Cross-correlations of center of mass and center of pressure displacements reveal multiple balance strategies in response to sinusoidal platform perturbations.

Compared to static balance, dynamic balance requires a more complex strategy that goes beyond keeping the center of mass (COM) within the base of support, as established by the range of foot center of pressure (COP) displacement. Instead, neuromechanics must accommodate changing support conditions and inertial effects. Therefore, because they represent body's position and changes in applied moments, relative COM and COP displacements may also reveal dynamic postural strategies. To investigate this concept, kinetics and kinematics were recorded during three 12 cm, 1.25 Hz, sagittal perturbations. Forty-one individual trials were classified according to averaged cross-correlation lag between COM and COP displacement (lag(COM:COP)) and relative head-to-ankle displacement (Δ(head)/Δ(ankle)) using a k-means analysis. This process revealed two dominant patterns, one for which the lag(COM:COP) was positive (Group 1 (n=6)) and another for which it was negative (Group 2 (n=5)) . Group 1 (G1) absorbed power from the platform over most of the cycle, except during transitions in platform direction. Conversely, Group 2 (G2) participants applied power to the platform to maintain a larger margin between COM and COP position and also had larger knee flexion and ankle dorsiflexion, resulting in a lower stance. By the third repetition, the only kinematic differences were a slightly larger G2 linear knee displacement (p=0.008) and an antiphasic relationship of pelvis (linear) and trunk (angular) displacements. Therefore, it is likely that the strategy differences were detected by including COP in the initial screening method, because it reflects the pattern of force application that is not detectable by tracking body movements.

Initial electro-mechanical response to rearward perturbation.

The objective of this study was to examine the combined electromyographic (EMG) and mechanical response to a rearward perturbation and to separate the response into three categories: preset properties of the muscle, reflex changes to the muscle, and active changes to the muscle. We hypothesized that an active response is required to maintain balance on a moving platform. Eleven healthy adult subjects stood on a platform oscillating at three frequencies (0.75, 1.0, and 1.25 Hz). Ankle extensor EMG activity and ankle moment were analyzed and compared for initial movement cycles. Timing of events in EMG and moment data were examined to separate observed changes into the three categories. Results showed an initial rise in ankle moment as the platform started to move backwards, followed by a more rapid reflex increase. After a slight drop, ankle moment again rose due to active response. By the third cycle of platform movement, the EMG and moment were synchronized with the platform movement, maintaining the body in a desired posture. Initial preset properties of the ankle extensor muscles combined with reflex activity were not sufficient to maintain balance. Following an initial reflex reaction, further active control was required to match the timing of the ankle moment and the platform motion and avoid a loss of balance. This study provides new insight for the rehabilitation of postural deficits.

Effect of joint stiffness on standing stability.

Standing balance depends on the effective control of the torques at the ankle, knee, and hip. Stiffness at each joint and feedback proportional to joint angle contributes to these torques and to postural stability. This study examines the interaction of multiple joints on the minimum effective joint stiffnesses needed to maintain quiet standing and determines the inherent patterns of sway motion based on dynamic calculations of a four-link, three-joint, sagittal plane model. The equations of motion for quiet standing are solved to obtain the limits of stability for an individual (75 kg, 1.753 m tall) considering different combinations of joint stiffness. These calculations demonstrate that the single-link inverted pendulum model provides a less conservative estimate of minimum stiffness. That is, more stiffness is required at each joint to preserve stability when rotation is permitted at the knee and hip joints. Based on these analyses, the well recognized ankle and hip balance strategies appear to correspond to variations of the inherent patterns of motion of the lowest frequency mode. Additional calculations show that the stability decreases with an increase in body mass index. The present results quantify the interaction of the combined active and passive stiffnesses at the ankle, knee, and hip, and identify the minimum conditions needed for quiet standing. These criteria define standing-balance stability thresholds needed to assess the risk of falling and to guide rehabilitation.

Preamputation pain and acute pain predict chronic pain after lower extremity amputation.

UNLABELLED: Although previous research suggests that preamputation pain is a risk factor for pain after amputation, little is known about the association between acute postsurgical pain and chronic amputation-related pain. The current prospective study examined the associations of preamputation pain and acute postamputation pain with chronic amputation-related pain. The sample consisted of patients with lower limb amputation (N = 57) who provided both preamputation and postamputation data during a 2-year study period. Preamputation pain intensity and duration were assessed before amputation; acute phantom limb pain (PLP) and residual limb pain (RLP) intensity were assessed on postsurgical days 4 and 5. Acute PLP intensity was the only significant independent predictor of chronic PLP intensity at 6 and 12 months after amputation, whereas preamputation pain intensity was the only significant predictor of chronic PLP intensity at 24 months. Similarly, acute RLP was found to be the best overall predictor of chronic RLP. Other variables (age, gender, level and etiology of amputation, amount of postsurgical pain medication, and duration of preamputation pain) were not associated with chronic pain. These results suggest that higher levels of pain either before or soon after amputation might help to identify individuals at greatest risk for chronic pain problems and most in need of early, intensive pain interventions. PERSPECTIVE: This study suggests that both preamputation pain and acute pain soon after amputation might be associated with bothersome chronic pain. The results support further research on acute pain mechanisms and the effectiveness of early interventions aimed at preventing or managing amputation-related pain.

Measurement and analyses of the effects of adjacent end plate curvatures on vertebral stresses.

BACKGROUND: Vertebral end plates of the lumbosacral spine have various degrees of concavity and convexity. It is believed that the shape of the end plates alters the distribution of loads transferred along the spine, between the vertebrae. Animal models have been regularly used in the design and development of vertebral disc implants and cages; to date, very little information is known about the animal vertebral end plate curvature. PURPOSE: The purpose was to measure and analyze the end plate curvature in the cadaver human male-female, chimpanzee, and canine lumbar vertebral bones. STUDY DESIGN/SETTING: Nondestructive and nontouching scanning method was designed to obtain curvature in anterior-posterior and medial-lateral directions in the cadaver bones. Statistical analysis was performed on the data collected, and this data was then used to create a biomechanical model to evaluate the load transmission. METHOD: Measurements in anterior-posterior and medial-lateral directions were performed on human, canine, and chimpanzee cadaver lumbar bones to obtain accurate data for the end plate curvatures. Six sets of measurements (on human male-female L4 lower to S1 upper end plates) were performed. A parametric vertebral motion segment model (with and without posterior elements) that includes the experimental curvature information was developed. The characteristic kidney-shaped cross-sectional model was created using a parametric equation. This model was used to perform finite element analyses investigating the effects of the location of maximum curvature on the stress distributions. RESULTS: The measurements for different species showed that the canine and chimpanzees, the quadrupeds, have entirely different curvature of their upper end plates compared with those in humans, the bipeds. Also, the curvatures of the human S1 upper end plates are significantly different from the rest of the vertebrae. This is a very useful piece of information in the comparison of these species. The stress distribution varied as the location of the maximum curvature shifted from the center to a more posterior position. The stresses in the vertebral core were found to decrease, with the shell taking more loads. CONCLUSIONS: This provides essential information for rehabilitation and surgical techniques, including designs for various interbody devices such as fusion cages, bone grafts, and disc prosthesis.

Pain management.

This article provides information regarding treatments for the management of moderate to severe pain in patients who are at the end of life. Discussion focuses on the use of strong opioids and adjuvant analgesics. Special attention also is given to the most frequently used forms of interventional pain management. Although pain in terminally ill patients is not always related to cancer, many of the studies cited in this article were performed in cancer patients, a model that informs much of what is presented.

Trial of amitriptyline for relief of pain in amputees: results of a randomized controlled study.

OBJECTIVE: To evaluate whether amitriptyline is more effective than placebo in improving phantom limb pain or residual limb pain. DESIGN: Randomized controlled trial of amitriptyline for 6 weeks. SETTING: University hospital. PARTICIPANTS: Thirty-nine persons with amputation-related pain lasting more than 6 months. INTERVENTION: Six-week trial of amitriptyline (titrated up to 125 mg/d) or an active placebo (benztropine mesylate). MAIN OUTCOME MEASURES: Analyses were conducted to examine whether there was a medication group effect on the primary outcomes (average pain intensity) and secondary outcome measures (disability, satisfaction with life, handicap). RESULTS: No significant differences were found between the treatment groups in outcome variables when controlling for initial pain scores. CONCLUSIONS: Our findings do not support the use of amitriptyline in the treatment of postamputation pain.

Stiffness between different directions of transpedicular screws and vertebra.

OBJECTIVE: This study was undertaken to evaluate the variation in bone density within the vertebral body and to determine the biomechanical stiffness of the screw-bone interface for different superior-inferior transpedicular screw orientations in the vertebral body. DESIGN: The stiffness of three directions of screw placement (upper, middle, and lower) were measured in two modes of loading (flexion and torsion). All screws were inserted to 80% of the distance along the path of the screw from the point of insertion to the anterior vertebral cortex. BACKGROUND: The placement of transpedicular screws within the vertebral body is a fundamental determinant the stiffness and strength of the bone-screw interface and consequently the stability of an implant construct. The bone stiffness within the pedicle and vertebra must be sufficient to resist spine forces and moments. METHODS: The stiffness of pedicle-screw fixation was tested for three different screw orientations based on the position of the screw tip in the vertebral body (upper, middle, and lower). Fixation rigidity was measured for two modes of loading (flexion and torsion). In all, eighteen individual vertebrae (L(2)-L(4)) from 6 cadaveric spine segments were examined. All the screws were inserted to 80% of the depth of the posterior to anterior vertebral cortex dimension. Quantitative bone density measurements were obtained from vertebral mid-sagittal cross sections of the additional vertebral bodies using an image analysis system to determine the distribution of bone density in the mid-sagittal plane. RESULTS: Based on area fraction measurements of cancellous bone in seventeen L1 mid-sagittal regions, highest densities were observed adjacent to the endplates (area fractions > 0.29). Regions of lowest density were found in the central portions of the vertebral bodies, above the mid-line of the body (area fractions < 0.20). In flexion, the stiffness of all three groups of screws were similar, ranging from 4.0-4.2 Nm deg(-1). In torsion the upper and middle directions were significantly more rigid than lower position (p < 0.04). CONCLUSION: Pedicle screw insertion aimed toward the superior-anterior aspect of the vertebral body, adjacent to the superior endplate provides the best overall rigidity considering both directions of loading. Screws placed in the mid-portion of the vertebral body may provide good lateral torsion rigidity, but would not be optimal for flexion stability if bone density is reduced in that region.