Static versus dynamic loading in the mechanical modulation of vertebral growth

Anterior vertebral tethering for adolescent idiopathic scoliosis: our initial ten year clinical experience

BACKGROUND

Anterior vertebral tethering (AVT) is a minimally invasive alternative to fusion surgery for adolescent idiopathic scoliosis (AIS) that offers the potential for definitive scoliosis treatment with the possibility of preservation of the growth, motion, function and overall health of the spine. This study represents our first ten years using AVT to treat AIS.

METHODS

In this retrospective review we analyzed our first 74 AIS patients treated with AVT 2010-2020. Multiple Lenke curve types 33-70° were treated with skeletal maturity spanning Risser -1 to 5.

RESULTS

Of 74 consecutive AIS patients treated with AVT, 52 patients (47 female, 5 male) had sufficient 2-year follow-up for inclusion. Forty-six of these 52 patients (88%) with 65 curves (35T, 30TL/L) were satisfactorily treated with AVT demonstrating curve correction from 48.6° pre-op (range 33°-70°) at age 15.1 years (range 9.2-18.8) and skeletal maturity of Risser 2.8 (range -1 to 5) to 23.2° post-op (range 0°-54°) and 24.0° final (range 0°-49°) at 3.3 years follow-up (range 2-10 years). Curve corrections from pre-op to post-op and pre-op to final were both significant (p < 0.001). The 0.8° change from post-op to final was not significant but did represent good control of scoliosis correction over time. Thoracic kyphosis and lumbar lordosis were maintained in a normal range throughout while axial rotation demonstrated a slight trend toward improvement. Skeletal maturity of Risser 4 or greater was achieved in all but one patient. Four of the 52 patients (8%) required additional procedures for tether rupture (3 replacements) or overcorrection (1 removal) to achieve satisfactory treatment status after AVT. An additional 6 of the 52 patients (12%), however, were not satisfactorily treated with AVT, requiring fusion for overcorrection (2) or inadequate correction (4).

CONCLUSIONS

In this study, AIS was satisfactorily treated with AVT in the majority of patients over a broad range of curve magnitudes, curve types, and skeletal maturity. Though late revision surgery for overcorrection, inadequate correction, or tether rupture was not uncommon, the complication of overcorrection was eliminated after our first ten patients by a refinement of indications.

LEVEL OF EVIDENCE

IV.

Static and dynamic stress analysis of different crown materials on a titanium base abutment in an implant-supported single crown: a 3D finite element analysis

BACKGROUND

This Finite Element Analysis was conducted to analyze the biomechanical behaviors of titanium base abutments and several crown materials with respect to fatigue lifetime and stress distribution in implants and prosthetic components.

METHODS

Five distinct designs of implant-supported single crowns were modeled, including a polyetheretherketone (PEEK), polymer-infiltrated ceramic network, monolithic lithium disilicate, and precrystallized and crystallized zirconia-reinforced lithium silicates supported by a titanium base abutment. For the static load, a 100 N oblique load was applied to the buccal incline of the palatal cusp of the maxillary right first premolar. The dynamic load was applied in the same way as in static loading with a frequency of 1 Hz. The principal stresses in the peripheral bone as well as the von Mises stresses and fatigue strength of the implants, abutments, prosthetic screws, and crowns were assessed.

RESULTS

All of the models had comparable von Mises stress values from the implants and abutments, as well as comparable maximum and minimum principal stress values from the cortical and trabecular bones. The PEEK crown showed the lowest stress (46.89 MPa) in the cervical region. The prosthetic screws and implants exhibited the highest von Mises stress among the models. The lithium disilicate crown model had approximately 9.5 times more cycles to fatique values for implants and 1.7 times more cycles to fatique values for abutments than for the lowest ones.

CONCLUSIONS

With the promise of at least ten years of clinical success and favorable stress distributions in implants and prosthetic components, clinicians can suggest using an implant-supported lithium disilicate crown with a titanium base abutment.

Short term outcomes and complications of distal ulnar ostectomy in 23 juvenile dogs with carpal valgus secondary to discordant radial-ulnar physeal growth

Objective

The goal of this study was to report short term clinical and radiographic outcomes after distal ulnar ostectomy in dogs with carpal valgus due to discordant radial-ulnar growth.

Study design

Retrospective case study.

Sample group

Client owned dogs under 1 year of age with carpal valgus and open distal radial physes pre-operatively.

Methods

Medical records from four veterinary referral centers were searched from January 1, 2015 to January 1, 2022 for juvenile dogs that had been treated with distal ulnar ostectomy for carpal valgus due to premature closure of the distal ulnar physis. Patients were excluded if they were skeletally mature at the time of ostectomy; medical records were incomplete; radial physis was closed at surgery; or definitive corrective osteotomy was performed. Radiographs were evaluated pre-operatively and for short term follow up at ~8 weeks. Complications and short term clinical outcomes were evaluated also.

Results

31 limbs from 23 dogs were evaluated. Patients ranged from 4 to 10.8 months of age. All dogs presented for visible carpal valgus and varying degrees of thoracic limb lameness. Sixty-four percent of patients showed resolution of lameness while an additional 13% showed an improvement in clinical lameness without complete resolution. Complications were seen in 32% of patients with 70% percent of those being minor, bandage related complications. Radiographically, 38% of limbs showed bridging callus formation of the ostectomy at an average of 7.5 weeks post operatively and 75% percent of patients with elbow incongruity improved radiographically. There was no significant difference in radial joint angles pre-operatively and at the time of follow up.

Conclusion

Distal ulnar ostectomy ameliorates lameness in juvenile dogs with premature distal ulnar physeal closure and shows lack of progression of distal carpal valgus deformity, but does not improve joint angulation.

Clinical significance

Distal ulnar ostectomy is associated with mild bandage-related complications and halting of progressive limb deformity within the time frame evaluated, and should therefore be considered a treatment for premature closure of the distal ulnar physis. It does not lead to deformity correction at 8 weeks following surgery but is associated with improved elbow congruity.

Biomechanical analysis of fixation methods in acetabular fractures: a systematic review of test setups

Purpose

Optimal anatomical reduction and stable fixation of acetabular fractures are important in avoiding secondary dislocation and osteoarthritis. Biomechanical studies of treatment options of acetabular fractures aim to evaluate the biomechanical properties of different fixation methods. As the setup of the biomechanical test can influence the experimental results, this review aimed to analyze the characteristics, comparability and clinical implications of studies on biomechanical test setups and finite element analyses in the fixation of acetabular fractures.

Methods

A systematic literature research was conducted according to the PRISMA guidelines, using the PubMed/MEDLINE and Web of Science databases. 44 studies conducting biomechanical analyses of fixation of acetabular fractures were identified, which met the predefined inclusion and exclusion criteria and which were published in English between 2000 and April 16, 2021. The studies were analyzed with respect to distinct parameters, including fracture type, material of pelvis model, investigated fixation construct, loading direction, loading protocol, maximum loading force, outcome parameter and measurement method.

Results

In summary, there was no standardized test setup within the studies on fixation constructs for acetabular fractures. It is therefore difficult to compare the studies directly, as they employ a variety of different test parameters. Furthermore, the clinical implications of the biomechanical studies should be scrutinized, since several test parameters were not based on observations of the human physiology.

Conclusion

The limited comparability and restricted clinical implications should be kept in mind when interpreting the results of biomechanical studies and when designing test setups to evaluate fixation methods for acetabular fractures.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00068-022-01936-9.

Contribution of postoperative vertebral remodeling to reversal of vertebral wedging and prevention of correction loss in patients with adolescent Scheuermann's kyphosis

OBJECTIVE

This study aimed to investigate reversal of vertebral wedging and to evaluate the contribution of vertebral remodeling to correction maintenance in patients with adolescent Scheuermann's kyphosis (SK) after posterior-only instrumented correction.

METHODS

A retrospective cohort study of patients with SK was performed. In total, 45 SK patients aged 10-20 years at surgery were included. All patients received at least 24 months of follow-up and had Risser sign greater than grade 4 at latest follow-up. Patients with Risser grade 3 or less at surgery were assigned to the low-Risser group, whereas those with Risser grade 4 or 5 were assigned to the high-Risser group. Radiographic data and patient-reported outcomes were collected preoperatively, immediately postoperatively, and at latest follow-up and compared between the two groups.

RESULTS

Remarkable postoperative correction of global kyphosis was observed, with similar correction rates between the two groups (p = 0.380). However, correction loss was slightly but significantly less in the low-Risser group during follow-up (p < 0.001). The ratio between anterior vertebral body height (AVBH) and posterior vertebral body height (PVBH) of deformed vertebrae notably increased in SK patients from postoperation to latest follow-up (p < 0.05). Loss of correction of global kyphosis was significantly and negatively correlated with increased AVBH/PVBH ratio. Compared with the high-Risser group, the low-Risser group had significantly greater increase in AVBH/PVBH ratio during follow-up (p < 0.05). The two groups had similar preoperative and postoperative Scoliosis Research Society-22 questionnaire scores for all domains.

CONCLUSIONS

Obvious reversal in wedge deformation of vertebrae was observed in adolescent SK patients. Patients with substantial growth potential had greater vertebral remodeling and less correction loss. Structural remodeling of vertebral bodies has a positive effect and protects against correction loss. These results could be help guide treatment decision-making.

Part 1. Review and meta-analysis of studies on modulation of longitudinal bone growth and growth plate activity: A macro-scale perspective

Growth modulation is an emerging method for treatment of angular skeletal deformities such as adolescent idiopathic scoliosis (AIS). The Hueter-Volkmann law, by which growth is stimulated in tension and inhibited in compression, is widely understood, and applied in current growth-modulating interventions such as anterior vertebral body tethering (AVBT) for AIS. However, without quantification of the growth rate effects of tension or compression, the possibility of under- or over- correction exists. A definitive mechanical growth modulation relationship relating to treatment of such skeletal deformities is yet to exist, and the mechanisms by which growth rate is regulated and altered are not fully defined. Review of current literature demonstrates that longitudinal (i.e., lengthwise) growth rate in multiple animal models depend on load magnitude, anatomical location, and species. Additionally, alterations in growth plate morphology and viability vary by loading parameters such as magnitude, frequency, and whether the load was applied persistently or intermittently. The aggregate findings of the reviewed studies will assist in work towards increasingly precise and clinically successful growth modulation methods. Part 1 of this review focuses on the effects of mechanical loading, species, age, and anatomical location on the macro-scale alterations in longitudinal bone growth, as well as factors that affect growth plate material properties. Part 2 considers the effects on micro-scale alterations in growth plate morphology such as zone heights and proportions, chondrocyte viability, and related gene and protein expression.

Relationship between bone strength index of the hemiparetic tibial diaphysis and muscle strength in people with chronic stroke: influence of muscle contraction type and speed

This study was conducted to examine the association between the tibial bone strength index and leg muscle strength of different contraction types and speeds among people with chronic stroke. We found that concentric muscle power at moderate speed was more associated with tibial bone strength index than other types.

INTRODUCTION

To compare the influence of muscle strength of different contraction types and speeds on the bone strength index of tibial diaphysis in people with chronic stroke.

METHODS

Eighty individuals with chronic stroke (age: 62.6 ± 8.0 years; men/women: 46/34; post-stroke duration: 9.0 ± 5.4 years) underwent scanning of the tibia at the 66% site on both sides using peripheral quantitative computed tomography. Each participant was also evaluated for isometric and dynamic (at 60°/s and 120°/s) strength of knee flexors/extensors and ankle dorsiflexors/plantarflexors using an isokinetic dynamometer. For a given contraction type and speed, the strength values of the four muscle groups were summed to yield a composite score. Multiple regression analysis was used to identify the association of the percent side-to-side difference (%SSD) in tibial polar-stress-strain index (p-SSI) with %SSD in muscle strength of different contraction types and speeds.

RESULTS

The p-SSI and all muscle strength parameters on the paretic side had lower values than the non-paretic side (p ≤ 0.001). The %SSD in concentric muscle power at angular speed of 60°/s (R² = 0.317, p = 0.006) and 120°/s (R² = 0.298, p = 0.020) remained independently associated with that in p-SSI, after controlling for age, sex, body mass index, post-stroke duration, motor impairment, spasticity, and physical activity level. The effect of isometric strength and eccentric muscle power was not significant in multivariate analysis.

CONCLUSIONS

Concentric muscle power was more predictive of %SSD in p-SSI at the tibial diaphysis than other contraction types and may be an important target for intervention to promote bone health in people with chronic stroke.

Clinical Outcome of Magnetically Controlled Growing Rod in Early-onset Scoliosis: A Systematic Review

STUDY DESIGN

This was a systematic literature review.

OBJECTIVE

To evaluate the efficacy and safety of magnetically controlled growing rod (MCGR) in treating early-onset scoliosis.

SUMMARY OF BACKGROUND DATA

Preliminary studies reported the promising potential of MCGR in deformity correction and spine growth preservation with noninvasive distraction. This study was conducted on the basis of data from published literatures.

MATERIALS AND METHODS

A systematic search from multimedical database was conducted for studies of MCGR. Involved studies should provide at least one of the followings: deformity correction, spine growth, and complication. During treatment period, improvement or progression of scoliosis/kyphosis, annual T1-S1/T1-T12 extension, incidence of complication, and unplanned surgery were summarized and the correlations with term of follow-up were investigated.

RESULTS

In total, 13 studies (n=249) were involved, with an average of 22.2-month follow-up. Scoliosis correction was well maintained, improving from 36.4 degrees at postinitial to 37.1 degrees at last follow-up (P=0.794). Kyphosis progressed from 28.8 degrees at postinitial to 34.4 degrees at last follow-up (P=0.024). Annual T1-S1 and T1-T12 longitudinal extensions were 8.7 and 4.7 mm/year, respectively. As follow-up went on, no significant decline was observed in annual extension of T1-S1 (P=0.4680) or T1-T12 (P=0.8053). The incidences of alignment-related, implant-related, and wound-related complication were 3.5%, 30.1%, and 6.9%, respectively. No significant correlation was observed between occurrence of complication and term of follow-up. The rate of receiving unplanned surgery was 24.3%.

CONCLUSIONS

MCGR is efficient in the maintenance of coronal correction and preservation of spine growth, with loss of sagittal correction and a 24.3% unplanned revision rate.

Cyclically controlled vertebral body tethering for scoliosis: an in vivo verification in a pig model of the pressure exerted on vertebral end plates

STUDY DESIGN

Experimental in vivo study of the pressure exerted on the spine of a pig by a new cyclic anterior vertebral body tethering (AVBT) prototype.

OBJECTIVES

To evaluate the relationship between the tether tension and the pressures transmitted onto the vertebral end plates by a cyclic AVBT prototype. AVBT is a recent surgical technique for the treatment of pediatric scoliosis that compresses the convex side of the spine with a sustained tension, to modulate the growth to progressively correct the deformity over time. Previous studies demonstrated that cyclic compression has similar growth modulation capacity but with less detrimental effects on the integrity of the discs and growth plates.

METHODS

A 3-month-old healthy Duroc pig was anesthetized and a lateral thoracotomy was performed. The T7-T10 segment was instrumented and compressed during 50 s with the load oscillating (0.2 Hz) from + 30 to - 30% of the following mean tensions: 29, 35, 40, 44, and 49 N. The pressure exerted on T9 superior vertebral end plate was monitored during the cyclic loading. Three repetitions of each test were performed.

RESULTS

The resulting mean pressure exerted on the vertebral end plate was linearly correlated with the mean tether tension (r² = 0.86). Each cycle translated in a hysteresis profile of the measured pressure and tension, with amplitudes varying between ± 11.5 and ± 29.9%.

CONCLUSIONS

This experimental study documented the relationship between the tether tension and the pressure. This study confirmed the feasibility of cyclic AVBT principle to transfer varying pressures on the vertebral end plates, which is intended to control vertebral growth, while keeping the spine flexibility and preserving the health of soft tissues such as the intervertebral discs and the growth plate but remained to be further verified.

LEVEL OF EVIDENCE

Level IV.

Mechanical stimulation of growth plate chondrocytes: Previous approaches and future directions

Growth plate cartilage resides near the ends of long bones and is the primary driver of skeletal growth. During growth, both intrinsically and extrinsically generated mechanical stresses act on chondrocytes in the growth plate. Although the role of mechanical stresses in promoting tissue growth and homeostasis has been strongly demonstrated in articular cartilage of the major skeletal joints, effects of stresses on growth plate cartilage and bone growth are not as well established. Here, we review the literature on mechanobiology in growth plate cartilage at macroscopic and microscopic scales, with particular emphasis on comparison of results obtained using different methodological approaches, as well as from whole animal and in vitro experiments. To answer these questions, macroscopic mechanical stimulators have been developed and applied to study mechanobiology of growth plate cartilage and chondrocytes. However, the previous approaches have tested a limited number of stress conditions, and the mechanobiology of a single chondrocyte has not been well studied due to limitations of the macroscopic mechanical stimulators. We explore how microfluidics devices can overcome these limitations and improve current understanding of growth plate chondrocyte mechanobiology. In particular, microfluidic devices can generate multiple stress conditions in a single platform and enable real-time monitoring of metabolism and cellular behavior using optical microscopy. Systematic characterization of the chondrocytes using microfluidics will enhance our understanding of how to use mechanical stresses to control the bone growth and the properties of tissue-engineered growth plate cartilage.

A Microfluidic Platform for Stimulating Chondrocytes with Dynamic Compression

Mechanical stimuli are known to modulate biological functions of cells and tissues. Recent studies have suggested that compressive stress alters growth plate cartilage architecture and results in growth modulation of long bones of children. To determine the role of compressive stress in bone growth, we created a microfluidic device actuated by pneumatic pressure, to dynamically (or statically) compress growth plate chondrocytes embedded in alginate hydrogel cylinders. In this article, we describe detailed methods for fabricating and characterizing this device. The advantages of our protocol are: 1) Five different magnitudes of compressive stress can be generated on five technical replicates in a single platform, 2) It is easy to visualize cell morphology via a conventional light microscope, 3) Cells can be rapidly isolated from the device after compression to facilitate downstream assays, and 4) The platform can be applied to study mechanobiology of any cell type that can grow in hydrogels.

Practical Aspects of Finite Element Method Applications in Dentistry

The use of numerical methods, such as finite element method (FEM), has been widely adopted in solving structural problems with complex geometry under external loads when analytical solutions are unachievable. Basic idea behind FEM is to divide the complex body geometry into smaller and simpler domains, called finite elements, and then to formulate solution for each element instead of seeking a solution for the entire domain. After finding the solutions for all elements they can be combined to obtain a solution for the whole domain. This numerical method is mostly used in engineering, but it is also useful for studying the biomechanical properties of materials used in medicine and the influence of mechanical forces on the biological systems. Since its introduction in dentistry four decades ago, FEM became powerful tool for the predictions of stress and strain distribution on teeth, dentures, implants and surrounding bone. FEM can indicate aspects of biomaterials and human tissues that can hardly be measured in vivo and can predict the stress distribution in the contact areas which are not accessible, such as areas between the implant and cortical bone, denture and gingiva, or around the apex of the implant in trabecular bone. Aim of this paper is to present - using results of several successful FEM studies - the usefulness of this method in solving dentistry problems, as well as discussing practical aspects of FEM applications in dentistry. Some of the method limitations, such as impossibility of complete replication of clinical conditions and need for simplified assumptions regarding loads and materials modeling, are also presented. However, the emphasis is on FE modelling of teeth, bone, dentures and implants and their modifications according to the requirements. All presented studies have been carried out in commercial software for FE analysis ANSYS Workbench.

Spinal growth modulation with posterior unilateral elastic tether in immature swine model

BACKGROUND CONTEXT

Fusionless scoliosis surgery is frequently performed in children. Many studies have analyzed the effects of spinal growth modulation by tethering the anterior and anterolateral aspects of the spine in animal models. However, few studies have reported the disc health and spinal motion in spines with posterior unilateral elastic tethering.

PURPOSE

To analyze the regional radiography, biochemistry, and histology of spinal motion segments fixed by posterior unilateral elastic tethering.

STUDY DESIGN

A randomized controlled trial.

OUTCOME MEASURES

Preoperative and postoperative radiographs of the spines were taken. After an 8-week recovery period, the spines were harvested en bloc and underwent radiographic, biochemical, and histologic analyses.

METHODS

Fifteen 3-month-old swine were randomly divided into three groups. Instrumentation was performed posteriorly in the swine. In the elastic fixation (EF) group, five swine were instrumented on the left side of the lumbar vertebrae from L1 to L5 with pedicle screws that were connected with a unilateral elastic tether with tension to produce a curve on the spine. The same surgery was performed in the five animals of the metal rod fixation (MF) group, in which the screws were connected with metal rods and curves were established. In the control group, five animals were instrumented with five screws with no connecting cable.

RESULTS

Scoliosis and lordosis were created in the coronal and sagittal planes in both the EF and MF groups. On average, the Cobb angles were 12.16°±1.37° and 9.10°±2.02° (p=.023) in the coronal plane and 17.44°±11.29° and 5.32°±3.06° (p=.049) in the sagittal plane in the two groups, respectively. The vertebrae and discs wedged on the tethered side in the two groups showed no significant differences (p>.05). The thickness of end-plate epiphysis on the fixed side was significantly decreased in the two groups (p=.032 and p=.024). No apparent change was found in the gross morphology of the discs in the two groups. The distribution of collagen types I and II decreased and that of matrix metalloprotease-3 (MMP-3) increased in both the EF and MF groups. Additionally, the proteoglycan synthesis decreased in the two groups.

CONCLUSIONS

Unilateral elastic tethering resulted in vertebral wedging and scoliosis. Although changes in collagen and MMP-3 distribution, proteoglycan synthesis, end-plate epiphysis, and disc thickness were observed, the tethered discs and end plates did not demonstrate gross morphologic signs of degeneration.

In vivo dynamic loading reduces bone growth without histomorphometric changes of the growth plate

This in vivo study aimed at investigating the effects of dynamic compression on the growth plate. Rats (28 days old) were divided into three dynamically loaded groups, compared with two groups (control, sham). A device was implanted on the 6th and 8th caudal vertebrae for 15 days. Controls (n = 4) did not undergo surgery. Shams (n = 4) were operated but not loaded. Dynamic groups had sinusoidal compression with a mean value of 0.2 MPa: 1.0 Hz and ± 0.06 MPa (group a, n = 4); 0.1 Hz and ± 0.2 MPa (group b, n = 4); 1.0 Hz and ± 0.14 MPa (group c, n = 3). Growth rates (µm/day) of dynamic groups (a) and (b) were lower than shams (p < 0.01). Growth plate heights, hypertrophic cell heights and proliferative cell counts per column did not change in dynamic (a) and (b) groups compared with shams (p > 0.01). Rats from dynamic group (c) had repeated inflammations damaging tissues; consequently, their analysis was unachievable. Increasing magnitude or frequency leads to growth reduction without histomorphometric changes. However, the combined augmentation of magnitude and frequency alter drastically growth plate integrity. Appropriate loading parameters could be leveraged for developing novel growth modulation implants to treat skeletal deformities.

Potential regenerative rehabilitation technology: implications of mechanical stimuli to tissue health

BACKGROUND

Mechanical loads induced through muscle contraction, vibration, or compressive forces are thought to modulate tissue plasticity. With the emergence of regenerative medicine, there is a need to understand the optimal mechanical environment (vibration, load, or muscle force) that promotes cellular health. To our knowledge no mechanical system has been proposed to deliver these isolated mechanical stimuli in human tissue. We present the design, performance, and utilization of a new technology that may be used to study localized mechanical stimuli on human tissues. A servo-controlled vibration and limb loading system were developed and integrated into a single instrument to deliver vibration, compression, or muscle contractile loads to a single limb (tibia) in humans. The accuracy, repeatability, transmissibility, and safety of the mechanical delivery system were evaluated on eight individuals with spinal cord injury (SCI).

FINDINGS

The limb loading system was linear, repeatable, and accurate to less than 5, 1, and 1 percent of full scale, respectively, and transmissibility was excellent. The between session tests on individuals with spinal cord injury (SCI) showed high intra-class correlations (>0.9).

CONCLUSIONS

All tests supported that therapeutic loads can be delivered to a lower limb (tibia) in a safe, accurate, and measureable manner. Future collaborations between engineers and cellular physiologists will be important as research programs strive to determine the optimal mechanical environment for developing cells and tissues in humans.

Large animal models in fusionless scoliosis correction research: a literature review

BACKGROUND CONTEXT

Numerous prenatal, systemic, or local procedures have been described that have created an experimental scoliosis within different animal species. Compression-based fusionless scoliosis correction devices have been used to induce scoliosis (inverse approach) as an indication for their potential corrective efficacy in large animals. Deformities that most closely approximate the three-dimensional nature of an idiopathic-like scoliosis have been created in large animals using a posterior spinal tether. Fusionless scoliosis correction devices have subsequently been tested in these models.

PURPOSE

To provide an overview of large animal models used for preclinical testing of fusionless scoliosis correction devices and to describe recent advances in the creation of an idiopathic-like scoliosis large animal model.

STUDY DESIGN

Literature review of large animal models in fusionless scoliosis correction research.

METHODS

MEDLINE electronic database was searched for studies in which large animal models for spinal or vertebral growth modulation or the creation of an experimental scoliosis were described. The literature search was limited to articles written in the English language.

RESULTS

The pig appears to be the most suitable animal species for preclinical testing of fusionless scoliosis correction devices because of its large growth potential and the possibility for early weaning. With the inverse approach, it is difficult to gain insight into the possible corrective efficacy of the tested device, and therefore, a two-step approach is preferred. Using a posterior spinal tether, persistent spinal deformities are attained when the deformity has approximately doubled in comparison to the postoperative measure in a time span of approximately 12 weeks. Sufficient tether midline offset is required to render rib procedures unnecessary.

CONCLUSIONS

An idiopathic-like scoliosis animal model can be created using a posterior spinal tether in a fully reversible procedure. Experimental results will need to be reproduced to establish a standard idiopathic-like scoliosis large animal model.

In vivo dynamic compressive stresses in the disc annulus: a pilot study of bilateral differences due to hemiepiphyseal implant in a quadruped model

STUDY DESIGN

In vivo biomechanical study in quadruped model.

OBJECTIVE

To develop in vivo model capable of determining physiological compressive stresses bilaterally in the intervertebral disc annulus and preliminarily assess effects of a hemiepiphyseal implant.

SUMMARY OF BACKGROUND DATA

Spine growth modification alters stress distributions in vertebral growth plates and discs. Quantification of stresses is required to help assess implant efficacy and disc health. More generally, despite widespread and necessary use of animals in preclinical studies of spine instrumentation, limited quantitative information is available on mechanobiological conditions in quadruped spines for comparisons with those of humans.

METHODS

Skeletally immature domestic pigs were instrumented with an implant and 4 stress sensors. Sensors were inserted into left and right sides of the annulus at 2 thoracic levels. A titanium staple-screw construct was implanted at 1 level. Signals were acquired intraoperatively, postoperatively during normal activities, and biweekly with the animal under anesthesia, for up to 8 weeks.

RESULTS

Stresses varied by sensor location relative to implant, postoperative time, activity, and animal. Intraoperatively, the mean peak stress due to staple insertion was 1.6 MPa at the sensor nearest the staple. Mean stress at the end of surgery was 0.23 MPa. Mean stress standing the first day was 0.38 MPa. Dynamic stresses were recorded at all locations, including the location nearest the staple. Highest mean stresses were those nearest the implant. With the animal under anesthesia, the dynamic stress range in the resting prone position was 0.1 MPa, whereas this range was 0.9 MPa when the spine was manually flexed.

CONCLUSION

Compressive stresses were dynamic at both control and stapled levels, which indicated that the disc was not immobilized by the implant. These pilot results suggested that mean disc compression was increased within the first postoperative week. Stresses ranged up to levels measured in humans.

In vivo dynamic bone growth modulation is less detrimental but as effective as static growth modulation

Longitudinal bone growth, which occurs in growth plates, has important implications in pediatric orthopedics. Mechanical loads are essential to normal bone growth, but excessive loads can lead to progressive deformities. In order to compare the effects of in vivo static and dynamic loading on bone growth rate and growth plate histomorphometry, a finely controlled, normalized and equivalent compression was applied for a period of two weeks on the seventh caudal vertebra (Cd7) of rats during their pubertal growth spurt. The load was sustained (0.2MPa, 0.0Hz) in the static group and sinusoidally oscillating (0.2MPa±30%, 0.1Hz) in the dynamic group. Control and sham (operated but no load applied) groups were also studied. Cd7 growth rate was statistically reduced by 19% (p<0.001) for both static and dynamic groups when compared to the sham group. Loading effects on growth plate histomorphometry were greater in the static than dynamic groups with significant reductions (p<0.001) observed for growth plate thickness, proliferative chondrocyte number per column and hypertrophic chondrocyte height in the static group when compared to the sham group. Significant differences (p<0.01) were also found between static and dynamic groups for growth plate thickness and proliferative chondrocyte number per column while the difference nearly reached significance (p=0.014) for hypertrophic chondrocyte height. This in vivo study shows that static and dynamic loading are equally effective in modulating bone growth of rat caudal vertebrae. However, dynamic loading causes less detrimental effects on growth plate histomorphometry compared to static loading. This knowledge is greatly relevant for the improvement and/or development of new minimally invasive approaches, which are based on the local modulation of bone growth, to correct several progressive musculoskeletal deformities.

The creation of scoliosis by scapula-to-contralateral ilium tethering procedure in bipedal rats: a kyphoscoliosis model

STUDY DESIGN

Randomized trial.

OBJECTIVE

To create a new scoliotic model.

SUMMARY OF BACKGROUND DATA

Although there were a lot of modeling techniques producing scoliosis, failed was the creation of a scoliotic animal model all characterized by the evident axial rotation of vertebrae body, the simulation of the human erect posture, and avoiding direct traumas to the spine, the spinal cord, ribs, or glands in modeling techniques.

METHODS

A total of 45 4-week-old female wistar rats were randomly divided into three groups. Group 1 underwent subcutaneous left scapula-to-contralateral ilium tethering procedure with a nonadsorbable suture, which made the spine convex toward right side, and then removed forelimbs and tails of rats to create the bipedal rats. Tethering sutures were cut at postoperative eighth week, and the spines of rats were then observed during 2 weeks. Group 2 was the same as group 1 but in which scapula-to-ipsilateral ilium tethering procedure was performed. Group 3 was the same as group 1 except that the bipedal rats were not created. All postoperative rats were fed separately in special high cages for groups 1 and 2 or in standard cages for group 3.

RESULTS

At 2 weeks after tether release, the incidence of vertebral rotation was significantly higher in group 1 than in group 2 (P = 0.004). The differences in degrees of scoliosis and kyphosis between groups at the time of initial tethering were not found to be significant (P > 0.05), whereas those at 2 weeks after tether release were significantly larger in group 1 than in group 3 (P < 0.01). There were no significant differences in postoperative first food-taking duration, body weight, spinal relative length, modeling mortality, the incidences of reoperation, and scoliosis between groups (all P > 0.05).

CONCLUSION

The scoliotic model created by scapula-to-contralateral ilium tethering procedure in bipedal rats can preferably simulate the human scoliosis.

Growth plate explants respond differently to in vitro static and dynamic loadings

This study aimed at investigating the effects of static and dynamic compression applied on growth plate explants using matched compressive strains. Growth plate explants from 4-week-old swine ulnae were submitted to in vitro static (10% strain) or dynamic (oscillating between 7% and 13% at 0.1 Hz) unconfined compression for 48 h. The total growth plate height, the combined proliferative and hypertrophic thickness and the resulting ratio between these two thicknesses were evaluated. Standard immunohistochemistry was used to analyze the protein expression of key components of the extracellular matrix: aggrecan, type II collagen, type X collagen, and MMP13. In the statically loaded samples, the columnar organization of the cells was preserved but with slight columns deviation from the growth axis. Decreases in all histomorphological parameters were important and a notable loss of aggrecan, type II and type X collagens expressions was denoted. In the dynamically loaded samples, a severe loss of columnar arrangement was observed in the proliferative and hypertrophic zones. However, dynamic compressive loads preserved the proliferative and hypertrophic zones ratio and contributed to the synthesis of aggrecan and type II collagen in the extracellular matrix. The exact response of the growth plate to mechanical stresses along with optimal loading parameters could help improve the current treatment approaches or develop new treatment approaches for the underlying progressive musculoskeletal deformities.

Nonfusion treatment of adolescent idiopathic scoliosis by growth modulation and remodeling

BACKGROUND

Adolescent idiopathic scoliosis (AIS) is a common disorder in which the spine gradually develops a curvature that is first detected in patients between 11 and 17 years of age. The only accepted treatment methods are bracing and surgery. Whether brace treatment alters the natural history is being questioned, and patient compliance is low. Surgery usually includes a spinal fusion that creates a rigid spine and concentrates stresses at the ends.

METHODS

This study focuses on correlating the laboratory results with clinical reports for treating patients with AIS. In the laboratory, scoliosis with vertebral wedging has been created by asymmetric mechanical loading and has been corrected by reversing the loading. In the clinic, bracing and derotational casting have been successful in some reports, but compliance has been a problem with bracing and derotational casts have mainly been used in young children. Operative treatment has been successful, but a nonfusion operation remains elusive. FINDINGS AND RESULTS: In the laboratory, axial loading of growth plates altered growth according to the Hueter-Volkmann law, which states that compression decreases and distraction increases growth. Asymmetric loading of the spine caused asymmetric growth resulting in scoliosis with vertebral wedging. Asymmetric loading of tail vertebrae created vertebral wedging according to Wolff's law, which states that the bone remodels over time in response to prevailing mechanical demands. In the clinic, studies have shown that bracing may work if patients wore the brace as prescribed. Derotational casting in young children has been shown to prevent progression and even correct the scoliosis in some patients. Convex vertebral stapling has been successful in mild curves, but the results in larger curves have been disappointing. Anterolateral tethering has been successful in mild curves in young patients, but there is limited experience with this technique in patients with large curves.

CONCLUSIONS

A brace that applies the appropriate loading and is worn as prescribed may dramatically improve the results of brace treatment. A procedure using external fixation or adjustable anterolateral tethering may achieve a nonfusion correction of AIS.

LEVEL OF EVIDENCE

Level II.

Coronal plane segmental flexibility in thoracic adolescent idiopathic scoliosis assessed by fulcrum-bending radiographs

Knowledge about segmental flexibility in adolescent idiopathic scoliosis is crucial for a better biomechanical understanding, particularly for the development of fusionless, growth-guiding techniques. Currently, there is lack of data in this field. The objective of this study was, therefore, to compute segmental flexibility indices (standing angle minus corrected angle/standing angle). We compared segmental disc angles in 76 preoperative sets of standing and fulcrum-bending radiographs of thoracic curves (paired, two-tailed t tests, p < 0.05). The mean standing Cobb angle was 59.7 degrees (range 41.3 degrees -95 degrees ) and the flexibility index of the curve was 48.6% (range 16.6-78.8%). The disc angles showed symmetric periapical distribution with significant decrease (all p values <0.0001) for every cephalad (+) and caudad (-) level change. The periapical levels +1 and -1 wedged at 8.3 degrees and 8.7 degrees (range 3.5 degrees -14.8 degrees ), respectively. All angles were significantly smaller on the-bending views (p values <0.0001). We noted mean periapical flexibility indices of 46% (+1), 49% (-1), 57% (+2) and 81% (-2), which were significantly less (p < 0.001) than for the group of remote levels 105% (+3), 149% (-3), 231% (+4) and 300% (-4). The discal and bony wedging was 60 and 40%, respectively, and mean values 35 degrees and 24 degrees (p < 0.0001). Their relationship with the Cobb angle showed a moderate correlation (r = 0.56 and 0.45). Functional, radiographic analysis of idiopathic thoracic scoliosis revealed significant, homogenous segmental tethering confined to four periapical levels. Future research will aim at in vivo segmental measurements in three planes under defined load to provide in-depth data for novel therapeutic strategies.

Growth plate mechanics and mechanobiology. A survey of present understanding

The longitudinal growth of long bones occurs in growth plates where chondrocytes synthesize cartilage that is subsequently ossified. Altered growth and subsequent deformity resulting from abnormal mechanical loading is often referred to as mechanical modulation of bone growth. This phenomenon has key implications in the progression of infant and juvenile musculoskeletal deformities, such as adolescent idiopathic scoliosis, hyperkyphosis, genu varus/valgus and tibia vara/valga, as well as neuromuscular diseases. Clinical management of these deformities is often directed at modifying the mechanical environment of affected bones. However, there is limited quantitative and physiological understanding of how bone growth is regulated in response to mechanical loading. This review of published work addresses the state of knowledge concerning key questions about mechanisms underlying biomechanical modulation of bone growth. The longitudinal growth of bones is apparently controlled by modifying the numbers of growth plate chondrocytes in the proliferative zone, their rate of proliferation, the amount of chondrocytic hypertrophy and the controlled synthesis and degradation of matrix throughout the growth plate. These variables may be modulated to produce a change in growth rate in the presence of sustained or cyclic mechanical load. Tissue and cellular deformations involved in the transduction of mechanical stimuli depend on the growth plate tissue material properties that are highly anisotropic, time-dependent, and that differ in different zones of the growth plate and with developmental stages. There is little information about the effects of time-varying changes in volume, water content, osmolarity of matrix, etc. on differentiation, maturation and metabolic activity of chondrocytes. Also, the effects of shear forces and torsion on the growth plate are incompletely characterized. Future work on growth plate mechanobiology should distinguish between changes in the regulation of bone growth resulting from different processes, such as direct stimulation of the cell nuclei, physico-chemical stimuli, mechanical degradation of matrix or cellular components and possible alterations of local blood supply.

A porcine model for progressive thoracic scoliosis

STUDY DESIGN

An IACUC-approved study to create a scoliotic deformity representative of adolescent idiopathic scoliosis.

OBJECTIVE

The goal of this study was to develop a reliable porcine scoliosis model and to evaluate the three-dimensional progression of the deformity.

SUMMARY OF BACKGROUND DATA

Optimal development of nonfusion techniques for treatment of adolescent idiopathic scoliosis requires a reliable large animal model that achieves a progressive three-dimensional (frontal, sagittal, axial) deformity. Limitations in previous work have led our team to the development of a porcine model.

METHODS

This IACUC-approved study included 18 Yorkshire pigs, obtained at 11 weeks old. Scoliosis was induced through unilateral posterior ligament tethering of the spine via pedicle screw fixation, and ipsilateral ribcage tethering. Progressive deformity was documented with biweekly radiographs. Frontal, sagittal, and axial modifications were assessed using the Cobb method. Animals were observed until severe deformity (>50 degrees) developed, then killed.

RESULTS

Animals were observed for a mean 11 weeks. The mean coronal Cobb angle was 25 degrees immediately postoperatively and 55 degrees at 11 weeks. The mean lordosis increased from 4 degrees postoperative to 24 degrees at final follow-up. Apical axial rotation (posterior elements into concavity) increased from 4% postoperative to 27% at 11 weeks. Rate of coronal curve progression was significantly correlated with the initial Cobb index.

CONCLUSION

This study establishes a porcine scoliosis model. With placement of a unilateral ligamentous spinal tether combined with concave ribcage ligament tethering a three-dimensional (frontal, sagittal, and axial) spinal deformity can be obtained. The speed of the progressive deformity leaves significant remaining skeletal growth to assess growth modulating therapies for correction. This work forms the basis for a number of investigative efforts at developing new fusionless therapies for patients suffering from adolescent scoliosis.

Growth modulation in the management of growing spine deformities

The Hueter-Volkmann law explains the physiological response of the growth plate under mechanical loading. This law mainly explains the pathological mechanism for growing long-bone deformities. Vertebral endplates also show a similar response under mechanical loading. Experimental studies have provided information about spinal growth modulation and, now, it is possible to explain the mechanism of the curvature progression. Convex growth arrest is shown to successfully treat deformities of the growing spine and unnecessary growth arrest of the whole spine is prevented. Both anterior and posterior parts of the convexity should be addressed to achieve a satisfactory improvement in the deformity, albeit epiphysiodesis effect cannot be stipulated at all times. Anterior vertebral body stapling without fusion yielded better results with new shape memory alloys and techniques. This method can be used with minimally invasive techniques and has the potential advantage of producing reversible physeal arrest. Instrumented posterior hemiepiphysiodesis seems to be as effective as classical combined anterior and posterior arthrodesis, where it is less invasive and morbid. Convex hemiepiphysiodesis with concave-side distraction through growing rod techniques provide a better control of the curve immediately after surgery. This method has the advantages of posterior instrumented hemiepiphysiodesis, but necessitates additional surgeries. Concave-side rib shortening and/or convex-side lengthening is an experimental method with an indirect effect on spinal growth. To conclude, whatever the cause of the spinal deformity, growth modulation can be used to manage the growing spine deformities with no or shorter segment fusions.

PRINCIPLES OF GROWTH MODULATION IN THE TREATMENT OF SCOLIOTIC DEFORMITIES

INTRAOPERATIVE MANIPULATION TO correct scoliotic deformities relies upon spinal instrumentation for stabilization and fusion. However, novel strategies and innovative implant biotechnologies have emerged, applying natural growth and elongation of the immature spine for the treatment of scoliosis in young patients. In this work, we review the principles of growth modulation and the Hueter-Volkmann law as it applies to experimental models of scoliosis formation and correction. Current implant technologies, including shape memory alloy vertebral staples, growing rods, and vertical expandable titanium prosthetic ribs, are explored, with regards to implant design, surgical techniques, and clinical investigations. An exciting area of spinal implant technology is now becoming available to expand the surgical armamentarium for treating severe scoliotic deformity in young patients.

A novel fusionless vertebral physeal device inducing spinal growth modulation for the correction of spinal deformities

Current fusionless scoliosis surgical techniques span the intervertebral disc. This alters the spine stiffness, disc pressure equilibrium and possibly may lead to disc degeneration. A new fusionless physeal device was developed that locally modulates vertebral growth by compressing the physeal ring, while maintaining maximum segmental spinal mobility without spanning the intervertebral disc. This study's objective was to test the feasibility of the device on a small animal model by inducing a scoliotic deformity (inverse approach) while analyzing the growth modifications. This study was conducted on caudal vertebrae of 21 rats (26-day-old) divided into 3 groups: (1) "experimental" (n = 11) with 4 instrumented vertebrae, (2) sham (n = 5) and (3) control (n = 5). Radiographs were taken at regular intervals during the 7-week experimental period. Tissues were embedded in methyl metacrylate (MMA), prepared by the cutting/grinding method, and then stained (Toluidine blue). The discs physiological alterations were qualitatively assessed and classified by inspection of the histological sections. A mean maximum Cobb angle of 30 masculine (+/-6 masculine) and a mean maximum vertebral wedge angle of 10 masculine (+/-3 masculine) were obtained between the 23rd and 35th day postoperative in the subgroup that underwent a long-term response from the device. The sham group underwent no growth alterations when compared to the control group. Descriptive histological analyses of the operated segments showed that 69% had no alterations to the intervertebral disc. This study presents experimental evidence that the device induces a significant and controlled wedging of the vertebrae while maintaining regular flexibility. In most discs, there were no visible morphological alterations induced. Further analysis of the discs and testing of this device on a larger animal is recommended with the long-term objective of developing an early treatment of progressive idiopathic scoliosis.

Fusionless treatment of scoliosis

The recent investigations of convex anterior vertebral body stapling have offered promising early results with use of improved implants and techniques. The use of a shape memory alloy staple tailored to the size of the vertebral body, the application of several staples per level, the instrumentation of the Cobb levels of all curves, and the employment of minimally invasive thoracoscopic approaches all offer substantial improvements over previous fusionless techniques. Patient selection may also play a role in the current success of these fusionless treatments, with perhaps the ideal candidates for this intervention possessing smaller and more flexible curves. Long-term results of the effects on the instrumented motion segments and adjacent spine are not yet available.