Experimentelle Spondylodese der Schafshalswirbelsäule Teil 1: Der Effekt des Cage-Designs auf die intervertebrale Fusion

PEEK Versus Ti Interbody Fusion Devices: Resultant Fusion, Bone Apposition, Initial and 26-Week Biomechanics

STUDY DESIGN

Comparative evaluation of in vitro and in vivo biomechanics, resulting fusion and histomorphometric aspects of polyetheretherketone (PEEK) versus titanium (Ti) interbody fusion devices in an animal model with similar volumes of bone graft.

OBJECTIVE

Identify differences in the characteristics of fusion and biomechanics immediately following implantation (time 0) and at 26 weeks with each interbody implant.

SUMMARY OF BACKGROUND DATA

PEEK has been well accepted in spinal surgery, it provides a closer match to the mechanical properties of bone than metallic implants such as Ti. This is thought to reduce graft stress shielding and subsidence of interbody fusion devices. There remains controversy as to the overall influence of this as a factor influencing resultant fusion and initial stability. Although material modulus is 1 factor of importance, other design factors are likely to play a large role determining overall performance of an interbody implant.

METHODS

A Ti and PEEK device of similar size with a central void to accommodate graft material were compared. The PEEK device had a ridged surface on the caudal and cephalad surfaces, whereas Ti device allowed axial compliance and had bone ingrowth endplates and polished internal surfaces. A 2-level ALIF was performed in 9 sheep and fusion, biomechanics, and bone apposition were evaluated at 26 weeks. Time 0 in vitro biomechanical tests were performed to establish initial stability immediately after implantation.

RESULTS

No differences were detected in the biomechanical measures of each of the devices in in vitro time 0 tests. All levels were fused by 26 weeks with considerably lower range of motion when compared with in vitro tests. Range of motion in all modes of bending was reduced by over 70% when compared with intact values for axial rotation (Ti-74%, PEEK-71%), lateral bending (Ti-90%, PEEK-88%), and flexion/extension (Ti-92%, PEEK-91%). Mechanical properties of fusions formed with each implant did not differ; however, bone apposition was variable with polished internal Ti surfaces being lower than PEEK and treated Ti endplates showing the greatest levels. Graft material displayed axial trabecular alignment with both implants.

CONCLUSIONS

Although material properties and surface characteristics resulted in differing amounts of biological integration from the host, both implants were capable of producing excellent fusion results using similar volumes of bone graft.

[Fusion criteria for cages as vertebral body replacement in thoracolumbar fractures]

BACKGROUND

No commonly accepted criteria to evaluate bony incorporation of cages as vertebral body replacement in thoracolumbar fractures exist. The goal of this study was a thorough radiological evaluation of the fusion process in posterior-anterior stabilized fractures.

PATIENTS AND METHODS

In this study 35 patients were evaluated radiologically including computed tomography (CT) scanning and bone mineral density measurement inside the cages. Correction loss, cage subsidence and tilting, bone growth in and around the cages as well as bone mineral density were assessed. Fusion grading was assessed with defined criteria (i.e. bridging bone, bone growth through the cage, stability in functional X-rays and no radiolucent lines).

RESULTS

After 12 months minor subsidence and tilting of the cages had caused significant correction loss of the basal plate angle of 2.4° on average. Of the patients 20 (57%) fulfilled the criteria for complete or incomplete fusion and 5 (14%) showed no signs of bony fusion. Bone mineral density measurements were unreliable due to metallic artefacts.

CONCLUSIONS

The advocated criteria allow accurate assessment of bony incorporation of cages. Bony incorporation can be detected in and around the cages over time; however, only 57% of patients showed signs of bony fusion after 1 year.

Bone regeneration in long-bone defects: tissue compartmentalisation? In vivo study on bone defects in sheep

Regeneration of living tissue varies with species, age and type of tissue, and undoubtedly with the biological and mechanical environment of the precise tissue. Autologous cancellous bone grafting is a well-known technique that provides bony regeneration. We investigated the efficiency of autologous bone grafting in a well-vascularised muscle environment, and additionally when isolated from the muscle and connected only to the bony environment. We designed a reproducible animal model producing a stable 3cm middiaphyseal bone and periosteal defect on sheep femurs and created a foreign-body membrane with a temporary poly-methylmethacrylate spacer. The foreign-body membrane had the outer dimension of the removed bone segment. We then ascertained the bony regeneration potential within the bone defect using autologous cancellous bone graft. Regeneration of bone is enhanced considerably by an autologous foreign-body membrane that separates the interfragmentary space from the muscular environment. This effect is independent of the autologous bone graft. The results suggest that bone behaves like a compartment that protects its specific humoral or its cellular environment, or both. Regeneration of bone can be enhanced by compartmentalisation of the bone defect.

Histological evaluation of the sheep's cervical spine segment 3/4--a histomorphological and histomorphometrical study

The 2-year-old sheep is a frequently used model in spinal orthopaedic research; however, so far no investigations have been performed on physiological tissue of untreated animals. In this study, the cervical spine segment 3/4 of eight 2-year-old female Merino sheep was evaluated histologically and histomorphometrically. Specimens were stained with Masson-Goldner-Trichrome, Safranin-Orange/Lightgreen, Safranin-Orange/von Kossa and Astrablue. Analysis of bone mass, mass of cartilage and mineralized cartilage in a defined region of interest was performed. A new finding of the study was that--although 2-year-old sheep is regarded as adult--tissue of the growth plate was still detected. Moreover, this study revealed that in the nucleus pulposus of the 2-year-old sheep notochordal cells are still existent, suggesting a complex remodelling of the nucleus pulposus in sheep.

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.

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.