Autograft Cellular Contribution to Spinal Fusion and Effects of Intraoperative Storage Conditions.

STUDY DESIGN: Controlled animal study. OBJECTIVE: To assess the cellular contribution of autograft to spinal fusion and determine the effects of intraoperative storage conditions on fusion. SUMMARY OF BACKGROUND DATA: Autograft is considered the gold standard graft material in spinal fusion, purportedly due to its osteogenic properties. Autograft consists of adherent and non-adherent cellular components within a cancellous bone scaffold. However, neither the contribution of each component to bone healing is well understood nor are the effects of intraoperative storage of autograft. MATERIALS AND METHODS: Posterolateral spinal fusion was performed in 48 rabbits. Autograft groups evaluated included: (1) Viable, (2) partially devitalized, (3) devitalized, (4) dried, and (5) hydrated iliac crest. Partially devitalized and devitalized grafts were rinsed with saline, removing nonadherent cells. Devitalized graft was, in addition, freeze/thawed, lysing adherent cells. For 90 minutes before implantation, air dried iliac crest was left on the back table whereas the hydrated iliac crest was immersed in saline. At 8 weeks, fusion was assessed through manual palpation, radiography, and microcomputed tomography. In addition, the cellular viability of cancellous bone was assayed over 4 hours. RESULTS: Spinal fusion rates by manual palpation were not statistically different between viable (58%) and partially devitalized (86%) autografts ( P = 0.19). Both rates were significantly higher than devitalized and dried autograft (both 0%, P < 0.001). In vitro bone cell viability was reduced by 37% after 1 hour and by 63% after 4 hours when the bone was left dry ( P < 0.001). Bone cell viability and fusion performance (88%, P < 0.001 vs . dried autograft) were maintained when the graft was stored in saline. CONCLUSIONS: The cellular component of autograft is important for spinal fusion. Adherent graft cells seem to be the more important cellular component in the rabbit model. Autograft left dry on the back table showed a rapid decline in cell viability and fusion but was maintained with storage in saline.

Preliminary experience with a novel facet-based lateral mass drill guide for the placement of lateral mass screws compared to freehand technique: a cadaveric study.

BACKGROUND CONTEXT: Lateral mass screw fixation is the standard for posterior subaxial cervical fixation. Several freehand surgical techniques for placing lateral mass screws have been described which rely on anatomical landmarks and surgeon mastery of the technique to safely place screws. The accuracy of these freehand techniques is inherently variable and can be influenced by a surgeon's level of clinical experience. A novel technique was developed that utilizes the plane of the facet joint to create lateral mass screw pilot holes parallel with the joint line to improve the safety and accuracy of lateral mass screw placement regardless of experience. PURPOSE: To assess the safety and accuracy of lateral mass screw placement using a novel lateral mass drill guide instrument (LM Guide), compared to standard freehand technique. STUDY DESIGN: Randomized cadaveric study utilizing multiple surgeon evaluators to compare the safety and accuracy of guided cervical lateral mass placement compared to traditional freehand techniques. MATERIALS AND METHODS: Lateral mass screws were placed from C3 to C7 in 20 cadaver specimens by 8 spine surgeons of varying levels of clinical experience (4 attendings, 4 fellows). Screws were placed bilaterally using standard anatomic landmarks ("freehand") randomly allocated on one side and using the LM Guide on the other. Cadaveric specimens were imaged with high-resolution CT to assess screw placement. Zone grading for safety was conducted based on screw tip position and clinical severity of screw breach was based on proximity to surrounding neurovascular anatomy. Screws were graded as safe, at-risk, or critical, with at-risk and critical screws considered malpositioned. To assess the accuracy of screw trajectory placed using the LM Guide compared to freehand, sagittal screw angle was measured and compared to an "ideal" screw path parallel to the facet joint line. Freehand and LM Guide groups were compared using Pearson's chi-square correlation. RESULTS: Screw placement using the LM guide yielded a significantly lower rate of screw malpositioning, with 7 of 91 (7.7%) compared with 18 of 99 (18.2%) screws placed in the At-Risk or Critical Zones, p<.05. Of the 91 screws inserted using the LM Guide, 84 (92.3%) were in the Safe Zone, 7 (7.7%) were At-Risk, and 0 were in Critical zones. There was no incidence of neural or transverse foramen breaches with the LM Guide. In comparison, for the 99 screws inserted freehand, 81 (81.8%) were Safe, 14 (14.1%) were At-Risk, and 4 (4.1%) were in Critical zones. The 4 Critical zone freehand screw breaches included 1 neural foramen breach, 2 transverse foramen breaches, and 1 facet breach. The LM Guide also resulted in higher accuracy of screw trajectory, as indicated by a significant reduction in sagittal screw angle compared with freehand, p<.01. Notably, in the less-experienced surgeon cohort, the LM Guide significantly reduced the sagittal screw angle and resulted in no critical screw breaches compared to 3 critical breaches with freehand technique suggesting there might be a benefit in decreasing the learning curve associated with lateral mass screw placement. CONCLUSIONS: Lateral mass screw placement with a novel LM Guide that uses the facet joint to control screw trajectory improved the accuracy and reproducibility of screw placement with a significant reduction in screw breach rate and sagittal screw angle compared to freehand techniques regardless of surgeon experience level. CLINICAL SIGNIFICANCE: The inherent variability of freehand lateral mass screw placement can increase the risk of clinical complications associated with screw malpositioning. The technique presented in this cadaveric study may be a viable alternative to standard freehand technique that can improve the overall safety of lateral mass screw placement.

Examination of the Role of Cells in Commercially Available Cellular Allografts in Spine Fusion: An in Vivo Animal Study.

BACKGROUND: Despite the extensive use of cellular bone matrices (CBMs) in spine surgery, there is little evidence to support the contribution of cells within CBMs to bone formation. The objective of this study was to determine the contribution of cells to spinal fusion by direct comparisons among viable CBMs, devitalized CBMs, and cell-free demineralized bone matrix (DBM). METHODS: Three commercially available grafts were tested: a CBM containing particulate DBM (CBM-particulate), a CBM containing DBM fibers (CBM-fiber), and a cell-free product with DBM fibers only (DBM-fiber). CBMs were used in viable states (CBM-particulatev and CBM-fiberv) and devitalized (lyophilized) states (CBM-particulated and CBM-fiberd), resulting in 5 groups. Viable cell counts and bone morphogenetic protein-2 (BMP-2) content on enzyme-linked immunosorbent assay (ELISA) within each graft material were measured. A single-level posterolateral lumbar fusion was performed on 45 athymic rats with 3 lots of each product implanted into 9 animals per group. After 6 weeks, fusion was assessed using manual palpation, micro-computed tomography (µ-CT), and histological analysis. RESULTS: The 2 groups with viable cells were comparable with respect to cell counts, and pairwise comparisons showed no significant differences in BMP-2 content across the 5 groups. Manual palpation demonstrated fusion rates of 9 of 9 in the DBM-fiber specimens, 9 of 9 in the CBM-fiberd specimens, 8 of 9 in the CBM-fiberv specimens, and 0 of 9 in both CBM-particulate groups. The µ-CT maturity grade was significantly higher in the DBM-fiber group (2.78 ± 0.55) compared with the other groups (p < 0.0001), while none of the CBM-particulate samples demonstrated intertransverse fusion in qualitative assessments. The viable and devitalized samples in each CBM group were comparable with regard to fusion rates, bone volume fraction, µ-CT maturity grade, and histological features. CONCLUSIONS: The cellular component of 2 commercially available CBMs yielded no additional benefits in terms of spinal fusion. Meanwhile, the groups with a fiber-based DBM demonstrated significantly higher fusion outcomes compared with the CBM groups with particulate DBM, indicating that the DBM component is probably the key determinant of fusion. CLINICAL RELEVANCE: Data from the current study demonstrate that cells yielded no additional benefit in spinal fusion and emphasize the need for well-designed clinical studies on cellular graft materials.

Undercut macrostructure topography on and within an interbody cage improves biomechanical stability and interbody fusion.

BACKGROUND CONTEXT: The interface and interactions between an interbody cage, graft material, and host bone can all participate in the fusion. Shortcomings of Poly(aryl-ether-ether-ketone) interbody cages have been addressed with novel titanium surfaces. Titanium surfaces paired with macroscale topography features on the endplates and within the aperture may provide additional benefits. PURPOSE: To evaluate the influence of cage design parameters on interbody fusion in a large animal preclinical model. STUDY DESIGN/SETTING: A comparative preclinical large animal model was performed to evaluate how macroscale topography features of an interbody cage can facilitate early integration between the host bone, graft material, and interbody cage and these effects on biomechanical stability and fusion. METHODS: Forty single level interbody fusions (L4-L5) using iliac crest autograft and bilateral pedicle screw fixation were performed in adult sheep to evaluate the effect of undercut macrostructure topography features of an interbody cage on the endplates and within the aperture. Fusions were evaluated at 6 and 12 weeks (n=10 per group) using radiography, microcomputed tomography, biomechanical integrity, and histology endpoints. RESULTS: The presence of the undercut macrostructures present on the endplates and within the aperture statistically improved biomechanical integrity at 6 and 12 weeks compared with controls. Microcomputed tomography and histology demonstrated bony interdigitation within the endplate and aperture features contributing to the improvement in properties. CONCLUSIONS: The present study demonstrates that Poly(aryl-ether-ether-ketone) implants with titanium surfaces can be augmented by undercut macrostructures present on the endplates and within the aperture to provide opportunities for a series of anchoring points that, with new bone formation and remodelling, result in earlier and improved biomechanical integrity of the treated level. CLINICAL SIGNIFICANCE: This preclinical study showed that bone interdigitation with the undercut macrostructures present on the endplates and within the aperture resulted in improved fusion and biomechanical stability in a clinically relevant spinal fusion model. Future clinical study is warranted to evaluate such implants' performance in humans.

In-vivo Performance of Seven Commercially Available Demineralized Bone Matrix Fiber and Putty Products in a Rat Posterolateral Fusion Model.

Introduction: Demineralized bone matrix (DBM) is a widely used bone graft in spinal fusion. Most commercial DBMs are composed of demineralized bone particles (~125-800 microns) suspended in a carrier that provides improved handling but dilutes the osteoinductive component. DBM fibers (DBF) provide improved osteoconductivity and do not require a carrier. It has been suggested that 100% DBF may offer improved performance over particulate-based DBMs with carrier. Study Design: Seven commercially available DBM products were tested in an athymic rat posterolateral fusion model. There were four 100% DBFs, two DBFs containing a carrier, and one particulate-based DBM containing carrier. Objective: The study objectives were to evaluate the in vivo performance: (1) compare fusion rate and fusion maturity of six commercially available DBFs and one particulate-based DBM, and (2) assess the effect of carrier on fusion outcomes for DBFs in a posterolateral fusion model. Methods: The DBF/DBM products evaluated were: StrandTM Family, Propel® DBM Fibers, Vesuvius® Demineralized Fibers, Optium® DBM Putty, Grafton® DBF, Grafton Flex, and DBX® Putty. Single-level posterolateral fusion was performed in 69 athymic rats. Fusion was assessed bilaterally after 4 weeks by manual palpation, radiograph and CT for bridging bone. Fusion mass maturity was assessed with a CT maturity grading scale and by histology. Statistical analysis was performed using Fishers Exact Test for categorical data and Kruskal-Wallis Test for non-parametric data. Results: Strand Family achieved 100% fusion (18/18) by manual palpation, radiographic and CT evaluation, significantly higher than Propel Fibers, Vesuvius Fibers, Optium Putty, and DBX Putty, and not statistically higher than Grafton DBF and Grafton Flex. Strand Family provided the highest fusion maturity, with CT maturity grade of 2.3/3.0 and 89% mature fusion rate. Fusion results suggest a detrimental effect of carrier on fusion performance. Conclusions: There were large variations in fusion performance for seven commercially available DBM products in an established preclinical fusion model. There were even significant differences between different 100% DBF products, suggesting that composition alone does not guarantee in vivo performance. In the absence of definitive clinical evidence, surgeons should carefully consider available data in valid animal models when selecting demineralized allograft options.

The in vivo response to a novel Ti coating compared with polyether ether ketone: evaluation of the periphery and inner surfaces of an implant.

BACKGROUND CONTEXT: Increasing bone ongrowth and ingrowth of polyether ether ketone (PEEK) interbody fusion devices has the potential to improve clinical outcomes. PURPOSE: This study evaluated the in vivo response of promoting new bone growth and bone apposition with NanoMetalene (NM) compared with PEEK alone in a cancellous implantation site with an empty aperture. STUDY DESIGN: This is a randomized control animal study. METHODS: Implants and funding for this study were provided by SeaSpine (60,000 USD). Cylindrical dowels with two apertures were prepared as PEEK with a sub-micron layer of the titanium (NM). The titanium coating was applied over the entire implant (Group 1) or just the apertures (Group 2). Polyether ether ketone implants with no coating served as controls (Group 3). Implants were placed in the cancellous bone of the distal femur or proximal tibia with no graft material placed in the apertures in eight adult sheep. Bone ongrowth to the surface of the implant and ingrowth into the apertures was assessed at 4 and 8 weeks after surgery with micro-computed tomography (CT) and undecalcified histology. RESULTS: The apertures in the implants were notably empty in the PEEK group at 4 and 8 weeks. In contrast, new bone formation into the apertures was found in samples coated with NM even though no graft material was placed into the defect. The bone growing into the aperture tracked along the titanium layer. Apertures with the titanium coating demonstrated significantly more bone by micro-CT qualitative grading compared with PEEK with average bone coverage scores of Group 1 (NM) 1.62±0.89, Group 2 (NM apertures only) 1.62±0.77, and Group 3 (PEEK) 0.43±0.51, respectively, at 4 weeks (p<.01) and Group 1 (NM) 1.79±1.19, Group 2 (NM apertures only) 1.98±1.18, and Group 3 (PEEK) 0.69±0.87, respectively, at 8 weeks (p<.05). The amount of bone in the apertures (ingrowth) quantified using the volumetric data from the micro-CT supported an overall increase in bone volume inside the apertures with the titanium coating compared with PEEK. Histology showed newly formed woven bone tracked along the surface of the titanium in the apertures. The PEEK interface presented the typical nonreactive fibrous tissue inside the apertures at 4 weeks and some focal contact with bone on the outside at 4 weeks and 8 weeks. CONCLUSIONS: Micro-CT and histology demonstrated bone ongrowth to the surfaces coated with NM where the newly formed bone tracked along the thin titanium-coated surfaces. Polyether ether ketone surfaces presented the nonreactive fibrous tissue at the interface as previously reported in preclinical scenarios.

Thermal Analysis of the Tibial Cement Interface with Modern Cementing Technique.

BACKGROUND: The major cause of cemented Total Knee Arthroplasty (TKA) failure is aseptic loosening of the tibial component necessitating revision surgery. Recently, multiple techniques have been described to maximize cement penetration depth and density in the proximal tibia during TKA to potentially avoid early loosening. While cement polymerisation is an exothermic reaction, minimal investigation into the proximal tibial thermal safety margin during cement polymerisation has been undertaken. In animal models osseous injury occurs at temperatures greater than 47 °C when applied for one minute. The aim of this study was to investigate the cement bone interface temperatures in TKA using modern tibial cementing techniques with a cadaveric tibial tray model. METHODS: Eight adult cadavers were obtained with the proximal tibial surface prepared by a fellowship trained arthroplasty surgeon. Third generation cementation techniques were used and temperatures during cement polymerization on cadaveric knee arthroplasty models were recorded using thermocouples. RESULTS: The results showed that no tibial cement temperature exceeded 44 °C for more than 1 minute. Two of the eight cadaveric tibias recorded maximum temperatures greater than 44 °C for 55 seconds and 33 seconds, just less than the 60 seconds reported to cause thermal injury. Average maximum polymerization temperatures did not correlate with deeper cement penetration or tray material. Maximum mantle temperatures were not statistically different between metal and all polyethylene tibial trays. CONCLUSION: Our investigation suggests that modern cementing techniques result in maximum mantle temperatures that are less than previously recorded temperatures required to cause thermal osseous injury, although this thermal injury safety margin is quite narrow at an average of 4.95 °C (95% confidence interval ± 4.31).

Tibiofemoral contact areas and pressures in six high flexion knees.

The tibiofemoral articulating interfaces of six high flexion knee designs were examined using a standard testing protocol developed by Harris et al. [J Biomech 32:951-958 (1999)] to investigate the polyethylene insert contact areas and pressures. A load of 3600 N was applied for 10 s at 0, 30, 60, 90, 110, 135 and 155 degrees of flexion. Contact areas and pressures at the femoral-polyethylene insert interface were measured with a I-scan 4000 system. Up to 110 degrees of flexion, the VANGUARD RP HI-FLEX showed the highest contact area and lowest pressure. At the deep flexion angles, contact area decreased and contact pressure increased significantly in all knees. The NexGen series showed a constant contact area throughout the various flexion angles. In general, all high flexion knees could result in almost point contact in an extremely high range of motion.

Stress relaxation and creep: viscoelastic properties of common suture materials used for flexor tendon repair.

PURPOSE: The time-dependent mechanical behavior of common suture materials may have a pronounced influence on the quality of flexor tendon repairs with respect to gap formation. METHODS: Sutures commonly used in plastic surgery, particularly for hand tendon repair, were tested in tension for analysis of stress relaxation and creep properties. The three materials tested were (1) Prolene, a monofilament polypropylene(Ethicon, Somerville, NJ); (2) Ethilon, a monofilament nylon(Ethicon, Somerville, NJ); and (3) Ticron, a braided polyester fiber (Sherwood, Davis & Geck, St. Louis, MO). All measurements were made with 1.5-mm (4-0) varieties of the sutures, in physiological buffered solution (0.9% phosphate-buffered saline [PBS]) at both room (22 degrees C) and body temperature (37 degrees C) conditions. RESULTS: Ticron was the stiffest suture at both room and body temperature, followed by Prolene and Ethilon. Both Prolene and Ethilon showed statistically (p< .05) reduced stiffness at body temperature. Prolene exhibited statistically larger (p< .05) relaxation and creep ratios than the other two sutures, and it was the only material to show temperature effects in creep and relaxation. CONCLUSIONS: Based on static and viscoelastic mechanical properties, Ticron is the most suitable suture for flexor tendon repairs when compared to Prolene and Ethilon.

Effect of low intensity pulsed ultrasound on healing of an ulna defect filled with a bone graft substitute.

A 1.5 cm unilateral rabbit ulna defect model was performed in 18 adult NZ white rabbits. The defects were filled with a beta-tricalcium phosphate bone graft substitute (JAX TCP). The surgical site in half the animals was treated daily with 20 min of low intensity pulsed ultrasound (LIPUS). Animals were sacrificed at 4 weeks (n = 3 per group) or 12 weeks (n = 6 per group) following surgery for radiographic and histologic endpoints. Radiography revealed some resorption of the JAX TCP by 12 weeks in the control and LIPUS treated groups. LIPUS treatment did not accelerate this resorption. Some new bone formation was noted in the control groups at the defect margins while little bone formed in the center of the defect at 4 and 12 weeks. In contrast, radiographs revealed more new bone at 4 and 12 weeks in the LIPUS treated animals throughout the section. Bone mineral density (DEXA) revealed a statistically significant difference at 4 weeks with LIPUS while no differences were found at 12 weeks. Histology of the LIPUS treated sections demonstrated new woven bone formation on and between the JAX TCP bone graft substitute particles across the defect. VEGF expression was increased with LIPUS treatment at 4 weeks and remained elevated at 12 weeks compared with controls. CBFA-1 expression levels were elevated with LIPUS treatment at both time points. LIPUS treatment increased bone formation in ulna defect healing with a beta-tricalcium phosphate bone graft substitute.

Beta-TCP bone graft substitutes in a bilateral rabbit tibial defect model.

The use of artificial bone graft substitutes has increased as the surgical applications widen and the availability of allograft bone decreases. The ideal graft substitute should reabsorb with time to allow and encourage new bone formation whilst maintaining its properties as an osteoconductive scaffold until it is no longer required. A potential disadvantage of some synthetic substitutes is their long dissolution time. Beta-tricalcium phosphates (beta-TCPs) have some advantages when compared to hydroxyapatite (HA), when used as a filler, in that it is more rapidly reabsorbed. Three commercially available and clinically used beta-TCP bone graft substitutes with the same chemistry (Vitoss, Osferion, Chronos) but with varying macro and microscopic characteristics were investigated using a bilateral tibial metaphyseal defect model in New Zealand white rabbits. When placed into tibial defects all three materials performed similarly in terms of mechanical properties of the healing defects. A decrease in properties was found at 12 weeks where implant resorption was nearly achieved while remodelling of the anteromedial cortex had yet to be completed. All materials were osteoconductive and supported new bone formation while implant resorption with time differed between materials. Vitoss resorbed faster than the other materials and is likely to differences in particle geometry, pore structure and interconnectivity.

Comparison of poly-L-lactide and polylactide carbonate interference screws in an ovine anterior cruciate ligament reconstruction model.

PURPOSE: The purpose of this study was to compare polylactide carbonate (PLC) interference screws with poly-L-lactide (PLLA) screws in an ovine anterior cruciate ligament reconstruction model. METHODS: A PLC screw or PLLA screw was placed in the center of a 4-strand soft-tissue autograft fixating the graft within the tibial tunnel. Assessments were made at 6 and 12 weeks for fixation strength and at time points of 6, 12, 26, and 52 weeks via computed tomography and histology. RESULTS: No adverse or inflammatory reactions were noted for either material at any time point. Mechanical fixation strength increased from 6 to 12 weeks for both the PLC and PLLA screws, with no significant differences in fixation strength being found between the 2 groups. By 26 weeks, the PLC screw was partially replaced by new bone, a process that was completed by 52 weeks. The PLLA screws were intact and surrounded by a fibrous layer at 52 weeks with no obvious resorption. New bone formation within the tendon construct located in the bone tunnel proximal to the interference screw was also noted in the PLC screw group but was not observed in the PLLA group. CONCLUSIONS: This study has supported the hypothesis that this bioabsorbable composite has sufficient mechanical properties and strength retention to function successfully as an interference screw but also stimulates a biologic healing response, enabling replacement by bone and tunnel healing. CLINICAL RELEVANCE: This study shows both the satisfactory mechanical characteristics and osteoconductive nature of PLC used in an interference screw in an ovine anterior cruciate ligament reconstruction model.

Effects of low-intensity pulsed ultrasound on tendon-bone healing in an intra-articular sheep knee model.

PURPOSE: This study reports the mechanical and histologic properties of intra-articular tendon-bone healing with the application of low-intensity pulsed ultrasound (LIPUS) in an ovine knee model. METHODS: A single digital extensor tendon autograft from the right hoof was used as the graft in 89 adult sheep. Femoral fixation was achieved with an EndoButton (Smith & Nephew Endoscopy, Andover, MA) and tibial fixation by tying over a bony post. LIPUS treatment was performed daily for 20 minutes over the femoral and tibial tunnels until sacrifice in all groups, apart from the 26-week group, which was treated only for the first 12 weeks. Histology was performed at 3, 6, 12, and 26 weeks. Mechanical testing was performed at 6, 12, and 26 weeks. RESULTS: The LIPUS-treated group showed increased cellular activity at the tendon-bone interface and general improvement in tendon-bone integration and vascularity. Stiffness and peak load were greater compared with the control group at 26 weeks after surgery (P < .05). CONCLUSIONS: The application of LIPUS appears to improve healing at the tendon-bone interface for soft tissue grafts fixed with a suspensory fixation technique. Histology supports a benefit based on increased integration between tendon and bone and a biologically more active interface, which would account for the improved mechanical properties. CLINICAL RELEVANCE: The indications of LIPUS may be expanded to include tendon-bone healing, for example, in anterior cruciate ligament reconstruction.