Loads in the spinal structures during lifting: development of a three-dimensional comprehensive biomechanical model

Fragility of L5 Vertebral Fracture After Rod Fracture at the Lumbosacral Junction Following Long-Segment Spinal Fusion Surgery for Adult Spine Deformity

We report a case of vertebral fracture in a patient with rod fractures after adult spinal deformity surgery, which occurred at the same level as the rod fractures, even though intervertebral bone fusion in the fusion range had been achieved. A 77-year-old female underwent corrective spinal surgery for adult spinal deformity from T12 to the pelvis but had a subsequent uppermost instrumented vertebral fracture, resulting in pseudarthrosis and severe kyphosis. The patient underwent proximal fusion extension to the T4, which improved alignment. A right-sided rod fracture at the lumbosacral junction occurred after 18 months; however, it showed no symptoms. After a month, the patient experienced severe low back pain with left leg pain and was diagnosed with bilateral rod fractures associated with L5 hyperextension vertebral fracture. The patient underwent revision surgery to repair the fractured rods with a multiple-rod construct. Rod fractures can occur even when bone fusion is achieved within the fusion range. When rod fractures are detected at the lumbosacral junction even if the interbody fusion was achieved, a hyperextension vertebral fracture may occur.

High-resolution, three-dimensional magnetic resonance imaging axial load dynamic study improves diagnostics of the lumbar spine in clinical practice

BACKGROUND

The response to axial physiological pressure due to load transfer to the lumbar spine structures is among the various back pain mechanisms. Understanding the spine adaptation to cumulative compressive forces can influence the choice of personalized treatment strategies.

AIM

To analyze the impact of axial load on the spinal canal’s size, intervertebral foramina, ligamenta flava and lumbosacral alignment.

METHODS

We assessed 90 patients using three-dimensional isotropic magnetic resonance imaging acquisition in a supine position with or without applying an axial compression load. Anatomical structures were measured in the lumbosacral region from L1 to S1 in lying and axially-loaded magnetic resonance images. A paired t test at α = 0.05 was used to calculate the observed differences.

RESULTS

After axial loading, the dural sac area decreased significantly, by 5.2% on average (4.1%, 6.2%, P < 0.001). The intervertebral foramina decreased by 3.4% (2.7%, 4.1%, P < 0.001), except for L5-S1. Ligamenta flava increased by 3.8% (2.5%, 5.2%, P < 0.001), and the lumbosacral angle increased.

CONCLUSION

Axial load exacerbates the narrowing of the spinal canal and intervertebral foramina from L1-L2 to L4-L5. Cumulative compressive forces thicken ligamenta flava and exaggerate lumbar lordosis.

Pseudarthrosis and Rod Fracture Rates After Transforaminal Lumbar Interbody Fusion at the Caudal Levels of Long Constructs for Adult Spinal Deformity Surgery

BACKGROUND

Interbody fusion at the caudal levels of long constructs for adult spinal deformity (ASD) surgery is used to promote fusion and secure a solid foundation for maintenance of deformity correction. We sought to evaluate long-term pseudarthrosis, rod fracture, and revision rates for TLIF performed at the base of a long construct for ASD.

METHODS

We reviewed 316 patients who underwent TLIF as a component of ASD surgery for medical comorbidities, surgical characteristics, and rate of unplanned reoperation for pseudarthrosis or instrumentation failure at the TLIF level. Fusion grading was assessed after revision surgery for pseudarthrosis at the TLIF level.

RESULTS

Rate of pseudarthrosis at the TLIF level was 9.8% (31/316), and rate of rod fractures was 7.9% (25/316). The rate of revision surgery at the TLIF level was 8.9% (28/316), and surgery was performed at a mean of 20.4 ± 16 months from the index procedure. Current smoking status (odds ratio 3.34, P = 0.037) was predictive of pseudarthrosis at the TLIF site. At a mean follow-up of 43 ± 12 months after revision surgery, all patients had achieved bony union at the TLIF site.

CONCLUSIONS

At 3-year follow-up, the rate of pseudarthrosis after TLIF performed at the base of a long fusion for ASD was 9.8%, and the rate of revision surgery to address pseudarthrosis and/or rod fracture was 8.9%. All patients were successfully treated with revision interbody fusion or posterior augmentation of the fusion mass, without need for further revision procedures at the TLIF level.

People With Low Back Pain Display a Different Distribution of Erector Spinae Activity During a Singular Mono-Planar Lifting Task

This study aimed to investigate the variation in muscle activity and movement in the lumbar and lumbothoracic region during a singular mono-planar lifting task, and how this is altered in individuals experiencing low back pain (LBP). Muscle activity from the lumbar and lumbothoracic erector spinae of 14 control and 11 LBP participants was recorded using four 13 × 5 high-density surface electromyography (HDEMG) grids. Root mean squared HDEMG signals were used to create spatial maps of the distribution of muscle activity. Three-dimensional kinematic data were recorded focusing on the relationship between lumbar and thoracic movements. In the task, participants lifted a 5 kg box from knee height to sternal height, and then returned the box to the starting position. The center of muscle activity for LBP participants was found to be systematically more cranial throughout the task compared to the control participants (P < 0.05). Participants with LBP also had lower signal entropy (P < 0.05) and lower absolute root mean squared values (P < 0.05). However, there were no differences between groups in kinematic variables, with no difference in contributions between lumbar and thoracic motion segments (P > 0.05). These results indicate that participants with LBP utilize an altered motor control strategy to complete a singular lifting task which is not reflected in their movement strategy. While no differences were identified between groups in the motion between lumbar and thoracic motion segments, participants with LBP utilized a less homogenous, less diffuse and more cranially focussed contraction of their erector spinae to complete the lifting movement. These results may have relevance for the persistence of LBP symptoms and the development of new treatments focussing on muscle retraining in LBP.

Cell-Seeded Adhesive Biomaterial for Repair of Annulus Fibrosus Defects in Intervertebral Discs

Defects in the annulus fibrosus (AF) of intervertebral discs allow nucleus pulposus tissue to herniate causing painful disability. Microdiscectomy procedures remove herniated tissue fragments, but unrepaired defects remain allowing reherniation or progressive degeneration. Cell therapies show promise to enhance repair, but methods are undeveloped and carriers are required to prevent cell leakage. To address this challenge, this study developed and evaluated genipin-crosslinked fibrin (FibGen) as an adhesive cell carrier optimized for AF repair that can deliver cells, match AF material properties, and have low risk of extrusion during loading. Part 1 determined that feasibility of bovine AF cells encapsulated in high concentration FibGen (F140G6: 140 mg/mL fibrinogen; 6 mg/mL genipin) for 7 weeks could maintain high viability, but had little proliferation or matrix deposition. Part 2 screened tissue mechanics and in situ failure testing of nine FibGen formulations (fibrin: 35-140 mg/mL; genipin: 1-6 mg/mL). F140G6 formulation matched AF shear and compressive properties and significantly improved failure strength in situ. Formulations with reduced genipin also exhibited satisfactory material properties and failure behaviors warranting further biological screening. Part 3 screened AF cells encapsulated in four FibGen formulations for 1 week and found that reduced genipin concentrations increased cell viability and glycosaminoglycan production. F70G1 (70 mg/mL fibrinogen; 1 mg/mL genipin) demonstrated balanced biological and biomechanical performance warranting further testing. We conclude that FibGen has potential to serve as an adhesive cell carrier to repair AF defects with formulations that can be tuned to enhance biomechanical and biological performance; future studies are required to develop strategies to enhance matrix production.

A subject-specific biomechanical control model for the prediction of cervical spine muscle forces

BACKGROUND

The aim of the present study is to propose a subject-specific biomechanical control model for the estimation of active cervical spine muscle forces.

METHODS

The proprioception-based regulation model developed by Pomero et al. (2004) for the lumbar spine was adapted to the cervical spine. The model assumption is that the control strategy drives muscular activation to maintain the spinal joint load below the physiological threshold, thus avoiding excessive intervertebral displacements. Model evaluation was based on the comparison with the results of two reference studies. The effect of the uncertainty on the main model input parameters on the predicted force pattern was assessed. The feasibility of building this subject-specific model was illustrated with a case study of one subject.

FINDINGS

The model muscle force predictions, although independent from EMG recordings, were consistent with the available literature, with mean differences of 20%. Spinal loads generally remained below the physiological thresholds. Moreover, the model behavior was found robust against the uncertainty on the muscle orientation, with a maximum coefficient of variation (CV) of 10%.

INTERPRETATION

After full validation, this model should offer a relevant and efficient tool for the biomechanical and clinical study of the cervical spine, which might improve the understanding of cervical spine disorders.

Multi-Rod Constructs Can Prevent Rod Breakage and Pseudarthrosis at the Lumbosacral Junction in Adult Spinal Deformity

STUDY DESIGN

Retrospective cohort study.

OBJECTIVE

To determine if patients fused with multi-rod constructs to the pelvis have a lower incidence of lumbosacral rod failure and pseudarthrosis than those fused with dual-rod constructs.

METHODS

We performed a retrospective review of consecutive adult spinal deformity patients who underwent long fusion to the pelvis. Inclusion criteria were >5 levels, primary fusion or revision for L5-S1 pseudarthrosis, and minimum 1-year follow-up. Revision patients with indications other than L5-S1 pseudarthrosis were excluded. One-year follow-up plain radiographs were reviewed for rod integrity, and computed tomography scan (CT) was obtained whenever rod breakage was observed. Dual-rod and multi-rod (3 or 4 rods) cohorts were statistically compared.

RESULTS

There were 31 patients with 15 in the dual-rod group and 16 in the multi-rod group, with average ages of 68 ± 9 and 63 ± 12 years, respectively. No patients in the multi-rod group experienced rod fracture, whereas 6 in the dual-rod group fractured a rod (P = .007), with 4 occurring at the lumbosacral junction (P = .04). CT scan in the 4 lumbosacral rod fracture cases, and surgical exploration in 3, confirmed pseudarthrosis and hypertrophic nonunion at the L5-S1 junction.

CONCLUSION

Patients with dual-rod constructs had a statistically greater incidence of lumbosacral pseudarthrosis with implant failure than those with multi-rod constructs. CT and surgical exploration showed hypertrophic nonunion as opposed to oligo- or atrophic nonunion. This suggests that mechanical instability, not biology, is the main reason for failure, and could be addressed with the use of multi-rods.

The Effect of Lumbar Disc Herniation on Musculoskeletal Loadings in the Spinal Region During Level Walking and Stair Climbing

Background

People with low back pain (LBP) alter their motion patterns during level walking and stair climbing due to pain or fear. However, the alternations of load sharing during the two activities are largely unknown. The objective of this study was to investigate the effect of LBP caused by lumbar disc herniation (LDH) on the muscle activities of 17 main trunk muscle groups and the intradiscal forces acting on the five lumbar discs.

Material/Methods

Twenty-six healthy adults and seven LDH patients were recruited to perform level walking and stair climbing in the Gait Analysis Laboratory. Eight optical markers were placed on the bony landmarks of the spinous process and pelvis, and the coordinates of these markers were captured during the two activities using motion capture system. The coordinates of the captured markers were applied to developed musculoskeletal model to calculate the kinetic variables.

Results

LDH patients demonstrated higher muscle activities in most trunk muscle groups during both level walking and stair climbing. There were decreases in anteroposterior shear forces on the discs in the pathological region and increases in the compressive forces on all the lumbar discs during level walking. The symmetry of mediolateral shear forces was worse in LDH patients than healthy adults during stair climbing.

Conclusions

LDH patients exhibited different kinetic alternations during level walking and stair climbing. However, both adaptive strategies added extra burdens to the trunk system and further increased the risk for development of LDH.

The Effect of Lumbar Disc Herniation on Spine Loading Characteristics during Trunk Flexion and Two Types of Picking Up Activities

The main purpose of this study was to investigate the compensatory response of the muscle activities of seventeen major muscle groups in the spinal region, intradiscal forces of the five lumbar motion segment units (MSUs), and facet forces acting on the ten lumbar facet joints in patients with lumbar disc herniation (LDH). Twenty-six healthy adults and seven LDH patients performed trunk flexion, ipsilateral picking up, and contralateral picking up in sequence. Eight optical markers were placed on the landmarks of the pelvis and spinal process. The coordinates of these markers were captured to drive a musculoskeletal model to calculate the muscle activities, intradiscal forces, and facet forces. The muscle activities of the majority of the seventeen major muscle groups were found increases in LDH patients. In addition, the LDH patients displayed larger compressive forces and anteroposterior forces on all the five lumbar MSUs and more lumbar facet inventions on most facet joints. These findings suggest that the LDH patients demonstrate compensatory increases in the most trunk muscle activities and all spinal loads. These negative compensatory responses increase the risk of the aggravation of disc herniation. Therefore, treatment should intervene as earlier as possible for the severe LDH patients.

The activL(®) Artificial Disc: a next-generation motion-preserving implant for chronic lumbar discogenic pain

Degeneration of the lumbar intervertebral discs is a leading cause of chronic low back pain in adults. Treatment options for patients with chronic lumbar discogenic pain unresponsive to conservative management include total disc replacement (TDR) or lumbar fusion. Until recently, only two lumbar TDRs had been approved by the US Food and Drug Administration - the Charité Artificial Disc in 2004 and the ProDisc-L Total Disc Replacement in 2006. In June 2015, a next-generation lumbar TDR received Food and Drug Administration approval - the activL(®) Artificial Disc (Aesculap Implant Systems). Compared to previous-generation lumbar TDRs, the activL(®) Artificial Disc incorporates specific design enhancements that result in a more precise anatomical match and allow a range of motion that better mimics the healthy spine. The results of mechanical and clinical studies demonstrate that the activL(®) Artificial Disc results in improved mechanical and clinical outcomes versus earlier-generation artificial discs and compares favorably to lumbar fusion. The purpose of this report is to describe the activL(®) Artificial Disc including implant characteristics, intended use, surgical technique, postoperative care, mechanical testing, and clinical experience to date.

In vivo loads in the lumbar L3-4 disc during a weight lifting extension

BACKGROUND

Knowledge of in vivo human lumbar loading is critical for understanding the lumbar function and for improving surgical treatments of lumbar pathology. Although numerous experimental measurements and computational simulations have been reported, non-invasive determination of in vivo spinal disc loads is still a challenge in biomedical engineering. The object of the study is to investigate the in vivo human lumbar disc loads using a subject-specific and kinematic driven finite element approach.

METHODS

Three dimensional lumbar spine models of three living subjects were created using MR images. Finite element model of the L3-4 disc was built for each subject. The endplate kinematics of the L3-4 segment of each subject during a dynamic weight lifting extension was determined using a dual fluoroscopic imaging technique. The endplate kinematics was used as displacement boundary conditions to calculate the in-vivo disc forces and moments during the weight lifting activity.

FINDINGS

During the weight lifting extension, the L3-4 disc experienced maximum shear load of about 230 N or 0.34 bodyweight at the flexion position and maximum compressive load of 1500 N or 2.28 bodyweight at the upright position. The disc experienced a primary flexion-extension moment during the motion which reached a maximum of 4.2 Nm at upright position with stretched arms holding the weight.

INTERPRETATION

This study provided quantitative data on in vivo disc loading that could help understand intrinsic biomechanics of the spine and improve surgical treatment of pathological discs using fusion or arthroplasty techniques.

Relaxation response of lumbar segments undergoing disc-space distraction: implications to the stability of anterior lumbar interbody implants

STUDY DESIGN

A biomechanical study of human cadaveric lumbar spine segments undergoing disc-space distraction for insertion of anterior lumbar interbody implants.

OBJECTIVE

To measure the distraction force and its relaxation during a period of up to 3 hours after disc-space distraction as a function of the distraction magnitude and disc level.

SUMMARY OF BACKGROUND DATA

Interbody implants depend on compressive preload produced by disc-space distraction (annular pretension) for initial stabilization of the implant-bone interface. However, the amount of preload produced by disc-space distraction due to insertion of the implant and its subsequent relaxation have not been quantified.

METHODS

Twenty-two fresh human lumbar motion segments (age: 51 ± 14.8 years) were used. An anterior lumbar discectomy was performed. The distraction test battery consisted of a tension stiffness test performed before and after each relaxation test, 2 distraction magnitudes of 2 and 4 mm, and a recovery period before each distraction input. The distraction forces and lordosis angles were measured. RESULTS.: Peak distraction force was significantly larger for the 4-mm distraction (431.8 ± 116.4 N) than for the 2-mm distraction (204.9 ± 55.5 N) (P < 0.01). The distraction force significantly decreased over time (P < 0.01), approximating steady-state values of 146.1 ± 47.3 N at 2-mm distraction and 289.8 ± 92.8 N at 4-mm distraction, respectively. The distraction force reduced in magnitude by more than 20% of peak value in the first 15 minutes and reduced by approximately 30% of the peak value at the end of the testing period. The spine segment relaxed by the same amount of force, regardless of the disc level (P > 0.05).

CONCLUSION

The "tightness of fit" that the surgeon notes immediately after interbody device insertion in the disc space degrades in the very early postoperative period, which could compromise the stability of the bone-implant interface.

Mechanical Characterization of a Viscoelastic Disc for Lumbar Total Disc Replacement

A viscoelastic artificial disc may more closely replicate normal stiffness characteristics of the healthy human disc compared with first-generation total disc replacement (TDR) devices, which do not utilize viscoelastic materials and are based on a ball and socket design that does not allow loading compliance. Mechanical testing was performed to characterize the durability and range of motion (ROM) of an investigational viscoelastic TDR (VTDR) device for the lumbar spine, the Freedom® Lumbar Disc. ROM data were compared with data reported for the human lumbar disc in the clinical literature. Flexibility and stiffness of the VTDR in compression, rotation, and flexion/extension were within the parameters associated with the normal human lumbar disc. The device constrained motion to physiologic ranges and replicated normal stress/strain dynamics. No mechanical or functional failures occurred within the loads and ROM experienced by the human disc. Fatigue testing of the worst case VTDR device size demonstrated a fatigue life of 50 years of simulated walking and 240 years of simulated significant bends in both flexion/extension and lateral bending coupled with axial rotation, with no functional failures. These results indicate that the VTDR evaluated in this mechanical study is durable and has the ability to replicate the stiffness and mechanics of the natural, healthy human lumbar disc.

Can experimental data in humans verify the finite element-based bone remodeling algorithm?

STUDY DESIGN

A finite element analysis-based bone remodeling study in human was conducted in the lumbar spine operated on with pedicle screws. Bone remodeling results were compared to prospective experimental bone mineral content data of patients operated on with pedicle screws.

OBJECTIVE

The validity of 2 bone remodeling algorithms was evaluated by comparing against prospective bone mineral content measurements. Also, the potential stress shielding effect was examined using the 2 bone remodeling algorithms and the experimental bone mineral data.

SUMMARY OF BACKGROUND DATA

In previous studies, in the human spine, the bone remodeling algorithms have neither been evaluated experimentally nor been examined by comparing to unsystematic experimental data.

METHODS

The site-specific and nonsite-specific iterative bone remodeling algorithms were applied to a finite element model of the lumbar spine operated on with pedicle screws between L4 and L5. The stress shielding effect was also examined. The bone remodeling results were compared with prospective bone mineral content measurements of 4 patients. They were measured after surgery, 3-, 6- and 12-months postoperatively.

RESULTS

After 1 year, there was an average experimental bone loss of 9.78% below the positions of pedicle screws, and the results for the 2 bone remodeling algorithms showed an average bone gain of 8.41% and 1.61%. There were no similarities between the bone remodeling and experimental data.

CONCLUSION

The bone remodeling data showed no resemblances when compared to the prospective data of BMC measurements. There was no basis for confirming the validity of the bone remodeling algorithms in this study.

Enhancing the stability of anterior lumbar interbody fusion: a biomechanical comparison of anterior plate versus posterior transpedicular instrumentation

STUDY DESIGN

Biomechanical study using human cadaver spines.

OBJECTIVE

To assess the stabilizing effect of a supplemental anterior tension band (ATB, Synthes) plate on L5-S1 anterior lumbar interbody fusion (ALIF) using a femoral ring allograft (FRA) under physiologic compressive preloads, and to compare the results with the stability achieved using FRA with supplemental transpedicular instrumentation.

SUMMARY OF BACKGROUND DATA

Posterior instrumentation can improve the stability of ALIF cages. Anterior plates have been proposed as an alternative to avoid the additional posterior approach.

METHODS

Eight human specimens (L3 to sacrum) were tested in the following sequence: (i) intact, (ii) after anterior insertion of an FRA at L5-S1, (iii) after instrumentation with the ATB plate, and (iv) after removal of the plate and adding transpedicular instrumentation at the same level. Specimens were tested in flexion-extension, lateral bending, and axial rotation. Flexion-extension was tested under 0 N, 400 N, and 800 N compressive follower preload to simulate physiologic compressive preloads on the lumbar spine.

RESULTS

Stand-alone FRAs significantly decreased the range of motion (ROM) in all tested directions (P < 0.05); however, the resultant ROM was large in flexion-extension ranging between 6.1 +/- 3.1 degrees and 5.1 +/- 2.2 degrees under 0 N to 800 N preloads. The ATB plate resulted in a significant additional decrease in flexion-extension ROM under 400 N and 800 N preloads (P < 0.05). The flexion-extension ROM with the ATB plate was 4.1 +/- 2.3 under 0 N preload and ranged from 3.1 +/- 1.8 to 2.4 +/- 1.3 under 400 N to 800 N preloads. The plate did not significantly decrease lateral bending or axial rotation ROM compared with stand-alone FRA (P > 0.05), but the resultant ROM was 2.7 +/-1.9 degrees and 0.9 +/- 0.6 degrees , respectively. Compared with the ATB plate, the transpedicular instrumentation resulted in significantly less ROM in flexion-extension and lateral bending (P < 0.05), but not in axial rotation (P > 0.05).

CONCLUSION

The ATB plate can significantly increase the stability of the anterior FRA at L5-S1 level. Although supplemental transpedicular instrumentation results in a more stable biomechanical environment, the resultant ROM with the addition of a plate is small, especially under physiologic preload, suggesting that the plate can sufficiently resist motion. Therefore, clinical assessment of the ATB plate as an alternative to transpedicular instrumentation to enhance ALIF cage stability is considered reasonable.

Rapid and reliable biomechanical screening of injectable bone cements for autonomous augmentation of osteoporotic vertebral bodies: Appropriate values of elastic constants for finite element models

We performed finite element analysis studies on 3 three-dimensional representations of a single vertebral body: a regular cube, made of low-density polyurethane foam (foam cube analog); a regular cube considered composed of cancellous bone only (bone cube analog)); and the body of the L2 vertebra (full anatomical body model). Each finite element model was subjected to a compressive load of 2300 N, uniformly distributed over its superior surface. The cancellous and cortical bones were assigned anisotropic elastic properties, while the foam and the endplate material were considered to have isotropic properties. In each representation, the elastic properties of the material(s) were adjusted (from the initial values that were used) to give a stiffness of the representation that was equal to that of the mean result for fresh cadaveric osteoporotic single vertebral bodies, as obtained from ex vivo experimental studies reported in the literature (1226 +/- 996 N mm(-1)). Thus, any one of these representations, when used with the final adjusted value(s) of the elastic constants and modified to include a cylindrical hole filled with a specific volume of bolus of an injected bone cement, may be utilized in the rapid and reliable experimental ex vivo and/or numerical screening of these cements for use in autonomous vertebral body augmentation. This approach has many advantages over those that are currently being used, which are either characterization of the cement in isolation from the vertebral body or use of cadaveric vertebral bodies.

Metastatic Burst Fracture Risk Assessment Based on Complex Loading of the Thoracic Spine

The mechanical integrity of vertebral bone is compromised when metastatic cancer cells migrate to the spine, rendering it susceptible to burst fracture under physiologic loading. Risk of burst fracture has been shown to be dependent on the magnitude of the applied load, however limited work has been conducted to determine the effect of load type on the stability of the metastatic spine. The objective of this study was to use biphasic finite element modeling to evaluate the effect of multiple loading conditions on a metastatically-involved thoracic spinal motion segment. Fifteen loading scenarios were analyzed, including axial compression, flexion, extension, lateral bending, torsion, and combined loads. Additional analyses were conducted to assess the impact of the ribcage on the stability of the thoracic spine. Results demonstrate that axial loading is the predominant load type leading to increased risk of burst fracture initiation, while rotational loading led to only moderate increases in risk. Inclusion of the ribcage was found to reduce the potential for burst fracture by 27%. These findings are important in developing a more comprehensive understanding of burst fracture mechanics and in directing future modeling efforts. The results in this study may also be useful in advising less harmful activities for patients affected by lytic spinal metastases.

Direct real-time measurement of in vivo forces in the lumbar spine

BACKGROUND CONTEXT

Accurate knowledge of the mechanical loads in the lumbar spine is critical to understanding the causes of degenerative disc disease and to developing suitable treatment options and functional disc replacements. To date, only indirect methods have been used to measure the forces developed in the spine in vivo. These methods are fraught with error, and results have never been validated using direct experimental measurements.

PURPOSE

The major aims of this study were to develop a methodology to directly measure, in real time, the in vivo loading in the lumbar spine, to determine if the forces developed in the lumbar spine are dependent on activity and/or posture and to assess the baboon as an animal model for human lumbar spine research based on in vivo mechanical loading.

STUDY DESIGN

Real-time telemetered data were collected from sensor-imbedded implants that were placed in the interbody space of the lumbar spines of two baboons.

METHODS

An interbody spinal implant was designed and instrumented with strain gauges to be used as a load cell. The implant was placed anteriorly in the lumbar spine of the baboon. Strain data were collected in vivo during normal activities and transmitted by means of a telemetry system to a receiver. The forces transmitted through the implant were calculated from the measured strain based on precalibration of the load cell. Measured forces were correlated to videotaped activities to elucidate trends in force level as a function of activity and posture over a 6-week period. The procedure was repeated in a second baboon, and data were recorded for similar activities.

RESULTS

Implants measured in vivo forces developed in the lumbar spine with less than 10% error. Loads in the lumbar spine are dependent on activity and posture. The maximum loads developed in the lumbar spine during normal (baboon) activities exceeded four times body weight and were recorded while animals were sitting flexed. Force data indicate similar trends between the human lumbar spine and the baboon lumbar spine.

CONCLUSIONS

It is possible to monitor the real-time forces present in the lumbar spine. Force data correlate well to trends previously reported for in vivo pressure data. Results also indicate that the baboon may be an appropriate animal model for study of the human lumbar spine.

A Proprioception Based Regulation Model to Estimate the Trunk Muscle Forces

Evaluation of loads acting on the spine requires the knowledge of the muscular forces acting on it, but muscles redundancy necessitates developing a muscle forces attribution strategy. Optimisation, EMG, or hybrid models allow evaluating muscle force patterns, yielding a unique muscular arrangement or/and requiring EMG data collection. This paper presents a regulation model of the trunk muscles based on a proprioception hypothesis, which searches to avoid the spinal joint overloading. The model is also compared to other existing models for evaluation. Compared to an optimisation model, the proposed alternative muscle pattern yielded a significant spine postero-anterior shear decrease. Compared to a model based on combination of optimisation criteria, present model better fits muscle activation observed using EMG (38% improvement). Such results suggest that the proposed model, based on regulation of all spinal components, may be more relevant from a physiologic point of view.

Effect of supplemental translaminar facet screw fixation on the stability of stand-alone anterior lumbar interbody fusion cages under physiologic compressive preloads

STUDY DESIGN

A biomechanical study of lumbar threaded interbody cage construct under varying compressive preloads of similar magnitudes to those experienced in vivo during daily activities.

OBJECTIVES

To test the hypothesis that supplemental translaminar facet screws would enhance the stability (ability to reduce segmental angular motion) of threaded interbody cages in flexion-extension during activities in which the spine is subjected to low compressive preloads, and therefore the stand-alone interbody cage construct is least stable.

SUMMARY OF BACKGROUND DATA

Controversy exists over whether threaded anteriorly placed interbody cages can be routinely used as "stand-alone" devices or whether they require supplemental posterior stabilization to achieve successful fusion. Biomechanical studies suggest that under conditions of low preloads, the motion segment treated with stand-alone cages might be less stable, particularly in extension. METHODS.: Eight human lumbar spine specimens (from L1 to sacrum) were tested intact, after insertion of 2 threaded cylindrical cages (BAK) at L5-S1 and after supplemental translaminar facet screw fixation. They were subjected to flexion and extension moments under progressively increasing magnitude of externally applied compressive follower preload from 0 to 1200 N. The range of angular motion in flexion-extension at L5-S1 was analyzed to assess the effect of translaminar facet screws on the stability of the cage construct for different compressive preloads.

RESULTS

In flexion, over 0 to 400 N preload, the supplemental translaminar facet screw fixation reduced the L5-S1 angular motion relative to intact by 71% to 74% as compared to 40% to 44% for the cages alone. This difference was statistically significant (P < 0.05). In extension at 0 N preload, the cages allowed more angular motion than the intact segment, whereas with translaminar facet screw fixation, the motion was reduced to the level of the intact segment. At 400 N preload, supplemental TLFS fixation significantly increased the stability of the cages, reducing the extension angular motion by 60% of intact (P = 0.04). Supplemental translaminar facet screw fixation did not significantly increase the stability provided by the cages in flexion or extension at the 1200 N preload magnitude.

CONCLUSIONS

In vivo during activities of daily living, interbody cage constructs are subject to varying compressive preloads due to external loads generated by paraspinal musculature, and our results suggest that the stability created by the cage (reduction in segmental angular motion) is not constant. The cage construct is likely to be least stable in extension during activities that impart low compressive preloads to the lumbar spine. Supplemental translaminar facet screw fixation will enhance stability of the motion segment treated with threaded cages, particularly during conditions of low compressive preloads, the very condition in which the cage alone is least effective in providing stability.

Dynamic stiffness and damping of human intervertebral disc using axial oscillatory displacement under a free mass system

The aim of this study was to analyse the dynamic response of the human intervertebral disc to vibration in a physiologically relevant frequency spectrum. Eight lumbar intervertebral discs were harvested. After preparation, each sample was subjected to a pre-loading and then dynamic compression (from 5 to 30 Hz). The dynamic compression was applied using an experimental set-up comprising a free weight loading from above and a driving oscillatory displacement from below (closest to the in vivo loading). A viscoelastic model enabled the calculation of stiffness and damping from the transfer function. From 5 Hz to 30 Hz the stiffness values are between 0.19 and 3.66 (MN/m) and the damping values between 32 and 2094 (Ns/m). The mean resonant frequency was found at 8.7 Hz. These dynamic characteristics of the intervertebral disc could be used in a three-dimensional finite elements model of the human body to study its response to vibration in the driving position.

Compressive preload improves the stability of anterior lumbar interbody fusion cage constructs

BACKGROUND

Insertion of an anterior lumbar interbody fusion cage has been shown to reduce motion in a human spine segment in all loading directions except extension. The "stand-alone" cages depend on compressive preload produced by anular pretensioning and muscle forces for initial stabilization. However, the effect that the in vivo compressive preload generated during activities of daily living has on the construct is not fully understood. This study tested the hypothesis that the ability of the cages to reduce the segmental motions in flexion and extension is significantly affected by the magnitude of the externally applied compressive preload.

METHODS

Fourteen specimens from human lumbar spines were tested intact and after insertion of two threaded cylindrical cages at level L5-Sl. They were subjected to flexion and extension moments under progressively increasing magnitudes of externally applied compressive follower preload from 0 to 1200 N. The range of motion at level L5-S1 after cage insertion was compared with the value achieved in the intact specimens at each compressive preload magnitude.

RESULTS

The cages significantly reduced the L5-S1 flexion motion at all preloads (p < 0.05). They decreased flexion motion by 29% to 43% of that of the intact specimens for low preloads (0 to 400 N) and by 69% to 79% of that of the intact specimens under preloads of 800 to 1200 N. In extension, in the absence of an externally applied preload, the cages permitted 24% more motion than the intact segment (p < 0.05). In contrast, they reduced the extension motion at preloads from 200 to 1200 N. Under preloads of 800 to 1200 N, the reduction in extension motion after cage placement was 42% to 48% of that of the intact segment (p < 0.05). The reduction of motion in both flexion and extension after cage placement was significantly greater at preloads of 800 to 1200 N compared with the motion reductions at preloads of < or =400 N (p < 0.05).

CONCLUSIONS

In contrast to the observed extension instability under anular tension preload only, the two-cage construct exerted a stabilizing effect on the motion segment (a reduction in segmental motion) in flexion as well as extension under externally applied compressive preloads of physiologic magnitudes. The external compressive preload significantly affected the stabilization provided by the cages. The cages provided substantially more stabilization, both in flexion and in extension, at larger preloads than at smaller preloads.

CLINICAL RELEVANCE

The study suggests that the segment treated with an anterior lumbar interbody fusion cage is relatively less stable under conditions of low external compressive preload. The magnitude of preload required to achieve stabilization with stand-alone cages may be only partially achieved by anular pretensioning. Since the magnitude of the preload across the disc space due to muscle activity can vary with activities of daily living, supplemental stabilization of the cage construct may provide a more predictably stable environment for lumbar spine fusion.

Representation of passive spinal element contributions to in vitro flexion-extension using a polynomial model: illustration using the porcine lumbar spine

A polynomial modeling approach was developed to describe the contribution of individual passive spinal elements to the lumbar spinal motion segment flexion-extension motion. Flexion-extension moment-angle curves from porcine lumbar spines tested using a robotic testing system were described using sixth-order polynomials; the polynomials describing different dissection conditions were subtracted to describe the contribution of individual spinal elements to the motion segment flexion-extension properties. This modeling approach is a powerful and straightforward method for representing the mechanics of individual spinal tissues in biomechanical models and could easily be expanded to incorporate other features such as axial load.

The impact of mental processing and pacing on spine loading: 2002 Volvo Award in biomechanics

STUDY DESIGN

The impact of various levels of mental processing and pacing (during lifting) on spine loading was monitored under laboratory conditions.

OBJECTIVES

To explore how mental demands and pacing influence the biomechanical response and subsequent spine loading and, to determine whether individual characteristics have a modifying role in the responses.

SUMMARY OF BACKGROUND DATA

Modern work often requires rapid physical exertions along with demands of mental processing (both psychosocial stressors). While the effect of physical workplace factors on spine loading has been widely documented, few studies have investigated the impact that interaction of psychosocial factors and individual factors has on spine loads.

METHODS

For this study, 60 subjects lifted boxes while completing two types of mental processing tasks: 1) series tasks with decisions occurring before the act of lifting, and 2) simultaneous tasks with decisions occurring concurrently with the lift. For both of these mental processing conditions, two intensities of mental load were evaluated: simple and complex. Task pacing was also adjusted under slow and fast conditions. Finally, individual characteristics (personality and gender) were evaluated as potential modifiers. An electromyographically assisted model evaluated the three-dimensional spine loads under the experimental conditions.

RESULTS

Simultaneous mental processing had the largest impact on the spine loads, with the complex intensity resulting in increases of 160 N with lateral shear, 80 N with anteroposterior shear, and 700 N with compression. Increased task pace produced greater lateral shear (by 20 N), anteroposterior shear (by 60 N), and compression loads (by 410 N). Gender and personality also influenced loadings by as much as 17%.

CONCLUSIONS

Mental processing stress acted as a catalyst for the biomechanical responses, leading to intensified spine loading. Mental stress appeared to occur as a function of time pressures on task performance and resulted in less controlled movements and increases in trunk muscle coactivation. These adjustments significantly increased spine loading. These results suggest a potential mechanism for the increase in low back pain risk resulting from psychosocial stress caused by modern work demands.

Effects of varying backpack loads on peak forces in the lumbosacral spine during walking

OBJECTIVE: To compare the differences in lumbosacral spine forces under varying backpack loads. DESIGN: A biomechanical model was used to determine the changes in peak forces in the L5/S1 joint with increasing backpack loads during level walking. BACKGROUND: Most studies involving varying external backpack loads have been concerned mainly with kinematic and physiological measurements. To the author's knowledge, there has been no investigation of the change in peak forces in the lumbosacral joint during the carriage of such loads. METHOD: Data acquisition was carried out using a 5-camera Vicon motion analysis system and two Kistler force plates. Ten male subjects with similar weights, height and age were recruited for this study. Three different backpack loading conditions were studied, that is walking with no load, with 15% BW and with 30% BW. RESULTS: It was observed that all the ten subjects while walking with heavier backpack load adopted a compensatory trunk flexion posture. However, kinematic gait parameters such as walking speed and stride length remained unchanged with the increasing loads. Walking with backpack load of 15%BW and 30%BW resulted in corresponding increase in lumbosacral force of 26.7% and 64% respectively when compared to walking without backpack load. CONCLUSION: In carrying a given packload during walking, it will give rise to a disproportionate force increase acting on the L5/S1 joint.

Real-time in vivo loading in the lumbar spine: part 1. Interbody implant: load cell design and preliminary results

STUDY DESIGN

Instrumented interbody implants were placed into the disc space of a motion segment in two baboons. During the animal's activities, implants directly measured in vivo loads in the lumbar spine by telemetry transmitter.

OBJECTIVES

Develop and test an interbody implant-load cell and use the implant to measure directly loads imposed on the lumbar spine of the baboon, a semiupright animal.

SUMMARY OF BACKGROUND DATA

In vivo forces in the lumbar spine have been estimated using body weight calculations, moment arm models, dynamic chain models, electromyogram measurements, and intervertebral disc pressure measurements.

METHODS

An analytical model was used to determine the force-strain relation in a customized interbody implant. After validation by finite element modeling, strain gauges were mounted onto the implant and connected to a telemetry transmitter. Implants were placed surgically into the L4-L5 disc space of skeletally mature baboons and the transmitter in the flank. After surgery, load data were collected from the animals during activities. Radiographs were taken monthly to assess fusion.

RESULTS

The implant-load cell is sufficiently sensitive to monitor dynamic changes in strain and load. During extreme activity, highest measurable strain values were indicative of loads in excess of 2.8 times body weight.

CONCLUSIONS

The study technique and technology are efficacious for measuring real-time in vivo loads in the spine. Measuring load on an intradiscal implant over the course of healing provides key information about the mechanics of this process. Loads may be used to indicate performance demands on the intervertebral disc and interbody implants for subsequent implant design.

The in vivo dynamic response of the spine to perturbations causing rapid flexion: effects of pre-load and step input magnitude

OBJECTIVE

To evaluate the impact of muscle pre-activation levels and load magnitude on the response of the trunk to loading conditions causing rapid flexion.

DESIGN

Eight male subjects were asked to maintain an upright standing posture while resisting the application of forward flexion moments produced by four different loading conditions consisting of combinations of two pre-loads (4% or 16% of the maximum extensor moment) and two added loads (12% or 24%). Pre-loading was used to develop different initial levels of trunk muscle activity prior to the application of the added loads. Of special interest were the two conditions that resulted in total final loads of 28%.

BACKGROUND

Cocontraction of the antagonistic and agonistic muscles of the trunk are required to provide stability during normal physiological loading conditions. In several in vivo studies, levels of trunk muscle cocontraction have been observed prior to the application of unexpected or sudden loads. Forces from the abdominal muscles have been proposed to provide stability when extensor moments are generated. The response of trunk muscles to rapid flexor moments would provide further insight into the dynamic stability mechanisms of the spine.

METHODS

Measurements were made of the trunk extensor moments, angular displacement of the trunk and unilateral surface EMG amplitudes of three abdominal and three trunk extensor muscles. Values were recorded during the isometric pre-load and for the maximum magnitude of each variable in response to the added load.

RESULTS

Higher pre-loads resulted in lower flexion rotations of the spine and higher added loads caused larger rotations. With increasing magnitudes of final loads, a corresponding increase in trunk extensor moments and trunk muscle cocontraction was observed. The largest activations were observed in the lumbar erector spinae and thoracic erector spinae muscles, while smaller yet substantial EMG activity was observed in the internal oblique and external oblique. A comparison of the 28% loading conditions showed an increased response of the trunk to the [4 + 24] loading condition (with lower initial trunk stiffness) when compared to the [16 + 12] loading condition.

CONCLUSIONS

Pre-activation of trunk extensor muscles can serve to reduce the flexion displacements caused by rapid loading. The abdominal oblique muscles, especially external oblique, will rapidly increase their activation levels in response to rapid loading. These changes are more pronounced when pre-activation levels are low, resulting in lower initial trunk stiffness and spine compression force. It is proposed that these factors will ultimately affect spine stability and the risk of injury.

RELEVANCE

The results of this study provide insight into several mechanisms involved in the dynamic stability of the spine. Injuries can be caused by unexpected and rapid loading of the spine. A study of the mechanisms available to respond to such perturbations is important to an understanding of spine mechanics and the etiology of low back injury.

Prediction of biomechanical parameters in the lumbar spine during static sagittal plane lifting

A combined approach involving optimization and the finite element technique was used to predict biomechanical parameters in the lumbar spine during static lifting in the sagittal plane. Forces in muscle fascicles of the lumbar region were first predicted using an optimization-based force model including the entire lumbar spine. These muscle forces as well as the distributed upper body weight and the lifted load were then applied to a three-dimensional finite element model of the thoracolumbar spine and rib cage to predict deformation, the intradiskal pressure, strains, stresses, and load transfer paths in the spine. The predicted intradiskal pressures in the L3-4 disk at the most deviated from the in vivo measurements by 8.2 percent for the four lifting cases analyzed. The lumbosacral joint flexed, while the other lumbar joints extended for all of the four lifting cases studied (rotation of a joint is the relative rotation between its two vertebral bodies). High stresses were predicted in the posterolateral regions of the endplates and at the junctions of the pedicles and vertebral bodies. High interlaminar shear stresses were found in the posterolateral regions of the lumbar disks. While the facet joints of the upper two lumbar segments did not transmit any load, the facet joints of the lower two lumbar segments experienced significant loads. The ligaments of all lumbar motion segments except the lumbosacral junction provided only marginal moments. The limitations of the current model and possible improvements are discussed.

Effects of muscle dysfunction on lumbar spine mechanics. A finite element study based on a two motion segments model

STUDY DESIGN

A combined finite element and optimization approach was developed to investigate the clinically relevant biomechanical parameters of the muscular lumbar spine under five quasistatic back-lifting conditions.

OBJECTIVES

To quantify the effects of muscle "dysfunction" on the mechanical behavior of the lumbar spine.

SUMMARY OF BACKGROUND DATA

Trunk muscles have been proven to play an important role in the normal functioning of the spine. Although passive structures of the spine are believed to be subjected increasingly to mechanical stresses when muscular support is inadequate, supportive quantitative data have been lacking.

METHODS

External loads at L3-L4 for various lifting tasks were estimated experimentally and partitioned to the disc and muscles across the L3-L4 segment using an optimization scheme. These forces were incorporated into a finite element model of the ligamentous L3-L5 lumbar spine. Muscle "dysfunction" was simulated by decreasing the computed muscle forces.

RESULTS

The range of motion intradiscal pressure forces in ligaments, and load across facets increased nonlinearly with the increases in trunk flexion and the load held in hands. At higher loads or at larger flexed postures, muscles were found to play a more crucial role in stabilizing the spine compared with the passive structures. Muscle "dysfunction" destabilized the spine, reduced the role of facet joints in transmitting load, and shifted loads to the discs and ligaments.

CONCLUSIONS

Muscle dysfunction disturbs the normal functioning of other spinal components and may cause spinal disorders.