Li Qun Zhang

The Effect of Graft Height Mismatch on Contact Pressure Following Osteochondral Grafting A Biomechanical Study

Hypothesis Incongruity of the articular cartilage following osteochondral transplantation affects surface contact pressure. Study Design An 80 N load was applied for 120 seconds to the femoral condyles of 10 swine knees. Contact pressures were measured using Fuji prescale film. Seven conditions were tested: (1) intact articular surface; (2) 4.5-mm diameter defect; (3) grafted with 4.5-mm diameter plug elevated 1 mm above adjacent cartilage; (4) plug elevated 0.5 mm; (5) plug flush; (6) plug sunk 0.5 mm below surface; and (7) sunk 1.0 mm. Conclusions Peak contact pressures were significantly (P< .001) elevated by ~20% after defect creation and were reduced to normal when plugs were flush. There were large and significant (P< .001) increases in pressure with plugs elevated 1 and 0.5 mm. Contact pressures with plugs sunk 0.5 and 1 mm were significantly (P< .01) higher than intact cartilage but were significantly (P < .01) lower than an empty defect. Clinical Relevance Normal contact pressures and patterns can be duplicated with flush articular surface grafts. However, small incongruities, particularly when the plug is elevated, can lead to significantly increased pressure. This reinforces the importance of articular surface congruity in the initial biomechanical state following osteochondral implantation.

Two-part surgical neck fractures of the proximal part of the humerus. A biomechanical evaluation of two fixation techniques.

BACKGROUND Successful internal fixation of fractures of the surgical neck of the humerus can be difficult to achieve because of osteopenia of the proximal aspect of the humerus. The purpose of this study was to compare the biomechanical stability of a proximal humeral intramedullary nail and a locking plate for the treatment of a comminuted two-part fracture of the surgical neck in a human cadaver model. METHODS Twenty-four cadaveric humeri were instrumented with use of either a titanium proximal humeral nail (PHN) or a 3.5-mm locking compression plate for the proximal part of the humerus (LCP-PH). The specimens were matched by bone mineral density and were separated into four experimental groups with six humeri in each: PHN bending, LCP-PH bending, PHN torsion, or LCP-PH torsion. Comminuted fractures of the surgical neck were simulated by excising a 10-mm wedge of bone. Bending specimens were cyclically loaded from 0 to 7.5 Nm of varus bending moment at the fracture site. Torsion specimens were cyclically loaded to +/-2 Nm of axial torque. The mean and maximum displacement in bending, mean and maximum angular rotation in torsion, and stiffness of the bone-implant constructs were compared. RESULTS In bending, the LCP-PH group demonstrated significantly less mean displacement of the distal fragment than did the PHN group over 5000 cycles (p = 0.002). In torsion, the LCP-PH group demonstrated significantly less mean angular rotation than did the PHN group over 5000 cycles (p = 0.04). A significant number of specimens in the PHN group failed prior to reaching 5000 cycles (p = 0.04). The LCP-PH implant created a significantly stiffer bone-implant construct than did the PHN implant (p = 0.007). CONCLUSIONS The LCP proximal humeral plate demonstrated superior biomechanical characteristics compared with the proximal humeral nail when tested cyclically in both cantilevered varus bending and torsion. The rate of early failure of the proximal humeral nail could reflect the high moment transmitted to the locking proximal screw-bone interface in this implant. CLINICAL RELEVANCE The high failure rate in torsion of the proximal humeral nail-bone construct is concerning, and, with relatively osteoporotic bone and early motion, the results could be poor.

Simultaneous and nonlinear identification of mechanical and reflex properties of human elbow joint muscles

The naturally coexisting intrinsic mechanical and reflex properties of the human elbow joint were identified simultaneously using nonlinear, time-delay, continuous-time, and dynamic models. Angular random perturbations of small amplitude and low bandwidth were applied to the joint using a computer-controlled servomotor, while the subject maintained various levels of mean background muscle torque. Joint neuromuscular dynamics were identified from the measured elbow angle and torque. Stretch reflexes were modeled nonlinearly with both dynamic and static reflex gains. A continuous-time system identification method was developed to estimate parameters of the nonlinear models directly from sampled data while retaining realistic physical or physiological interpretations. Results from six subjects showed that dynamic stretch reflex gains, joint stiffness, and viscosity generally increased with mean background muscle torque; and that dynamic stretch reflex gain was higher during muscle stretch than that during muscle shortening. More importantly, the study provided realistic simultaneous estimates of the relative contributions of intrinsic mechanical and reflex actions to net joint torque. In particular, reflexively-mediated stiffness generated a significant portion of the total joint stiffness and the percentage varied systematically with background muscle torque.

Intelligent stretching of ankle joints with contracture/spasticity

An intelligent stretching device was developed to treat the spastic/contractured ankle of neurologically impaired patients. The device stretched the ankle safely throughout the range of motion (ROM) to extreme dorsiflexion and plantarflexion until a specified peak resistance torque was reached with the stretching velocity controlled based on the resistance torque. The ankle was held at the extreme position for a period of time to let stress relaxation occur before it was rotated back to the other extreme position. Stretching was slow at the joint extreme positions, making it possible to reach a larger ROM safely and it was fast in the middle ROM so the majority of the treatment was spent in stretching the problematic extreme ROM. Furthermore, the device evaluated treatment outcome quantitatively in multiple aspects, including active and passive ROM, joint stiffness and viscous damping and reflex excitability. The stretching resulted in considerable changes in joint passive ROM, stiffness, viscous damping and reflex gain. The intelligent control and yet simple design of the device suggest that with appropriate simplification, the device can be made portable and low cost, making it available to patients and therapists for frequent use in clinics/home and allowing more effective treatment and long-term improvement.

Vertically oriented femoral neck fractures: Mechanical analysis of four fixation techniques

Objective: Femoral neck fractures in young individuals are typically high angled shear fractures. These injuries are difficult to stabilize due to a strong varus displacement force across the hip with weight bearing. The purpose of this study was to compare the biomechanical stability of four differing fixation techniques for stabilizing vertical shear femoral neck fractures. Methods: Vertical femoral neck fracture stability was assessed using 4 surgical constructs in 32 cadaveric femurs: 7.3 mm cannulated screws placed in a triangular configuration (group 1), a 135-degree dynamic hip screw (group 2), a 95-degree dynamic condylar screw (group 3), and a locking proximal femoral plate (group 4). The 4 groups were matched for mean bone density and each specimen was tested under incremental loading, cyclical loading, and loading to failure. The modes of fixation failure were recorded for each specimen and the mean group stiffness, failure loads, and failure energies were calculated. Results: All 8 specimens failed during incremental loading in group 1. Five of 8 constructs failed with incremental loading, and 3 failed with cyclical testing in group 2. The combined 16 specimens in groups 3 and 4 survived both incremental and cyclical loading. The differences in stiffness, failure loads, and failure energies between the 4 groups were statistically significant (P < 0.001). The strongest construct was the locking plate and the weakest construct was the 7.3-mm cannulated screw configuration. The cannulated screw configuration group failed as the screws backed out of the femoral head and by varus collapse of the osteotomy; the fixed angled devices all failed at the bone-implant interface. Conclusions: The strongest construct for stabilizing a vertical shear femoral neck fracture is the proximal femoral locking plate, followed in descending order by the dynamic condylar screw, the dynamic hip screw, and the 3 cannulated screw configuration.

Changes in Passive Mechanical Properties of the Gastrocnemius Muscle at the Muscle Fascicle and Joint Levels in Stroke Survivors

OBJECTIVES To investigate the ankle joint-level and muscle fascicle-level changes and their correlations in stroke survivors with spasticity, contracture, and/or muscle weakness at the ankle. DESIGN To investigate the fascicular changes of the medial gastrocnemius muscle using ultrasonography and the biomechanical changes at the ankle joint across 0 degrees, 30 degrees, 60 degrees, and 90 degrees knee flexion in a case-control manner. SETTING Research laboratory in a rehabilitation hospital. PARTICIPANTS Stroke survivors (n=10) with ankle spasticity/contracture and healthy control subjects (n=10). INTERVENTIONS Not applicable. MAIN OUTCOME MEASUREMENTS At the muscle fascicle level, medial gastrocnemius muscle architecture including the fascicular length, pennation angle, and thickness were evaluated in vivo with the knee and ankle flexion changed systematically. At the joint level, the ankle range of motion (ROM) and stiffness were determined across the range of 0 degrees to 90 degrees knee flexion. RESULTS At comparable joint positions, stroke survivors showed reduced muscle fascicle length, especially in ankle dorsiflexion (P< or =.048) and smaller pennation angle, especially for more extended knee positions (P< or =.049) than those of healthy control subjects. At comparable passive gastrocnemius force, stroke survivors showed higher fascicular stiffness (P< or =.044) and shorter fascicle length (P< or =.025) than controls. The fascicle-level changes of decreased muscle fascicle length and pennation angle and increased medial gastrocnemius fascicle stiffness in stroke were correlated with the joint level changes of increased joint stiffness and decreased ROM (P<.05). CONCLUSIONS This study evaluated specific muscle fascicular changes as mechanisms underlying spasticity, contracture, and joint-level impairments, which may help improve stroke rehabilitation and outcome evaluation.

In vivo human knee joint dynamic properties as functions of muscle contraction and joint position

Information on the dynamic properties (joint stiffness, viscosity and limb inertia) of the human knee joint is scarce in the literature, especially for actively contracting knee musculature. A joint driving device was developed to apply small-amplitude random perturbations to the human knee at several flexion angles with the subject maintaining various levels of muscle contraction. It was found that joint stiffness and viscosity increased with muscle contraction substantially, while limb inertia was constant. Stiffness produced by the quadriceps was highest at 30 degrees flexion and decreased with increasing or decreasing flexion angle, while knee flexors produced highest stiffness at 90 degree flexion. When knee flexion was < 60 degrees, stiffness produced by the quadriceps was higher than that of the hamstrings and gastrocnemius at the same level of background muscle torque, while knee flexor muscles produced higher stiffnesses than the quadriceps at 90 degree flexion. Similar but less obvious trends were observed for joint viscosity. Passive joint stiffness at full knee extension was significantly higher than in more flexed positions. Surprisingly, as the knee joint musculature changed from relaxed to contracting at 50% MVC, system damping ratio remained at about 0.2. This outcome potentially simplifies neuromuscular control of the knee joint. In contrast, the natural undamped frequency increased more than twofold, potentially making the knee joint respond more quickly to the central nervous system commands. The approach described here provides us with a potentially valuable tool to quantify in vivo dynamic properties of normal and pathological human knee joints.

Sitting with adjustable ischial and back supports: biomechanical changes.

Study Design. The seat and back contact force, pressure distribution, lumbar lordosis, and low back muscle activities associated with a new seat design with adjustable ischial support and backrest were investigated using kinematic, kinetic, electromyographic, and radiographic measurements. Objectives. To investigate the biomechanical effects of adjusting ischial and backrest supports during sitting. Summary of the Background Data. Sitting may induce posterior rotation of the pelvis, reduction of lumbar lordosis, and increases in muscle tension, disc pressure, and pressure on the ischium and coccyx, which may be associated with low back pain. A device that reduces the ischial load and maintains lumbar lordosis may help increase seating comfort and reduce low back pain. Methods. Fifteen office workers with no known low back pain history were tested. Contact pressure distributions, reaction forces between the buttock-thighs and seat and between the back and backrest, load carried by the seat pan and backrest, sacral inclination, lumbar lordosis, intervertebral space of lumbar spine, and muscular activity in stabilizing the trunk were measured for sitting with and without ischial support and with adjustable back support. Results. When the ischial support was relieved, the center of the force on the seat and on the legs of the chair, and the peak center of pressure on the seat, were significantly (P < 0.002) shifted forward toward the thighs. The total contact area on the seat pan and on the backrest was significantly decreased and increased, respectively (P < 0.001). The sacral inclination, total and segmental lumbar lordosis, and lumbar spine disc height were significantly increased for sitting upright with backrest, with the lumbar curve close to that during standing. Conclusions. Sitting with reduced ischial support and fitted backrest to the lower spine altered the contact area, reduced peak pressure under the ischia, reduced muscular activity, maintained total and segmental lumbar lordosis, rotated the sacrum forward, and increased lumbar intervertebral disc heights, which could potentially reduce low back pain.

In vivo noninvasive evaluation of abnormal patellar tracking during squatting in patients with patellofemoral pain

BACKGROUND Patellofemoral pain syndrome is one of the most common knee problems and may be related to abnormal patellar tracking. Our purpose was to compare, in vivo and noninvasively, the patellar tracking patterns in symptomatic patients with patellofemoral pain and those in healthy subjects during squatting. We tested the hypothesis that patients with patellofemoral pain exhibit characteristic patterns of patellar tracking that are different from those of healthy subjects. METHODS Three-dimensional patellar kinematics were recorded in vivo with use of a custom-molded patellar clamp and an optoelectronic motion capture system in ten healthy subjects and nine subjects with patellofemoral pain. The position of osseous knee landmarks was digitized while subjects stood upright, and then patellofemoral kinematics were recorded during squatting. The tracking technique was validated with use of both in vitro and in vivo methodologies, and the average absolute error was <1.2 degrees and <1.1 mm. RESULTS At 90 degrees of knee flexion, the patella showed lateral spin (the distal pole of the patella rotated laterally) in subjects with patellofemoral pain (mean and standard deviation, -10.13 degrees +/- 2.24 degrees) and medial spin in healthy subjects (mean, 4.71 degrees +/- 1.17 degrees) (p < 0.001). At 90 degrees of knee flexion, the patella demonstrated significantly more lateral translation in subjects with patellofemoral pain (mean, 5.05 +/- 3.73 mm) than in healthy subjects (mean, -4.93 +/- 3.93 mm) (p < 0.001). CONCLUSIONS Kinematic differences between healthy subjects and subjects with patellofemoral pain were demonstrated through a large, dynamic range of knee flexion angles. Increased lateral patellar translation and lateral patellar spin in subjects with patellofemoral pain suggest that the patella is not adequately balanced during functional activities in this group. Prospective studies are needed to identify when patellofemoral pain-related changes begin to occur and to determine the risk for the development of patellofemoral pain in individuals with abnormal kinematics.

Comparison of lateral locking plate and antiglide plate for fixation of distal fibular fractures in osteoporotic bone: a biomechanical study.

Objectives: The purpose of this study was to compare the biomechanical properties of posterolateral antiglide plating and lateral locked plating for fixation of displaced short oblique fractures of the fibula in osteoporotic bone. Methods: Short oblique fractures of the distal fibula at the level of the syndesmosis were simulated with a fibular osteotomy and ligamentous sectioning in 18 paired fresh frozen ankles. The fractures were fixed with either a lateral locking plate with an independent lag screw or a posterolateral antiglide plate with a lag screw through the plate. The specimens were tested under a torsional load to failure. The torque to failure, angular rotation at failure, and construct stiffness of the two groups were compared. Results: The torque to failure and construct stiffness were significantly greater on the side with the posterolateral antiglide plate than on the side with the the lateral locking plate (P = 0.01 and 0.005, respectively). Conclusions: The posterolateral antiglide plate demonstrated improved biomechanical stability as compared to the lateral locking plate in osteoporotic bone. In situations where fixation needs to be optimized, use of an antiglide plate may be favored over a lateral locking plate construct.