Instrumented transforaminal lumbar interbody fusion with bioresorbable polymer implants and iliac crest autograft

Biodegradable interbody cages for lumbar spine fusion: Current concepts and future directions

Lumbar fusion often remains the last treatment option for various acute and chronic spinal conditions, including infectious and degenerative diseases. Placement of a cage in the intervertebral space has become a routine clinical treatment for spinal fusion surgery to provide sufficient biomechanical stability, which is required to achieve bony ingrowth of the implant. Routinely used cages for clinical application are made of titanium (Ti) or polyetheretherketone (PEEK). Ti has been used since the 1980s; however, its shortcomings, such as impaired radiographical opacity and higher elastic modulus compared to bone, have led to the development of PEEK cages, which are associated with reduced stress shielding as well as no radiographical artefacts. Since PEEK is bioinert, its osteointegration capacity is limited, which in turn enhances fibrotic tissue formation and peri-implant infections. To address shortcomings of both of these biomaterials, interdisciplinary teams have developed biodegradable cages. Rooted in promising preclinical large animal studies, a hollow cylindrical cage (Hydrosorb™) made of 70:30 poly-l-lactide-co-d, l-lactide acid (PLDLLA) was clinically studied. However, reduced bony integration and unfavourable long-term clinical outcomes prohibited its routine clinical application. More recently, scaffold-guided bone regeneration (SGBR) with application of highly porous biodegradable constructs is emerging. Advancements in additive manufacturing technology now allow the cage designs that match requirements, such as stiffness of surrounding tissues, while providing long-term biomechanical stability. A favourable clinical outcome has been observed in the treatment of various bone defects, particularly for 3D-printed composite scaffolds made of medical-grade polycaprolactone (mPCL) in combination with a ceramic filler material. Therefore, advanced cage design made of mPCL and ceramic may also carry initial high spinal forces up to the time of bony fusion and subsequently resorb without clinical side effects. Furthermore, surface modification of implants is an effective approach to simultaneously reduce microbial infection and improve tissue integration. We present a design concept for a scaffold surface which result in osteoconductive and antimicrobial properties that have the potential to achieve higher rates of fusion and less clinical complications. In this review, we explore the preclinical and clinical studies which used bioresorbable cages. Furthermore, we critically discuss the need for a cutting-edge research program that includes comprehensive preclinical in vitro and in vivo studies to enable successful translation from bench to bedside. We develop such a conceptual framework by examining the state-of-the-art literature and posing the questions that will guide this field in the coming years.

The effectiveness of biodegradable instrumentation in the treatment of spinal fractures

INTRODUCTION

A variety of biodegradable implants (screws, rods, plates and cages) are available which are composed of many different biodegradable polymers with varying characteristics. The present review of animal and clinical studies examines the efficacy and safety of biodegradable implants in spinal fracture intervention.

METHODS

A review of the literature through March 2018 was performed using PubMed and Cochrane databases. Success rates were calculated according to sufficient tissue biocompatibility, solid clinical fusion and propensity for osseointegration.

RESULTS

49 articles (24 animal and 25 human studies) were included. In animal experiments, the overall success rate for spinal fusion was 60.3%, while the mean success rate regarding the cervical spine was 51.8% compared to 68.1% for the lumbar spine (p = 0.002). In studies involving control group(s): the mean bioabsorbable implant success rate for spinal fusion was 42% compared to 57% for conventional implants (p = 0.0016). In the lumbar spine pL-lactide acid (PLLA) had 75.2% success rate compared to poly (L-lactide-co-DL-lactide) (PLDLLA) at 53.4% (p = 0.003). In clinical studies, the overall mean success rate was 89%, while the mean success rate regarding the cervical spine was 92%, as compared to 83.6% for the lumbar spine (p = 0.001). In studies involving control group(s): the mean bioabsorbable implant success rate was 75% compared to a conventional implant mean success rate of 97% (p<0.0001). In the cervical spine PLLA had a 98.7% success rate compared to 90% with PLDLLA (p = 0.015). In the lumbar spine PLDLLA had 84.7% success rate compared to 63.6% for poly-glycolic acid (PGA) (p = 0.085).

DISCUSSION

Studies combined biodegradable and conventional implants. Polymers were used in various combinations and surface modification of the implants also varied. Comparison studies were of small sample size. Animal and clinical studies diverged. The current data are not encouraging. The end-point of assessing osseointegration varies in the studies and is indeterminate. In early stages the structure comparison of osseous restoration using biodegradable implants appears inferior to utilization of conventional cages and instrumentation. There is no statistically significant evidence supporting the efficacy of biodegradable implants replacing traditional instrumentation. There is a lack of prospective clinical trials with long-term follow-up regarding utilization of biodegradable implants and the available data does not support their routine use in spinal fracture intervention.

Bioabsorbable self-retaining PLA/nano-sized β-TCP cervical spine interbody fusion cage in goat models: an in vivo study

STUDY DESIGN

This is an experimental animal study.

OBJECTIVE

The objective of this study was to compare an anterior cervical discectomy and interbody fusion of a novel polylactide/nano-sized β-tricalcium phosphate (PLA/nβ-TCP) bioabsorbable self-retaining cervical fusion cage (BCFC) with an autologous bone graft and polyetheretherketone (PEEK) cages.

BACKGROUND

Although PLA cervical cages have potential advantages compared with traditional materials, they are not currently routinely used in spine surgery because of undesirable effects such as the lack of osteoconductivity and osteolysis around the implant. This study involved the manufacturing of a bioabsorbable cage from PLA/nβ-TCP that was then used as a device for anterior cervical discectomy and fusion (ACDF) on a goat cervical spine fusion model.

MATERIALS AND METHODS

Eighteen goats underwent C3/C4 discectomy and were randomly divided into three groups based on the following methods: Group A (n=6), an autologous bone graft; Group B (n=6), PEEK cage filled with an autologous graft; and Group C (n=6), BCFC filled with an autologous iliac bone. Radiography was performed preoperatively and postoperatively and at 1, 4, 8, and 12 weeks after the operation. Disc space height (DSH) was measured at the same time. After 12 weeks, the fused segments were harvested and evaluated with functional radiographic views, biomechanical testing, and histological analyses.

RESULTS

Over a 12-week period, the BCFC and PEEK cage groups exhibited significantly higher DSH values than the bone graft group. Additionally, the BCFC group yielded a significantly lower range of motion in axial rotation than both the autologous bone graft and PEEK cage groups. A histologic evaluation revealed an increased intervertebral bone volume/total volume ratio and better interbody fusion in the BCFC group than in the other groups.

CONCLUSION

The BCFC device exhibited better results than the autologous bone graft and PEEK cages in single-level ACDF models in vivo. This device may be a potential alternative to the current PEEK cages.

Mechanical properties of bioresorbable self-reinforced posterior cervical rods

STUDY DESIGN

A biomechanical study.

OBJECTIVE

To test the mechanical and physical properties of self-reinforced copolymer bioresorbable posterior cervical rods and compare their mechanical properties to commonly used Irene titanium alloy rods.

SUMMARY OF BACKGROUND DATA

Bioresorbable instrumentation is becoming increasingly common in surgical spine procedures. Compared with metallic implants, bioresorbable implants are gradually reabsorbed as the bone heals, transferring the load from the instrumentation to bone, eliminating the need for hardware removal. In addition, bioresorbable implants produce less stress shielding due to a more physiological modulus of elasticity.

METHODS

Three types of rods were used: (1) 5.5 mm copolymer rods and (2) 3.5 mm and (3) 5.5 mm titanium alloy rods. Four tests were used on each rod: (1) 3-point bending test, (2) 4-point bending test, (3) shear test, and (4) differential scanning calorimeter test. The outcomes were recorded: Young modulus (E), stiffness, maximum load, deflection at maximum load, load at 1.0% strain of the rod's outer surface, and maximum bending stress.

RESULTS

The Young modulus (E) for the copolymer rods (mean range, 6.4-6.8 GPa) was significantly lower than the 3.5 mm titanium rods (106 GPa) and the 5.5 mm titanium rods (95 GPa). The stiffness of the copolymer rods (mean range, 16.6-21.4 N/mm) was also significantly lower than the 3.5 mm titanium alloy rods (43.6 N/mm) and the 5.5 mm titanium alloy rods (239.6 N/mm). The mean maximum shear load of the copolymer rods was 2735 N and they had significantly lower mean maximum loads than the titanium rods.

CONCLUSIONS

Copolymer rods have adequate shear resistance, but less load resistance and stiffness compared with titanium rods. Their stiffness is closer to that of bone, causing less stress shielding and better gradual dynamic loading. Their use in semirigid posterior stabilization of the cervical spine may be considered.

Evaluation of bioabsorbable multiamino acid copolymer/α-tri-calcium phosphate interbody fusion cages in a goat model

STUDY DESIGN

A study of cervical interbody fusion using polyamino acid-based bioabsorbable fusion cages in a goat model.

OBJECTIVE

To compare interbody fusion of a bioabsorbable multiamino acid copolymer/α-tri-calcium phosphate (MAACP/α-TCP) fusion cage with an autologous tricortical iliac-crest bone graft and a titanium cage.

SUMMARY OF BACKGROUND DATA

Polyamino acid is widely used as a carrier for drug delivery. To our knowledge, no study investigates interbody fusion cage made of polyamino acid.

METHODS

A total of 15 sheep underwent C3/C4 discectomy and fusion. The following stabilization techniques were used: group A, autologous tricortical iliac crest bone graft (n = 5); group B, MAACP/α-TCP cage filled with autologous cancellous bone graft (n = 5); group C, titanium cage filled with autologous cancellous bone graft (n = 5). Radiographic scans to determine disc space height were performed before and after surgery and after 4, 8, and 12 weeks, respectively. After 12 weeks, the C3/C4 motion segment was isolated and sectioned to create a 5-mm thick parasagittal section from which lateral radiographs were obtained. All the radiographs were encoded and reviewed in a blinded fashion to evaluate interbody fusion within the cage devices according to a three-point radiographic score. Biomechanical testing was performed in flexion, extension, axial rotation, and lateral bending to determine range of motion (ROM). Histomorphological and histomorphometrical analyses were performed to evaluate fusion and foreign-body reactions associated with the bioabsorbable cages.

RESULTS

Radiographic results showed that the disc space height (DSH) in MAACP/α-TCP cage group was better than that of bone graft group and the best radiographic score was found in MAACP/α-TCP cage group. Biomechanical test showed that no significant difference was found in ROM between MAACP/α-TCP cage group and titanium cage group whereas the value of ROM in bone graft group was the largest. Histologic evaluation showed a higher intervertebral bone volume/total volume ratio and a better interbody fusion in the MAACP/α-TCP cage group than in the other two groups. Two MAACP/α-TCP cages showed microcracks and the other three cages had maintained their original geometry. All MAACP/α-TCP cages showed excellent biocompatibility.

CONCLUSION

After 12 weeks, there was no significant difference between the MAACP/α-TCP cage and the titanium cage in distractive properties and biomechanical properties. Compared with titanium cages, MAACP/α-TCP cages showed an advanced interbody fusion. Although MAACP/α-TCP cages developed cracks after only 12 weeks, they showed significantly better distractive properties, biomechanical properties, and an advanced interbody fusion than the tricortical iliac crest bone graft. Improvement should be made to insure the strength of MAACP/α-TCP cage last at least 6 month after implantion.

Posterior lumbar interbody fusion using non resorbable poly-ether-ether-ketone versus resorbable poly-L-lactide-co-D,L-lactide fusion devices. Clinical outcome at a minimum of 2-year follow-up

Previous papers on resorbable poly-L-lactide-co-D,L-lactide (PLDLLA) cages in spinal fusion have failed to report adequately on patient-centred clinical outcome measures. Also comparison of PLDLLA cage with a traditionally applicable counterpart has not been previously reported. This is the first randomized prospective study that assesses clinical outcome of PLDLLA cage compared with a poly-ether-ether-ketone (PEEK) implant. Twenty-six patients were randomly assigned to undergo instrumented posterior lumbar interbody fusion (PLIF) whereby either a PEEK cage or a PLDLLA cage was implanted. Clinical outcome based on visual analogue scale scores for leg pain and back pain, as well as Oswestry Disability Index (ODI) and SF-36 questionnaires were documented and analysed. When compared with preoperative values, all clinical parameters have significantly improved in the PEEK group at 2 years after surgery with the exception of SF-36 general health, SF-36 mental health and SF-36 role emotional scores. No clinical parameter showed significant improvement at 2 years after surgery compared with preoperative values in the PLDLLA patient group. Only six patients (50%) in the PLDLLA group showed improvement in the VAS scores for leg and back pain as well as the ODI, as opposed to 10 patients (71%) in the PEEK group. One-third of the patients in the PLDLLA group actually reported worsening of their pain scores and ODI. Three cases of mild to moderate osteolysis were seen in the PLDLLA group. Following up on our preliminary report, these 2-year results confirm the superiority of the PEEK implant to the resorbable PLDLLA implant in aiding spinal fusion and alleviating symptoms following PLIF in patients with degenerative spondylolisthesis associated with either canal stenosis or foramen stenosis or both and emanating from a single lumbar segment.

Posterior lumbar interbody fusion using nonresorbable poly-ether-ether-ketone versus resorbable poly-L-lactide-co-D,L-lactide fusion devices: a prospective, randomized study to assess fusion and clinical outcome

STUDY DESIGN

A prospective randomized clinical study. OBJECTIVE.: To assess fusion, clinical outcome, and complications.

SUMMARY OF BACKGROUND DATA

Resorbable poly-L- lactide-co-D,L-lactide (PLDLLA) cages intended to aid spinal interbody fusion have been introduced into clinical practice within the last decade. Although early case series show promising results with respect to fusion rate, worries persist with regards to efficacy and potential risks of early failure of these implants. Despite widespread clinical application this is the first randomized prospective study to assess clinical and radiologic outcomes of PLDLLA cage compared with a traditionally applicable counterpart.

METHODS

Twenty-six patients were randomly assigned to undergo instrumented posterior lumbar interbody fusion whereby either a nonresorbable poly-ether-ether-ketone (PEEK) cage or a resorbable PLDLLA cage was implanted to aid fusion. Fusion rate, subsidence, and clinical outcome based on visual analog scale scores for leg pain and back pain, as well as Oswestry Disability Index and SF-36 questionnaires were documented and analyzed. Complications and adverse events were recorded.

RESULTS

Fusion rate was significantly higher with the PEEK cage compared with PLDLLA cage (Fisher exact test, P = 0.0302). Rate of subsidence was significantly higher with the PLDLLA cage compared to PEEK cage (Fisher exact test, P = 0.0414). The PEEK group demonstrated greater improvement in the Oswestry Disability Index compared with the PLDLLA group (Fisher exact test, P = 0.1414). Two cases of mild to moderate osteolysis were seen in the PLDLLA group.

CONCLUSION

Our results strongly suggest that PLDLLA cage proffers a lower rate of fusion compared with PEEK cage, and also confirms that the efficacy of PLDLLA cage in enhancing interbody spinal fusion is yet to be established. The higher rate of subsidence and occurrence of osteolysis seen in association with PLDLLA cages in this study remain worrisome.

Position of interbody spacer in transforaminal lumbar interbody fusion: effect on 3-dimensional stability and sagittal lumbar contour

STUDY DESIGN

Biomechanical study.

OBJECTIVE

To test 2 different intervertebral positions of a semilunar cage and their effects on 3-dimensional stability and segmental lordosis in a model of transforaminal lumbar interbody fusion (TLIF).

SUMMARY OF BACKGROUND DATA

In his original TLIF description, Harms recommended decortication of endplates, followed by placement of mesh cages in the middle-posterior intervertebral third. Subsequent studies presented conflicting recommendations: anterior placement of the spacer-cage for better load-sharing versus placement on the stronger posterolateral endplate regions.

METHODS

Six human lumbar spinal functional units were first tested intact. TLIF was performed using a semilunar poly-ether-ether-ketone cage randomly inserted in the anterior (TLIF-A) or posterior (TLIF-P) disc space. Pedicle screws and rods were added. Unconstrained pure moments in axial-torsion, lateral-bending (LB), and flexion-extension (FE) were applied under 0.05 Hz and +/-5 Nm sinusoidal waveform. Segmental motions were recorded. Range of motion (ROM) and neutral zone (NZ) were calculated. Pairwise comparisons were made using nonparametric Wilcoxon-matched pairs signed rank sum test with statistical significance set at P<0.05.

RESULTS

TLIF-A and TLIF-P significantly decreased ROM (P<0.05) of the intact spinal functional unit, in FE and LB. In axial-torsion, decrease of ROM after TLIF procedures was not significant (P>0.05). Delta-ROM between TLIF-A and TLIF-P was not significant (P>0.05). TLIF-A and TLIF-P significantly decreased NZ in LB (P<0.05). In FE, TLIF-P significantly decreased NZ (P<0.05); TLIF-A showed a trend toward significance (P=0.09). Delta-NZ between TLIF-A and TLIF-P was not significant (P>0.05). Segmental lordosis of TLIF-A and TLIF-P on C-arm views showed angle differences within the range of measurement error of Cobb angles.

CONCLUSIONS

Difference in ROM and NZ between anterior (TLIF-A) or posterior (TLIF-P) positions was not statistically significant. Similarly, both positions did not influence segmental lordosis.

Time-dependent mechanical strength of 70/30 Poly(L, DL-lactide): shedding light on the premature failure of degradable spinal cages

STUDY DESIGN

In vitro studies on the mechanical strength of 70/30 poly(l,dl-lactic acid) (70/30 PLDLLA) cages.

OBJECTIVE

To evaluate the effect of loading rate, humidity, temperature, and continuous static loading on the strength of 70/30 PLDLLA, to elucidate the mechanism of premature failure of degradable spinal cages observed in earlier studies.

SUMMARY OF BACKGROUND DATA

Degradable 70/30 PLDLLA cages have been designed to withstand mechanical loads in a goat lumbar spine for at least 6 months. Yet mechanical failure was observed after only 3 months in vivo. We hypothesize that this observation can be related to the time-dependent nature of the polymer.

METHODS

Degradable 70/30 PLDLLA cages were loaded to failure at loading rates between 10 and 10 mm/s under standard loading conditions (in air at room temperature: +/-23 degrees C). The experiments were also done at body temperature (37 degrees C) and under wet conditions. Furthermore, we determined the time-to-failure for 70/30 PLDLLA cages subjected to loads well below their instantaneous mechanical strength.

RESULTS

The mechanical strength of 70/30 PLDLLA cages was lower for lower loading rates, higher temperature, and higher humidity. The cages already failed within less than 5 minutes when statically loaded at 75% of their strength, and within 1 day when loaded at about 50% of their strength. Extrapolation predicts cage failure at 3 months when loaded at 25% of their strength.

CONCLUSION

Premature failure of 70/30 PLDLLA cages, as observed in vivo in earlier studies, is owing to mechanical loading and the time-dependent mechanical properties of the material. The standards for mechanical testing of implants made of strongly time-dependent materials like polylactide should be reconsidered.

Resorbable Anterior Cervical Plates for Single-Level Degenerative Disc Disease

Objective:

Resorbable spinal implants have generated increasing interest. There are minimal data in the literature on the use of resorbable anterior cervical plates. The purpose of this study is twofold. First, we present clinical outcomes of anterior cervical decompression and fusion (ACDF) for single-level degenerative disease using the first commercially available resorbable cervical plate. Second, we compare x-ray outcomes, including pseudarthrosis rates and angulation of healing, between patients receiving resorbable plates and a control group of patients receiving titanium plates.

Methods:

Twenty-four consecutive patients treated with single-level ACDF using a resorbable anterior cervical plate were prospectively evaluated with minimum 12-month follow-up (mean, 15 mo). As a control group, 93 consecutive patients treated with single-level ACDF using a titanium plate with minimum 1-year follow-up were assessed.

Results:

Overall outcomes were good in the resorbable plate group at a mean 15 months follow-up. Neck Disability Index scores decreased from a mean of 64.0 to 20.1% at follow-up (P &lt; 0.001). Numerical Rating Scale pain scores decreased from a mean of 8.0 preoperatively to 2.9 at follow-up (P &lt; 0.001). Three cases of pseudarthrosis [three of 24 (12.5%)] were identified in the resorbable plate group compared with two cases in the control group [two of 93 (2.2%)]. This difference was significant (P = 0.047). Angulation was measured on final x-rays using Cobb angles. Patients in the resorbable plate group healed with a mean 4.52-degree kyphosis, whereas patients in the titanium plate group healed with a mean 2.41-degree lordosis (P &lt; 0.001).

Conclusion:

This report describes, to our knowledge, the first series of patients to undergo ACDF with the first commercially available resorbable cervical plate. Healing in kyphosis and pseudarthrosis was significantly more likely with resorbable implants as compared with the use of titanium plating. Although ultimate clinical outcomes were good, x-ray outcomes may be better with the use of titanium plating.

Sterilization and strength of 70/30 polylactide cages: e-beam versus ethylene oxide

STUDY DESIGN

In vitro and in vivo studies on the degradation of 70/30 poly(L,DL-lactide) (PLDLLA) cages.

OBJECTIVE

To evaluate the effect of e-beam and ethylene oxide sterilization on degradation and strength.

SUMMARY OF BACKGROUND DATA

e-beam-sterilized PLDLLA cages were shown to maintain mechanical strength for at least 6 months during degradation studies in vitro. Yet failure of the cages was observed after only 3 months in vivo. We hypothesized that degradation characteristics and mechanical strength could be improved by sterilizing the cages through ethylene oxide (EtO) instead of e-beam.

METHODS

PLDLLA cages were sterilized either by e-beam or EtO, and degraded in phosphate-buffered saline. Each month, cages were compressed until failure. Inherent viscosity was determined as a measure of degradation. For the in vivo evaluation, e-beam- or EtO-sterilized cages were implanted at L3-L4 in a standardized goat model. After 3 or 6 months, retrieved segments were scanned by high-resolution magnetic resonance imaging. Also, inherent viscosity of the polymer was measured.

RESULTS

e-beam sterilization strongly decreased inherent viscosity of PLDLLA compared with EtO sterilization, but initial strength was only affected marginally. After 6 months, the strength of the e-beam-sterilized cages dropped, while that of EtO-sterilized cages was maintained. Degradation in vivo was slightly faster than in vitro. In both groups, however, mechanical failure occurred at 3 months after implantation.

CONCLUSIONS

Inherent viscosity decreases with degradation time, but strength only decreases when inherent viscosity is below a certain threshold. Above this threshold, mechanical strength is a property of the polymer and independent of inherent viscosity. e-beam sterilization strongly decreases inherent viscosity and thus advances mechanical degradation. EtO sterilization delays degradation but does not increase initial strength. Early failure of PLDLLA cages in the goat model thus is unrelated to sterilization method and requires further study.

Is One Cage Enough in Posterior Lumbar Interbody Fusion: A Comparison of Unilateral Single Cage Interbody Fusion to Bilateral Cages

Posterior lumbar interbody fusion (PLIF), as recommended with bilateral lumbar interbody cages and pedicle screw fixation, has increased the successful fusion rate to nearly 100%. Presently, a unilateral approach to the disc space with a variant of PLIF, the trans-foraminal interbody fusion is often used. There are few clinical studies of unilateral interbody fusion. The clinical and fusion results of unilateral interbody fusion are important as the usage of trans-foraminal interbody fusion procedure increases. This retrospective study of 26 consecutive patients treated with a unilateral cage asks whether fusion healing and clinical outcome is comparable with that obtained with bilateral cages. In this study, there were no pseudarthroses, instrumentation failures, or significant subsidence at any of the single cage levels. Disc space height and foraminal height were restored by the surgery and maintained at last follow-up. Using Prolo scores, 23/26 patients had clinical success (88%), and 3 were unsuccessful. Fusion was successful at all single cage fusion levels and overall in 23/26 (88%) reviewing all levels of fusion. In conclusion, fusion and clinical success rates were not diminished by the use of a unilateral interbody cage rather than the recommended 2 cages. This retrospective comparative study is a Level III-2 Therapeutic Study investigating the results of unilateral PLIF with a single interbody cage compared with historical series with interbody cages.

Lumbar body fusion with a bioresorbable cage in a goat model is delayed by the use of a carboxymethylcellulose-stabilized collagenous rhOP-1 device

The purpose of this study was to evaluate the efficacy of recombinant human osteogenic protein-1 (rhOP-1) with a carboxymethylcellulose-stabilized collagenous carrier as a bone graft substitute for instrumented lumbar spinal fusion in an established goat model. Twenty goats received a resorbable poly-L-lactic acid (PLLA) interbody cage packed with either rhOP-1 and its carrier or autologous bone graft. The carrier material was bovine collagen type-1 stabilized with carboxymethylcellulose. The fusion segments were retrieved at 3 or 6 months postimplantation and evaluated by radiographic and histologic analyses. The rhOP-1 graft substitute, used in combination with the resorbable PLLA cage, showed inferior results as compared to autologous bone graft in the goat lumbar fusion model. Whereas four out of five segments from the autograft group were fused after 6 months, none of the four segments receiving the rhOP-1 graft substitute were fused at this time point. Bone ingrowth into the cage was delayed or absent in the experimental group, whereas all autograft specimens showed advanced bone ingrowth (3 months) or fusion (6 months). We suggest that the fusion process was inhibited, because cells were unable to penetrate the rhOP-1 graft material. This led to delayed bone formation and in some cases inadequate tissue formation.