Evaluation of porous biphasic calcium phosphate ceramics for anterior cervical interbody fusion in a caprine model

[Application of bone-plastic materials for mastoidoplasty]

The aim of this study is comparative assessment of of different bone-plastic materials for mastoidoplasty. The study included 30 patients who underwent 'canal wall down' mastoid surgery with mastoidoplasty. The following criteria were used for comparing: 1) simplicity of modeling of the material 2) the wound healing process 3) osteointegration - the process of formation of 'new bone' tissue (CT of temporal bones 6 and 12 months after oreretion). It was confirmed that the main groups of bone-plastic material (bioglass, composite materials and β-tricalcium phosphate) is suitable for mastoidoplasty after 'canal wall down' mastoid surgery.

Load-sharing through elastic micro-motion accelerates bone formation and interbody fusion

BACKGROUND CONTEXT

Achieving a successful spinal fusion requires the proper biological and biomechanical environment. Optimizing load-sharing in the interbody space can enhance bone formation. For anterior cervical discectomy and fusion (ACDF), loading and motion are largely dictated by the stiffness of the plate, which can facilitate a balance between stability and load-sharing. The advantages of load-sharing may be substantial for patients with comorbidities and in multilevel procedures where pseudarthrosis rates are significant.

PURPOSE

We aimed to evaluate the efficacy of a novel elastically deformable, continuously load-sharing anterior cervical spinal plate for promotion of bone formation and interbody fusion relative to a translationally dynamic plate.

STUDY DESIGN/SETTING

An in vivo animal model was used to evaluate the effects of an elastically deformable spinal plate on bone formation and spine fusion.

METHODS

Fourteen goats underwent an ACDF and received either a translationally dynamic or elastically deformable plate. Animals were followed up until 18 weeks and were evaluated by plain x-ray, computed tomography scan, and undecalcified histology to evaluate the rate and quality of bone formation and interbody fusion.

RESULTS

Animals treated with the elastically deformable plate demonstrated statistically significantly superior early bone formation relative to the translationally dynamic plate. Trends in the data from 8 to 18 weeks postoperatively suggest that the elastically deformable implant enhanced bony bridging and fusion, but these enhancements were not statistically significant.

CONCLUSIONS

Load-sharing through elastic micro-motion accelerates bone formation in the challenging goat ACDF model. The elastically deformable implant used in this study may promote early bony bridging and increased rates of fusion, but future studies will be necessary to comprehensively characterize the advantages of load-sharing through micro-motion.

CERVICAL SPINOLAMINOPLASTY WITH NEWLY DESIGNED TITANIUM MINI-PLATES

One of the complication of cervical laminoplasty is the restenosis of the opened laminae. Weakness of the screws placed on laminae may cause restenosis. Here, we describe a new technic ‘spinolaminoplasty (Turkish Open-door laminoplasty)’ with newly designed titanium mini plate that placed one side to lateral mass, and the other on spinous process to keep the laminae opened. Two different fixation materials were used for axial compression tests. One was Ultra high molecular weight polyethylene block with cervical vertebrae geometry and fresh ovine cervical vertebrae. In the first group, mini plates were fixed on laminae as in the conventional method described by Hirabayashi. In the second group, mini plates were fixed on spinous process to perform spinolaminoplasty with single and double screws. New fixation method with double screw provides 26% higher stiffness than the closest group, namely new fixation on polyethylene block. And new fixation method with double screw was exhibited significantly higher ([Formula: see text]) performance between the Ovine groups. As in the spinolaminoplasty technic fixing the mini plates to spinous process with longer screws instead of laminae, strengthens the system compared to the conventional method. This proves that rigidity of new construction model is more stable than the conventional method. Tight fixed laminae may prevent restenosis. Also applying the screw through spinous process instead of laminae may prevent the possible cord injuries.

Preliminary results in anterior cervical discectomy and fusion with an experimental bioabsorbable cage - clinical and radiological findings in an ovine animal model

BACKGROUND

Bioabsorbable implants are not widely used in spine surgery. This study investigated the clinical and radiological findings after anterior cervical discectomy and fusion (ACDF) in an ovine animal model with an experimental bioabsorbable cage consisting of magnesium and polymer (poly-ϵ-caprolactone, PCL) in comparison to a tricortical bone graft as the gold standard procedure.

MATERIALS AND METHODS

24 full-grown sheep had ACDF of C3/4 and C5/6 with an experimental bioabsorbable implant (magnesium and PCL) in one level and an autologous tricortical bone graft in the second level. The sheep were divided into 4 groups (6 sheep each). After 3, 6, 12, or 24 weeks postoperatively, the cervical spines were harvested and conventional x-rays of each operated segment were conducted. The progress of interbody fusion was classified according to a three-point scoring system.

RESULTS

There were no operation related complications except for one intraoperative fracture of the anterior superior iliac spine and two cases of screw loosening and sinking, respectively. In particular, no vascular, neurologic, wound healing or infectious problems were observed. According to the time of follow-up, both interbody fusion devices showed similar behaviour with increasing intervertebral osseointegration and complete arthrodesis in 10 of 12 (83.3%) motion segments after 24 weeks.

CONCLUSIONS

The bioabsorbable magnesium-PCL cage used in this experimental animal study showed clinically no signs of incompatibility such as infectious or wound healing problems. The radiographic results regarding the osseointegration are comparable between the cage and the bone graft group.

Current application of β-tricalcium phosphate composites in orthopaedics

Presently, bioceramic materials have been extensively used in spinal surgery as bone grafts; however, there are some limitations for bioceramic materials. Calcium sulfate is rapidly absorbed in vivo, the degradation of which often occurs prior to the formation of new bones. Hydroxyapatite (HA) is hardly absorbed, which blocks the formation of new bones and remodeling, and results in poor local stability or permanent stress concentration. Only β-tricalcium phosphate (β-TCP) is relatively balanced between scaffold absorption and bone formation. And it is a good biodegradable ceramic material that could supply a large quantity of calcium ion and sulfate ion as well as scaffold structure for bone regeneration. However, the problem of single β-TCP is lack of osteoinductivity and osteogenicity, which restricts its application. Therefore β-TCP composite materials have been used in the field of orthopaedics in recent decades, which fully use excellent properties of other bone repairing materials, such as biodegradability, osteoinductivity, osteogenicity and osteoconductivity. These materials make up for the deficiencies of single β-TCP and endow β-TCP with more biological and physical properties.

Biphasic, triphasic and multiphasic calcium orthophosphates

Biphasic, triphasic and multiphasic (polyphasic) calcium orthophosphates have been sought as biomaterials for reconstruction of bone defects in maxillofacial, dental and orthopedic applications. In general, this concept is determined by advantageous balances of more stable (frequently hydroxyapatite) and more resorbable (typically tricalcium orthophosphates) phases of calcium orthophosphates, while the optimum ratios depend on the particular applications. Therefore, all currently known biphasic, triphasic and multiphasic formulations of calcium orthophosphate bioceramics are sparingly soluble in water and, thus, after being implanted they are gradually resorbed inside the body, releasing calcium and orthophosphate ions into the biological medium and, hence, seeding new bone formation. The available formulations have already demonstrated proven biocompatibility, osteoconductivity, safety and predictability in vitro, in vivo, as well as in clinical models. More recently, in vitro and in vivo studies have shown that some of them might possess osteoinductive properties. Hence, in the field of tissue engineering biphasic, triphasic and multiphasic calcium orthophosphates represent promising biomaterials to construct various scaffolds capable of carrying and/or modulating the behavior of cells. Furthermore, such scaffolds are also suitable for drug delivery applications. This review summarizes the available information on biphasic, triphasic and multiphasic calcium orthophosphates, including their biomedical applications. New formulations are also proposed.

[Bone substitutes: Classification and concerns]

Autograft is considered as the "gold standard" for bone reconstruction. It provides osteoinductive factors, osteogenic cells, and appropriate osteoconductive scaffold. Donor site morbidity is the main limitation of autograft. Donor disease transmission limits the use of allograft. Synthetic bone substitutes still lack osteoinductive or osteogenic properties. Composite bone substitutes combining synthetic scaffold and biochemical substances initiating proliferation and cell differentiation, and possibly osteogenesis. Bone substitutes and grafts intended for clinical use are listed.

Comparative radiopacity of bone graft materials

The aim of this study was to investigate the radiopacity of bone graft materials (BGMs) in comparison with bovine mandibular cortical bone and human dentine. Eight samples of each material (8 mm in diameter and 3 mm in thickness) were prepared from Dexabone(®) (DB), Bio - Oss(®) (BO), 4BONE SBS (4B), KASIOS(®) TCP (KA), S.C. PONETI (PO), and Apatite-Wollastonite (AW). The optical densities of each material, along with one tooth section (human canine tooth 1 mm slice), bovine mandibular cortical bone (BC) samples, and an aluminum step wedge, were measured from radiographic images using a transmission densitometer. The data were analyzed by nonparametric one-way ANOVA (Kruskal-Wallis) and Duncan's multiple range tests for post hoc comparison (α = 0.05). BC and AW had statistically lower optical density values than BO, 4B and human dentine (p < 0.05). Among BGMs, AW (3.681 ± 0.409 mm eq Al) had the highest radiopacity values whereas BO (1.925 ± 0.176 mm eq Al) had the lowest one. The radiopacity values of DB and KA did not reveal a statistically significant difference when compared with other materials (p > 0.05). The radiopacity of all BGMs investigated seemed to be too low to be detected radiographically when placed in the mandibular cortical bone.

Anterior cervical interbody fusion with hydroxyapatite graft: clinical and radiological analysis of graft breakage

STUDY DESIGN

Prospective longitudinal cohort.

OBJECTIVE

To evaluate the efficacy of hydroxyapatite (HA) grafts in combination with cervical plates in terms of fusion, restoration, and maintenance of cervical lordosis and to compare clinical and radiologic outcomes of patients who experienced graft breakage with patients who did not.

SUMMARY OF BACKGROUND DATA

The most common complication related to the use of HA graft for cervical anterior fusion is graft breakage. However, the implication of graft breakage in terms of loss of graft height, cervical alignment, plate migration, and clinical outcomes has not been adequately evaluated.

METHODOLOGY

A prospective study of 40 patients who underwent anterior cervical fusion in which HA graft and plate systems were used. Clinical and radiologic assessments were made 1 month after surgery and again at the final follow-up.

RESULTS

At the final follow-up evaluation, 80% of patients had an excellent clinical outcome, 15% had a good outcome, and 5% had a fair outcome based on Odom's classification. All patients achieved lordotic alignment in the immediate postoperative period. Graft breakage was observed in 25% of cases. Patients who experienced HA block breakage have 21 times more chance to have intervertebral height loss greater than 2 mm, 4.9 times more likely to undergo loss of cervical alignment exceeding 3 degrees, and 12.4 times more likely to present migration of the plates when compared to patients who had normal HA grafts.

CONCLUSION

Despite the positive clinical results observed in this study, breakage of HA grafts was a common complication occurring in 25% of patients. Graft breakage was associated with strut height loss of more than 2 mm, loss of cervical alignment exceeding 3 degrees and a higher rate of plate migration. These changes related to the HA graft breakage demonstrate the necessity to continue searching for better grafting methods to perform cervical interbody fusion.

Clinical and radiological outcomes of anterior cervical interbody fusion using hydroxyapatite spacer

OBJECTIVE

This is retrospective study of clinical and radiological outcomes of anterior cervical fusion using Bongros-HA (BioAlpha, Seongnam, Korea) which is a type of synthetic hydroxyapatite (HA) spacer to evaluate the efficacy in its clinical application and usefulness as a reliable alternative to autograft bone.

METHODS

Twenty-nine patients were enrolled in this study and 40 segments were involved. All patients were performed anterior cervical interbody fusion using HA spacer and plating system. Indications for surgery were radiculopathy caused by soft-disc herniation or spondylosis in 18 patients, spondylotic myelopathy in 1 patient, and spinal trauma in 10 patients. Cervical spine radiographs were obtained on postoperative 1day, 1week, and then at 1, 2, 6, and 12 months in all patients to evaluate intervertebral disc height, and the degrees of lordosis. Cervical computed tomography was done at postoperative 12 month in all patients to confirm the fusion status. The mean period of clinical follow-up was 17 months.

RESULTS

Complete interbody fusion was achieved in 100% of patients. Preoperative kyphotic deformities were corrected in all cases after surgery. Intervertebral disc height was well maintained during follow up period. There were no cases of graft extrusion, graft deterioration and graft fracture.

CONCLUSION

HA spacer is very efficient in achieving cervical fusion, maintaining intervertebral disc height, and restoring lordosis. When combined with the placement of a cervical plate, immediate stability can be achieved and graft related complication can be prevented.

Evaluation of hydroxyapatite and beta-tricalcium phosphate mixed with bone marrow aspirate as a bone graft substitute for posterolateral spinal fusion

BACKGROUND

Autologous cancellous bone is the most effective biological graft material. However, harvest of autologous bone is associated with significant morbidity. Since porous hydroxyapatite and beta-tricalcium phosphate are biodegradable materials and can be replaced by bone tissue, but it lacks osteogenic property. We conducted a study to assess their use as a scaffold and combine them with bone marrow aspirate for bone regeneration using its osteogenic property for posterolateral spinal fusion on one side and autologous bone graft on the other side and compare them radiologically in terms of graft incorporation and fusion.

MATERIALS AND METHODS

Thirty patients with unstable dorsal and lumbar spinal injuries who needed posterior stabilization and fusion were evaluated in this prospective study from October 2005 to March 2008. The posterior stabilization was done using pedicle screw and rod assembly, and fusion was done using hydroxyapatite and beta-tricalcium phosphate mixed with bone marrow aspirate as a bone graft substitute over one side of spine and autologous bone graft obtained from iliac crest over other side of spine. The patients were followed up to a minimum of 12 months. Serial radiographs were done at an interval of 3, 6, and 12 months and CT scan was done at one year follow-up. Graft incorporation and fusion were assessed at each follow-up. The study was subjected to statistical analysis using chi-square and kappa test to assess graft incorporation and fusion.

RESULTS

At the end of the study, radiological graft incorporation and fusion was evident in all the patients on the bone graft substitute side and in 29 patients on the autologous bone graft side of the spine (P > 0.05). One patient showed lucency and breakage of distal pedicle screw in autologous bone graft side. The interobserver agreement (kappa) had an average of 0.72 for graft incorporation, 0.75 for fusion on radiographs, and 0.88 for the CT scan findings.

CONCLUSION

Hydroxyapatite and beta-tricalcium phosphate mixed with bone marrow aspirate seems to be a promising alternative to conventional autologous iliac bone graft for posterolateral spinal fusion.

Bryan cervical disc prosthesis: 12-month clinical outcome

A prospective observational clinical study was carried out to determine whether Bryan disc replacement surgery is a suitable alternative to arthrodesis for cervical disc disease.

Use of 3D beta-tricalcium phosphate (Vitoss) scaffolds in repairing bone defects

Vitoss is the most porous (90%) of a number of beta-tricalcium phosphate osteoconductive bone fillers. Its inherent limitations are those of the calcium phosphate class, being a purely osteoconductive product without inherent structural stability and with a moderate resorption rate. Currently, a number of additives, composites and related compounds are under study at various stages. In animal experiments, Vitoss performs well in comparison with other synthetic grafts, and with marrow added in various ways, it rivals autograft. Clinical efficacy is established for Vitoss as a spinal graft extender, as well as for periodontal, dental and orthopedic tumor defects. Apart from recombinant human platelet-derived growth factor, clinical data is lacking on the addition of bone marrow, stem cells and growth factors.

Refinement of interbody implant testing in goats: a surgical and morphometric rationale for selection of a cervical level. Laboratory investigation

OBJECT

The authors provide a surgical description of the ventral approach to the cervical spine in a goat model and identify selection of the most appropriate level for testing interbody devices. These constructs are designed for implantation in humans during anterior cervical discectomy and fusion. Such description and guidelines for level selection have never been published in either the medical or veterinarian literature.

METHODS

The study comprised three phases: surgical, anatomical, and morphometric. Six goats underwent ventral approaches and were later killed; their necks were dissected and the cervical spines were processed to obtain clean specimens of the vertebral bodies. Measurements were made at each level using a contact digitizer.

RESULTS

The anterolateral bone spurs, called alar processes, and the increased thickness of the longus colli muscle are the surgically relevant characteristics in the goat. The morphometric analysis showed that C2-3 is the most suitable level for implantation of interbody devices. The vertebral endplates at the C2-3 level are relatively flat and parallel to each other, and are perpendicular to the spinal canal axis. More distally, the endplates adopt a more curved arrangement, and the endplate angle becomes significantly greater than 90 degrees. The authors describe anatomical landmarks that are important to safely and effectively perform a ventral cervical spinal approach in the goat.

CONCLUSIONS

The authors' model identifies C2-3 as the most appropriate level for animal testing of cervical implants because of its similarity to human anatomy. Further study with rigorous biomechanical range of motion evaluation of each caprine cervical level is needed.

Multiphasic Biomaterials: A Concept for Bone Substitutes Developed in the "Pays de la Loire"

This paper reports on the research into multiphase bone substitutes carried out by laboratories from the ‘Pays de la Loire’ region in France. This collaborative research was funded by both the French Government and the Regional Council in the period 2000-2007. Calcium phosphate bioceramics, polymers and combinations have been developed as bone substitutes for various maxillofacial and orthopaedic applications. These bone substitutes should support and regenerate bone tissue and resorb after implantation. In the bone tissue engineering area, they have been combined with autologous bone marrow cells or bioactive factors. The bone substitutes were tested in various animal models mimicking clinical situations or under pathological conditions (osteoporosis). In order to complete our research, the multiphase materials were also evaluated in clinical trials.

Evaluation of hydroxyapatite ceramic vertebral spacers with different porosities and their binding capability to the vertebral body: an experimental study in sheep

Object

The aim of this study was to evaluate the degree of bone ingrowth and bonding stiffness at the surface of hydroxyapatite ceramic (HAC) spacers with different porosities in an animal model and to discuss the ideal porous characteristics of these spacers for anterior spinal reconstruction.

Methods

Twenty-one adult sheep (age 1–2 years, mean weight 70 kg) were used in this experiment. Surgery consisted of anterior lumbar interbody fusion at L2–3 and L4–5, insertion of an HAC spacer (10 × 13 × 24 mm) with three different porosities (0, 3, and 15%), and single-rod anterior instrumentation. At 4 and 6 months postoperatively, the lumbar spines were harvested. Bonding conditions at the bone–HAC spacer interface were evaluated using neuroimages and biomechanically. A histological evaluation was also conducted to examine the state of bone ingrowth at the surface of the HAC spacer.

Biomechanical testing showed that the bonding strength of HAC at 6 months postoperatively was 0.047 MPa in 0% porosity spacers, 0.39 MPa in 3%, and 0.49 MPa in 15% porosity spacers. The histological study showed that there was a soft-tissue layer at the surface of the HAC spacer with 0% porosity. Direct bonding was observed between bone and spacers with 3 or 15% porosity. Micro–computed tomography scans showed direct bonding between the bone and HAC with 3 or 15% porosity. No direct bonding was observed in HAC with 0% porosity.

Conclusions

Dense (0%) HAC anterior vertebral spacers did not achieve direct bonding to the bone in the sheep model. The HAC vertebral spacers with 3 or 15% porosity showed proof of direct bonding to the bone at 6 months postoperatively. The higher porosity HAC spacer showed better bonding stiffness to the bone.

Expandable cylindrical cages in the cervical spine: a review of 22 cases

Object

Expandable cylindrical cages (ECCs) have been utilized successfully to reconstruct the thoracic and lumbar spine. Their advantages include ease of insertion, reduced endplate trauma, direct application/maintenance of interbody distraction force, and one-step kyphosis correction. The authors present their experience with ECCs in the reconstruction of the cervical spine in patients with various pathological conditions.

Methods

Data obtained in 22 patients were reviewed retrospectively. A standard anterior cervical corpectomy was performed in all cases. Local vertebral body bone was harvested for use as graft material. Patients underwent pre- and postoperative assessment involving the visual analog scale (VAS), Nurick grading system for determining myelopathy disability, and radiographic studies to determine cervical kyphosis/lordosis and cage subsidence. Fusion was defined as the absence of motion on flexion–extension x-ray films.

Sixteen patients presented with spondylotic myelopathy, two with osteomyelitis, two with fracture, one with tumor metastasis, and one with severe stenosis. Fourteen patients underwent supplemental posterior spinal fusion, seven underwent single-level corpectomy, and 15 patients underwent multilevel corpectomy. No perioperative complications occurred. The mean follow-up period was 22 months. In 11 patients with preexisting kyphosis (mean deformity +19°), the mean correction was 22°. There was no statistically significant difference in subsidence between single- and multilevel corpectomy or between 360º fusion and anterior fusion alone. The VAS scores improved by 35%, and the Nurick grade improved by 31%. The fusion rate was 100%.

Conclusions

The preliminary results support the use of ECCs in the cervical spine in the treatment of patients with various disease processes. No significant subsidence was noted, and pain and functional scores improved in all cases. Expandable cylindrical cages appear to be well suited for cervical reconstruction and for correcting sagittal malalignment.

Animal models for spinal fusion

Animal models for spinal fusion are essential for preclinical testing of new fusion methods and adjuncts. They allow for control of individual variables and quantification of outcome measures. Model characteristics are considered. Preclinical experiments to evaluate proof of concept, feasibility, and efficacy are generally studied in an orderly progression from smaller to larger animal models with an evolving cascade of evidence which has become known as the "burden of proof". Methods of fusion analysis include manual palpation, radiographs, computed tomography, histology, biomechanical testing, and molecular analysis. Models which have been developed in specific species are reviewed. This sets the stage for the interpretation of studies evaluating bone graft materials such as allograft, demineralized bone matrices, bone morphogenetic proteins, ceramics, and others with consideration of the variables affecting their success. As evidence accumulates, clinical trials and applications are defined.

Primary stability of anterior lumbar stabilization: interdependence of implant type and endplate retention or removal

This is a comparative in vitro biomechanical study of the primary stability of an anterior lumbar interbody stabilization. The objective was to compare the stability of a interbody stabilizing titanium cage with and without the retention of the bordering vertebral endplates, as well as to compare the titanium cage with a tricalcium phosphate block when the endplates are removed. An adequate blood supply is critical for interbody fusion, which suggests surgical treatment of the bordering endplates. On the other hand, primary stability is improved by the retention of the endplates. Furthermore, bone substitute materials are finding more frequent use due to complications associated with autologous bone grafts. Ten bovine lumbar spine motion segments (average age 6 months) were investigated. Pure bending loadings as well as eccentric axial compression loadings were applied. A titanium cage and tricalcium phosphate block, were tested in conjunction with an anterior augmentation (MACS). Range of motion, neutral zone (NZ) and bending stiffness were measured under pure bending to 10 Nm, and bending stiffness under axial loads of up to 1,500 N. Range of motion of both implants in flexion-extension was significantly smaller than physiologic (cage without endplates 4.3 degrees , cage with 2.8 degrees , block without 3.4 degrees , and physiologic 6.6 degrees , all p<0.001). The cage with endplates and the block without endplates were both significantly stiffer than physiologic in all directions except left lateral bending. The block without endplates and the cage with endplates were both stiffer than the cage without endplates. The results suggest that the use of the bone substitute block provides better stability than the cage when the endplates are removed.

Use of osteopromotive growth factors, demineralized bone matrix, and ceramics to enhance spinal fusion

Recently developed materials that can enhance fusion rates for posterolateral lumbar arthrodesis may be used alone or in combination with autogenous bone grafts. Novel osteopromotive growth factor preparations are currently under scrutiny; these include autogenous growth factor concentrate, bovine bone-derived osteoinductive protein, and recombinant human MP52. Demineralized bone matrix products may enhance or extend grafts. However, few studies, especially prospective randomized clinical trials, have assessed their efficacy, so it is difficult to compare formulations. Ceramics have been evaluated in animal studies and human clinical trials for a variety of applications in spinal surgery. These materials function best as bone graft extenders or as bioactive osteoinductive material carriers in posterolateral lumbar fusions. They have the advantage of variable porosity, low cost, and ease of manufacture. Hydroxyapatite/tricalcium phosphate ceramics have been shown to perform as well as autogenous bone grafts but with fewer complications.

Anwendung eines Bandscheibenersatzimplantats aus einer neuartigen porösen TiO2/Glas-Keramik – Teil 2: Biomechanische Untersuchungen nach Implantation in die Schafs-Halswirbelsäule / Application of a Stand-Alone Interbody Fusion Cage Based on Porous TiO2/glass Composites – Part 2: Biomechanical Evaluation after Implantation in the Sheep Cervical Spine

Animals are becoming more and more common as in vivo models for the human spine. Especially the sheep cervical spine is stated to be of good comparability and usefulness in the evaluation of in vivo radiological, biomechanical and histological behaviour of new bone replacement materials, implants and cages for cervical spine interbody fusion. In preceding biomechanical in vitro examinations human cervical spine specimens were tested after fusion with either a cubical stand-alone interbody fusion cage manufactured from a new porous TiO2/glass composite (Ecopore) or polymethylmethacrylate (PMMA) after discectomy. Following our first experience with the use of the new material and its influence on the primary stability after in vitro application we carried out fusions of 20 sheep cervical spines levels with either PMMA or an Ecopore-cage, and performed radiological examinations during the following 2-4 months. In this second part of the study we intended the biomechanical evaluation of the spine segments with reference to the previously determined morphological findings, like subsidence of the implants, significant increase of the kyphosis angle and degree of the bony fusion along with the interpretation of the results. 20 sheep cervical spines segments with either PMMA- or Ecopore-fusion in the levels C2/3 and C4/5 were tested, in comparison to 10 native corresponding sheep cervical spine segments. Non-destructive biomechanical testing was performed, including flexion/extension, lateral bending and axial rotation using a spine testing apparatus. Three-dimensional range of motion (ROM) was evaluated using an ultrasound measurement system. In the native spine segments C2/3 and C4/5 the ROM increased in cranio-caudal direction particulary in flexion/extension, less pronounced in lateral flexion and axial rotation (p < 0.05). The overall ROM of both tested segments was greatest in lateral flexion, reduced to 52% in flexion/extension and to 16% in axial rotation. After 2 months C2/3- and C4/5-segments with PMMA-fusion and C2/3-segments with Ecopore-interposition showed decrease of ROM in lateral flexion in comparison to the native segments, indicating increasing stiffening. However, after 4 months all operated segments, independent from level or implanted material, were stiffer than the comparable native segments. The decrease of the ROM correlated with the radiological-morphological degree of fusion. Our evaluation of the new porous TiO2/glass composite as interbody fusion cage has shown satisfactory radiological results as well as distinct biomechanical stability and fusion of the segments after 4 months in comparison to PMMA. After histological analysis of the bone-biomaterial-interface, further examinations of this biomaterial previous to an application as alternative to other customary cages in humans are necessary.

Maintenance of interbody space in one- and two-level anterior cervical interbody fusion: comparison of the effectiveness of autograft, allograft, and cage

The use of allografts, autologous iliac crest grafts, and cages for anterior cervical fusion is well documented, however there is no comparison regarding the effectiveness of maintaining the interbody space with the three approaches. We retrospectively measured the rate and amount of interspace collapse, segmental sagittal angulations, clinical results, and radiographic fusion success rates to determine which is the best fusion material. We assessed 73 patients who had one- and two-level cervical discectomies and interbody fusions without instrumentation. The three groups had similar clinical results and fusion rates. However, in the autograft group union occurred in 4 months. In the allograft group, union did not occur until 5.54 months. Moreover, the loss of cervical lordosis (2.75 degrees) was less in the cage group than in the allograft group (9.23 degrees). Additionally, the anterior interspace collapse (1.73 mm) in the cage group was less than the collapse recorded in the autograft group (2.82 mm) and in the allograft group (4 mm). An interspace collapse of 3 mm or greater was observed in 56.1% of the patients in the allograft group, compared with only 19% of the patients in the cage group. We showed that the cage is superior to the allograft and autograft in maintaining cervical interspace height and cervical lordosis after one-level and two-level anterior cervical decompression procedures.

LEVEL OF EVIDENCE

Therapeutic study, Level III-2 (retrospective cohort study).

Anwendung eines Bandscheibenersatzimplantats aus einer neuartigen porösen TiO2/Glas-Keramik – Teil 1: Einsatz in der Schafs-Halswirbelsäule und radiologische Verlaufsuntersuchungen / Application of a Stand-Alone Interbody Fusion Cage Based on a Novel Porous TiO2/glass Composite – Part 1: Implantation in the Sheep Cervical Spine and Radiological Evaluation

Zur Beurteilung des radiologischen, biomechanischen und histologischen Einwachsverhaltens neuer Materialien, Implantate und Cages für die zervikale interkorporelle Fusion, bieten sich Tiermodelle und hier insbesondere das Schafs-Halswirbelsäulenmodell an.

In biomechanischen In-vitro-Versuchen an humanen Kadaver-Halswirbelsäulen wurden erste Erfahrungen hinsichtlich Primärstabilität eines Cage aus einer neuartigen, porösen TiO

Zur entsprechenden In-vivo-Beurteilung fusionierten wir 10 Schafs-Halswirbelsäulen in den Höhen C2/3 und C4/5 jeweils mit PMMA und einem Ecopore-Keramik-Cage und führten nativradiologische, sowie computertomographische Verlaufsuntersuchungen direkt post-operativ und alle 4 Wochen in den folgenden 2 bzw. 4 Monaten durch. Neben der Etablierung des Tiermodells, wurden die radiologischen Veränderungen im Verlauf und die Fusion der operierten Segmente analysiert. Darüberhinaus wurden Messungen der entsprechenden Bandscheibenfachhöhen (DSH) und Intervertebralwinkel (IVA) durchgeführt und verglichen.

Nach Einbringen der Implantate in die Bandscheibenfächer nahm zunächst in beiden Gruppen die mittlere Bandscheibenfachhöhe und der Intervertebralwinkel zu (34,8%; 53,9%). In den folgenden Monaten verringerte sich die Bandscheibenfachhöhe nicht signifikant, deutlicher nach Ecopore-Fusion als nach PMMA-Interposition bis auf Werte unterhalb der Ausgangswerte. Ebenso nahm der Intervertebralwinkel im postoperativen Verlauf, deutlicher in der Ecopore-Gruppe als in der PMMA-Gruppe, ab (p < 0,05). Diese Veränderungen im Sinne einer Einsinterung der Implantate, konnte in den radiologischen Verlaufskontrollen morphologisch bestätigt werden. Die radiologisch beurteilbare Fusion, d.h. solide knöcherne Überbauung des operierten Segments, war nach Implantation eines Ecopore-Cage ausgeprägter (83%) als nach PMMA-Interposition (50%) (nicht statistisch signifikant).

In diesem ersten Teil unserer In-vivo-Untersuchungen zu dem Einsatz des neuartigen Cage-Materials wurde die Anwendung im Spondylodesemodell der Schafs-Halswirbelsäule aufgezeigt. Es zeigten sich radiologische Unterschiede, in Bezug auf die ausgeprägtere Einsinterung des Ecopore-Cage und die deutlichere, nachweisbare Fusion des mit dem neuen Material operierten Segments. In dem ersten Teil dieser Studie wurden die radiologischen Veränderungen der fusionierten Segmente über mehrere Monate dargestellt und morphologisch analysiert, bevor die biomechanischen Analysen und Vergleiche in einem weiteren Teil präsentiert werden sollen.

 

Animals are becoming more and more common as in vitro and in vivo models for the human spine. Especially the sheep cervical spine is stated to be of good comparability and usefulness in the evaluation of in vivo radiological, biomechanical and histological behaviour of new bone replacement materials, implants and cages for cervical spine interbody fusion.

In preceding biomechanical in vitro examination human cervical spine specimens were tested after fusion with either a cubical stand-alone interbody fusion cage manufactured from a new porous TiO

Immediately after placement of both implants in the disc spaces the mean DSH and IVA increased (34.8% and 53.9%, respectively). During the following months DSH decreased to a greater extent in the Ecopore-segments than in the PMMA-segments, even to a value below the initial value (p > 0,05). Similarly, the IVA decreased in both groups in the postoperative time lapse, but more distinct in the Ecopore-segments (p < 0,05). These changes in terms of a subsidence of the implants, were confirmed morphologically in the radiological examination in the course. The radiologically evaluated fusion, i.e. bony bridging of the operated segments, was more pronounced after implantation of an Ecopore-cage (83%), than after PMMA interposition (50%), but did not gain statistical significance.

In this first in vivo examination of our new porous ceramic bone replacement material we showed its application in the spondylodesis model of the sheep cervical spine. Distinct radiological changes regarding evident subsidence and detectable fusion of the segments, operated on with the new biomaterial, were seen. We demonstrated the radiological changes of the fused segments during several months and analysed them morphologically, before the biomechanical evaluation will be presented in a subsequent publication.

Beta-tricalcium phosphate combined with recombinant human bone morphogenetic protein-2: a substitute for autograft, used for packing interbody fusion cages in the canine lumbar spine

Beta-tricalcium phosphate (beta-TCP) combined with recombinant human bone morphogenetic protein-2 (BMP-2) was examined as a substitute for autograft for packing into interbody fusion cages in the canine lumbar spine model. Discectomy and interbody cage fusion were performed at three disc spaces in eight dogs. Examination of microradiographs and histological sections of the lumbar spine at 16 weeks postsurgery revealed three fusions in the autograft cages (Group A), three in the beta-TCP cages (Group B), and five in the beta-TCP-BMP-2 cages (Group C). The mean percentage of trabecular bone area in the cages was 51.9% in Group A, 48.8% in Group B, and 65.6% in Group C. Mean percentage of trabecular bone formation and mechanical stiffness were highest in the cages filled with beta-TCP and BMP-2. Combination of BMP to beta-TCP may act as an osteoconductive and osteoinductive bone graft substitute in clinical spine surgery.

Accuracy and interobserver agreement for determinations of rabbit posterolateral spinal fusion

STUDY DESIGN

The accuracy and interobserver agreement of fine detail radiography and computed tomography (CT) determination of spinal fusion were evaluated in an established animal spine fusion model.

OBJECTIVE

To determine the accuracy and interobserver agreement of radiographic determinations of spinal fusion in rabbit posterolateral spine fusion.

SUMMARY OF BACKGROUND DATA

The rabbit posterolateral intertransverse process spine fusion model is an established animal model for evaluating bone graft alternatives for spinal fusion. However, little is known regarding the accuracy and interobserver agreement of radiographic determinations of spondylodesis in this model.

METHODS

Forty-two New Zealand White rabbits underwent posterolateral spinal fusion. The animals were killed at 9 weeks and the lumbar spine harvested. Manual manipulation, fine detail radiography, and CT images were used to assess spinal fusion.

RESULTS

Using manual palpation testing as the standard by which to assess fusion, there was high sensitivity and negative predictive value for both radiographic methods. Positive predictive value, however, was poor (26% fine detail radiography, 61% CT scan). CT correlated better with manual palpation testing when compared with fine detail radiographs. There was substantial interobserver agreement of successful fusion using CT scan imaging (kappa = 0.63) and moderate interobserver agreement radiographs (kappa = 0.52).

CONCLUSIONS

Both radiographic techniques used in the study recorded high sensitivity and negative predictive value. However, positive predictive value was poor, especially with fine detail radiographs. Nevertheless, CT with reformatted images did appear to be superior to fine detail radiographs in accurately identifying nonunions in this animal model.

Bone tissue engineering and spinal fusion: the potential of hybrid constructs by combining osteoprogenitor cells and scaffolds

In this paper, we discuss the current knowledge and achievements on bone tissue engineering with regard to spinal fusion and highlight the technique that employs hybrid constructs of porous scaffolds with bone marrow stromal cells. These hybrid constructs potentially function in a way comparable to the present golden standard, the autologous bone graft, which comprises besides many other factors, a construct of an optimal biological scaffold with osteoprogenitor cells. However, little is known about the role of the cells in autologous grafts, and especially survival of these cells is questionable. Therefore, more research will be needed to establish a level of functioning of hybrid constructs to equal the autologous bone graft. Spinal fusion models are relevant because of the increasing demand for graft material related to this procedure. Furthermore, they offer a very challenging environment to further investigate the technique. Anterior and posterolateral animal models of spinal fusion are discussed together with recommendations on design and assessment of outcome parameters.

Modification of gene expression induced in human osteogenic and osteosarcoma cells by culture on a biphasic calcium phosphate bone substitute

Bone hybrids made of bioceramics seeded with mesenchymal or osteoblastic cells are very promising alternatives to autologous bone graft. Along this line, the development of in vitro models, dedicated to analyze the influence of these biomaterials on osteogenic cells, will help to improve the performance of these bone substitutes. In the present work we analyzed the effects of a macroporous biphasic calcium phosphate ceramic (BCP, Triosite) on three different human osteosarcoma cell lines and on human primary osteogenic cells and compared this culture substratum to traditional culture on plastic. We showed that all these osteoblastic cells adhere and proliferate on the trabecular BCP blocks, with a different spatial organization for osteosarcoma cells compared to normal osteogenic cells. We also demonstrated that osteoblastic marker genes such as Cbfa1, type I collagen, osteonectin, osteopontin, and osteocalcin were expressed at similar levels by these cells cultured on either substratum, suggesting that adhesion to BCP does maintain the osteoblastic phenotype of these cells. Next, we provided the first evidence of differences of cytokine expression profiles revealed on this Ca-P ceramic as compared to expression in classical culture. These modifications affected the expression of cytokines such as TGF-beta1, G-CSF, and IL-3 and were quantitatively different between osteosarcoma cells and normal osteogenic cells. Given the role of these cytokines in bone biology and in hematopoiesis, these results obtained in vitro suggest that the BCP ceramic studied here could stimulate osteogenesis in vivo by activating cellular processes during bone formation and healing. This study highlights the notion that the nature of the culture substratum must be taken into account when studying bone cell biology in vitro. Owing to the nature and spatial organization of the BCP, our hypothesis is that culture on BCP is closer to the physiological situation than culture on plastic.

Macroporous biphasic calcium phosphate scaffold with high permeability/porosity ratio

Macroporous biphasic calcium phosphate (BCP) with channel-shaped pores was produced by a novel dual-phase mixing method. The processing route includes mixing water-based BCP slurry and polymethylmethacrylate resin; shaping in a mold; and polymerization, drying, pyrolyzing, and sintering. After comparison with two other commercial macroporous BCP materials, which were produced along different routes, it was found that conventional parameters such as porosity and pore size cannot describe a macroporous structure precisely enough for the application as tissue-engineering scaffold. Instead, permeability can be seen as an intrinsic and quantitative parameter to describe the macroporous structure of various scaffolds, because it is independent of sample size and fluid used in the test. Another parameter, the permeability/porosity ratio, provides an indication of the percolative efficiency per unit porous volume of a scaffold. Structural characterizations and permeability studies of other macroporous scaffold materials were also performed, and it was found that permeability could reflect a combination of five important parameters for scaffold: (1) porosity, (2) pore size and distribution, (3) interconnectivity, (4) fenestration size and distribution, and (5) orientation of pores. Finally, the implications of relating permeability with biological performances are also discussed.

Suitability of bioresorbable cages for anterior cervical fusion

OBJECT

The authors conducted a pilot study to determine whether a bioresorbable intervertebral fusion device composed of 85/15 polylactide-polyglycolide (PLA-PGA) copolymer packed with bone autograft is a suitable alternative to promote arthrodesis after anterior cervical discectomy (ACD) in a caprine model.

METHODS

The caprine cervical spine model has been used to evaluate interbody healing and fusion after application of bone grafts and instrumentation. Whether a bioresorbable device is suitable for facilitating intervertebral bone union has not been determined. Twelve goats underwent two-level ACD and fusion; eight received bioresorbable cages packed with autologous bone, and four received autologous bone alone. Goats were maintained without an orthosis and after 12 weeks underwent physical, radiographic, and histological evaluation. Cages had structurally degraded, and two had become extruded. Stable intervertebral union developed in three (19%) of 16 cage-implanted interspaces, and one (14%) of seven bone autograft-implanted interspaces; each was judged manually to be rigid (Grade 2), radiographically to be bridged by new osseous densities (Grade 2), and histologically to have marked new bone formation (Grade 3). A primarily fibrous union, however, stabilized the cage-implanted interspaces, and eight (50%) had developed a 4 to 6-mm foreign body granuloma. These interposed soft tissues were not present in the stable autologous bone-implanted interspace, which had successfully become fused.

CONCLUSIONS

Interbody cages composed of 85/15 PLA-PGA copolymer contributed to a stable fibrous union, degraded. and produced granuloma after 12 weeks. Additional evaluations are necessary to determine whether other copolymer mixtures, or other bioresorbable materials, can contribute to an arthodesis without deleterious consequences.

A prospective randomized comparison of coralline hydroxyapatite with autograft in cervical interbody fusion

STUDY DESIGN

A prospective randomized trial with independent clinical and radiographic outcome review of patients receiving either hydroxyapatite or tricortical iliac crest graft for cervical interbody fusion was conducted.

OBJECTIVE

To determine whether coralline-derived hydroxyapatite is a suitable bone graft substitute in cervical interbody fusion.

SUMMARY OF BACKGROUND DATA

Tricortical iliac crest bone is the "gold standard" graft material for cervical interbody fusion. Various bone substitutes have been used for this procedure to avoid potential donor site morbidity. ProOsteon 200 is a coralline-derived hydroxyapatite product, the use of which remains unclear for cervical interbody fusion.

METHODS

In this study, 29 patients undergoing anterior cervical fusion and plating were randomized to receive either ProOsteon 200 or iliac crest grafts. The SF-36 and Oswestry Disability Index were used to measure clinical outcome. Postoperative radiographs were analyzed for graft fragmentation, loss of height, angular alignment, and hardware failure to assess structural integrity of the graft material. Plain radiographs and computed tomography scans were used to evaluate fusion.

RESULTS

Both the ProOsteon 200 and iliac crest groups demonstrated significant improvement in clinical outcome scores. There was no significant difference in clinical outcome or fusion rates between the two groups. Graft fragmentation occurred in 89% of the hydroxyapatite grafts and 11% of the autografts (P = 0.001). Significant graft settling occurred in 50% of the hydroxyapatite grafts, as compared with 11% of the autografts (P = 0. 009). One patient in the ProOsteon 200 group required revision surgery for graft failure.

CONCLUSIONS

ProOsteon 200 does not possess adequate structural integrity to resist axial loading and maintain disc height or segmental lordosis during cervical interbody fusion.

Experimental determination of adult and pediatric neck scale factors

The purpose of this study was to determine scale factors for small, mid-size and large adults using a caprine model. In a previous study conducted in our lab, scaling relationships were developed to define cervical spine tolerance values of children using caprine specimens. In that study, tolerances were normalized with respect to an average adult. Because airbag-related injuries are associated with out-of-position children and small adult females, additional experimental data are needed to better estimate human tolerance. In the present study, cervical spine radiographs from the 5th, 50th and 95th percentile human adults were used to determine vertebral body heights for small, mid-size and large anthropometries. Mean human vertebral body heights were computed for each anthropometry and were normalized with respect to mid-size anthropometry. Similar measurements were calculated from caprine cervical spine radiographs and each caprine specimen was grouped into one of the three categories based upon vertebral body size. Seventy-two motion segments (OC-C2, C3-C4, C5-C6 and C7-T1) from 18 adult caprine cadavers were subjected to pure moment and distraction loads. Pure moment testing resulted in bending stiffness, and distraction testing resulted in failure force and linear stiffness. Data were normalized with respect to the mid-size anthropometric category. For the small, mid-size and large adult categories, tensile failure force yielded scaling ratios of 0.74, 1.00 and 1.13, linear stiffness yielded ratios of 0.78, 1.00 and 1.10 and bending stiffness resulted in ratios of 0.89, 1.00 and 1.03. For the one-year-old, three-year-old, six-year-old and 12-year-old, scaling ratios were 0.10, 0.16, 0.30 and 0.62 for the tension force, 0.13, 0.18, 0.38 and 0.66 for the linear stiffness and 0.13, 0.19, 0.42 and 0.76 for the bending stiffness. These scale factors are compared with FMVSS 208.

Beta-tricalcium phosphate as a substitute for autograft in interbody fusion cages in the canine lumbar spine

OBJECT

An interbody fusion cage has been introduced for cervical anterior interbody fusion. Autogenetic bone is packed into the cage to increase the rate of union between adjacent vertebral bodies. Thus, donor site-related complications can still occur. In this study a synthetic ceramic, beta-tricalcium phosphate (TCP), was examined as a substitute for autograft bone in a canine lumbar spine model.

METHODS

In 12 dogs L-1 to L-4 vertebrae were exposed via a posterolateral approach, and discectomy and placement of interbody fusion cages were performed at two intervertebral disc spaces. One cage was filled with autograft (Group A) and the other with TCP (Group B). The lumbar spine was excised at 16 weeks postsurgery, and biomechanical, microradiographic, and histological examinations were performed. Both the microradiographic and histological examinations revealed that fusion occurred in five (41.7%) of 12 operations performed in Group A and in six (50%) of 12 operations performed in Group B. The mean percentage of trabecular bone area in the cages was 54.6% in Group A and 53.8% in Group B. There were no significant intergroup differences in functional unit stiffness.

CONCLUSIONS

Good histological and biomechanical results were obtained for TCP-filled interbody fusion cages. The results were comparable with those obtained using autograft-filled cages, suggesting that there is no need to harvest iliac bone or to use allo- or xenografts to increase the interlocking strength between the cage and vertebral bone to achieve anterior cervical interbody fusion.

Complications related to hydroxyapatite vertebral spacer in anterior cervical spine surgery

STUDY DESIGN

This is a report of complications related to the hydroxyapatite vertebral spacer used for anterior cervical reconstructive surgery. Compression of the spinal cord by broken fragments of hydroxyapatite spacer as well as its surrounding radiolucent clear zone were observed in seven patients.

OBJECTIVES

To report complications related to the use of hydroxyapatite vertebral spacer for anterior cervical reconstructive surgery and to discuss how to prevent these complications.

SUMMARY OF BACKGROUND DATA

Despite previous articles reporting the clinical applications of hydroxyapatite vertebral spacer for the cervical spine, clinical reports regarding the long-term results of hydroxyapatite spacer for anterior cervical surgery and its complications have been limited.

METHODS

The authors reviewed patients who underwent anterior reconstructive surgery using the hydroxyapatite spacer at other hospitals and had postoperative complications related to hydroxyapatite spacer.

RESULTS

Seven patients previously treated by anterior cervical spine surgery using the hydroxyapatite vertebral spacer were referred to the authors because of unsatisfactory surgical outcomes. All the patients had a radiolucent clear zone around the spacer and experienced severe neck pain. Four had fracture of the hydroxyapatite spacer, and two had compression of the spinal cord by retropulsed fragments of broken hydroxyapatite spacers.

CONCLUSIONS

Although hydroxyapatite has been used in many medical fields because of its bioactive characteristics, its mechanical properties should be improved to lessen the risks of breakage and subsequent spinal cord compression. Gentle insertion maneuvers are also important to avoid the production of cracks inside the spacer.

The use of carbon fiber cages in anterior cervical interbody fusion

Object

Cage devices were introduced in spinal fusion to overcome the shortcomings of autograft, allograft, and biocompatible implants. The aim of this study was to assess the short-term results of anterior cervical discectomy and fusion (ACDF) in which an interbody carbon fiber cage (CFC) and local osteophyte–derived bone graft were implanted.

Methods

A retrospective review was conducted of 100 consecutive patients treated by ACDF in which a CFC was packed with bone fragments obtained from osteophytes at the surgical site. Plain radiographs with dynamic lateral views obtained 1 year postoperatively were used to assess bone fusion, alignment of the cervical spine, and stability. Dynamic radiographs were also obtained at last follow up to determine whether loss of cervical alignment or collapse at the fused disc had occurred.

The mean follow-up period was 25 months. In all cases the cervical lordosis was maintained or corrected to different extents and disc height was restored. Solid fusion was achieved in 98% of the cases. There were no cage-related complications and no cases of cage failure.

Conclusions

The authors conclude that application of the CFC for ACDF is safe, effective, and technically feasible. Osteophytes resected during surgery may be a good alternative material for bone grafting in cage-assisted cervical interbody fusion.

Lumbar posterolateral fusion with biphasic calcium phosphate ceramic

The authors conducted a retrospective observational study of patients who needed lumbar posterolateral fusion (PLF) using a biphasic calcium phosphate ceramic implant as a substitute for bone graft. The findings of clinical, radiographic, and histologic examinations were reviewed. Thirty-two patients underwent single-level PLF with instrumentation. In all cases, to decrease the occurrence of donor-site complications and morbidity, locally harvested morselized bone from the decompressive site was mixed with hydroxyapatite and beta-tricalcium phosphate (HAP-TCP) granules and sticks and used for fusion at the posterolateral aspect of the lumbar spine. The histologic findings of three biopsy specimens obtained during second operations for metallic implant removal were reviewed. The minimum follow-up period was 26 months. There was no evidence of instrument loosening or breakage. However, bone-graft incorporation was difficult to evaluate radiographically, because image quality was inferior to that with conventional autogenous iliac bone graft. Partial graft bone resorption was revealed on radiographs in 75% of cases. The results showed clinical improvement in all but one case. Solid fusion was observed during the second operation in all three cases. Histologic analysis showed excellent bone incorporation around the HAP-TCP granules. These findings suggest that, although the bulk of the fusion mass with HAP-TCP was smaller than that with autogenous bone, this technique combined with rigid instrumentation is a safe and effective procedure.

Effect of endplate conditions and bone mineral density on the compressive strength of the graft-endplate interface in anterior cervical spine fusion

STUDY DESIGN

Destructive compression tests and finite element analyses were conducted to investigate the biomechanical strength at the graft-endplate interface in anterior cervical fusion.

OBJECTIVES

To investigate the effect of endplate thickness, endplate holes, and bone mineral density of the vertebral body on the biomechanical strength of the endplate-graft interface in an anterior interbody fusion of the cervical spine.

SUMMARY OF BACKGROUND

Subsidence of the graft into the vertebral body is a well-known complication in anterior cervical fusion. However, there is no information in the literature regarding the compressive strength of the graft-endplate interface in relation to the endplate thickness, holes in the endplate, and bone mineral density of the vertebral body.

METHODS

Biomechanical destructive compression tests and finite element analyses were performed in this study. Cervical vertebral bodies (C3-C7) isolated from seven cadaveric cervical spines (age at death 69-86 years, mean 79 years) were used for compression tests. Bone mineral density of each vertebral body was measured using a dual energy radiograph absorptiometry unit. Endplate thickness was measured using three coronal computed tomography images of the middle portion of the vertebral body obtained using a computer-assisted imaging analysis. Then each vertebral body was cut into halves through the horizontal plane. A total of 54 specimens, consisting of one endplate and half of the vertebral body, were obtained after excluding eight vertebrae with gross pathology on plain radiograph. Specimens were assigned to one of three groups with different endplate conditions (Group I, intact; Group II, partial removal; and Group III, complete removal) so that group mean bone mineral density became similar. Each endplate was slowly compressed until failure using an 8-mm-diameter metal indenter, and the load to failure was determined as a maximum force on a recorded force-displacement curve. The effect on the strength of the graft-endplate interface of various hole patterns in the endplate was studied using a finite element technique. The simulatedhole patterns included the following: one large central hole, two lateral holes, two holes in the anterior and posterior portion of the endplate, and four holes evenly distributed from the center of the endplate. Stress distribution in the endplate was predicted in response to an axial compressive force of 110 N, and the elements with von Mises stress greater than 4.0 MPa were determined as failed.

RESULTS

The endplate thickness and bone mineral density were similar at all cervical levels, and the superior and inferior endplates had similar thickness at all cervical levels. There was no significant association between bone mineral density and endplate thickness. Load to failure was found to have a significant association with bone mineral density but not with endplate thickness. However, load to failure tends to decrease with incremental removal of the endplate, and load to failure of the specimens with an intact endplate was significantly greater than that of the specimens with no endplate. Finite element model predictions showed significant influence of the hole pattern on the fraction of the upper endplate exposed to fracture stress. A large hole was predicted to be more effective than the other patterns at distributing a compressive load across the remaining area and thus minimizing the potential fracture area.

CONCLUSION

Results of this study suggest that it is important to preserve the endplate as much as possible to prevent graft subsidence into the vertebral body, particularly in patients with poor bone quality. It is preferable to make one central hole rather than multiple smaller holes in the endplate for vascularity of the bone graft because it reduces the surface area exposed to fracture stresses.

Nonautologous interbody fusion materials in cervical spine surgery: how strong is the evidence to justify their use?

STUDY DESIGN

A review of the literature concerning the use of interbody fusion devices and materials in anterior cervical surgery.

OBJECTIVES

To examine the evidence supporting the use of interbody fusion devices as an alternative to autologous bone after anterior cervical discectomy.

SUMMARY OF BACKGROUND DATA

Concerns over the morbidity associated with harvesting autologous bone and the risk of transmissible infectious from allografts and xenografts have prompted the search for alternative methods of achieving interbody fusion. Several of these methods have been associated with an unacceptable rate of complications. The clinical and health economic implications of the widespread introduction of interbody fusion devices in the absence of sound evidence cannot be ignored.

METHODS

A systematic review of the literature relating to cervical interbody fusion was undertaken. Studies were assessed critically with respect to their methodology, results, and conclusions.

RESULTS

Thirty-two clinical studies and 10 laboratory studies were analyzed. Methodologic weaknesses were identified in the majority. Only four clinical reports were either randomized or blinded or involved independent assessment of their outcomes. Fewer than half of the studies included a valid statistical analysis. Radiologic evidence of fusion was limited in many cases. There was little evidence that nonautologous fusion devices offered a reduction in the length of hospital stay. Autologous bone was as effective as, or superior to, many other fusion devices. The early results of some new fusion techniques used alone or in combination showed promise.

CONCLUSIONS

There is limited evidence supporting the use of a cervical interbody fusion device in place of autologous bone. There is a need to standardize the testing of implants with good quality laboratory work preceding clinical use. Certain devices including cages, some forms of hydroxyapatite, and bone morphogenic proteins merit further study.

Effects of neck movements on stability and subsidence in cervical interbody fusion: an in vitro study

OBJECT

The aim of this in vitro study was to determine the influence of simulated postoperative neck movements on the stabilizing effect and subsidence of four different anterior cervical interbody fusion devices. Emphasis was placed on the relation between subsidence and spinal stability.

METHODS

The flexibility of 24 human cervical spine specimens was tested before and directly after being stabilized with a WING, BAK/C, AcroMed I/F cage, or with bone cement in standard flexibility tests under 50 N axial preload. Thereafter, 700 pure moment loading cycles (+/- 2 Nm) were applied in randomized directions to simulate physiological neck movements. Additional flexibility tests in combination with measurements of the subsidence depth were conducted after 50, 100, 200, 300, 500, and 700 loading cycles. In all four groups, simulated postoperative neck movements caused an increase of the range of motion (ROM) ranging from 0.4 to 3.1 degrees and of the neutral zone from 0.1 to 4.2 degrees. This increase in flexibility was most distinct in extension followed by flexion, lateral bending, and axial rotation. After cyclic loading, ROM tended to be lower in the group fitted with AcroMed cages (3.3 degrees in right lateral bending, 3.5 degrees in left axial rotation, 7.8 degrees in flexion, 8.3 degrees in extension) and in the group in which bone cement was applied (5.4 degrees, 2.5 degrees, 7.4 degrees, and 8.8 degrees, respectively) than in those fixed with the WING (6.3 degrees, 5.4 degrees, 9.7 degrees, and 6.9 degrees, respectively) and BAK cages (6.2 degrees, 4.5 degrees, 10.2 degrees, and 11.6 degrees, respectively).

CONCLUSIONS

Simulated repeated neck movements not only caused an increase of the flexibility but also subsidence of the implants into the adjacent vertebrae. The relation between flexibility increase and subsidence seemed to depend on the implant design: subsiding BAK/C cages partially supported stability whereas subsiding WING cages and AcroMed cages did not.

Subsidence resulting from simulated postoperative neck movements: an in vitro investigation with a new cervical fusion cage

STUDY DESIGN

A biomechanical in vitro subsidence test of different cervical interbody fusion devices was performed using a new testing protocol that simulates physiologic conditions.

OBJECTIVES

To investigate the effect of simulated postoperative neck movements on the subsidence of the new WING cervical interbody fusion cage in comparison with two other cages and bone cement.

SUMMARY OF BACKGROUND DATA

Cervical interbody fusion cages sometimes cause complications because of subsidence into the adjacent vertebrae with collapse of the intervertebral space. Complications such as cage dislocation or nonunion with instability also have been reported. To prevent such complications, the new WING cervical interbody fusion cage (Medinorm AG, Quierschied, Germany) has been developed. Its area of contact with the adjacent vertebrae is supposed to be large enough to resist excessive subsidence and small enough to prevent stress protection of the tissue growing in the cage.

METHODS

In this study, 24 human cervical spine specimens were tested after stabilization with either a WING, BAK/C, AcroMed I/F cage or bone cement. Then, in a new testing protocol, 700 pure-moment loading cycles (+/-2 Nm) were applied in randomized directions (lateral bending, flexion-extension, and axial rotation alone or in combination with each other) to simulate the patient's neck movements during the first few postoperative days. Measurements of the subsidence depth (total height loss) in combination with flexibility tests (+/-2.5 Nm) were performed before cyclic loading and after 50, 100, 200, 300, 500, and 700 loading cycles.

RESULTS

Cyclic loading caused subsidence in all four device groups, most distinct with BAK/C-cages (1.63 mm after 700 loading cycles) followed by the new WING (0.90 mm) and the AcroMed (0.82 mm) cages. No statistically significant difference could be found among the three cage designs. However, all three cage types showed a significantly higher subsidence depth than bone cement (0.48 mm;P = 0.023 between each of the three cage-types and bone cement). A moderate correlation between bone mineral density and subsidence depth could be found only in the BAK/C group (r2 = 0.495). A large subsidence depth after 700 loading cycles was associated with a large flexibility increase in the WING (r2 = 0.786) and AcroMed groups (r2 = 0.21), but with a small flexibility increase in the BAK/C group (r2 = 0.58).

CONCLUSIONS

Postoperative neck movements caused subsidence in all cervical interbody implant types. The new WING cage and the AcroMed cage seemed to have a better resistance against subsidence than the BAK/C cage. However, all three cage types had a significantly higher subsidence tendency than bone cement.