Internal Spinal Fixation Devices Research Articles

Loads on a Telemeterized Vertebral Body Replacement Measured in Two Patients

The loads acting on a vertebral body replacement (VBR) were measured in vivo. To measure the implant loads for different activities within the first 6 months after surgery. Mathematical models exist for predicting spinal loads for various activities. The intradiscal pressure has been measured in vivo for many activities. Loads on internal spinal fixation devices have been measured in 10 patients. However, only little information exists regarding the loads acting on a VBR. Telemeterized VBRs were implanted into 2 patients with a fractured L1 vertebral body. The implant allows the in vivo measurement of 3 force and 3 moment components acting on the implant. For several activities, implant loads were measured in the first 6 months after surgery. One month after surgery, the resultant force during standing was about 270 N in 1 patient and 300 N in the other. When the patients were lying in relaxed positions, resultant forces were less than 30% of the values during standing. In one patient, implant loads were slightly lower during sitting than during standing whereas in the other patient higher loads were measured during sitting. In both patients, flexion of the upper body and walking upstairs caused implant loads, which were more than twice as high as those during standing. Force direction varied only slightly for forces higher than 100 N. High forces may act on a VBR especially in the first postoperative month. Flexion of the upper body and going upstairs cause high implant loads and should be avoided in the first few months after stabilizing the spine.

In-vitro measurement of loading using an instrumented vertebral internal fixator

Only little is known about the loads acting on internal spinal fixation devices. The forces and moments for different external loads were now measured using instrumented implants. Measurements were performed on intact cadaver spines and after a corpectomy. Additionally the influence of the mounting accuracy on the distribution of the loads in the fixators was investigated. It could be shown that the implants are not loaded symmetrically. After a corpectomy the loads on the implants are significantly higher than for an intact spine.

Loads on a telemeterized vertebral body replacement measured in three patients within the first postoperative month

Background Little is known about the loads acting on a vertebral body replacement within the first month postoperatively. Among other things, such data are required for choosing the optimal rehabilitation program for a patient and for evaluating the efficacy of aids like crutches and braces. Methods Telemeterized vertebral body replacements were implanted into three patients, replacing parts of their fractured vertebral body L1. Six load components were measured for several exercises in upright and lying positions within the first postoperative month. The fractured vertebra was additionally stabilized with an internal spinal fixation device. Findings The highest force component acted in the direction defined by a line connecting the centres of the adjacent vertebral bodies. The resultant force was always less than 100 N in a lying position. Standing and sitting caused forces between about 150 and 450 N. Support with the arms or hands reduced the force considerably. Flexion of the upper body caused resultant forces higher than 420 N in all three patients. Elevation of both arms in the sagittal plane to about 90° also led to high resultant forces, especially when carrying an additional weight in the hands or when working against the resistance of a physiotherapist. In the latter case, forces higher than 700 N were measured. The force direction was close to the axial direction of the treated spinal segment in upright body positions and varied only slightly for most exercises. The highest resultant bending and torsional moments measured were less than 4 Nm. For most exercises the resultant moment was below 2 Nm. Interpretation High loads act on a vertebral body replacement during several exercises already in the first month postoperatively. These activities should be avoided, especially in patients with osteoporotic vertebrae in order to prevent implant subsidence. Using e.g. a wheeled invalid walker or supporting the upper body by hands when sitting was found to reduce implant loads.

Effect of bone graft characteristics on the mechanical behavior of the lumbar spine

There is little information about the influence of bone graft size, position and elasticity on the mechanical behavior of the lumbar spine. Intersegmental motion, intradiscal pressure and stresses in the lumbar spine were calculated using a three-dimensional, nonlinear finite element model which included an internal spinal fixation device and a bone graft. Cross-sectional area, position, and elastic modulus of the graft were varied in this study. Bone grafts, especially very stiff ones, increase stresses on adjacent endplates. Though larger grafts lead to less contact pressure, it is difficult to judge the quality of different bone graft positions. In general, ventral flexion results in lower maximum contact pressure than lateral bending. There is always little intersegmental rotation in the bridged region compared with that of an intact spine. A larger graft with low stiffness should be favored from a mechanical point of view. Patients should avoid lateral bending of the upper body shortly after surgery.

Loads on an internal spinal fixation device during physical therapy.

Modified internal spinal fixation devices allow the measurement of the forces and moments acting on the implants. The aim of this study was to measure the loads on internal fixation devices for selected body positions and movements during physical therapy. Loads on an internal spinal fixation device were measured in 10 patients with degenerative instability or compression fractures using a telemeterized implant. Relatively low implant loads were found in the recumbent body positions. Most exercises performed in a lying position caused implant loads less than that measured for standing and are therefore not likely to increase the risk of screw breakage. Fixation device loads were lower for sitting relaxed than for standing. The highest implant loads (128% of the value for standing) were measured during walking. Standing up, sitting down, and lateral bending and axial rotation of the upper body while standing led to fixation device loads between 111% and 120% related to the value for standing. Even higher fixation device loads were measured for ventral flexion and extension of the upper body while standing. Kneeling on hands and knees, and flexing and extending the back in this position, caused implant loads that were lower than for standing. Standing up, sitting down, and lateral bending and axial rotation of the upper body while standing may slightly increase the risk of pedicle screw breakage, whereas ventral flexion and extension of the upper body while standing may increase this risk considerably if the region bridged by the implant is distracted (the distance between upper and lower screws was increased) during surgery. However, walking is the exercise that plays the major role concerning pedicle screw breakage because it causes the highest bending moments of all exercises studied and it loads the fixation devices most frequently.

Effect of a double intervertebral cage on the mechanical behavior of the lumbar spine

A three-dimensional nonlinear finite element model of the lumbar spine was developed. Paired threaded cages and a monosegmental internal spinal fixation device were integrated into the computer model. The model was loaded with such forces as apply during standing, as well as with pure moments in the three main anatomical planes, plus an additional preload. The latter was generated by shortening the distance between the pedicle screws on the longitudinal rod of the fixator. With the exception of torsional loading, an implant appreciably reduces the mobility in the segment concerned. At the loads studied, cages had only a minor impact on the movements and stresses in the adjacent regions, but a strong influence on the stresses in the endplates in contact with them. A preload increases these stresses dramatically. Contact conditions between vertebral body and cages also have a marked effect on the stress distribution in the corresponding vertebral endplate, especially in the case of extension loading. Owing to the preload, maximum stresses were higher for the rigid bond than when contact elements were used.

Loads on an internal spinal fixation device during sitting

Sitting is often assumed to involve high loads on the spine as well as on implants for stabilising the spine. Loads on internal spinal fixation devices were therefore measured in ten patients sitting on several types of seats, including a stool, a stool with a padded wedge, a chair, a physiotherapy ball, a knee-stool, and a bench. The patients also successively sat relaxed and erect on a stool. In addition, six of them sat on a special chair allowing different inclinations of the backrest. Implant loads were also measured for standing up and sitting down. There were only minor differences in fixator loads for sitting on the different types of seats. Sitting erect caused an average of 11% higher implant loads than sitting relaxed. Implant loads decreased with increasing inclination of the upper body while sitting on a chair with an adjustable backrest. Implant loads were about 27% higher for standing up and sitting down than for sitting.

Comparison of intradiscal pressures and spinal fixator loads for different body positions and exercises

Loading of the spine is still not well understood. The most reliable results seemed to come from the intradiscal pressure measurements from studies by Nachemson, 1966. A new similar study by Wilke et al. (1999) complemented the present study and confirmed some of the earlier data, although it contradicted others. The new data did not confirm that the load on the spine is higher in sitting compared with standing and did not find distinct differences between positions in which subjects were lying down. The objective of this paper was to compare results from two independent in vivo studies (applying different methods) to provide information about spinal loading. In one of these studies (Wilke 1999), intradiscal pressure was measured in one volunteer in different postures and exercises, and in the other study (Rohlmann et al. 1994) the loads on an internal spinal fixation device (an implant for stabilising unstable spines) were determined in 10 patients. The absolute values of the results from both studies were normalized and compared for many body positions and dynamic exercises. The relative differences in intradiscal pressure and flexion bending moments in the fixators corresponded in most cases. Both studies showed slightly lower loads for sitting than for standing and comparatively low loads in all lying positions. High loads were measured for jogging, jumping on a trampoline and skipping. Differences between trends for intradiscal pressure and for flexion bending moments in the fixators were found when the load was predominantly carried by the anterior spinal column, as during flexion of the upper part of the body or when lifting and carrying weights. The combination of the results from these two methods may improve the understanding of the biomechanical behaviour of the lumbar spine and may be used to validate models and theories of spinal loading.

Comparison of intradiscal pressures and spinal fixator loads for different body positions and exercises

Loading of the spine is still not well understood. The most reliable results seemed to come from the intradiscal pressure measurements from studies by Nachemson, 1966. A new similar study by Wilke et al. (1999) complemented the present study and confirmed some of the earlier data, although it contradicted others. The new data did not confirm that the load on the spine is higher in sitting compared with standing and did not find distinct differences between positions in which subjects were lying down. The objective of this paper was to compare results from two independent in vivo studies (applying different methods) to provide information about spinal loading. In one of these studies (Wilke 1999), intradiscal pressure was measured in one volunteer in different postures and exercises, and in the other study (Rohlmann et al. 1994) the loads on an internal spinal fixation device (an implant for stabilising unstable spines) were determined in 10 patients. The absolute values of the results from both studies were normalized and compared for many body positions and dynamic exercises. The relative differences in intradiscal pressure and flexion bending moments in the fixators corresponded in most cases. Both studies showed slightly lower loads for sitting than for standing and comparatively low loads in all lying positions. High loads were measured for jogging, jumping on a trampoline and skipping. Differences between trends for intradiscal pressure and for flexion bending moments in the fixators were found when the load was predominantly carried by the anterior spinal column, as during flexion of the upper part of the body or when lifting and carrying weights. The combination of the results from these two methods may improve the understanding of the biomechanical behaviour of the lumbar spine and may be used to validate models and theories of spinal loading.

109 脊柱固定用スクリューの引抜き特性に関する研究(OS4-2 生体材料)(OS4 生体材料力学の新展開)

Internal spinal fixation devices are commonly used for stabilizing spines with degenerative, traumatic, or tumorous disorder. However screw pullout is a possibility, especially if low density osteoporotic bone is encounterd. The final goal of this study is to propose optimum configuration of a screw for spinal fixation. Pullout tests were carried out using three types of polyurethane foam with different values of density and compressive strength. Pullout strength was changed depending on density and compressive strength of polyurethane foam. The effect of pre-bending moment on pullout strength was also discussed.

Monitoring In Vivo Implant Loads With a Telemeterized Internal Spinal Fixation Device

Implant loads were measured in 10 patients using telemeterized internal spinal fixation devices. To determine the postoperative temporal course of implant loads. Little information exists regarding the temporal course of loads on internal spinal fixation devices. The telemeterized internal spinal fixator allows the measurement of three force components and three moments acting in the fixator. Implant loads were determined in up to 20 measuring sessions for different activities, including walking, standing, sitting, lying in the supine position, and lifting an extended leg while in the supine position. Implant loads often increased shortly after anterior interbody fusion was performed. Several patients retained the same high level even after fusion had taken place. This explains the reason why screw breakage sometimes occurs more than half a year after implantation. The time of fusion could not be pinpointed from the loading curves. The results show that fixators may be highly loaded even after fusion has occurred. A flexion bending moment acts on the implant even with the body in a relaxed lying position. This means that already shortly after the anterior procedure, the shape of the spine is not neutral and unloaded, but slightly deformed, which loads the fixators. Pedicle screw breakage more than half a year after insertion does not prove that anterior interbody fusion has not occurred.

Influence of load carrying on loads in internal spinal fixators

After spinal stabilization with an implant, patients want to know whether carrying loads will endanger their spine and their implant. Therefore, the effect of carrying a weight on the loading of internal spinal fixation devices was determined in 10 patients using instrumented, telemeterized internal fixators. Patients carried a 5 kg dumbbell in one hand as well as up to 20 kg in both hands. Compared to normal standing carrying a weight caused only a slight increase of the fixator loads in all patients. The maximum flexion bending moment in the fixators was then lower than during walking. Carrying a weight even decreased the axial compression force in the fixators of the patients with the T11 or T12 vertebra bridged. Patients also raised their arms in anteversion while holding weights of up to 5 kg with both hands. This increased the flexion bending moment in the fixators. Only a small part of a carried weight is taken over by internal fixators. Carrying a shopping bag only slightly increases the risk of pedicle screw breakage, however, since the spine has to support most of the additional weight, it represents a risk with regard to correction loss and bone sintering.

Changes in the loads on an internal spinal fixator after iliac-crest autograft

Spines are often stabilised posteriorly by internal fixation and anteriorly by a bone graft. The effect of an autologous bone graft from the iliac crest on implant loads is unknown. We used an internal spinal fixation device with telemetry to measure implant loads for several body positions and activities in nine patients before and after anterior interbody fusion. With the body upright, implant loads were often higher after than before fusion using a bone graft. Distraction of the bridged region led to high implant loads in patients with a fractured vertebra and to marked changes in load in those with degenerative instability. Leaving the lower of the bridged intervertebral discs intact led to only small changes in fixator load after anterior interbody fusion. A bone graft alone does not guarantee a reduction of implant loads.

Changes in the loads on an internal spinal fixator after iliac-crest autograft.

Spines are often stabilised posteriorly by internal fixation and anteriorly by a bone graft. The effect of an autologous bone graft from the iliac crest on implant loads is unknown. We used an internal spinal fixation device with telemetry to measure implant loads for several body positions and activities in nine patients before and after anterior interbody fusion. With the body upright, implant loads were often higher after than before fusion using a bone graft. Distraction of the bridged region led to high implant loads in patients with a fractured vertebra and to marked changes in load in those with degenerative instability. Leaving the lower of the bridged intervertebral discs intact led to only small changes in fixator load after anterior interbody fusion. A bone graft alone does not guarantee a reduction of implant loads.

Loads on internal spinal fixators measured in different body positions.

Telemeterized internal spinal fixation devices were implanted in ten patients. The loads acting on the fixators were compared for different body positions, including standing, sitting, and lying in a supine, prone, and lateral position. Implant loads differed considerably from patient to patient depending, for example, on the indication for surgery and the surgical procedure. They were altered by anterior interbody fusion. Mostly, only small differences in implant loads were found for the various lying positions. Flexion bending moments were significantly higher in upright than in lying body positions. Loads on the fixators were not higher for sitting than for standing. Patients who have undergone mono- or bisegmental spine stabilization should therefore be allowed to sit as soon as they can leave the bed.

Estimation of trunk muscle forces using the finite element method and in vivo loads measured by telemeterized internal spinal fixation devices

Direct quantitative measurement of muscle forces is not possible. Forces in the trunk muscles were estimated for standing and flexion of the upper body using three-dimensional, nonlinear finite element models of the lumbar spine with and without an internal spinal fixation device. Muscle forces assumed were two pairs dorsally and one ventrally, each representing several muscles. Muscle forces in the model with internal fixators were varied in discrete steps until the implant loads calculated closely corresponded to those measured in a patient with an instrumented implant. The calculated angles between adjacent lumbar vertebrae were compared with corresponding values measured on X-ray films of a patient as well as with literature values and served as a second criterion for predicting muscle forces. For the model without an implant, the muscle forces of the first model were slightly varied until the lumbar spine shape and the intradiscal pressure were physiological. The abdomen was shown to have a considerable supporting function for flexion.

Internal spinal fixator stiffness has only a minor influence on stresses in the adjacent discs.

Stresses in vertebral endplates and discs were calculated using the three-dimensional nonlinear finite-element model of a lumbar spine with an internal spinal fixation device. To determine the influence of fixator stiffness on stresses in the adjacent discs. There are few computer models of the lumbar spine with a fixator. Most of these models neglect the muscle forces. Fixator stiffness is assumed to influence greatly the stresses in the adjacent discs. Two three-dimensional nonlinear finite-element models were used to determine stresses in the lumbar spine for standing and 60 degrees flexion of the upper body. One model had an internal spinal fixator, the other did not. In a parameter study, the diameters of the longitudinal rod of the fixator were assumed to be 3, 5, 7, and 10 mm. In the computer model, the forces of the trunk muscles were simulated. The diameter of the longitudinal rod strongly affected the fixator loads but hardly influenced the stresses in the vertebral endplates. The stresses in the bridged discs were strongly reduced. However, the internal fixator had only a minor influence on the stresses in the anulus fibrosus and the pressure in the nucleus pulposus of the adjacent discs. The stiffness of an internal spinal fixation device has only a minor influence on stresses in the adjacent discs.

Loads on internal spinal fixation devices.

The loads acting on internal spinal fixation devices were measured for different activities in ten patients using telemeterized bisegmental implants. The highest loads were found for walking and lateral bending of the upper body while standing. When bending forwards the upper body, the fixator loads were only slightly altered. The forces and moments were not higher during sitting than during standing. Therefore, sitting should be allowed for patients with instrumented spines as soon as getting up is allowed. The forces and moments in the fixators were often altered due to anterior interbody fusion. Especially in patients with degenerative instability, the implant loads were higher after anterior interbody fusion than before. Braces were not able to markedly reduce the fixator loads. Therefore, it does not seem helpful to brace patients after mono- or bisegmental stabilization of the thoracic or lumbar spine.

Braces do not reduce loads on internal spinal fixation devices

Objective. To determine the effect of a brace or harness on loads on internal spinal fixation devices. Design. The implant loads were measured in vivo using telemeterized internal spinal fixators. Background. Only limited information exists regarding the load reduction due to a brace or harness. Methods. A Boston overlap brace, a reclination brace, and a lumbotrain harness were examined to determine how they affect the loads on internal spinal fixation devices. The implant loads were measured using telemeterized fixators in six patients for several positions and activities, including sitting, standing, walking, bending forward, and lifting an extended leg in a supine position. Results. None of the braces studied were able to markedly reduce the loads on the fixators. Frequently even higher fixator loads were measured when wearing a brace or harness. Conclusions. It does not seem helpful to brace patients after mono- or bisegmental stabilization of the lumbar spine.

Placing a bone graft more posteriorly may reduce the risk of pedicle screw breakage: analysis of an unexpected case of pedicle screw breakage

Telemeterized internal spinal fixation devices were implanted in a patient with degenerative instability and a narrow spinal canal in order to measure the fixator loads during daily activities. Anterior interbody fusion was performed three weeks later. During walking, the typical maximum flexion bending moments were 10 N m in the left and 5 N m in the right fixator. On removal of the implants three months later, a fatigue fracture was found not on the high loaded left side but in the upper right pedicle screw. The crack started on the caudal side of the cross-sectional area and progressed cranially. Upper vertebral tilting in the sagittal plane must have caused the screw breakage. This would probably have been prevented by a more posteriorly placed bone graft.