The role of spinal instrumentation in augmenting lumbar posterolateral fusion

Polyetheretherketone (PEEK) rods versus titanium rods for posterior lumbar fusion surgery: a systematic review and meta-analysis

BACKGROUND

Rigid fixation, represented by titanium rods, is a widely used fixation technique for lumbar fusion. However, this technique carries the risk of degeneration of adjacent segments. In recent years, the semi-rigid fixation technique represented by PEEK rods has gradually matured, and its effectiveness has been verified by numerous studies. The aim of this study was to systematically evaluate the effectiveness of these two fixation modalities in posterior lumbar fusion surgery.

METHODS

Studies meeting the inclusion criteria were searched in PubMed, Cochrane Library, ScienceDirect, Embase, CNKI, and Wanfang databases. After data extraction and quality assessment of included studies, meta-analysis was performed using STATA 15.1 software. The protocol for this systematic review was registered on INPLASY (2021110049) and is available in full on the inplasy.com ( https://inplasy.com/inplasy-2021-11-0049/ ).

RESULTS

Fifteen relevant studies were finally included, including eight prospective studies and seven retrospective studies. The results of meta-analysis showed that in ODI (P = 0.000), JOA score (P = 0.017), VAS score for lower limb pain (P = 0.027), fusion rate of bone graft at week 12 (P = 0.001), fusion rate of bone graft at last follow-up (P = 0.028), there was a statistical difference between the two groups. The PEEK rod group was superior to the titanium rod group in the above aspects. While in VAS score for LBP (P = 0.396), there was no statistical difference between the two groups.

CONCLUSION

Both PEEK rods and titanium rods are effective fixation materials in lumbar fusion surgery. PEEK rods may be superior to titanium rods in improving postoperative function and improving bone graft fusion rates. However, given the limitations of this study, whether these conclusions are applicable needs further research.

Biphasic calcium phosphate with submicron surface topography in an Ovine model of instrumented posterolateral spinal fusion

As spinal fusions require large volumes of bone graft, different bone graft substitutes are being investigated as alternatives. A subclass of calcium phosphate materials with submicron surface topography has been shown to be a highly effective bone graft substitute. In this work, a commercially available biphasic calcium phosphate (BCP) with submicron surface topography (MagnetOs; Kuros Biosciences BV) was evaluated in an Ovine model of instrumented posterolateral fusion. The material was implanted stand-alone, either as granules (BCPgranules) or as granules embedded within a fast-resorbing polymeric carrier (BCPputty) and compared to autograft bone (AG). Twenty-five adult, female Merino sheep underwent posterolateral fusion at L2-3 and L4-5 levels with instrumentation. After 6, 12, and 26 weeks, outcomes were evaluated by manual palpation, range of motion (ROM) testing, micro-computed tomography, histology and histomorphometry. Fusion assessment by manual palpation 12 weeks after implantation revealed 100% fusion rates in all treatment groups. The three treatment groups showed a significant decrease in lateral bending at the fusion levels at 12 weeks (P < 0.05) and 26 weeks (P < 0.001) compared to the 6 week time-point. Flexion-extension and axial rotation were also reduced over time, but statistical significance was only reached in flexion-extension for AG and BCPputty between the 6 and 26 week time-points (P < 0.05). No significant differences in ROM were observed between the treatment groups at any of the time-points investigated. Histological assessment at 12 weeks showed fusion rates of 75%, 92%, and 83% for AG, BCPgranules and BCPputty, respectively. The fusion rates were further increased 26 weeks postimplantation. Similar trends of bone growth were observed by histomorphometry. The fusion mass consisted of at least 55% bone for all treatment groups 26 weeks after implantation. These results suggest that this BCP with submicron surface topography, in granules or putty form, is a promising alternative to autograft for spinal fusion.

The Ovariectomized Ewe Model in the Evaluation of Biomaterials for Prosthetic Devices in Spinal Fixation

The effect of surgical ovariectomy on cancellous bone was investigated by comparing mechanical properties and microarchitectural characteristics of the lumbar vertebrae in ovariectomized and sham-operated ewes. Eighteen mongrel ewes, 4±1 years old, were randomly divided into three groups: 6 animals served as a control group (Baseline), 6 were bilaterally ovariectomized (OVX), and the others were used as a sham-operated group (SHAM). OVX and SHAM ewes were euthanized 24 months after surgery; the L5 vertebrae were processed for mechanical and histomorphometric analyses. Maximum load, maximum strength (p<0.0005) and elastic modulus (p <0.005) decreased by about 28% in the OVX group in comparison with the other groups. In the OVX group, vertebral cancellous bone volume, trabecular thickness and trabecular number decreased by about 32% (p<0.0005), 15% (p=0.001) and 20% (p=0.019), respectively. An overall decrease in the bone turnover rate of the OVX group was registered in terms of bone formation rate (p= 0.007) and activation frequency (p<0.0005). The variations observed in cancellous bone mechanics and histomorphometry would suggest the development of an osteopenic state in ewe vertebrae at 24 months. Such findings may be useful for future experimental investigations on biomaterials and prosthetic devices to be implanted in the osteopenic spine.

Biomaterials in Spinal Fixation. An Experimental Animal Study to Improve the Performance

Different pedicle screws were biomechanically and morphologically studied and compared through the use of an animal model to determine their efficacy and resistance in spinal fixation. The principal objective was to compare biomechanical and histomorphological aspects of HA-coated screws to uncoated ones.

Fourty-eight cylindrical transpedicular self-tapping screws divided into three groups of sixteen each were employed; Group A: stainless steel screws; Group B: titanium screws; Group C: HA-coated titanium screws. The screws were implanted bilaterally and randomly into the L3, L4, and L5 pedicles of eight adult mongrel sheep. The final insertion torque was measured in all the implants. After one and four months, upon euthanization, four samples per group were extracted from the surrounding bone and the screw extraction torque was measured. The remaining samples were examined and processed for histological and histomorphological evaluations.

No differences were observed at one month among the extraction torque of the three groups. After four months the only significance between insertion and extraction values was for the HA group, i.e. p=0.001. Comparing the extraction torque values of the three groups after four months of healing, the HA-coated group showed a greater than twofold increase (p<0.0005). No differences were observed at one month among the percentages of bone-implant contact in the three groups. After four months the percentage was significant only for the C group (p<0.0005). At four months a correlation was found between the morphological and the biomechanical data of group C (p<0.0005).

The use of hydroxyapatite-coated screws could act as an effective method to improve the bone-implant interface, thus obtaining a strong fixation of the implant independently of the arthrodesis achieved with bone graft.

Impact of spinal rod stiffness on porcine lumbar biomechanics: Finite element model validation and parametric study

A three-dimensional finite element model of the porcine lumbar spine (L1-L6) was used to assess the effect of spinal rod stiffness on lumbar biomechanics. The model was validated through a comparison with in vitro measurements performed on six porcine spine specimens. The validation metrics employed included intervertebral rotations and the nucleus pressure in the first instrumented intervertebral disc. The numerical results obtained suggest that rod stiffness values as low as 0.1 GPa are required to reduce the mobility gradient between the adjacent and instrumented segments and the nucleus pressures across the porcine lumbar spine significantly. Stiffness variations above this threshold value have no significant effect on spine biomechanics. For such low-stiffness rods, intervertebral rotations in the instrumented zone must be monitored closely in order to guarantee solid fusion. Looking ahead, the proposed model will serve to examine the transverse process hooks and variable stiffness rods in order to further smooth the transition between the adjacent and instrumented segments, while preserving the stability of the instrumented zone, which is needed for fusion.

A study to compare the efficacy of polyether ether ketone rod device with titanium devices in posterior spinal fusion in a canine model

BACKGROUND

The benefits of posterior lumbar fusion surgery with orthotopic paraspinal muscle-pediculated bone flaps are well established. However, the problem of non-union due to mechanical support is not completely resolved. The aim of the study was to compare the efficacy of polyether ether ketone (PEEK) rod device with conventional titanium devices in the posterior lumbar fusion surgery with orthotopic paraspinal muscle-pediculated bone flaps.

METHODS

This was a randomized controlled study with an experimental animal model. Thirty-two mongrel dogs were randomly divided into two groups-control group (n = 16), which received the titanium device and the treatment group (n = 16), which received PEEK rods. The animals were sacrificed 8 or 16 weeks after surgery. Lumbar spines of dogs in both groups were removed, harvested, and assessed for radiographic, biomechanical, and histological changes.

RESULTS

Results in the current study indicated that there was no significant difference in the lumbar spine of the control and treatment groups in terms of radiographic, manual palpation, and gross examination. However, certain parameters of biomechanical testing showed significant differences (p < 0.05) in stiffness and displacement, revealing a better fusion (treatment group showed decreased stiffness with decreased displacement) of the bone graft. Similarly, the histological analysis also revealed a significant fusion mass in both treatment and control groups (p < 0.05).

CONCLUSIONS

These findings revealed that fixation using PEEK connecting rod could improve the union of the bone graft in the posterior lumbar spine fusion surgery compared with that of the titanium rod fixation.

Clinical and Radiological Comparison of Semirigid (WavefleX) and Rigid System for the Lumbar Spine

OBJECTIVE

Spinal fusion operation is an effective treatment in the spinal pathology, but it could change the physiological distribution of load at the instrumented and adjacent segments. This retrospective study compared the radiological and clinical outcomes of patients undergoing lumbar fusion with semirigid rods versus rigid rods system.

METHODS

Using transpedicular fixation and posterior lumbar interbody fusion at the level of L4/L5, 20 patients were treated with semirigid rods (WavefleX, SR group), and 20 patients with rigid rods (titanium, RR group). Clinical and radiological outcomes were evaluated, including visual analog score for lower back pain and leg pain, Prolo functional and economic scores, statues of implanted instruments, fusion rate, and complications during 24-month follow-up.

RESULTS

Clinical scores were significantly improved until postoperative 24-month follow-up as compared with preoperative scores in both groups (p<0.05), with similar levels of improvement observed at the same time points postoperatively between the 2 groups. Prolo economic scores were significantly improved in SR group compared to RR until 12 months, but this improvement became similar after 18 months. The overall fusion rate was 94.1% until the 24-month follow-up for both groups. No significant complication was observed in both groups.

CONCLUSION

The results of the present study indicate that semirigid rods system with posterior lumbar interbody fusion showed similar clinical and radiological result with rigid rods system until 2 years after instrumentation. The WavefleX rods system, as a semirigid rods with unique characteristics, may be an effective alternative treatment for patients in lumbar fusion.

Flexible Stabilisation of the Degenerative Lumbar Spine Using PEEK Rods

Posterior lumbar interbody fusion using cages, titanium rods, and pedicle screws is considered today as the gold standard of surgical treatment of lumbar degenerative disease and has produced satisfying long-term fusion rates. However this rigid material could change the physiological distribution of load at the instrumental and adjacent segments, a main cause of implant failure and adjacent segment disease, responsible for a high rate of further surgery in the following years. More recently, semirigid instrumentation systems using rods made of polyetheretherketone (PEEK) have been introduced. This clinical study of 21 patients focuses on the clinical and radiological outcomes of patients with lumbar degenerative disease treated with Initial VEOS PEEK^®-Optima system (Innov'Spine, France) composed of rods made from PEEK-OPTIMA^® polymer (Invibio Biomaterial Solutions, UK) without arthrodesis. With an average follow-up of 2 years and half, the chances of reoperation were significantly reduced (4.8%), quality of life was improved (ODI = 16%), and the adjacent disc was preserved in more than 70% of cases. Based on these results, combined with the biomechanical and clinical data already published, PEEK rods systems can be considered as a safe and effective alternative solution to rigid ones.

Biomechanical assessment of the stabilization capacity of monolithic spinal rods with different flexural stiffness and anchoring arrangement

BACKGROUND

Spinal disorders can be treated by several means including fusion surgery. Rigid posterior instrumentations are used to obtain the stability needed for fusion. However, the abrupt stiffness variation between the stabilized and intact segments leads to proximal junctional kyphosis. The concept of spinal rods with variable flexural stiffness is proposed to create a more gradual transition at the end of the instrumentation.

METHOD

Biomechanical tests were conducted on porcine spine segments (L1-L6) to assess the stabilization capacity of spinal rods with different flexural stiffness. Dual-rod fusion constructs containing three kinds of rods (Ti, Ti-Ni superelastic, and Ti-Ni half stiff-half superelastic) were implanted using two anchor arrangements: pedicle screws at all levels or pedicle screws at all levels except for upper instrumented vertebra in which case pedicle screws were replaced with transverse process hooks. Specimens were loaded in forward flexion, extension, and lateral bending before and after implantation of the fusion constructs. The effects of different rods on specimen stiffness, vertebra mobility, intradiscal pressures, and anchor forces were evaluated.

FINDING

The differences in rod properties had a moderate impact on the biomechanics of the instrumented spine when only pedicle screws were used. However, this effect was amplified when transverse process hooks were used as proximal anchors.

INTERPRETATION

Combining transverse hooks and softer (Ti-Ni superelastic and Ti-Ni half stiff-half superelastic) rods provided more motion at the upper instrumented level and applied less force on the anchors, potentially improving the load sharing capacity of the instrumentation.

Monolithic superelastic rods with variable flexural stiffness for spinal fusion: simplified finite element analysis of an instrumented spine segment

Rigid instrumentations have been widely used for spinal fusion but they come with complications, such as adjacent disc degeneration. Dynamic instrumentations have been tested but their efficiency (stabilization capability) and reliability (mechanical integrity of the implant) have yet to be proven. A monolithic Ti-Ni spinal rod with variable flexural stiffness is proposed to reduce the risks associated with spinal fusion while maintaining adequate stabilization. This publication presents a simplified numerical model capable of evaluating the eventual benefits of a Ti-Ni spinal rod with variable flexural stiffness.

METHODS

A simplified instrumented spine segment model composed of six vertebrae and five discs has been developed. Two types of spinal rods were evaluated: Classic Ti instrumentation and Ti-Ni rods with variable stiffness. Both instrumentations were tested using two anchor configurations: pedicle screws only or a screws-cable combination.

FINDINGS AND DISCUSSION

The all-screws configuration does not allow much motion with either classic Ti or variable Ti-Ni rods. The combination of a Ti rod with screws-cable anchoring allows more motion and, therefore, lower adjacent disk pressure, but puts extremely high stresses on the rod and anchors. The combination of the variable Ti-Ni rod and screws-cable anchoring leads to a significant decrease in adjacent disk pressure, without increasing stresses and pullout forces in the spinal instrumentation.

In-vitro assessment of the stabilization capacity of monolithic spinal rods with variable flexural stiffness: Methodology and examples

The concept of a monolithic Ti-Ni spinal rod with variable flexural stiffness is proposed to reduce the risks associated with spinal fusion. The variable stiffness is conferred to the rod using the Joule-heating local annealing technique. To assess the stabilization capacity of such a spinal rod, in vitro experiments on porcine spine models are carried out. This paper describes the methodology followed to evaluate the effect of Ti-Ni rods compared to conventional titanium rods. Validation of the methodology and examples of results obtained are also presented.

Monolithic superelastic rods with variable flexural stiffness for spinal fusion: modeling of the processing-properties relationship

The concept of a monolithic Ti-Ni spinal rod with variable flexural stiffness is proposed to reduce the risks associated with spinal fusion. The variable stiffness is conferred to the rod using the Joule-heating local annealing technique. The annealing temperature and the mechanical properties' distributions resulted from this thermal treatment are numerically modeled and experimentally measured. To illustrate the possible applications of such a modeling approach, two case studies are presented: (a) optimization of the Joule-heating strategy to reduce annealing time, and (b) modulation of the rod's overall flexural stiffness using partial annealing. A numerical model of a human spine coupled with the model of the variable flexural stiffness spinal rod developed in this work can ultimately be used to maximize the stabilization capability of spinal instrumentation, while simultaneously decreasing the risks associated with spinal fusion.

Lumbar spinal fusion with β-TCP granules and variable Escherichia coli-derived rhBMP-2 dose

BACKGROUND CONTEXT

The ideal tissue-engineered solution for any bone graft substitute is to assist in the rapid formation of bone and facilitate fusion.

PURPOSE

The present study aims to evaluate this E-BMP-2 (Escherichia coli-derived human bone morphogenetic protein-2) in ovine posterolateral lumbar fusion (PLF) to examine the influence of dose and overall performance in a model with similar graft size and diffusive challenges to the human.

STUDY DESIGN/SETTING

In vivo large animal model study.

METHODS

An adult ovine PLF was performed in 30 animals with groups of E-BMP-2 with a beta-tricalcium phosphate (β-TCP) carrier at three different dosages, β-TCP alone, and autograft from the iliac crest. The fusions were assessed by radiography (X-ray and microcomputed tomography), mechanical testing, and hard-tissue histology with bone labels at 6, 8, and 10 weeks along with routine paraffin histology at 12 weeks.

RESULTS

Results showed increasing new bone and fusion rate with E-BMP-2 dose, whereas β-TCP alone was largely resorbed and did not achieve fusion in this model at 12 weeks. Autograft showed similar grading for the amount of bone between the transverse processes but a lower fusion rate than β-TCP/E-BMP-2 groups. Bone labels revealed new bone formation at all time points for the E-BMP2 groups, whereas the autograft group showed active bone formation at 10 weeks. Beta-tricalcium phosphate displayed reliable incorporation into the decorticated host bone, whereas limited new bone was found between the transverse processes. At the center of the fusion mass, increased E-BMP-2 dose led to increased incorporation of β-TCP by new bone.

CONCLUSIONS

These results suggest that E-BMP-2 was capable of producing posterolateral fusion in the ovine model that is equal to or superior to autologous graft in terms of fusion rate and mechanical strength. E-BMP-2 dose had considerable influence on β-TCP granule resorption.

Comparative effectiveness of PEEK rods versus titanium alloy rods in lumbar fusion: a preliminary report

BACKGROUND

Posterior lumbar instruments made of titanium and its alloys could change the physiological distribution of load at the instrumented and adjacent segments, which is a main cause of implant failure, non-fusion and adjacent segment degeneration. Posterior lumbar rods made of polyetheretherketone (PEEK) which is a semirigid alternative to titanium and its alloys have been introduced in lumbar fusion. This prospective study compared the radiological and clinical outcomes of patients undergoing lumbar fusion with PEEK rods versus titanium alloy rods.

METHODS

Using transpedicular fixation and lumbar fusion, 21 patients were treated with titanium alloy rods (TI group), and 20 patients with PEEK rods (PEEK group). Radiological and clinical outcomes were evaluated, including the status of the implanted instruments, fusion rate, lumbar lordosis angle (LA), disc space height (DH), visual analog score (VAS) for lower back pain (VAS-BP) and leg pain (VAS-LP), Japanese Orthopedic Association scoring system (JOA score) and complications.

RESULTS

Clinical VAS-BP, VAS-LP and JOA scores were significantly improved at 3 months, 6 months, and 1 year postoperatively as compared with preoperative scores in both groups (p < 0.05), with similar levels of improvement observed at the same time points postoperatively between the two groups. The overall fusion rate was 100 % at the 1-year follow-up for both groups. No significant differences in lumbar lordosis angle were found preoperatively, 1 week and 1 year postoperatively in both groups (p > 0.05). The postoperative increase of disc space height and loss of disc space height during the follow-up showed a similar extent of change between both groups (p > 0.05).

CONCLUSIONS

PEEK rods offer a similar radiological and clinical efficacy as titanium alloy rods. PEEK rods, as a semirigid implant with unique characteristics, may be an effective alternative treatment for patients with degenerative lumbar disease in lumbar fusion.

Biomechanical assessment of a PEEK rod system for semi-rigid fixation of lumbar fusion constructs

The concept of semi-rigid fixation (SRF) has driven the development of spinal implants that utilize nonmetallic materials and novel rod geometries in an effort to promote fusion via a balance of stability, intra- and inter-level load sharing, and durability. The purpose of this study was to characterize the mechanical and biomechanical properties of a pedicle screw-based polyetheretherketone (PEEK) SRF system for the lumbar spine to compare its kinematic, structural, and durability performance profile against that of traditional lumbar fusion systems. Performance of the SRF system was characterized using a validated spectrum of experimental, computational, and in vitro testing. Finite element models were first used to optimize the size and shape of the polymeric rods and bound their performance parameters. Subsequently, benchtop tests determined the static and dynamic performance threshold of PEEK rods in relevant loading modes (flexion-extension (F/E), axial rotation (AR), and lateral bending (LB)). Numerical analyses evaluated the amount of anteroposterior column load sharing provided by both metallic and PEEK rods. Finally, a cadaveric spine simulator was used to determine the level of stability that PEEK rods provide. Under physiological loading conditions, a 6.35 mm nominal diameter oval PEEK rod construct unloads the bone-screw interface and increases anterior column load (approx. 75% anterior, 25% posterior) when compared to titanium (Ti) rod constructs. The PEEK construct's stiffness demonstrated a value lower than that of all the metallic rod systems, regardless of diameter or metallic composition (78% < 5.5 mm Ti; 66% < 4.5 mm Ti; 38% < 3.6 mm Ti). The endurance limit of the PEEK construct was comparable to that of clinically successful metallic rod systems (135N at 5 × 10(6) cycles). Compared to the intact state, cadaveric spines implanted with PEEK constructs demonstrated a significant reduction of range of motion in all three loading directions (> 80% reduction in F/E, p < 0.001; > 70% reduction in LB, p < 0.001; > 54% reduction in AR, p < 0.001). There was no statistically significant difference in the stability provided by the PEEK rods and titanium rods in any mode (p = 0.769 for F/E; p = 0.085 for LB; p = 0.633 for AR). The CD HORIZON(®) LEGACY(™) PEEK Rod System provided intervertebral stability comparable to currently marketed titanium lumbar fusion constructs. PEEK rods also more closely approximated the physiologic anteroposterior column load sharing compared to results with titanium rods. The durability, stability, strength, and biomechanical profile of PEEK rods were demonstrated and the potential advantages of SRF were highlighted.

Goat bone tissue engineering: comparing an intramuscular with a posterolateral lumbar spine location

The aim of this study was to investigate the effect of implant location on bone formation in goats using autologous bone marrow-derived stromal cells in porous calcium phosphate scaffolds. Intramuscular locations were compared to posterolateral spine fusion locations in eight goats. As scaffolds, we used biphasic calcium phosphate porous blocks of 5 x 5 x 5 mm. Cell-seeded implants were compared to empty controls. Bone marrow-derived stromal cells were seeded at 8 million cells per cm(3) scaffold and cultured for 1 week. The follow-up time was 12 weeks. Fluorochromes were administered intravenously at 4, 6, and 8 weeks. Ectopic implants showed 21 +/- 3.6% bone formation for the cell seeded and 2.0 +/- 3.0% for the controls (p < 0.001). Paraspinal implants, however, showed 0.10 +/- 0.13% in the cell seeded compared to 0.023 +/- 0.027% in the control group (p = 0.09). A benefit of the cells was only found in the area closest to the paraspinal muscles (p < 0.01). Bone formation in the control samples was of later onset compared to the cell-seeded implants. In conclusion, cell-based bone tissue engineering in an ectopic environment was clearly effective. Similar constructs implanted in a posterolateral spine fusion location hardly showed any effect.

Surgical treatment for the painful motion segment: matching technology with the indications: posterior lumbar fusion

STUDY DESIGN

A convenience literature-based review of the different techniques of posterior lumbar fusion.

OBJECTIVE

To describe the history, specific techniques, and outcomes of different methods of posterior lumbar fusion. The specific methods that were described include 1) uninstrumented posterior, posterolateral, and facet fusion, and 2) instrumented fusion using pedicle screws or facet screws.

SUMMARY OF BACKGROUND DATA

There are various posterior fusion techniques available for the treatment of degenerative lumbar spine conditions. Each individual technique has specific technical demands, indications, advantages, and disadvantages which should be taken into consideration when performing these procedures.

METHODS

The published scientific literature on the different methods of posterior lumbar fusion was reviewed. The history, indications, advantages, disadvantages, and clinical and radiographic outcomes were described based on the literature search.

RESULTS/CONCLUSIONS

Posterior fusion techniques have been and will continue to be among the most commonly performed procedures in lumbar spine surgery. The different methods of fusion are well defined, as are the possible complications and outcomes. They are effective techniques when performed on appropriately selected patients by a surgeon knowledgeable in the techniques and indications. Further studies are needed regarding promising but relatively unproven developments such as minimally invasive surgery and the use of osteoinductive agents.

Osteoporosis and biomaterial osteointegration

Biomaterial osteointegration depends not only on the properties of the implanted material but also on the characteristics and regenerative capability of the host bone. For this reason, researchers involved in biomaterial evaluation now place great importance on the various pathologies often present in orthopaedic patients which can negatively affect the success of surgical implants. Osteoporosis is undoubtedly one of the most frequently encountered of such diseases. Models reproducing the osteoporotic condition can be useful to understand the influence of the pathology on cell behaviour, bone regeneration and osteointegration processes, thus increasing our basic knowledge and allowing the development of surgical techniques and implant biomaterials more suitable for use in the surgical treatment of fractures in osteoporotic patients. The present paper is a literature review and, after a short description of how the presence of osteoporosis could influence bone regenerative processes, the results of the main studies on biomaterial biocompatibility and osteointegration both in vitro and in vivo in the presence of osteoporotic condition are reported. Both cell cultures and animal models are able to demonstrate the different response of bone to biomaterials by comparing healthy and pathological conditions. The use of pathological bone-derived cells and pathological animals is therefore recommended to test candidate orthopaedic materials.

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.

Efficacy of osteogenic protein-1 in a challenging multilevel fusion model

STUDY DESIGN

A therapeutic study compared the influence of osteogenic protein-1 to autograft and collagen carrier in multilevel sheep spine fusions.

OBJECTIVE

To evaluate the efficacy of osteogenic protein-1 compared to autograft and collagen carrier in achieving fusion in a challenging multilevel lumbar spine ovine model.

SUMMARY OF BACKGROUND DATA

Bone morphogenetic proteins can successfully augment spinal fusion. To date, all the preclinical and clinical studies using bone morphogenetic proteins have evaluated single-level fusion. In practice, multiple level fusions are commonly required for various conditions, like spinal deformity.

METHODS

Eighteen sheep underwent three-level spine fusion. Six sheep were treated with osteogenic protein-1 and its carrier, autograft, or with the carrier alone. Specimens were analyzed for evidence of fusion by palpation, radiographic and histologic analysis, and biomechanical testing.

RESULTS

Manual palpation testing for the presence of fusion showed none of the specimens fused all three levels or fused at the lumbosacral junction. No statistically significant difference was found between the osteogenic protein-1 and autograft groups' fusion rates based on radiographic grading (P = 0.65) or biomechanical testing. Histologic analysis showed no qualitative difference in bone morphology or cellularity of fusion masses when comparing the autograft and osteogenic protein-1 specimens.

CONCLUSIONS

No model before this exists that tests the efficacy of bone morphogenic proteins in as challenging an environment. Extrapolation of single-level preclinical and clinical studies with bone morphogenic proteins for use in multilevel fusion requires careful review. Autograft and osteogenic protein-1 had similar rates of fusion. A high rate of nonunion is seen with this multiple level fusion to the sacrum using autograft or osteogenic protein-1. The biologic enhancement with osteogenic protein-1 is not able to overcome this mechanically rigorous model.

Pedicular fixation in the osteoporotic spine: a pilot in vivo study on long-term ovariectomized sheep

Spinal instrumentation success is greatly affected by the presence of osteoporosis. To date, however, no data exist on in vivo investigations on biomaterial and surgical techniques in the osteoporotic spine. In the present study 24 uncoated and 24 HA-coated screws were implanted in the L3, L4 and L5 pedicles of eight sheep (four ovariectomized, OVX Group; four sham-operated, Control Group). At four months, uncoated screws showed a significant decrease of about -22% in the extraction torque of the OVX Group as compared to the Control Group (p < 0.005). The extraction torque of HA-coated screws significantly (p < 0.0005) improved in both groups when compared to that of uncoated screws and showed increases ranging from 133% to 157%. Pedicle trabecular bone of OVX sheep showed a significant decrease in BV/TV (-30%; p < 0.05) and Tb.Th (-33%; p < 0.01). The affinity index (AI) results revealed significant (p < 0.0005) differences between uncoated and HA-coated screws for both groups: values were lower for uncoated than HA-coated screws by about -35%. A significant difference was also found for the AI data of uncoated screws between the OVX and Control Groups (-13%, p < 0.005). The current findings have demonstrated that long-term ovariectomized sheep can be used to study in vivo osteointegration in the osteoporotic spine. The HA coating has proven to improve bone purchase and bone-screw interface strength in healthy and osteopenic animals.

Characterization of a developing lumbar arthrodesis in a sheep model with quantitative instability

BACKGROUND CONTEXT

Mechanical forces have been considered responsible for stress shielding an arthrodesis, but the biology of a developing lumbar fusion has not been well characterized.

PURPOSE

A large animal model was used to test the hypothesis that mechanical forces modify the biological processes involved in a developing bony fusion.

STUDY DESIGN

Lumbar fusion was performed in an ovine model using custom instrumentation that permitted a controlled degree of anterior-posterior translation after surgery. Fusion sites were evaluated by radiography, microradiography, histology and histomorphometry at time points that corresponded with predicted early and later stages of bone healing.

METHODS

Fourteen skeletally mature ewes underwent lumbar spinal fusion under general anesthesia. In the control (stable) group, the spine was rigidly fixed with a cage anteriorly and pedicle screws posteriorly. In the experimental (unstable) group, the spine was destabilized by an annulectomy (with no anterior implant) and custom pedicle screws that allowed 2 mm of anterior-posterior translation. Animals were euthanized 6 and 12 weeks after surgery.

RESULTS

Radiographs confirmed that the fusion mass had not fully consolidated at either time point. Microradiographs revealed a trend toward increased bone formation at 6 weeks in the stable case as compared with the unstable, but by 12 weeks, this trend had reversed (p=.03). Intramembranous bone formation was the primary mechanism of healing near the transverse process in animals with both stable and unstable fixation. In the area between the two transverse processes, new bone formation occurred primarily through endochondral ossification. At 12 weeks, the stable case had significantly more cartilage formed (p=.023) but less newly formed bone (p=.07) as compared with the quantitatively unstable.

CONCLUSIONS

This clinically realistic animal model allowed characterization of the biology of the developing arthrodesis before fusion. Under stable or unstable conditions, endochondral ossification was the predominant mechanism of new bone formation within the intertransverse process region. This finding, which contrasts with previous reports from small animal models of spine fusion, may reflect a difference in biology that results from the increased size of the intertransverse space in sheep as compared with small animals. Interestingly, mechanical instability increased the formation of new bone within this region, but not at the transverse process. Endochondral ossification therefore appears to respond to mechanical factors in the fusion site. The ovine model shows promise as an alternative to the rabbit model and may provide a more stringent test for potential new surgical and nonsurgical strategies for spine fusion.

The effect of osteogenic protein-1 in instrumented and noninstrumented posterolateral fusion in rabbits

BACKGROUND CONTEXT

The use of rigid instrumentation combined with bone graft makes intuitive sense given the requirements for vascular ingrowth, bone formation and a stable environment for the cellular events of healing to develop. However, with the advances of potent osteoinductive growth factors, the role of internal fixation may come into question. Whether bone morphogenic proteins (BMPs) would benefit from a more "stable" spinal segment for bone production and modeling remains unknown. In addition, it is unknown whether BMP and rigid fixation may have an additive effect on fusion healing.

PURPOSE

This study is proposed to test the hypothesis that rigid fixation in the lumbar spine would be advantageous to achieve fusion for autogenous bone grafting, but fusion would occur regardless of fixation with the use of osteogenic protein (OP)-1.

STUDY DESIGN/SETTING

A histologic and radiographic analysis of BMP in a rabbit lumbar fusion model.

METHODS

Thirty-two rabbits were randomized into four groups: 1) control animals: in situ posterolateral L5-L6 arthrodesis using autogenous iliac crest bone graft; 2) fixation group: posterolateral arthrodesis L5-L6 with autogenous bone graft and interspinous fixation; 3) OP-1 group: in situ posterolateral L5-L6 arthrodesis using OP-1 and 4) combined OP-1 and fixation group. Radiographic fusion analysis was performed with computed tomography scans at 3 and 12 weeks after surgery. Decalcified histology was performed to assess tissue morphology and cellularity.

RESULTS

Minimal evidence of fusion was noted at 3 weeks with autograft or OP-1. By 12 weeks, all OP-1-treated animals had solid fusion, whereas no fusion was noted in autograft animals. The addition of fixation slightly increased radiographic fusion at 3 weeks in autograft and OP-1 groups but did not affect OP-1 animals at 12 weeks where all were fused. Decalcified histologic results confirmed the proliferative bone formation noted with OP-1 and the variable cellular response with autograft.

CONCLUSIONS

The results of the present study suggest that the osteoinductive effect of OP-1 may be only minimally enhanced early in the bone healing process but does not appear to be affected in the long term by spinal fixation in the rabbit intertransverse fusion model. Fixation appeared to enhance early fusion in the autograft group.

Artificial intervertebral disc replacement using bioactive three-dimensional fabric: design, development, and preliminary animal study

STUDY DESIGN

A new artificial intervertebral disc was developed, and its intrinsic biomechanical properties, bioactivity, and the effectiveness as a total disc replacement were evaluated in vitro and in vivo.

OBJECTIVES

To introduce a new artificial intervertebral disc and to evaluate the in vitro mechanical properties, fusion capacity to bone, and segmental biomechanics in the total intervertebral disc replacement using a sheep lumbar spine.

SUMMARY OF BACKGROUND DATA

The loss of biologic fusion at the bone-implant interface and prosthetic failures have been reported in previous artificial discs. There have been no clinically applicable discs with detailed experimental testing of in vivo mechanics and interface fusion capacity.

METHODS

The artificial intervertebral disc consists of a triaxial three-dimensional fabric (3-DF) woven with an ultra-high molecular weight polyethylene fiber, and spray-coated bioactive ceramics on the disc surface. The arrangement of weave properties was designed to produce mechanical behavior nearly equivalent to the natural intervertebral disc. Total intervertebral disc replacement at L2-L3 and L4-L5 was performed using 3-DF disc with or without internal fixation in a sheep lumbar spine model. The segmental biomechanics and interface histology were evaluated after surgery at 4 and 6 months.

RESULTS

The tensile-compressive and torsional properties of prototype 3-DF were nearly equivalent to those of human lumbar disc. The lumbar segments replaced with 3-DF disc alone showed a significant decrease of flexion-extension range of motion to 28% of control values as well as partial bony fusion at 6 months. However, the use of temporary fixation provided a nearly physiologic mobility of the spinal segment after implant removal as well as excellent bone-disc fusion at 6 months.

CONCLUSION

An artificial intervertebral disc using a three-dimensional fabric demonstrated excellent in vitro and in vivo performance in both biomechanics and interface histology. There is a potential for future clinical application.

Lumbosacral stability of consolidated anteroposterior fusion after instrumentation removal determined by roentgen stereophotogrammetric analysis and direct surgical exploration

STUDY DESIGN

The intervertebral stability of bony consolidated anteroposterior lumbosacral spondylodesis is evaluated by roentgen stereophotogrammetric analysis and direct surgical exploration before and after removal of the internal fixator.

OBJECTIVES

To determine the remaining in vivo stability of spinal arthrodesis solely retained by a bony integrated carbon fiber cage.

SUMMARY OF BACKGROUND DATA

Roentgen stereophotogrammetric analysis studies on posterolateral lumbar fusions demonstrate primary spinal stability after additional dorsal instrumentation, which is retained during bony fusion healing. Animal models show a persistent stabilizing effect of the fixator despite the presence of bony fusion. Although direct surgical inspection is the most reliable method to evaluate fused vertebrae, roentgen stereophotogrammetric analysis has also proven to be a highly accurate method to evaluate spinal stability.

METHODS

In 10 patients lumbosacral fusion was performed using carbon interbody implants and an internal fixator. Ten months after initial surgery (range 7-15 months) the internal fixation was removed to reduce local soft tissue impingement as soon as bony fusion was achieved. Fusion site exploration in the course of instrumentation removal was performed by applying distraction, compression, and torque to the grafted area under fluoroscopic control. Any motion indicated a pseudarthrosis. Lumbosacral stability was evaluated by serial roentgen stereophotogrammetric analysis after fusion and after instrumentation removal.

RESULTS

During instrumentation removal the mechanical stress test under fluoroscopic control did not indicate pseudarthrosis. After instrumentation removal, roentgen stereophotogrammetric analysis measurements revealed a nonsignificant increase in lumbosacral micromotions within the fused segment with 0.14, 0.31, and 0.44 mm in the transverse, vertical, and sagittal axes, respectively.

CONCLUSIONS

The internal fixator could be removed without endangering the stability of the fusion. Direct surgical exploration confirmed the adequacy of roentgen stereophotogrammetric analysis as a reliable in vivo method to evaluate lumbosacral stability after anteroposterior fusion.

Improved bone-screw interface with hydroxyapatite coating: an in vivo study of loaded pedicle screws in sheep

STUDY DESIGN

An in vivo sheep model with loaded pedicle screws was used, wherein each animal served as its own control.

OBJECTIVES

To examine the effects of hydroxyapatite (HA) coating on the bone-to-implant interface in loaded spinal instrumentations.

SUMMARY OF BACKGROUND DATA

Spinal instrumentation improves the healing rate in spinal fusion, but screw loosening constitutes a problem. HA coating of other implants has resulted in favorable effects on the bone-to-implant interface.

METHODS

Nine sheep were operated on with destabilizing laminectomies at two levels: L2-L3 and L4-L5. Each level was stabilized separately with a four-screw instrumentation. Uncoated screws (stainless steel) or the same type of screws coated with plasma-sprayed HA were used in either the upper or the lower instrumentation in a randomized fashion. The animals were killed at 6 or 12 weeks after surgery. The specimens were embedded in resin, ground to approximately 10 microm, and stained with toluidine blue. Histomorphometric evaluation was carried out in a Leitz Aristoplan (Wetzlar, Germany) light microscope equipped with a Leitz Microvid unit.

RESULTS

The average percentage of bone-to-implant contact after 6 weeks was 69 +/- 10 for the HA-coated screws and 18 +/- 11 for the uncoated screws (P < 0.03), and after 12 weeks 64 +/- 31 (HA-coated) and 9 +/- 13 (uncoated, P < 0.02). The average bone volume in the area close to the screw was significantly higher for the HA-coated screws at both 6 and 12 weeks.

CONCLUSIONS

HA coating improved the bone-to-implant interface significantly, indicating that HA coating can become useful for improving the purchase of pedicle screws.

Comparison between sheep and human cervical spines: an anatomic, radiographic, bone mineral density, and biomechanical study

STUDY DESIGN

The quantitative anatomic, radiographic, computerized tomographic, and biomechanical data of sheep and human cervical spines were evaluated.

OBJECTIVES

To compare the anatomic, radiographic, computerized tomographic, and biomechanical data of human and sheep cervical spines to determine whether the sheep spine is a suitable model for human spine research.

SUMMARY OF BACKGROUND DATA

Sheep spines have been used in several in vivo and in vitro experiments. Quantitative data of the normal sheep cervical spine are lacking, yet these data are crucial to discussion about the results of such animal studies.

METHODS

In this study, 20 fresh adult female Merino sheep cervical spines and 20 fresh human cadaver cervical spines were evaluated anatomically, radiographically, computerized tomographically, and biomechanically. Three linear and two angular parameters were evaluated on four digital radiographic views: anteroposterior, right lateral in neutral position, flexion, and extension. Quantitative computed tomography scans at the center of each vertebral body and 3 mm below both endplates were analyzed for bone mineral density measurements. Biomechanical testing was performed in flexion, extension, axial rotation, and lateral bending by a nondestructive stiffness method using a nonconstrained testing apparatus. Range of motion and stiffness of each motion segment were calculated. Additionally, 10 linear anatomic parameters of each vertebra were measured using a digital ruler.

RESULTS

Anterior and mean disc space height in the sheep cervical spine increased constantly from C2-C3 to C6-C7, whereas middle disc space height decreased and posterior disc space height remained unchanged. Anterior and mean disc space height were significantly higher in sheep. In both sheep and human cervical spines, intervertebral angles were not significantly different. Standard deviations of bone mineral density in the human cervical spine were fourfold higher than in the sheep cervical spine, yet no significant differences were found in bone mineral density values between the two species. Range of motion differed significantly between the two species except in flexion-extension of C3-C4, C5-C6, axial rotation of C2-C3, and lateral bending of C2-C3, C3-C4, and C4-C5. Stiffness also was significantly different except in flexion-extension of C2-C3, C4-C5, C5-C6, and lateral bending of C2-C3, C3-C4, and C4-C5. Anatomic evaluation showed no difference in upper endplate parameters for C4 and C5.

CONCLUSIONS

Although several differences were found between human and sheep cervical spines, the small intergroup standard deviations and the good comparability with the human spine encourage the use of the sheep cervical spine as a model for cervical spine research. On the basis of the quantitative data obtained in this study, the sheep motion segment C3-C4 seemed to be the most reliable model for the corresponding human motion segment.

A continuous pure moment loading apparatus for biomechanical testing of multi-segment spine specimens

An apparatus is described that enables the application of continuous pure moment loads to multi-segment spine specimens. This loading apparatus allows continuous cycling of the spine between specified flexion and extension (or right and left lateral bending) maximum load endpoints. Using a six-degree-of-freedom load cell and three-dimensional optoelectronic stereophotogrammetry, characteristic displacement versus load hysteresis curves can be generated and analyzed for different spinal constructs of interest. Unlike quasi-static loading, the use of continuous loading permits the analysis of the spine's behaviour within the neutral zone. This information is of particular clinical significance given that the instability of a spinal segment is related to its flexibility within the neutral zone. Representative curves for the porcine lumbar spine in flexion-extension and lateral bending are presented to illustrate the capabilities of this system.

The efficacy of interconnected porous hydroxyapatite in achieving posterolateral lumbar fusion in sheep

STUDY DESIGN

An animal study was performed to evaluate lumbar spinal fusion radiologically and mechanically.

OBJECTIVES

To assess the efficacy of interconnected porous hydroxyapatite in achieving posterolateral lumbar arthrodesis in sheep.

SUMMARY OF BACKGROUND DATA

Posterolateral spinal arthrodesis with autologous bone graft is the gold standard procedure for lumbar fusion. The procedure for harvesting bone from the iliac crest increases morbidity. Interconnected porous hydroxyapatite has been used effectively as an alternative to cancellous bone graft material in metaphyseal bone defects. Little is known about the efficacy of interconnected porous hydroxyapatite in achieving lumbar spinal fusion.

METHODS

Four groups of seven sheep underwent bisegmental posterolateral lumbar fusion with instrumentation using different intertransverse graft material. In group 1, no graft material was used. In group 2, autologous bone was used. Group 3 had interconnected porous hydroxyapatite. Group 4 had an equip of interconnected porous hydroxyapatite and autologous bone. The animals were killed at 20 weeks after surgery. Radiographs and computed tomography images were obtained. The fusion masses were graded for bone resorption and trabecular connectivity on the computed tomography images. Mechanical testing of the specimens was performed, and the three-dimensional segmental motion was measured in flexion/extension, axial rotation, and lateral bending.

RESULTS

The radiographic images were difficult to interpret because of the radiodense interconnected porous hydroxyapatite granules. According to mechanical stability criteria, the fusion rate for the different groups was as follows: 100% (14/14) for the autologous bone group, 72% (10/14) for the bone/interconnected porous hydroxyapatite group, 50% (7/14) for the pure interconnected porous hydroxyapatite group, and 15% (2/14) for the sham group.

CONCLUSIONS

Spinal arthrodesis using interconnected porous hydroxyapatite alone or mixed with bone as graft material reduced segmental motion. It was not, however, as effective as autologous bone graft material in achieving spinal arthrodesis. The sheep model using autologous bone achieved a 100% fusion rate. Because the nonunion rate for a single level in humans may be as high as 40%, the fusion rate with bone/interconnected porous hydroxyapatite in humans may be lower than the 72% found in the sheep model. The little resorption of the radiodense interconnected porous hydroxyapatite granules made the radiologic evaluation of the fusion masses difficult.

Does spinal kyphotic deformity influence the biomechanical characteristics of the adjacent motion segments? An in vivo animal model

STUDY DESIGN

In an in vivo sheep model, the effects of spinal fusion and kyphotic deformity on the neighboring motion segments were analyzed.

OBJECTIVES

To investigate the effects of spinal fusion and kyphotic deformity on the adjacent motion segment.

SUMMARY OF BACKGROUND DATA

The in vivo effects of kyphotic deformity on the neighboring motion segments have not been investigated in any studies.

METHODS

Eighteen sheep were equally randomized into three groups based on surgical procedure: L3-L5 in situ posterolateral fusion (n = 6) L3-L5 kyphotic posterolateral fusion (n = 6), and surgical exposure alone (n = 6). After a 16-week survival period, the adjacent motion segment changes were analyzed radiographically, biomechanically, and histologically.

RESULTS

The kyphosis group showed 5.0 degrees +/- 2.6 degrees and 1.7 degrees +/- 1.8 degrees compensatory hyperlordosis at L2-L3 and L5-L6, respectively, compared with surgical exposure and in situ posterolateral fusion, the kyphotic posterolateral fusion significantly influenced cranial adjacent motion segment biomechanics by inducing more stiffness in the posterior ligamentous complex (P < 0.05) and increasing lamina strain under flexion-extension loading (P < 0.05). Results of histologic analysis showed significant degenerative changes of the L2-L3 facet joints in the kyphosis group.

CONCLUSIONS

It is inferred that in the kyphosis group, compensatory hyperlordosis at the cranial adjacent level leads to lordotic contracture of the posterior ligamentous complex. The increased lamina strain, exhibited by the in situ group under flexion-extension, was further increased in the kyphosis group, indicating higher load transmission through the posterior column. Significant degenerative changes of the cephalad adjacent facet joints observed in the kyphosis group served to corroborate the biomechanical data. These results indicate that a kyphotic deformity may lead to facet joint contracture and facet arthritis and may serve as the origin of low back pain at the cranial adjacent level.

Biomechanical studies on two anterior thoracolumbar implants in cadaveric spines

STUDY DESIGN

A biomechanical comparison of two commonly used anterior spinal devices: the Smooth Rod Kaneda and the Synthes Anterior Thoracolumbar Spinal Plate.

OBJECTIVES

To compare the stability imparted to the human cadaveric spine by the Smooth Rod Kaneda and Synthes Anterior Spinal Plate, and to assess how well these devices withstand fatigue and uni- and bilateral facetectomy.

SUMMARY OF BACKGROUND DATA

Biomechanical studies on the aforementioned and similar devices have been performed using synthetic, porcine, calf, or dog spines. As of the time of this writing, studies comparing anterior spinal implants using human cadaveric spines are scarce.

METHODS

An L1 corpectomy was performed on 19 spines. Stabilization was accomplished by an interbody wooden graft and the application of the Smooth Rod Kaneda in 10 spines and the Synthes Anterior Spinal Plate in the remaining 9. Biomechanical testing of the spines was performed in six degrees of freedom before and after stabilization, and after fatiguing to 5000 cycles of +/- 3 Nm of flexion and extension. Testing was repeated after uni- and bilateral facetectomy.

RESULTS

After stabilization, the Smooth Rod Kaneda was significantly more rigid than the anterior thoracolumbar bar spinal plate in extension. After fatigue, the Smooth Rod Kaneda was significantly stiffer than the anterior thoracolumbar spinal plate in flexion, extension, right lateral bending, left lateral bending, and right axial rotation. A significant decrease in stiffness was noted with the Synthes device in flexion after bilateral facetectomy compared with the stabilized spine.

CONCLUSIONS

The smooth Rod Kaneda device tends to be stiffer than the anterior thoracolumbar spinal plate, particularly in extension, exceeding the anterior thoracolumbar spinal plate in fatigue tolerance. The spine stabilized with the anterior thoracolumbar spinal plate is more susceptible to the destabilizing effect of bilateral facetectomy than than that stabilized with the Smooth Rod Kaneda. The additional rigidity encountered with the Smooth Rod Kaneda must be weighed against the simplicity of anterior thoracolumbar spinal plate application.

Kinematics of cervical spine discectomy with and without bone grafting: quantitative evaluation of late fusion in a sheep model

OBJECTIVE

This study was conducted to evaluate the kinematic response of late fusion results for cervical spine discectomies with and without bone grafting.

MATERIALS AND METHODS

Fifteen Barbados Black Belly sheep underwent sham operations (Group A, n = 5), C2-C3 discectomies only (Group B, n = 5), and C2-C3 discectomies with autologous iliac bone grafting (Group C, n = 5). Ten months after surgery, the animals were killed. Fresh ligamentous spines (C1-C5) were subjected to the relevantly applied loads through a loading frame attached to the C1. Each vertebra (from C2 to C4) was attached with a set of three infrared light-emitting diodes to record the spatial location relating to each load application using a Selspot II system (Selcom Selective Electronics, Inc., Valdese, NC). The load-deformation data of the C2-C3 and C3-C4 motion segments were recorded and analyzed for the three groups.

RESULTS

At the C2-C3 motion segment, the results indicated that Group B displayed larger motion ranges of rotation and lateral bending loads than did the other two groups. Significantly larger motion ranges of rotation loads were found in Group B than in Group C (P<0.05, for both comparisons). In contrast, Group C had the smallest motion ranges of flexion, lateral bending, and rotation loads. At the C3-C4 motion segment, both groups that had undergone discectomies had a significantly larger motion range of flexion load compared with Group A (P<0.05, for both comparisons). A significant increase in the motion range of right axial rotation was found in Group B (P<0.05), but not in Group C, compared with Group A. Group B exhibited larger motion ranges responding to all six tested loads than did Group C.

CONCLUSION

The results indicate that anterior fusion after C2-C3 cervical discectomies, regardless of the presence or absence of bone grafting, decreases the motion range of flexion load at the C2-C3 motion segment, and contrary data were seen at the C3-C4 motion segment. For axial rotation loads, discectomies without bone grafting resulted in increased motion ranges of both C2-C3 and C3-C4 motion segments whereas discectomies with bone grafting did not. The data may have clinical relevance regarding the role of bone grafting in cases of cervical spine disease.

The biology of posterolateral lumbar spinal fusion

This article reviews the existing knowledge base concerning the biology of spinal fusion, with the understanding that the focus is weighted toward posterolateral lumbar spinal fusion because of a relative paucity of biologic information on healing of other types of fusions. The discussion focuses first on the basic science of spinal fusion healing from the standpoint of animal modeling. Next, the discussion centers on the multitude of local factors that can affect fusion healing. Finally, the numerous systemic factors known to affect fusion healing are discussed.

Complications in spinal fusion

Complications in spinal fusion can lead to less than desirable results. The complications of spinal fusion in the cervical and lumbar spine are discussed. Methods of avoiding and correcting complications also are reviewed. Through a better understanding, it is hoped that complications can be prevented.

The role of anteromedial foraminotomy and the uncovertebral joints in the stability of the cervical spine. A biomechanical study

STUDY DESIGN

The biomechanical role of the cervical uncovertebral joint was investigated using human cadaveric spines. Sequential resection of cervical uncovertebral joints, including clinical anteromedial foraminotomy, was conducted, followed by biomechanical testing after each stage of resection.

OBJECTIVES

To clarify the biomechanical role of uncovertebral joints and clinical anteromedial foraminotomy in the cervical spine and their effects on interbody bone graft stability.

SUMMARY OF BACKGROUND DATA

Although the biomechanical role of the cervical uncovertebral joints has been considered to be that of a guiding mechanism in flexion and extension and a limiting mechanism in posterior translation and lateral bending, there have been no studies quantifying this role. According to results in quantitative anatomic studies, anatomic variations exist in uncovertebral joints, depending on the vertebral level, articular angulation, and relative height of the joints.

METHODS

Fourteen human functional spinal units at C3-C4 and C6-C7 underwent sequential uncovertebral joint resection, with each stage of resection followed by biomechanical testing. The uncovertebral joint was divided anatomically into three parts on each side: the posterior foraminal part, the posterior half, and the anterior half. The loading modes included torsion, flexion, extension, and lateral bending. A simulated anterior bone graft construct was also tested after each uncovertebral joint resection procedure.

RESULTS

Significant changes in stability were observed after sequential uncovertebral joint resection in all loading modes (P < 0.05). The biomechanical contribution of uncovertebral joints decreased in the following order: the posterior foraminal part, the posterior half, and the anterior half. Unilateral and bilateral foraminotomy most affected the stability of the functional spinal unit during extension, causing a 30% and 36% decrease in stiffness of the functional spinal unit, respectively. The effect was less in torsion and lateral bending. After sequential resection, there was a statistically significant difference between decreases in torsional stiffness at C3-C4 and C6-C7 (P < 0.05). The stiffness of the simulated bone graft construct decreased progressively during flexion and lateral bending after each foraminotomy (P < 0.05). Increased bone graft height of 79% returned stability to the preforaminotomy level.

CONCLUSIONS

This is the first study to quantitate the biomechanical role of uncovertebral joints in cervical segmental stability and the effect at each intervertebral level. The effect differs because of anatomic variations in uncovertebral joints. The major biomechanical function of uncovertebral joints includes the regulation of extension and lateral bending motion, followed by torsion, which is mainly provided by the posterior uncovertebral joints. This study highlights the clinical assessment of additional segmental instability attributed to destruction of the uncovertebral joints during surgical procedures or by neoplastic lesions.

The effects of spinal fixation and destabilization on the biomechanical and histologic properties of spinal ligaments. An in vivo study

STUDY DESIGN

An animal study was conducted to assess whether different surgical procedures of spinal fixation and destabilization would influence the biomechanics and histology of lumbar spinal ligaments.

OBJECTIVES

To investigate the effects of spinal fixation and destabilization as well as surgical intervention itself on the biomechanical and histologic properties of lumbar spinal ligaments.

SUMMARY OF BACKGROUND DATA

Although several investigators have reported normal biomechanical properties of different spinal ligaments, there have been no studies in which changes in spinal ligament properties, secondary to the altered biomechanical environment provided by such surgical procedures as spinal fixation and destabilization, have been investigated.

METHODS

Thirty-six mature sheep were divided into four groups: Group I: nonsurgical control: Group II: sham operation consisting of bilateral posterolateral exposure at L4-L5; Group III: spinal fixation using transpedicular screws and plates and bilateral posterolateral bone graft at L4-L5; and Group IV: spinal destabilization consisting of bilateral facetectomy and anterior discectomy at L4-L5. Four months after surgery, the biomechanical analysis included destructive tensile testing of four different bone-ligament-bone complexes at the operative and proximal adjacent levels: anterior longitudinal ligament, posterior longitudinal ligament, ligamentum flavum, and supraspinous and interspinous ligaments combined. Histomorphometric analyses of the vertebral body and spinal ligaments were performed histomorphometrically.

RESULTS

Biomechanical analysis results demonstrated remarkable changes in the structural and mechanical ligament properties at the operative level. The fixation group's ligaments showed consistent decreases in the ultimate load and elastic modulus compared with those parameters in the control group (P < 0.05). Histologically, the fixation group's ligamentum flavum showed marked vacuolation in the ligament substance, whereas the interspinous ligament exhibited significant insertion changes compared with little change in substance. In all eight sheep in Group IV, unintentional bilateral facet fusions were obtained; and in all eight animals in Group III with pedicle instrumentation and posterolateral fusion, solid arthrodesis was exhibited. This allowed a distinction to be made between the stress-shielding effect of spinal instrumentation and arthrodesis (Group III) versus spinal fusion alone (Group IV) on both spinal ligament and vertebral body. Group II (sham) had a significant decrease in supraspinous and interspinous ligaments, but nonsignificant decreases in the stress-shielding effect of 10-12% in other ligaments.

CONCLUSIONS

Posterior spinal instrumentation and fusion led to decreased biomechanical properties of the ligamentum flavum, posterior longitudinal ligament, and interspinous and supraspinous ligaments. The stress-shielding effects were ligament dependent and were most pronounced on the posterior side. The altered biomechanical environment produced by spinal fixation, surgical intervention itself, or nonphysiologic mobilization can affect the ligamentous properties in vivo, possibly serving as the impetus for low back pain.

Are sheep spines a valid biomechanical model for human spines?

STUDY DESIGN

Range of motion, neutral zone, and stiffness parameters of the complete cervical, thoracic, and lumbar sheep spine were determined in flexion and extension, axial left/right rotation, and right/left lateral bending.

OBJECTIVES

To determine quantitative biomechanical properties of the sheep spine and compare them with those from the human spine.

SUMMARY OF BACKGROUND DATA

Sheep spines often serve as a model for experimental in vivo and in vitro studies in spine research, but few quantitative biomechanical data from sheep spines for comparison with human specimens are available.

METHODS

Complete spines were sectioned into single-joint segments and tested in a spine tester under pure moments in the three main anatomic planes.

RESULTS

The craniocaudal variation in range of motion in all load directions was qualitatively similar between sheep spines and values reported in the literature for human specimens.

CONCLUSIONS

Based on the biomechanical similarities of sheep and human spines demonstrated in this study, it appears that the use of the sheep spine, which already includes evaluation of surgical techniques and bone healing processes, might be extended to spinal implants.

Spondylodesis in the treatment of segmental instability of the lumbar spine with special reference to clinically verified instability

We analysed the outcome of 42 consecutive patients suffering from low back pain and segmental instability of the lumbar spine who underwent spondylodesis with transpedicular fixation. There were 29 (69%) females and 13 (31%) males with a mean age of 47 +/- 9 years (range 23 to 66 years). The presence of clinically verified instability was a basic requirement for operative treatment, with at least one clinical sign of instability positive in all patients. Radiological signs of instability were detected in only 74% of the patients. After a postoperative follow-up of one year, lumbar fusion was found in 38 (90%) patients. The fusion was classified as complete in 18 (43%) and as partial in 20 (47%) patients. No fusion there was detected in 4 (10%) patients. Low back pain was completely abolished in 19 (45%) and markedly diminished in 15 (36%) patients in (17%) patients, low back pain had remained unchanged and become worse in 1 (2%) patient. Clinical signs and symptoms of instability were still detected in 10 (24%) patients. As compared to the pre-operative situation, the decrease in clinical instability was statistically significant (p = 0.02). Moreover, a significant correlation was observed between the persistence of clinically verified instability and a poor postoperative outcome in daily activities. Of all patients, 17 (40%) returned to work during the period of follow up. Based on the good outcome of our patients, the results of this study show that clinical signs and symptoms of instability can be successfully used as selection criteria for lumbar spondylodesis.