role of ACLD in UKA.

METHODS

PubMed, the Web of Science, EMBASE, and Cochrane Central were queried for articles comparing the results of the ACLD and ACLI groups after UKA. Outcomes of interest included the Tegner Activity Scale, the Oxford Knee Score (OKS), postoperative slope of the implant (PSI), the Knee Injury and Osteoarthritis Outcomes Score (KOOS), the Lysholm score, and revision rate. There were eight studies included. The mean age was 66 years (range 49 to 87 year old) and the mean follow-up time was 6.9 years (range 1.3 to 16.6 years). There was baseline comparability regarding mean age, duration of follow-up, and body mass index (P > .5) between the ACLD and ACLI groups.

RESULTS

The ACLD and ACLI groups had improved postoperative functional indicators, and that postoperative revision rate (mean difference [MD], 1.24; 95% confidence interval [CI], 0.75 to 2.04; P = .4), Tegner score (MD, -0.1; 95% CI, -0.26 to 0.05; P = .19), and Lysholm score (95% CI, -2.46 to 7.32; P = .33) were similar between the groups, with no significant differences; however, the ACLD groups had significantly better KOOS Activities of Daily Living scores, with a significant difference (MD, 4.53; 95% CI, 1.75 to 7.3; P = .001). Also, there were no significant differences between two groups in the PSI, OKS, KOOS.

CONCLUSION

ACL deficiency is not always a contraindication for UKA. With correct patient selection, UKA could be considered for medial knee osteoarthritis with ACL deficiency without antero-posterior instability, especially these people over 60 years of age.

6DOF knee kinematic alterations due to increased load levels

Whether load carriage leads to six-degrees-of-freedom (6DOF) knee kinematic alterations remains unclear. Exploring this mechanism may reveal meaningful knee kinematic information that can be used to improve load carriage conditions, the design of protective devices, and the knowledge of the effects of load carriage on knees. We recruited 44 subjects to explore kinematic alterations from an unloaded state to 60% bodyweight (BW) load carriage. A three-dimensional gait analysis system was used to collect the knee kinematic data. One-way repeated analysis of variance (ANOVA) was used to explore the effects of load levels on knee kinematics. The effects of increasing load levels on knee kinematics were smooth with decreased or increased trends. We found that knees significantly exhibited increased lateral tibial translation (up to 1.2 mm), knee flexion angle (up to 1.4°), internal tibial rotation (up to 1.3°), and tibial proximal translation (up to 1.0 mm) when they went from an unloaded state to 60%BW load carriage during the stance phase (p < 0.05). Significant small knee adduction/abduction angle and posterior tibial translation alterations (<1°/mm) were also identified (p < 0.05). Load carriage can cause significant 6DOF knee kinematic alterations. The results showed that knee kinematic environments are challenging during increased load. Our results contain kinematic information that could be helpful for knee-protection-related activities, such as target muscle training to reduce abnormal knee kinematics and knee brace design.

Self-Healing of Hyaluronic Acid to Improve In Vivo Retention and Function

Convergent advances in the field of soft matter, macromolecular chemistry, and engineering have led to the development of biomaterials that possess autonomous, adaptive, and self-healing characteristics similar to living systems. These rationally designed biomaterials can surpass the capabilities of their parent material. Herein, the modification of hyaluronic acid (HA) to exhibit self-healing properties is described, and its physical and biological function both in vitro and in vivo is studied. The in vitro findings showed that self-healing HA designed to undergo self-repair improves lubrication, enhances free radical scavenging, and attenuates enzymatic degradation compared to unmodified HA. Longitudinal imaging following intraarticular injection of self-healing HA shows improved in vivo retention despite its low molecular weight. Concomitant with these functions, intraarticular injection of self-healing HA mitigates anterior cruciate ligament injury-mediated cartilage degeneration in rodents. This proof-of-concept study shows how incorporation of functional properties such as self-healing can be used to surpass the existing capabilities of biolubricants.

Effects of solvent osmolarity and viscosity on cartilage energy dissipation under high-frequency loading

Articular cartilage is a spatially heterogeneous, dissipative biological hydrogel with a high fluid volume fraction. Although energy dissipation is important in the context of delaying cartilage damage, the dynamic behavior of articular cartilage equilibrated in media of varied osmolarity and viscosity is not widely understood. This study investigated the mechanical behaviors of cartilage when equilibrated to media of varying osmolarity and viscosity. Dynamic moduli and phase shift were measured at both low (1 Hz) and high (75-300 Hz) frequency, with cartilage samples compressed to varied offset strain levels. Increasing solution osmolarity and viscosity both independently resulted in larger energy dissipation and decreased dynamic modulus of cartilage at both low and high frequency. Mechanical property alterations induced by varying osmolarity are likely due to the change in permeability and fluid volume fraction within the tissue. The effects of solution viscosity are likely due to frictional interactions at the solid-fluid interface, affecting energy dissipation. These findings highlight the significance of interstitial fluid on the energy dissipation capabilities of the tissue, which can influence the onset of cartilage damage.

Immune cell profiles in synovial fluid after anterior cruciate ligament and meniscus injuries

Background

Anterior cruciate ligament (ACL) and meniscus tears are common knee injuries. Despite the high rate of post-traumatic osteoarthritis (PTOA) following these injuries, the contributing factors remain unclear. In this study, we characterized the immune cell profiles of normal and injured joints at the time of ACL and meniscal surgeries.

Methods

Twenty-nine patients (14 meniscus-injured and 15 ACL-injured) undergoing ACL and/or meniscus surgery but with a normal contralateral knee were recruited. During surgery, synovial fluid was aspirated from both normal and injured knees. Synovial fluid cells were pelleted, washed, and stained with an antibody cocktail consisting of fluorescent antibodies for cell surface proteins. Analysis of immune cells in the synovial fluid was performed by polychromatic flow cytometry. A broad spectrum immune cell panel was used in the first 10 subjects. Based on these results, a T cell-specific panel was used in the subsequent 19 subjects.

Results

Using the broad spectrum immune cell panel, we detected significantly more total viable cells and CD3 T cells in the injured compared to the paired normal knees. In addition, there were significantly more injured knees with T cells above a 500-cell threshold. Within the injured knees, CD4 and CD8 T cells were able to be differentiated into subsets. The frequency of total CD4 T cells was significantly different among injury types, but no statistical differences were detected among CD4 and CD8 T cell subsets by injury type.

Conclusions

Our findings provide foundational data showing that ACL and meniscus injuries induce an immune cell-rich microenvironment that consists primarily of T cells with multiple T helper phenotypes. Future studies investigating the relationship between immune cells and joint degeneration may provide an enhanced understanding of the pathophysiology of PTOA following joint injury.

Biomechanical evaluation of bovine stifles stabilized with an innovative braided superelastic nitinol prosthesis after transection of the cranial cruciate ligament

OBJECTIVE

To determine the stability bovine stifles stabilized with nylon or nitinol superelastic prostheses after transection of the cranial cruciate ligament (CCL).

STUDY DESIGN

Ex vivo study.

SAMPLE POPULATION

Stifles (n = 15) harvested from adult bovine cadavers.

METHODS

The stifles were randomly assigned pairwise to a ligament reconstruction technique (n = 5): (1) and (2) Hamilton's technique using a prosthesis made of 24 nitinol strands (0.39 mm) braided at 40°or single 600-lb test nylon implant, and (3) nitinol prosthesis placed in femoral and tibial bone tunnels (bone-to-bone). Craniocaudal tibial translation at ±2000 N was applied to the tibia, and mediolateral angular displacement via measured under torsional tibial loading at ±60 Nm on three occasions: intact CCL, transected, and stabilized. Outcomes were evaluated with a mixed effect linear model for repeated measures.

RESULTS

Bone-to-bone using nitinol was the only repair that decreased tibial translation after CCL transection (p = .001) with a 23% change magnitude compared with intact CCL. Hamilton was the only stabilization reestablishing angular displacement, similar to intact CCL (p = .109 and .134 for nitinol and nylon). Bone-to-bone nitinol stabilization decreased angular displacement after CCL-transection with an 8% change magnitude (p = .040) without returning to normal values.

CONCLUSION

CCL replacement with nylon did restore joint stability. Nitinol prostheses passed through single femoral and tibial bone tunnels (bone-to-bone) were the only techniques reducing tibial translation.

CLINICAL SIGNIFICANCE/IMPACT

Bone-to-bone stabilization with a nitinol prosthesis may be considered as an alternative to nylon for CCL replacement in cattle. These results provide evidence to justify clinical evaluation in cattle undergoing CCL replacement.

Changes in anterior femoral articular cartilage structure in collegiate rugby athletes with and without a history of traumatic knee joint injury following a five-month competitive season

This study aimed to examine anterior femoral cartilage morphology before (pre-season) and after (post-season) a 5-month competitive season in collegiate ruby players with and without a previous history of traumatic injury to ligamentous, meniscus, and/or cartilage structures at the knee joint. Using a prospective cohort design, 42 male collegiate rugby players with a previous history of traumatic intracapsular knee joint injury and 124 players without knee injury history were included in this study. Ultrasonography assessments of anterior femoral cartilage were performed before (pre-season) and following a 5-month athletic season (post-season). Rugby players with a history of traumatic knee joint injury had greater lateral condylar thickness (2.37 ± 0.35 mm, p = 0.03), intercondylar thickness (2.51 ± 0.47 mm, p = 0.03), and partial area (44.67 ± 7.28mm², p = 0.02) compared to control players (lateral = 2.23 ± 0.35 mm, intercondylar = 2.32 ± 0.47 mm, partial area = 41.60 ± 7.26 mm²), regardless of pre-and post-season assessment time points. Pre-season ultrasonography assessment of lateral condylar thickness (2.34 ± 0.47 mm, p = 0.02), medial condylar thickness (2.05 ± 0.43 mm, p = 0.03), and partial area (44.10 ± 9.23 mm², p = 0.001) were significantly greater than the post-season ultrasonography assessment time point (lateral = 2.26 ± 0.43 mm, medial = 1.98 ± 0.43 mm, partial area = 42.17 ± 8.82 mm²), regardless of group membership. Rugby players with a history of intracapsular knee joint injury displayed altered anterior femoral cartilage size via ultrasonography assessments. Regardless of a presence of injury history, collegiate rugby players showed a decrease in cartilage thickness and partial area following a 5-month competitive season.

Osteoarthritis year in review 2020: imaging

This narrative "Year in Review" highlights a selection of articles published between January 2019 and April 2020, to be presented at the OARSI World Congress 2020 within the field of osteoarthritis (OA) imaging. Articles were obtained from a PubMed search covering the above period, utilizing a variety of relevant search terms. We then selected original and review studies on OA-related imaging in humans, particularly those with direct clinical relevance, with a focus on the knee. Topics selected encompassed clinically relevant models of early OA, particularly imaging applications on cruciate ligament rupture, as these are of direct clinical interest and provide potential opportunity to evaluate preventive therapy. Further, imaging applications on structural modification of articular tissues in patients with established OA, by non-pharmacological, pharmacological and surgical interventions are summarized. Finally, novel deep learning approaches to imaging are reviewed, as these facilitate implementation and scaling of quantitative imaging application in clinical trials and clinical practice. Methodological or observational studies outside these key focus areas were not included. Studies focused on biology, biomechanics, biomarkers, genetics and epigenetics, and clinical studies that did not contain an imaging component are covered in other articles within the OARSI "Year in Review" series. In conclusion, exciting progress has been made in clinically validating human models of early OA, and the field of automated articular tissue segmentation. Most importantly though, it has been shown that structure modification of articular cartilage is possible, and future research should focus on the translation of these structural findings to clinical benefit.

A synthetic mechanogenetic gene circuit for autonomous drug delivery in engineered tissues

Mechanobiologic signals regulate cellular responses under physiologic and pathologic conditions. Using synthetic biology and tissue engineering, we developed a mechanically responsive bioartificial tissue that responds to mechanical loading to produce a preprogrammed therapeutic biologic drug. By deconstructing the signaling networks induced by activation of the mechanically sensitive ion channel transient receptor potential vanilloid 4 (TRPV4), we created synthetic TRPV4-responsive genetic circuits in chondrocytes. We engineered these cells into living tissues that respond to mechanical loading by producing the anti-inflammatory biologic drug interleukin-1 receptor antagonist. Chondrocyte TRPV4 is activated by osmotic loading and not by direct cellular deformation, suggesting that tissue loading is transduced into an osmotic signal that activates TRPV4. Either osmotic or mechanical loading of tissues transduced with TRPV4-responsive circuits protected constructs from inflammatory degradation by interleukin-1α. This synthetic mechanobiology approach was used to develop a mechanogenetic system to enable long-term, autonomously regulated drug delivery driven by physiologically relevant loading.

Effect of walking on in vivo tibiofemoral cartilage strain in ACL-deficient versus intact knees

Anterior cruciate ligament (ACL) rupture alters knee kinematics and contributes to premature development of osteoarthritis. However, there is limited data regarding the in vivo biomechanical response of tibiofemoral cartilage to activities of daily living (ADLs) in ACL-deficient knees. In this study, eight otherwise healthy participants with chronic unilateral ACL deficiency completed a stress test to assess the effect of 20 min of level treadmill walking at a speed of 2.5 mph on tibiofemoral cartilage in their ACL-deficient and contralateral ACL-intact knees. Three-dimensional surface models developed from pre- and post-activity magnetic resonance (MR) images of the injured and uninjured knees were used to determine compressive strain across multiple regions of tibiofemoral cartilage (medial and lateral tibial plateaus, medial and lateral femoral condyles, medial aspect of femoral condyle adjacent to intercondylar notch of the femur). In the ACL-deficient knees, we observed significantly increased cartilage strain in the region of the medial femoral condyle adjacent to the intercondylar notch (6% in deficient vs. 2% in contralateral, p = 0.01) as well as across the medial and lateral tibial plateaus (4% vs. 3%, p = 0.01) relative to the contralateral ACL-intact knees. Increased compressive strain at the medial intercondylar notch and tibial plateau suggests alterations in mechanical loading or the response to load in these regions, presumably related to altered knee kinematics. These changes may disrupt cartilage homeostasis and contribute to subsequent development of osteoarthritis.

Combined Injury to the ACL and Lateral Meniscus Alters the Geometry of Articular Cartilage and Meniscus Soon After Initial Trauma

Combined injury to the anterior cruciate ligament (ACL) and meniscus is associated with earlier onset and increased rates of post-traumatic osteoarthritis compared with isolated ACL injury. However, little is known about the initial changes in joint structure associated with these different types of trauma. We hypothesized that trauma to the ACL and lateral meniscus has an immediate effect on morphometry of the articular cartilage and meniscus about the entire tibial plateau that is more pronounced than an ACL tear without meniscus injury. Subjects underwent magnetic resonance imaging scanning soon after injury and prior to surgery. Those that suffered injury to the ACL and lateral meniscus underwent changes in the lateral compartment (increases in the posterior-inferior directed slopes of the articular cartilage surface, and the wedge angle of the posterior horn of the meniscus) and medial compartment (the cartilage-to-bone height decreased in the region located under the posterior horn of the meniscus, and the thickness of cartilage increased and decreased in the mid and posterior regions of the plateau, respectively). Subjects that suffered an isolated ACL tear did not undergo the same magnitude of change to these articular structures. A majority of the changes in morphometry occurred in the lateral compartment of the knee; however, change in the medial compartment of the knee with a normal appearing meniscus also occurred. Statement of clinical significance: Knee injuries that involve combined trauma to the ACL and meniscus directly affect both compartments of the knee, even if the meniscus and articular cartilage appears normal upon arthroscopic examination. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 38:759-767, 2020.

In Vivo Anterior Cruciate Ligament Deformation During a Single-Legged Jump Measured by Magnetic Resonance Imaging and High-Speed Biplanar Radiography

BACKGROUND

The in vivo mechanics of the anterior cruciate ligament (ACL) and its bundles during dynamic activities are not completely understood. An improved understanding of how the ACL stabilizes the knee is likely to aid in the identification and prevention of injurious maneuvers.

PURPOSE/HYPOTHESIS

The purpose was to measure in vivo ACL strain during a single-legged jump through use of magnetic resonance imaging (MRI) and high-speed biplanar radiography. We hypothesized that ACL strain would increase with the knee near extension, and a peak in ACL strain would occur just before landing from the jump, potentially due to quadriceps contraction in anticipation of landing.

STUDY DESIGN

Descriptive laboratory study.

METHODS

Models of the femur, tibia, and ACL attachment sites of 8 male participants were generated from MRI scans through use of solid modeling. High-speed biplanar radiographs were obtained from these participants as they performed a single-legged jump. The bone models were registered to the biplanar radiographs, thereby reproducing the in vivo positions of the joint throughout the jump. ACL and bundle elongations were defined as the centroid to centroid distances between attachment sites for each knee position. ACL strain was defined as ACL length normalized to its length measured in the position of the knee at the time of MRI.

RESULTS

Peaks in ACL strain were observed before toe-off and 55 ± 35 milliseconds before initial ground contact. These peaks were associated with the knee positioned at low flexion angles. Mean ACL strain was inversely related to mean flexion angle (rho = -0.73, P < .001), such that ACL strain generally increased with knee extension throughout the jumping motion. ACL bundle lengths were significantly (rho > 0.85, P < .001) correlated with overall ACL length.

CONCLUSION

These findings provide insight into how landing in extension can increase the risk of ACL injury. Specifically, this study shows that peak ACL strain can occur just before landing from a single-legged jump. Thus, when an individual lands on an extended knee, the ACL is relatively taut, which may make it particularly vulnerable to injury, especially in the presence of a movement perturbation or unanticipated change in landing strategy.

CLINICAL RELEVANCE

This study provides a novel measurement of dynamic ACL strain during an athletic maneuver and lends insight into how landing in extension can increase the likelihood of ACL failure.

Magnetic resonance imaging (MRI) studies of knee joint under mechanical loading: Review

Osteoarthritis (OA) is a very common disease that affects the human knee joint, particularly the articular cartilage and meniscus components which are regularly under compressive mechanical loads. Early-stage OA diagnosis is essential as it allows for timely intervention. The primary non-invasive approaches currently available for OA diagnosis include magnetic resonance imaging (MRI), which provides excellent soft tissue contrast at high spatial resolution. MRI-based knee investigation is usually performed on joints at rest or in a non-weight-bearing condition that does not mimic the actual physiological condition of the joint. This discrepancy may lead to missed detections of early-stage OA or of minor lesions. The mechanical properties of degenerated musculoskeletal (MSK) tissues may vary markedly before any significant morphological or structural changes detectable by MRI. Recognizing distinct deformation characteristics of these tissues under known mechanical loads may reveal crucial joint lesions or mechanical malfunctions which result from early-stage OA. This review article summarizes the large number of MRI-based investigations on knee joints under mechanical loading which have been reported in the literature including the corresponding MRI measures, the MRI-compatible devices employed, and potential challenges due to the limitations of clinical MRI sequences.

A New Stress Test for Knee Joint Cartilage

Cartilage metabolism—both the synthesis and breakdown of cartilage constituents and architecture—is influenced by its mechanical loading. Therefore, physical activity is often recommended to maintain cartilage health and to treat or slow the progression of osteoarthritis, a debilitating joint disease causing cartilage degeneration. However, the appropriate exercise frequency, intensity, and duration cannot be prescribed because direct in vivo evaluation of cartilage following exercise has not yet been performed. To address this gap in knowledge, we developed a cartilage stress test to measure the in vivo strain response of healthy human subjects’ tibial cartilage to walking exercise. We varied both walk duration and speed in a dose-dependent manner to quantify how these variables affect cartilage strain. We found a nonlinear relationship between walk duration and in vivo compressive strain, with compressive strain initially increasing with increasing duration, then leveling off with longer durations. This work provides innovative measurements of cartilage creep behavior (which has been well-documented in vitro but not in vivo) during walking. This study showed that compressive strain increased with increasing walking speed for the speeds tested in this study (0.9–2.0 m/s). Furthermore, our data provide novel measurements of the in vivo strain response of tibial cartilage to various doses of walking as a mechanical stimulus, with maximal strains of 5.0% observed after 60 minutes of walking. These data describe physiological benchmarks for healthy articular cartilage behavior during walking and provide a much-needed baseline for studies investigating the effect of exercise on cartilage health.