Does the asymmetry multiplier in the 1991 NIOSH lifting equation adequately control the biomechanical loading of the spine?

The aim of this research was to evaluate whether the asymmetry multiplier incorporated in the 1991 National Institute for Occupational Safety and Health lifting equation adequately controls the biomechanical spine loads during asymmetric lifting. Sixteen male subjects lifted a box from four initial locations varying in terms of the angular deviation from the mid-sagittal plane (0, 30, 60 and 90 degrees). From each location, boxes that weighed the recommended weight limit (RWL) and three times the RWL were lifted at two qualitatively defined lifting speeds. Ground reaction forces were combined with kinematic data in a linked-segment model to quantify the 3-D moments at the base of the spine (L5/S1) and the spine compression forces. The results show that the twisting and lateral bending moments increased with task asymmetry despite the lessening of the RWL (p<0.01). The flexion moment and the spine compression decreased with asymmetry, although at a slower rate than the RWL. When the dynamics were removed from the linked segment spine model to approximate the assumption of slow and smooth lifting, the estimated compression remained approximately 3400 N across all asymmetry conditions. Thus, the reduction in the RWL due to asymmetry multiplier appears appropriate and should not be changed, as been suggested by recent psychophysical studies.

Relationship between disc injury and manual lifting: a poroelastic finite element model study.

Understanding how failure originates in a lumbar motion segment subjected to loading conditions that are representative of manual lifting is important because it will pave the way for a better formulation of the exposure-injury relationship. The aim of the current investigation was to use a poroelastic finite element model of a human lumbar disc to determine its biomechanical characteristics under loading conditions that corresponded to three different, commonly occurring lifting activities and to identify the most hazardous type of loading with regard to damage to the disc. The current study showed that asymmetric lifting may increase the risk of back injury and pain. Lifting that involved lateral bending (asymmetric lifting) of the trunk was found to produce stresses at a localized area in the annulus, annuluar fibres, end plates, and facet joints that were higher than their respective tissue failure strength. Thus asymmetric lifting, if performed over a large number of cycles, might help to propagate this localized failure of the disc tissue to a larger area, owing to fatigue. The analyses also showed that largest fluid exchange between the nucleus and the end plates occurred during asymmetric lifting. If the fluid exchange is restricted owing to end plate calcification or sclerosis of the subchondral bone, high intradiscal pressure might develop, leading to higher disc bulge causing back pain.

Effect of annular incision type on the change in biomechanical properties in a herniated lumbar intervertebral disc.

The technique used to incise the disc during discectomy may play a role in the subsequent healing and change in biomechanical stiffness of the disc. Several techniques of lumbar disc annulotomy have been described in clinical reports. The purpose of this paper was to study the influence of annulotomy technique on motion segment stiffness using a finite element model. Four incision methods (square, circular, cross, and slit) were compared. The analyses showed that each of the annular incisions produced increase in motions under axial moment loadings with circular incision producing the largest change in the corresponding rotational motion. Under shear loading mode, cross and slit-type annular incisions produced slightly larger changes in the principal motions of the disc than square and circular incisions. All other incision types considered in the current study produced negligibly small increase in motion under rest of the loading conditions. In addition to annulotomy, when nucleotomy was also included in the analyses, once again cross and slit incisions produced larger change in motion under shear loading mode as compared to the other two incision types. A comparison between the four types of annular incisions showed that cross incision produced an increase in motion larger than those produced by the other three incisions under flexion/extension and lateral moment loading and both shear force loadings. Circular incision produced the largest increase in motion under axial moment load in comparison to those produced by square, cross, and slit incisions. Sagittal plane symmetry was influenced by the incision injury to the motion segment leading to coupled motions as well as increased facet loads. From the study it can be concluded that the increase inflexibility of the disc due to annulotomy depends on the type of annulotomy and the annulotomy also produce asymmetrical deformations leading to increased facet loading.

Comparison of biomechanical response to surgical procedures used for cervical radiculopathy: posterior keyhole foraminotomy versus anterior foraminotomy and discectomy versus anterior discectomy with fusion.

The objective of this study was to compare the change in flexibility of C5-C6 caused by three procedures using a three-dimensional nonlinear finite element model: posterior foraminotomy (keyhole procedure), anterior foraminotomy with discectomy, and anterior discectomy with fusion. The keyhole procedure produced a minor increase in motion. The anterior foraminotomy and discectomy produced one to two times greater motion. Anterior discectomy with fusion produced 50% to 100% reduction in motion. The posterior keyhole foraminotomy has a much lesser effect on the stability of the cervical spine segment than does an anterior procedure, and fusion is a requisite part of the anterior decompression procedure.

Sensitivity of periprosthetic stress-shielding to load and the bone density-modulus relationship in subject-specific finite element models.

Subject-specific finite element (FE) computer models of the proximal femur in hip replacement could potentially predict stress-shielding and subsequent bone loss in individual patients. Before such predictions can be made, it is important first to determine if between subject differences in stress-shielding are sensitive to poorly defined parameters such as the load and the bone material properties. In this study we investigate if subject-specific FE models provide consistent stress-shielding patterns in the bone, independent of the choice of the loading conditions and the bone density-modulus relationship used in the computer model. FE models of two right canine femurs with and without implants were constructed based on contiguous computed tomography (CT) scans so that subject-specific estimates of stress-shielding could be calculated. Four different loading conditions and two bone density-modulus relationships were tested. Stress-shielding was defined as the decrease of strain energy per gram bone mass in the femur with the implant in place relative to the intact femur. The analyses showed that for the four loading conditions and two bone density-modulus relationships the difference in stress-shielding between the two subjects was essentially constant (1% variation) when the same loading condition and density-modulus relationship was used for both subjects. The severity of stress-shielding within a subject was sensitive to these input parameters, varying up to 20% in specific regions with a change in loading conditions and up to 10% for a change in the assumed density-modulus relationship. We conclude that although the choice of input parameters can substantially affect stress-shielding in an individual, this choice had virtually no effect on the relative differences in femoral periprosthetic stress-shielding between individuals. Thus, while care should be taken in the interpretation of the absolute value of stress-shielding calculated with these type of models, subject-specific FE models may be useful for explaining the variation in bone adaptation responsiveness between different subjects in experimental or clinical studies.

Anterior cervical fusion: a finite element model study on motion segment stability including the effect of osteoporosis.

STUDY DESIGN: Three-dimensional, nonlinear finite element models were used to evaluate the stability of the mid cervical spine after anterior fusion in patients with and without osteoporosis. OBJECTIVES: The objective of this study was to compare the change in flexibility of C5-C6 after anterior discectomy with both loose-fitting and tight-fitting fusion graft. SUMMARY OF BACKGROUND DATA: Many factors such as surgical technique, osteoporosis, and excess neck motion during the postoperative period may contribute to fusion failure. Knowledge about changes in biomechanical properties after the surgical procedure is important for selection of grafts with appropriate strength and for guiding patients in postoperative care and rehabilitation. METHODS: Analyses with anterior fusion models with both loose-fitting and tight-fitting graft were performed in a normal and an osteoporotic spine. The motion of the C5 vertebra in relation to the C6 vertebra were calculated, after multidirectional moment loads of 0.5 Nm combined with a compressive preload of 105 N. RESULTS: Loose-fitting graft produced both an increase and a decrease in motion under various external moment loads, with graft compressive stress below the compressive strength of the graft material. A reduction in motion was observed under all moment loads when a tight-fitting graft was used. The compressive stress in the tight-fitting graft was higher than the strength of the graft material. Osteoporosis increased the principal motions with both the loose-fitting graft and tight-fitting graft. Maximum increase in motion with a loose-fitting graft construct was observed under extension and axial torsion moment loads. CONCLUSIONS: Anterior discectomy and insertion of the loose graft resulted in increased motion. A tight-fitting graft is beneficial in reducing motion, but the stress within the graftincreases beyond the graft strength. The presence of osteoporosis was nominally significant when the graft was tight fitting.

The influence of lumbar disc height and cross-sectional area on the mechanical response of the disc to physiologic loading.

STUDY DESIGN: The influence of lumbar disc height and cross-sectional area on the mechanical response of the disc to physiologic loading was determined using a finite element model. OBJECTIVES: To identify which geometric characteristics are potentially related to motion segment mechanical response to applied load, such as flexibility, fiber stress, disc bulge, and nucleus pressure. SUMMARY OF BACKGROUND DATA: The height and area of the lumbar disc varies within the disc itself, between disc levels, between people, between men and women, with aging, and during the day. Mechanical theory dictates that the height and area influence the mechanical response of the disc to loading. This could have important consequences in risk of injury. METHODS: Three-dimensional finite-element models representing three disc heights (5.5 mm, 8.5 mm, and 10.5 mm) and three disc areas (1060 mm2, 1512 mm2, and 1885 mm2) were generated. The effect of disc geometry on the mechanical properties of the disc were studied for four moment loads (magnitude, 7.5 Nm) with compressive preload (400 N) and for three different direct forces. Commercially available finite-element software was used. RESULTS: Discs with a ratio of small disc area to disc height were more prone to larger motion, higher anular fiber stresses, and larger disc bulge. When the disc height alone was increased by a factor, its flexibility also increased, either by the same amount or by a much larger ratio. CONCLUSIONS: Discs with the most height and smallest area are exposed to much higher risk of failure than other combinations of disc height and geometry.

Study on effect of graded facetectomy on change in lumbar motion segment torsional flexibility using three-dimensional continuum contact representation for facet joints.

Facet joints provide rigidity to the lumbar motion segment and thus protect the disk, particularly against torsional injury. A surgical procedure that fully or partially removes the facet joints (facetectomy) will decrease the mechanical stiffness of the motion segment, and potentially place the disk at risk of injury. Analytical models can be used to understand the effect of facet joints on motion segment stability. Using a facet joint model that represents the contact area as contact between two surfaces rather than as point contact, it was concluded that a substantial sudden change in rotational motion, due to applied torsion moment, was observed after 75 percent of any one of the facet joints was removed. Applied torsional moment loading produced coupled extension motion in the intact motion segment. This coupled motion also experienced a large change following complete unilateral facetectomy. Clinically, the present study showed that surgical intervention in the form of unilateral or bilateral total facetectomy might require fusion to reduce the primary torsion motion.

The effects of lifting speed on the peak external forward bending, lateral bending, and twisting spine moments.

Lifting tasks that involve twisting have been repeatedly implicated as contributing to the onset of occupational back injuries in epidemiological studies. The objective of this work was to quantify the three directional external moments acting on the spine during a sagittally symmetric and two asymmetric lifting tasks. A total of 15 subjects participated in the three lifting tasks. All tasks were performed at two qualitatively defined lifting speeds, 'slow' and 'fast', and with two load magnitudes: 10 and 20% of the subject's body weight. The mid-sagittal plane lifts were performed using two horizontal reach distances: 40 and 60 cm. A four-camera, two-forceplate motion and force measurement system were used to obtain the kinematic and kinetic data as the lifts were performed. A dynamic link-segment biomechanical model was used to quantify the reaction forces and moments at the ankle, knee, and hip and L5/S1 joints. Results from all tasks showed increased sagittal plane (forward bending) spine moments with the heavier load and at the faster lifting speed (p < 0.001). Spine lateral bending and twisting moments increased during the mid-sagittal plane lifts with the greater reach distance and the faster lifting speed, respectively. The twisting moments on the spine were greatest as subjects lifted from in front and placed the load to the side but were dependent upon the lifting speed and the load magnitude. The lateral bending moments increased during this same task with the heavier load. However, the spine lateral bending moments were greatest when lifting from one side to the other.

A striated pattern of wear in ultrahigh-molecular-weight polyethylene components of Miller-Galante total knee arthroplasty.

Wear of the polyethylene tibial components is a potential cause of failure in total knee arthroplasty. In addition to pitting, burnishing, and scratching, subtle striations on the bearing portion of the tibial surface have been observed in components retrieved relatively early after implantation. The striated pattern most typically occurred in areas centrally located within the articulating surface. The striations were anteroposterior directed and were identified as local cold flow at the surface. There was a strong correlation between the medial and lateral striated areas, suggesting that these patterns are related to cyclic rolling of the knee. The general characteristics and alignment of the striations could be attributed to the compressive and tractive forces occurring during femoral rollback. For the clinician, these results suggest that kinematics, as well as contact stress, should be considered when evaluating wear of polyethylene components.

Initial in vitro stability of the tibial component in a canine model of cementless total knee replacement.

The tibial component of a canine cementless total knee replacement model was used to determine the degree to which pegs and screws contributed to the initial in vitro stability of the device. Three implant designs were investigated: (1) a four-peg implant in which cortical bone screws passed through the pegs, (2) the four-peg implant without adjuvant screw fixation, and (3) a flat implant with screws placed in the same positions as in the first design. For measuring the interface motion, the tibial component and proximal tibia were modeled as rigid bodies and an experimental method was developed which permitted all six degrees of freedom of the motion between these two objects to be determined. In tests performed to validate this methodological approach, the potential confounding influences of tibial deformation and differential amounts of tibial deformation with the use of screws or pegs were shown to be minimal, supporting the use of the rigid-body method. In general, the areas of greatest motion were at the periphery of the bone-implant interface, regardless of whether or not screws or pegs were used. The components secured with screws had up to five-fold reductions in interface motion compared to components which had pegs but lacked screw fixation. Components with pegs and screws and components with screws only had the same amount of interface motion. Thus, in the presence of screw fixation, the addition of pegs did not increase the stability of the tibial component.

Chondrocyte cells respond mechanically to compressive loads.

Many studies have illustrated the effect of mechanical loading on articular cartilage and the corresponding changes in chondrocyte metabolism, yet the mechanism through which the cells respond to loading still is unclear. The purpose of this study was to evaluate the change in shape of chondrocytes under a statically applied uniaxial compressive load. Isolated chondrocytes from rat chondrosarcoma were embedded in 2% agarose gel. Strains of 5, 10, and 15% were applied, and images of the cell were recorded from initial loading to equilibrium (15 minutes). A finite-element model was used to model the experimental setup and to estimate the mechanical properties of the chondrocyte at equilibrium. The transient behavior of the composite in the experiment was analyzed with use of a standard linear viscoelastic model. We found that all cells decreased in cross-sectional area under each of the applied compressive strains. In the finite-element model, the elasticity of the chondrocyte was similar to that of the surrounding agarose gel (4.0 kPa) and had a Poisson's ratio of 0.4. Viscoelastic analysis showed that the chondrocytes contributed a significant viscoelastic component to the behavior of the composite in comparison with the agarose gel alone. If a decrease in cell volume proportional to the decrease in cross-sectional area is assumed, the decrease observed was greater than would be predicted by a passive cellular response due to an equivalent osmotic pressure. This indicates that the chondrocyte may be altering its intracellular composition by cellular processes in response to mechanical loading.

Wear patterns on retrieved polyethylene tibial inserts and their relationship to technical considerations during total knee arthroplasty.

Fifty-five unconstrained polyethylene tibial inserts were retrieved at revision total knee arthroplasty and examined for evidence of wear after a mean implantation time of 34.2 months (2.5-80 months). Twenty inserts were ultra-high molecular weight polyethylene (UHMWPE) and 35 were carbon-reinforced polyethylene. Topographic maps of the articular and metal-backed surfaces of each component were constructed to characterize the extent and location of polyethylene degradation, identified visually by mode. In 32 of the retrieved inserts, pre- and postarthroplasty or prerevision radiographs were analyzed for component positioning, sizing, and extremity alignment. These factors then were compared with the patterns and severity of polyethylene wear on the inserts to establish correlations. Severe generalized articular wear was seen in inserts with third body wear from patellar metal-backed failure and cement debris. Severe localized delamination wear was seen in inserts with rotational-subluxation patterns of wear (p = 0.05). The external rotation subluxation wear pattern was strongly associated with knees that had lateral subluxation of the patella (p = 0.0002). Articular wear and cold flow into screw holes tended to be greater in the tightest prearthroplasty compartment (medial in the varus knee [p = 0.0157]; lateral in the valgus knees [p = 0.0226]). Fourteen of 16 knees with a preoperative varus deformities--even when corrected to a normal postarthroplasty anatomic axis--still had greater medial compartment articular wear (p = 0.001). Twelve of these knees did not have a medial release at the time of initial arthroplasty. Preoperative varus also was found to be related to the occurrence of posteromedial cold flow of polyethylene into tibial tray screw holes (p = 0.007). Increasing tibial insert posterior slope was associated with increasingly posterior articular wear track location (p = 0.03). This study indicates that unconstrained tibial component wear patterns and severity may be associated with clinical and mechanical factors under the surgeon's control, including component size and position, and knee alignment and ligament balance.

A model to study the disc degeneration process.

Initiation and propagation of different types of events that lead to disc degeneration and the effect of the degenerating process on the disc mechanical performance is difficult to study by experimental methods. This study aimed to develop and use a finite element model of a motion segment without posterior elements to study the disc degeneration process. The model was used to study the development of anular tears, nuclear clefts, and endplate fractures and subsequent propagation of these degenerative processes due to compressive and bending loads. The analyses showed that the failure always started at the end plates indicating that they are the weak link in the body-disc-body unit. The analyses also showed that anular injuries are unlikely to be produced by pure compressive loads. The model predicted that it would require a larger extension moment as compared to flexion moment to initiate and propagate failure in a motion segment, which leads to the conclusion that the motion segment is stiffer in extension. The model also suggested that the presence of discrete peripheral tears in the anulus fibrosus may have a role in the formation of concentric anular tears and in accelerating the degenerating process of the disc.