Changes to the articular cartilage thickness profile of the tibia following anterior cruciate ligament injury.

OBJECTIVES: We sought to determine if anterior cruciate ligament (ACL)-injured subjects demonstrated side-to-side differences in tibial cartilage thickness soon after injury, and if uninjured-control subjects displayed side-to-side symmetry in cartilage thickness. Second, we aimed to investigate associations between body mass index (BMI), cross-sectional area (CSA) of the proximal tibia, and articular cartilage thickness differences. METHODS: Bilateral Magnetic Resonance Images (MRIs) were obtained on 88 ACL-injured subjects (27 male; 61 female) a mean 27 days post-injury, and 88 matched uninjured control subjects. Within ACL-injured and uninjured control subjects, side-to-side differences in medial and lateral tibial articular cartilage thickness were analyzed with adjustment for tibial position relative to the femur during MRI acquisition. Associations between tibial CSA and cartilage thickness differences were tested within high and low BMI groups. RESULTS: Within the medial tibial compartment, ACL-injured females displayed significant increases: mean (confidence interval (CI)) = +0.18 mm (0.17, 0.19) and decreases: mean (CI) = -0.14 mm (-0.13, -0.15) in tibial cartilage thickness within the central and posterior cartilage regions respectively. Adjustment for tibial position revealed a decreased area of significant cartilage thickness differences, though 46% of points maintained significance. In the lateral compartment anterior region, there was a significantly different relationship between cartilage thickness differences and CSA, within high and low BMI groups (BMI group*CSA interaction, P = 0.007). Within the low BMI group, a significant negative correlation between cartilage thickness and CSA was identified (P = 0.03). CONCLUSIONS: ACL-injured females displayed cartilage thickness differences in the central, and posterior medial tibial cartilage regions. Tibial position effected thickness differences, but did not account for all significant differences.

Calcium phosphate particulates increase friction in the rat knee joint.

OBJECTIVE: Basic calcium phosphate (BCP) particulates are commonly found in cartilage and synovial fluid of osteoarthritis (OA) joints with the amount of BCP correlating with knee OA severity. How cartilage mineralization affects joint degeneration has yet to be determined. The objective of this study was to determine whether BCP in the synovial fluid affects the rat knee joint coefficient of friction (COF). METHODS: The COFs of knees from both hind limbs of four mature male rats were measured post mortem using a pendulum apparatus with an infrared tracking system. The three conditions evaluated were (1) the naïve state, (2) after the injection of 100 µL of phosphate buffered saline (PBS) (sham) and (3) after the injection of 100 µL of a 1 mg/mL BCP suspension. The decrease in the pendulum amplitude (decay) was fit using two friction models: (1) a one parameter Stanton linear decay model and (2) a two parameters combination Stanton linear decay and viscous damping exponential decay model. RESULTS: The COF increased 17.6% after injection of BCP compared to the naïve (P = 0.0012) and 16.0% compared to the saline injected (P = 0.0018) joints as derived from the one parameter model. The COF did not differ between naïve and saline injected joints. Results from the two parameters model showed a similar increase in COF after injection of BCP while the viscous damping was not significantly different between conditions. CONCLUSIONS: The increased joint friction with BCP particulates suggests BCPs may play a role in articular surface degradation and OA development.

Large residual strains are present in the intervertebral disc annulus fibrosus in the unloaded state.

The intervertebral disc annulus fibrosus (AF) is subjected to high circumferential tensile stresses resulting from nucleus pulposus pressurisation under axial compression. In other pressure containing tissues, such as blood vessel walls, residual compressive stresses along the inside surface of the tissues without pressurisation reduce peak tensile stresses under pressurisation. This study hypothesised that similar patterns of residual stress exist in the annulus fibrosus. Accurate characterisation of residual stresses is essential for both the incorporation of nonlinear material descriptions into models of the disc as well as the design of effective annulus repair strategies. By imaging nine bovine caudal discs before and after the release of residual stresses via incision, we measured a mean residual stretch of 0.86 ± 0.13 at the inner AF and 1.02 ± 0.08 at the outer AF. These stretch values were used to calculate a gradient of residual stress ranging from -230 ± 22 kPa of compression at the inner AF to 54 ± 0.2 kPa of tension at the outer AF. Material models of AF have assumed that the AF was in a stress free reference state when there are no external loads. However, this study documents that there are large residual stresses in the AF even without external loads. The release of residual tension in the outer AF by herniation, needle injection or incisions makes closure difficult and may accelerate degeneration of the surrounding tissue. Retention of these residual stresses may be essential to maintaining disc mechanical function and to producing viable AF repair techniques.

Trunk stiffness increases with steady-state effort.

Trunk stiffness was measured in healthy human subjects as a function of steady-state preload efforts in different horizontal loading directions. Since muscle stiffness increases with increased muscle activation associated with increasing effort, it is believed that coactivation of muscles helps to stiffen and stabilize the trunk. This paper tested whether increased steady-state preload effort increases trunk stiffness. Fourteen young healthy subjects each stood in an apparatus with the pelvis immobilized. They were loaded horizontally at directions of 0, 45, 90, 135 and 180 degrees to the forward direction via a thoracic harness. Subjects first equilibrated with a steady-state load of 20 or 40% of their maximum extension effort. Then a sine-wave force perturbation of nominal amplitude of 7.5 or 15% of maximum effort and nominal period of 250ms was applied. Both the applied force and subsequent motion were recorded. Effective trunk mass and trunk-driving point stiffness were estimated by fitting the experimental data to a second-order differential equation of the trunk dynamic behavior. The mean effective trunk mass was 14.1kg (s.d.=4.7). The trunk-driving point stiffness increased on average 36.8% (from 14.5 to 19.8N/mm) with an increase in the nominal steady-state preload effort from 20 to 40% (F(1,13)=204.96, p<0.001). There was a smaller, but significant variation in trunk stiffness with loading direction. The measured increase in trunk stiffness probably results from increased muscle stiffness with increased muscle activation at higher steady-state efforts.

Strategies used to stabilize the elbow joint challenged by inverted pendulum loading.

The stiffness of activated muscles may stabilize a loaded joint by preventing perturbations from causing large displacements and injuring the joint. Here the elbow muscle recruitment patterns were compared with the forearm loaded vertically (a potentially unstable inverted pendulum configuration) and with horizontal loading. Eighteen healthy subjects were studied with the forearm vertical and supinated and the elbow flexed approximately 90 degrees. In the first experiment EMG electrodes recorded activity of biceps, triceps, and brachioradialis muscles for joint torques produced (a) by voluntarily exerting a horizontal force isometrically (b) by voluntarily flexing and extending the elbow while the forearm was loaded vertically with 135N. The relationship between the EMG and the torque generated was quantified by the linear regression slope and zero-torque intercept. In a second experiment a vertical load increasing linearly with time up to 300N was applied. In experiment 1 the EMG-torque relationships for biceps and triceps had an intercept about 10% of maximum voluntary effort greater with the vertical compared to the horizontal force, the inverse was found for Brachioradialis, but the EMG-torque slopes for both agonist and antagonistic muscles were not different. In experiment 2 there were 29 trials with minimal elbow displacement and all the three muscles activated on the order of 11% of maximum activation to stabilize the elbow; 19 trials had small elbow extension and 14 trials small flexion requiring altered muscle forces for equilibrium; 7 trials ended in large unstable displacement or early termination of the test. An analysis indicate that the observed levels of muscle activation would only provide stability if the muscles' short-range stiffness was at the high end of the published range, hence the elbow was marginally stable. The stability analysis also indicated that the small elbow extension increased stability and flexion decreased stability.

The effects of abdominal muscle coactivation on lumbar spine stability.

STUDY DESIGN: A biomechanical model of the lumbar spine was used to calculate the effects of abdominal muscle coactivation on spinal stability. OBJECTIVES: To estimate the effects of abdominal muscle coactivation on lumbar spine stability, muscle fatigue rate, and lumbar spine compression forces. SUMMARY OF BACKGROUND DATA: The activation of human trunk muscles has been found to involve coactivation of antagonistic muscles, which has not been adequately predicted by biomechanical models. Antagonistic activation of abdominal muscles might produce flexion moments resulting from abdominal pressurization. Qualitatively, antagonistic activity also has been attributed to the need to stabilize the spine. METHODS: Spinal loads and spinal stability were calculated for maximum and submaximum (40%, 60% and 80%) efforts in extension and lateral bending using a previously published, anatomically realistic biomechanical model of the lumbar spine and its musculature. Three different antagonistic abdominal muscle coactivation patterns were imposed, and results were compared with those found in a model with no imposed coactivation. RESULTS: Results were quantified in terms of the sum of cubed muscle stresses (sigma sigma m3, which is related to the muscle fatigue rate), the maximum compressive loading on the lumbar spine, and the critical value of the muscle stiffness parameter (q) required for the spine to be stable. Forcing antagonistic coactivation increased stability, but at the cost of an increase in sigma sigma m3 and a small increase in maximum spinal compression. CONCLUSIONS: These analyses provide estimates of the effects of antagonistic abdominal muscle coactivation, indicating that its probable role is to stabilize the spine.

Incorporation of spinal flexibility measurements into finite element analysis.

This technical note demonstrates two methods of incorporating the experimental stiffness of spinal motion segments into a finite element analysis of the spine. The first method is to incorporate the experimental data directly as a stiffness matrix. The second method approximates the experimental data as a beam element.