Segmental stability and compressive strength of posterior lumbar interbody fusion implants

STUDY DESIGN: Human cadaveric study on initial segmental stability and compressive strength of posterior lumbar interbody fusion implants. OBJECTIVES: To compare the initial segmental stability and compressive strength of a posterior lumbar interbody fusion construct using a new cortical bone spacer machined from allograft to that of titanium threaded and nonthreaded posterior lumbar interbody fusion cages, tested as stand-alone and with supplemental pedicle screw fixation. SUMMARY OF BACKGROUND DATA: Cages were introduced to overcome the limitations of conventional allografts. Radiodense cage materials impede radiographic assessment of the fusion, however, and may cause stress shielding of the graft. METHODS: Multisegmental specimens were tested intact, with posterior lumbar interbody fusion implants inserted into the L4/L5 interbody space and with supplemental pedicle screw fixation. Three posterior lumbar interbody fusion implant constructs (Ray Threaded Fusion Cage, Contact Fusion Cage, and PLIF Allograft Spacer) were tested nondestructively in axial rotation, flexion-extension, and lateral bending. The implant-specimen constructs then were isolated and compressed to failure. Changes in the neutral zone, range of motion, yield strength, and ultimate compressive strength were analyzed. RESULTS: None of the stand-alone implant constructs reduced the neutral zone. Supplemental pedicle screw fixation decreased the neutral zone in flexion-extension and lateral bending. Stand-alone implant constructs decreased the range of motion in flexion and lateral bending. Differences in the range of motion between stand-alone cage constructs were found in flexion and extension (marginally significant). Supplemental posterior fixation further decreased the range of motion in all loading directions with no differences between implant constructs. The Contact Fusion Cage and PLIF Allograft Spacer constructs had a higher ultimate compressive strength than the Ray Threaded Fusion Cage. CONCLUSIONS: The biomechanical data did not suggest any implant construct to behave superiorly either as a stand-alone or with supplemental posterior fixation. The PLIF Allograph Spacer is biomechanically equivalent to titanium cages but is devoid of the deficiencies associated with other cage technologies. Therefore, the PLIF Allograft Spacer is a valid alternative to conventional cages.     

Compressive strength of interbody cages in the lumbar spine: the effect of cage shape, posterior instrumentation and bone density

One goal of interbody fusion is to increase the height of the degenerated disc space. Interbody cages in particular have been promoted with the claim that they can maintain the disc space better than other methods. There are many factors that can affect the disc height maintenance, including graft or cage design, the quality of the surrounding bone and the presence of supplementary posterior fixation. The present study is an in vitro biomechanical investigation of the compressive behaviour of three different interbody cage designs in a human cadaveric model. The effect of bone density and posterior instrumentation were assessed. Thirty-six lumbar functional spinal units were instrumented with one of three interbody cages: (1) a porous titanium implant with endplate fit (Stratec), (2) a porous, rectangular carbon-fibre implant (Brantigan) and (3) a porous, cylindrical threaded implant (Ray). Posterior instrumentation (USS) was applied to half of the specimens. All specimens were subjected to axial compression displacement until failure. Correlations between both the failure load and the load at 3 mm displacement with the bone density measurements were observed. Neither the cage design nor the presence of posterior instrumentation had a significant effect on the failure load. The loads at 3 mm were slightly less for the Stratec cage, implying lower axial stiffness, but were not different with posterior instrumentation. The large range of observed failure loads overlaps the potential in vivo compressive loads, implying that failure of the bone-implant interface may occur clinically. Preoperative measurements of bone density may be an effective tool to predict settling around interbody cages. Received: 3 July 1997 Revised: 18 September 1997 Accepted: 26 September 1997     

Mechanical studies of lumbar interbody fusion implants

In addition to autogenous or allogeneic bone grafts, fusion cages composed of metal or plastic are being used increasingly as spacers for interbody fusion of spinal segments. The goal of this study was the mechanical testing of carbon fiber reinforced plastic (CFRP) fusion cages used for anterior lumbar interbody fusion. With a special testing device according to American Society for Testing and Materials (ASTM) standards, the mechanical properties of the implants were determined under four different loading conditions. The implants (UNION cages, Medtronic Sofamor Danek) provide sufficient axial compression, shear, and torsional strength of the implant body. Ultimate axial compression load of the fins is less than the physiological compression loads at the lumbar spine. Therefore by means of an appropriate surgical technique parallel grooves have to be reamed into the endplates of the vertebral bodies according to the fin geometry. Thereby axial compression forces affect the implants body and the fins are protected from damaging loading. Using a supplementary anterior or posterior instrumentation, in vivo failure of the fins as a result of physiological shear and torsional spinal loads is unlikely. Due to specific complications related to autogenous or allogeneic bone grafts, fusion cages made of metal or carbon fiber reinforced plastic are an important alternative implant in interbody fusion.     

Intervertebral fixation: clinical results with anterior cages

Anterior lumbar interbody fusion has several clinical advantages over posterior or posterolateral lumbar fusion. Interbody fusion procedures place bone grafts within the disc space at the center of rotation of the vertebral motion segment. The intervertebral area is highly vascular, and the grafts have a wide contact area in the weight-bearing axis of the spinal motion segment. The high rates of fusion associated with the use of the threaded intervertebral fusion cages may be attributed, in part, to the following: (1) removal of the cartilagenous end plates and exposure of bleeding cancellous bony surfaces, (2) reestablishment of anatomic intradiscal height and tensioning of the annulus and ligamentous structures around the disc space, (3) use of appropriately sized implants to engage the peripheral apophysis of the vertebral end plates, and (4) use of autogenous grafts. Threaded interbody constructs provide adequate strength to ensure that no plastic deformation occurs within the maximum physiologic range. Dynamic testing of these implants also has shown that these implants are able to resist cyclic fatigue within typical normal daily physiologic loading. Stability testing has shown that when inserted anteriorly, these devices reduce intervertebral motion and increase spinal stiffness.     

The use of poly-L-lactic acid in lumbar interbody cages: design and biomechanical evaluation in vitro

Cage design and cage material may play a crucial role in the incidence of postoperative complications reported with current non-absorbable interbody cage devices. Bioabsorbable poly-L-lactic acid cage devices may have potential benefits. The purpose of this study was to determine the required strength of poly-L-lactic acid cages for use in experimental goat studies and to evaluate the mechanical properties of different cage designs in situ. The yield and ultimate strength of native goat motion segments (L1-L6) were determined; the yield strength was used as a design parameter for the cages. The mechanical behaviour of two types of poly-L-lactic acid cages, the influence of endplate perforation, differences between toothed and smooth cages, and the influence of cage filling were biomechanically tested and compared to native motion segments. Only axial compression until failure of the motion segments was performed. Dual energy X-ray absorptiometry was used to determine bone mineral content. The yield and ultimate strength of the native motion segments were 3.5 and 7.0 kN, respectively. Based on these data, flexible and stiff poly-L-lactic acid cages were designed with strengths of 3.5 and 7 kN, respectively. Poly-L-lactic acid cages, whether with or without bone graft and perforating the endplates, did not reduce the compressive strength of motion segments as compared to native segments. However, toothed titanium cages, with the same geometry, negatively influenced the segments' compressive strength, which effect was reduced using smooth titanium cages.     

Cages: designs and concepts

Many new interbody fusion cages have been recently developed, but clinical studies analyzing fusion outcome are still scarce. Radiological methods to assess fusion are not standardized and are often unreliable. Cages have been stated to provide good segmental distraction, provide axial load support and reduce segmental mobility, but there have been reports of failed fusions because of implant failure. This paper presents a critical opinion on current cage designs, stressing their clinical and biomechanical implications. Threaded cage designs compromise endplate integrity, and when placed in pairs have inherent limitations for distraction. Non-threaded cage designs usually preserve endplate integrity, but geometry may be inadequate to provide a good surface match to the endplate. The concept of an open frame type cage is believed to have biological advantages, because large graft volumes inside the cage can be in direct contact with host bone. Cadaveric tests suggest that open frame constructs have compressive strength similar to that of full surface contact cages. Restoration of segmental height, sagittal balance and increased neuroforaminal clearance are all functions of disc space distraction. The effect of cage instrumentation on axial load distribution, however, is not well understood. Biomechanical experiments strongly suggest supplementing cage instrumentation with posterior fixation, to achieve a marked increase in initial segmental stability. In the absence of gross segmental instability, micromotion at the host graft interface may still exist. As a result, fusion will never occur, instead a pseudoarthrosis will develop. For monitoring fusion, the use of non-metallic cages has distinct advantages, because no metal artifacts will disturb radiological assessment.     

Cages: designs and concepts

Many new interbody fusion cages have been recently developed, but clinical studies analyzing fusion outcome are still scarce. Radiological methods to assess fusion are not standardized and are often unreliable. Cages have been stated to provide good segmental distraction, provide axial load support and reduce segmental mobility, but there have been reports of failed fusions because of implant failure. This paper presents a critical opinion on current cage designs, stressing their clinical and biomechanical implications. Threaded cage designs compromise endplate integrity, and when placed in pairs have inherent limitations for distraction. Non-threaded cage designs usually preserve endplate integrity, but geometry may be inadequate to provide a good surface match to the endplate. The concept of an open frame type cage is believed to have biological advantages, because large graft volumes inside the cage can be in direct contact with host bone. Cadaveric tests suggest that open frame constructs have compressive strength similar to that of full surface contact cages. Restoration of segmental height, sagittal balance and increased neuroforaminal clearance are all functions of disc space distraction. The effect of cage instrumentation on axial load distribution, however, is not well understood. Biomechanical experiments strongly suggest supplementing cage instrumentation with posterior fixation, to achieve a marked increase in initial segmental stability. In the absence of gross segmental instability, micromotion at the host graft interface may still exist. As a result, fusion will never occur, instead a pseudoarthrosis will develop. For monitoring fusion, the use of non-metallic cages has distinct advantages, because no metal artifacts will disturb radiological assessment.     

椎间加压融合治疗胸腰椎骨折脱位

目的探讨椎弓根系统椎间加压融合治疗胸腰椎骨折脱位的临床效果，探讨促进早期椎间融合的机制。方法胸腰椎骨折脱位17例，男11例，女6例；年龄23～56岁，平均37．1岁。交通伤10例，高处坠落伤4例，横向挤压伤3例。损伤作用力主要为横向剪切力。A0分型：B1型2例，C1型12例，C2型3例。损伤部位：T10.11 1例，T11．12 1例，T12L13例，L1．2 3例，L2．3 1例，L3.4 2例，L4．5 2例，L5S1 4例。神经功能情况：完全瘫痪4例，不完全瘫痪10例，有不同程度神经根刺激表现3例。手术采用后路减压，植入四枚椎弓根钉，椎间孔充分扩大后撑开椎弓根钉，双侧椎间隙开窗，切除椎体间髓核，刮除中后部软骨板，椎体间植入碎皮质骨，椎弓根系统复位加压，使椎体后缘尽量接触固定，维持加压状态锁紧椎弓根钉系统。术后1周内、3个月、6个月、12个月、18个月复查X线片，测量椎体前后缘椎间高度，观察植骨融合情况。结果术中均无并发症发生。术后16例随访19～37个月，平均25．6个月。患者神经功能部分恢复5例，完全恢复9例。随访X线片显示，加压的椎体间距稳定，植骨均获骨性融合，椎间前部的纤维环骨化明显。无内固定物失败及神经根痛等并发症发生。结论椎弓根系统椎间加压融合治疗胸腰椎骨折脱位，可达到术后早期稳定和早期融合的疗效。     

Mapping the structural properties of the lumbosacral vertebral endplates

STUDY DESIGN: A biomechanical investigation using indentation tests in a human cadaveric model to seek variation in the structural properties across the lower lumbar and sacral endplates. OBJECTIVES: To determine 1) if there are regional differences in endplate strength and 2) whether any differences identified are affected by spinal level (lumbar spine vs. sacrum) or endplate (superior vs. inferior). SUMMARY OF BACKGROUND DATA: It has been postulated that some regions of the vertebral body may be stronger than others. Conclusive data, either supporting or disproving this theory, would be valuable for both spine surgeons and implant designers because one mode of failure of interbody implants is subsidence into one or both adjacent vertebrae. METHODS: Indentation tests were performed at 27 standardized test sites in 62 bony endplates of intact human vertebrae (L3-S1) using a 3-mm-diameter, hemispherical indenter with a test rate of 0.2 mm/sec to a depth of 3 mm. The failure load and stiffness at each test site were determined using the load-displacement curves. Three-way analyses of variance were used to analyze the resulting data. RESULTS: Both the failure load and stiffness varied significantly across the endplate surfaces (P < 0.0001), with posterolateral regions being stronger and stiffer than the central regions. Characteristic distributions were identified in the lumbar superior, lumbar inferior, and sacral endplates. The failure load distributions were found to differ in 1) the superior lumbar and sacral endplates (P = 0.0077), 2) the inferior lumbar and sacral endplates (P = 0.0014), and 3) the superior and inferior lumbar endplates (P < 0.0001). The sacral and inferior lumbar endplates were both found to be stronger than the superior lumbar endplates (sacrum, P = 0.054; inferior, P = 0.008) but were not themselves significantly different (P = 0.89). CONCLUSIONS: Highly significant regional strength and stiffness variations were identified in the lumbar and sacral endplates. The center of the bone, where implants are currently placed, is the weakest part of the lumbar endplates and is not the strongest region of the sacral endplate.     

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.     

Systemic alendronate treatment improves fixation of press-fit implants: a canine study using nonloaded implants.

Bone resorption associated with local trauma occurring during insertion of joint prostheses is recognized as an early event. Being an osteoclastic inhibitor, alendronate is a potential candidate means to decrease early periprosthetic bone resorption and thereby improve implant fixation. We investigated the influence of oral alendronate treatment on early implant fixation in two implant interface settings representing sites of an implant that are in contact with surrounding bone, and other sites without intimate bone contact. One plasma-sprayed cylindrical titanium implant (6 mm diameter) was inserted into each proximal tibia of 16 dogs. On one side the implant was inserted press-fit whereas on the contralateral side, the implants were surrounded by a 2 mm concentric gap. Oral alendronate (0.5 mg/kg/day) was given 2 weeks following surgery to eight dogs. The dogs were euthanized after 10 weeks of alendronate treatment. Bone ongrowth (bone in contact with implant surface) was estimated using the linear intercept technique and shear strength was calculated as the slope on a load-displacement curve. For the press fit implants, alendronate treatment significantly increased bone ongrowth from 24% to 29% and significantly increased ultimate shear strength from 1.26 to 3.72 MPa. Also, the fraction of periprosthetic bone significantly increased from 10% to 18%. For implants surrounded by a gap, alendronate neither stimulated nor impaired implant fixation, bone ingrowth, or new bone formation in the gaps. Because early implant stability is an important predictor of longevity, systemic alendronate treatment could be an important clinical tool to positively influence the early stages of implant incorporation.     

Fatigue response of lumbar intervertebral joints under axial cyclic loading

A low-cycle fatigue of 11 lumbar intervertebral joints under axial compression is reported. The magnitude of the maximum compressive load ranged from 37 to 80% of the failure load. The maximum deformation, as a function of the number of cycles, showed two distinct results: one group showed a gradual, stable increase, and the other an abrupt, unstable increase. The before- and after-test radiographs showed a one-to-one correspondence between unstable specimens and generalized bony failure. The radiographs of 5-mm thick transverse endplate slices show crack propagation from the periphery of the subchondral bone inward. Removal of the organic matrix from the cracked specimens produced its physical disintegration into small particles, while normal controls and stable specimens retained their size and shape.     

Anterior cervical interbody constructs: effect of a repetitive compressive force on the endplate

Graft subsidence following anterior cervical reconstruction can result in the loss of sagittal balance and recurring foraminal stenosis. This study examined the implant–endplate interface using a cyclic fatigue loading protocol in an attempt to model the subsidence seen in vivo. The superior endplate from 30 cervical vertebrae (C3 to T1) were harvested and biomechanically tested in axial compression with one of three implants: Fibular allograft; titanium mesh cage packed with cancellous chips; and trabecular metal. Each construct was cyclically loaded from 50 to 250 N for 10,000 cycles. Nondestructive cyclic loading of the cervical endplate–implant construct resulted in a stiffer construct independent of the type of the interbody implant tested. The trabecular metal construct demonstrated significantly more axial stability and significantly less subsidence in comparison to the titanium mesh construct. Although the allograft construct resulted in more subsidence than the trabecular metal construct, the difference was not significant and no difference was found when comparing axial stability. For all constructs, the majority of the subsidence during the cyclic testing occurred during the first 500 cycles and was followed by a more gradual settling in the remaining 9,500 cycles. © 2011 Orthopaedic Research Society. © 2011 Orthopaedic Research Society Published by Wiley Periodicals, Inc. J Orthop Res 30:587–592, 2012     

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

INTRODUCTION: There has been a rapid increase in the use of interbody fusion cages as an adjunct to spondylodesis, although experimental data are lacking. A sheep cervical spine interbody fusion model was used to determine the effect of different cage design parameters (endplate-implant contact area, maximum contiguous pore) on interbody fusion. MATERIAL AND METHOD: IN VITRO EVALUATION: 24 sheep cadaver specimens (C2-C5) were tested in flexion, extension, axial rotation, and lateral bending with a nondestructive flexibility method using a nonconstrained testing apparatus. Four different groups were examined: (1) control group (intact) ( n=24), (2) autologous tricortical iliac crest bone graft ( n=8), (3) Harms cage ( n=8), and (4) SynCage-C ( n=8). IN VIVO EVALUATION: 24 sheep underwent C3/4 discectomy and fusion: group 1: autologous tricortical iliac crest bone graft ( n=8), group 2: Harms cage filled with autologous cancellous iliac crest bone grafts ( n=8), and group 3: SynCage-C filled with autologous cancellous iliac crest bone grafts ( n=8). Radiographic scans were performed pre- and postoperatively and after 1, 2, 4, 8, and 12 weeks, respectively. At the same time points, disc space height (DSH), height index (HI), intervertebral angle (IVA), and endplate angle (EA) were measured. After 12 weeks the animals were killed and fusion sites were evaluated using biomechanical testing in flexion, extension, axial rotation, and lateral bending. Additionally, histomorphological and histomorphometrical analyses were performed. RESULTS: Over a 12-week period the cage groups showed significantly higher values for DSH, HI, IVA, and EA compared to the bone graft. In vivo stiffness was significantly higher for the tricortical iliac crest bone graft and Harms cage than in vitro stiffness. However, there was no difference between in vitro and in vivo stiffness of the SynCage-C. Histomorphometrical evaluation showed a more progressed bone matrix formation in the Harms cage group than in both other groups. CONCLUSION: The parameter endplate-implant contact area was not able to determine subsidence of cages. In contrast, the maximum contiguous pore of a cage significantly correlates with interbody bone matrix formation inside the cage. Additionally, there was no correlation between in vitro and in vivo stiffness of interbody fusion cages. Therefore, biomechanical in vitro studies are not able to determine in vivo outcome of fusion cages. Animal experimental evaluations of interbody fusion cages are essential prior to clinical use.     

Correlation of cervical endplate strength with CT measured subchondral bone density

Cervical interbody device subsidence can result in screw breakage, plate dislodgement, and/or kyphosis. Preoperative bone density measurement may be helpful in predicting the complications associated with anterior cervical surgery. This is especially important when a motion preserving device is implanted given the detrimental effect of subsidence on the postoperative segmental motion following disc replacement. To evaluate the structural properties of the cervical endplate and examine the correlation with CT measured trabecular bone density. Eight fresh human cadaver cervical spines (C2–T1) were CT scanned and the average trabecular bone densities of the vertebral bodies (C3–C7) were measured. Each endplate surface was biomechanically tested for regional yield load and stiffness using an indentation test method. Overall average density of the cervical vertebral body trabecular bone was 270 ± 74 mg/cm³. There was no significant difference between levels. The yield load and stiffness from the indentation test of the endplate averaged 139 ± 99 N and 156 ± 52 N/mm across all cervical levels, endplate surfaces, and regional locations. The posterior aspect of the endplate had significantly higher yield load and stiffness in comparison to the anterior aspect and the lateral aspect had significantly higher yield load in comparison to the midline aspect. There was a significant correlation between the average yield load and stiffness of the cervical endplate and the trabecular bone density on regression analysis. Although there are significant regional variations in the endplate structural properties, the average of the endplate yield loads and stiffnesses correlated with the trabecular bone density. Given the morbidity associated with subsidence of interbody devices, a reliable and predictive method of measuring endplate strength in the cervical spine is required. Bone density measures may be used preoperatively to assist in the prediction of the strength of the vertebral endplate. A threshold density measure has yet to be established where the probability of endplate fracture outweighs the benefit of anterior cervical procedure.     

Instrumented posterior interbody fusion in degenerative and multioperated lumbar spine

Abstract Lumbar interbody fusion is a valid technique for the treatment of disc diseases. We report a series of 37 patients who underwent posterior lumbar interbody fusion with titanium cylindric screwing-expansion cages. Clinical outcomes and radiological results were evaluated 3 years after surgery. After surgery, the majority of patients returned to their normal activities. Follow-up plain roentgenograms showed no loss of disc height and no signs of implant looseness. Computed tomography (CT) showed the presence of mineralized autologous bone grafts inside the interbody cages. Expandable interbody cages allow the restoration of disc space height, giving support to the anterior column, opening the neuroforaminal area and providing increased stability. The interpretation of fusion on the basis of roentgenograms is difficult; CT offers more information than radiography about the fusion process, but a bony arthrodesis cannot be demonstrated with certainty.     

The use of bioabsorbable implants in the spine.

BACKGROUND CONTEXT: Bioabsorbable implants are commonplace in sports medicine surgeries, especially in shoulder and knee ligamentous reconstruction. Their use is now expanding to the realm of spinal reconstructive surgery. Newer polymers offer reduced incidence of the side effects of aseptic sterile sinus formation and have appropriate resorption time parameters for spine use. These new bioabsorbable materials confer initial and intermediate-term stability that is adequate for stable bony healing in various applications. The majority of human clinical applications in the spine that have been documented involve bone graft harvest site reconstruction, posterior spinal graft containment, anterior interbody reconstruction and anterior cervical and lumbar spine tension band plating. PURPOSE: The purpose of this review article is to highlight the indications and outcomes of the use of bioabsorbable implants in specific spinal applications. STUDY DESIGN: A comprehensive literature review of the English and non-English literature on bioabsorbable implant technology. METHODS: A comprehensive literature review was performed to gather basic science, animal and human data on the use of bioabsorbable implants in spinal surgery. RESULTS: Bioabsorbable implants have demonstrated strength and resorption characteristics commensurate with the physiologic and biomechanical requirements of the human spinal axis. Histologic sampling has demonstrated successful time-patterned resorption accompanied by bony replacement and remodeling of intervertebral cage devices in the animal model. CONCLUSION: Bioresorbable compounds appear to have a role in specific spinal reconstructive procedures. Their radiolucent nature improves image assessment of fusion healing, and their time-engineered resorption characteristics allow controlled dynamization in interbody and plate applications. Their widespread use and acceptance may increase dramatically as further research and clinical studies report on their safety and efficacy.     

3D-printed titanium cages without bone graft outperform PEEK cages with autograft in an animal model

BACKGROUND CONTEXTModernization of 3D printing has allowed for the production of porous titanium interbody cages (3D-pTi) which purportedly optimize implant characteristics and increase osseointegration; however, this remains largely unstudied in vivo.PURPOSETo compare osseointegration of three-dimensional (3D) titanium cages without bone graft and Polyether-ether-ketone (PEEK) interbody cages with autologous iliac crest bone graft (AICBG).STUDY DESIGNAnimal study utilizing an ovine in vivo model of lumbar fusion.METHODSInterbody cages of PEEK or 3D-pTi supplied by Spineart SA (Geneva, Switzerland) were implanted in seven living sheep at L2-L3 and L4-L5, leaving the intervening disc space untreated. Both implant materials were used in each sheep and randomized to the aforementioned disc spaces. Computed tomography (CT) was obtained at 4 weeks and 8 weeks. MicroCT and histological sections were obtained to evaluate osseointegration.RESULTSMicroCT demonstrated osseous in-growth of native cancellous bone in the trabecular architecture of the 3D-pTi interbody cages and no interaction between the PEEK cages with the surrounding native bone. Qualitative histology revealed robust osseointegration in 3D-pTi implants and negligible osseointegration with localized fibrosis in PEEK implants. Evidence of intramembranous and endochondral ossification was apparent with the 3D-pTi cages. Quantitative histometric bone implant contact demonstrated significantly more contact in the 3D-pTi implants versus PEEK (p<.001); region of interest calculations also demonstrated significantly greater osseous and cartilaginous interdigitation at the implant-native bone interface with the 3D-pTi cages (p=.008 and p=.015, respectively).CONCLUSIONS3D-pTi interbody cages without bone graft outperform PEEK interbody cages with AICBG in terms of osseointegration at 4 and 8 weeks postoperatively in an ovine lumbar fusion model.CLINICAL SIGNIFICANCE3D-pTi interbody cages demonstrated early and robust osseointegration without any bone graft or additive osteoinductive agents. This may yield early stability in anterior lumbar arthrodesis and potentially bolster the rate of successful fusion. This could be of particular advantage in patients with spinal neoplasms needing post-ablative arthrodesis, where local autograft use would be ill advised.     

Reactions and complications after the implantation of Endobon including morphological examination of explants

In the study described here, the integration of hydroxyapatite (HA) ceramic implants (Endobon) was investigated. These implants have an interconnecting system of pores and are free from foreign protein. The material is not toxic, genotoxic, nor zytotoxic, and it is biocompatible. The progress of integration was investigated by means of clinical and radiological check-ups. From 10 patients, it was possible to obtain samples for histological analysis during a second operation (e.g., metal explantation). Microscopic examination showed bony integration with newly formed bone in direct contact with the HA ceramic; it also showed osteoblasts and osteoid seams. No second operation took place earlier than 4 months after the first operation, yet even after this relatively short period, bony integration was already evident. Clinical observation (based on X-rays, reports of pain, signs of inflammation) showed that in most cases healing was taking place without complications. More general operational complications such as thrombosis or nerve injury were observed in 4 patients. If the implant is not sufficiently protected from mechanical stress, bony integation will not take place, and the implant may fracture. HA ceramic, with a porosity between 30% and 80%, is not comparable to cortical bone but only to spongy bone. This factor must be taken into account when deciding whether a HA ceramic implant is indicated. Received: 13 March 1997     

Resistance of the Lumbar Spine Against Axial Compression Forces after Implantation of Three Different Posterior Lumbar Interbody Cages

Summary. Background: The aim of using interbody fusion cages is to distract the degeneratively decreased disc height to decompress the neural structures in the intervertebral foramina and allow bony fusion. Prerequisite for a successful fusion therapy is a high resistance against subsidence and breakage. Method: Three types of implants, a cylindrical threaded titanium cage (Ray^TM) (1c), a bullet shaped PEEK cage (Stryker^TM) (1a) and a rectangular titanium cage with an endplate anchorage device (Marquardt^TM) (1b) were implanted in eight monosegmental lumbar spine specimens (L 2/3 and L 4/5). Each specimen underwent a cyclic loading test with 40000 cycles at a rate of 5 Hz. A cyclic axial compression force ranging from 200 Newton [N] to 1000 N was applied and the axial translation recorded simultaneously to determine the subsidence tendency. After this procedure the specimens were tested with a progressive axial force until breakage. Findings: There were only small differences in the subsidence tendency for the three cage designs. The height reduction due to cyclic loading ranged between 0.9 mm (Marquardt^TM), 1.2 mm (Stryker^TM) and 1.4 mm (Ray^TM). The median break force ranged from 5486 N (Marquardt^TM), 8359 N (Stryker^TM) to 8413 N (Ray^TM). No correlation between bone mineral density and failure load could be detected. Interpretation: Endplate preparation and cage design of the tested implants do not seem to influence the resistance of the segment against cyclic axial compression. The compression with a continuously increasing load revealed that an implant-bone failure is not to be expected in physiological limits for all three cage types.