A Biocompatible Titanium Headpost for Stabilizing Behaving Monkeys

Many neurophysiological experiments involving monkeys require that the head be stabilized while the animal performs a task. Often a post is attached to the skull to accomplish this goal, using a headcap formed from dental acrylic. We describe a new headpost, developed by refinement of several prototypes, and supply an AutoCAD file to aid in machine shop production. This headpost is fabricated from a single piece of commercially pure titanium. It has a footplate consisting of four limbs arranged in the configuration of a “K.” These are bent during surgery to match the curvature of the skull and attached with specialized titanium bone screws. Headposts were implanted in seven rhesus monkeys ranging in age from 2 yr to adult. None has been rejected after up to 17 mo of regular use. They require little or no daily toilette and create only a 0.80-cm2 defect in the scalp. Computed tomography after implantation showed that the skull undergoes remodeling to embed the footplate in bone. This finding was confirmed by necropsy in two subjects. The outer table of the skull had grown over the titanium footplate, whereas the inner table had thickened to bury the tips of the titanium screws. The remarkable strength of the skull/implant bond was demonstrated by applying increasing amounts of torque to the headpost. At 26.3 Nm, the headpost tore from its metal footplate, but no screws came loose. The excellent performance of this implant is explained by integration of biocompatible titanium into remodeled bone tissue. The headpost is simpler to implant, more securely anchored, easier to maintain, and less obtrusive than devices attached with acrylic.

Ex vivo measurement of intravertebral pressure during vertebroplasty

STUDY DESIGN

Ex vivo biomechanical study using cadaver vertebral bodies. OBJECTIVE.: To measure the increase in internal vertebral body pressure from cement injection during vertebroplasty.

SUMMARY OF BACKGROUND DATA

Theoretically, the increased force required to inject polymerizing (viscous) cement into a vertebral body during vertebroplasty could produce a concomitant increase in intravertebral pressure and cause additional damage to the vertebral body. An alternative means of reducing intravertebral pressure during injection may be needed.

METHODS

We placed 11-gauge cannulas bipedicularly in six vertebral bodies from each of two fresh female cadaver spines (one osteoporotic, one normal). One cannula served as the injection route; a manometer was connected to the other. After immersion of the spines in a saline bath, the vertebral bodies were injected with 10 mL of Simplex P cement by depressing the syringe plunger at a rate of 7 mm/sec using a materials testing machine. Static pressure was measured before and after injection. Peak pressure was measured during injection. Maximum pressure elevation was calculated as peak pressure minus initial static pressure.

RESULTS

Maximum pressure elevation averaged (+/-SD) 9.4 +/- 8.5 mm Hg and 6.4 +/- 5.0 mm Hg in the osteoporotic and normal spines, respectively. In all cases (9 of 12) in which the pressure measurement system remained patent (i.e., not occluded by cement), postinjection pressure returned to the initial static pressure.

CONCLUSION

The increase in intravertebral body pressure from cement injection during vertebroplasty is minimal. Alternative means of reducing intervertebral pressure before injection may not be needed.

Mechanical performance of the new posterior spinal implant: effect of materials, connecting plate, and pedicle screw design

STUDY DESIGN

A newly designed spinal implant was tested to evaluate multicycle stiffness and fatigue resistance.

OBJECTIVES

To investigate the effect of different materials, connecting plate, and pedicle screw design on the mechanical performance of the spinal implant.

SUMMARY OF THE BACKGROUND DATA

The addition of cross-linkages did not significantly increase implant compression/flexion stiffness, but accelerated fatigue failure at the rod junctions. Both Ti-6Al-4V spinal implants and the 316L stainless-steel counterparts have been used extensively for clinical cases; however, design factors establishing the proposed superiority of the Ti-6Al-4V implant for fatigue resistance have not, as yet, been extensively studied.

METHODS

Twenty implants with connecting plates (two materials by two screw designs by five implants) and five implants without connecting plates were assembled to UHMWPE blocks and cyclically loaded from 60 N to 600 N at a frequency of 5 Hz.

RESULTS

Failure sites for the tested prototypes were at the cephalic screw hubs or rod-plate junctions. All Ti-6Al-4V implants demonstrated reduced stiffness compared to the structurally identical 316L analogs. The use of connecting plates raised the stiffness of the 316L prototypes without cross-links. However, elimination of the connecting plate avoided stress concentration at the rod/plate junctions and increased fatigue life. The Ti-6Al-4V new system with the minimal notch effect at the screw hubs achieved greater fatigue resistance than its 316L counterpart. By contrast, enlargement of the inner-hub diameter resulted in greater gains for fatigue resistance than for stiffness, especially for Ti-6Al-4V variants.

CONCLUSIONS

Although Ti-6Al-4V was superior to 316L for endurance-limit properties, structural design of the Ti-6Al-4V implant dramatically affects fatigue resistance. This may explain the differences between existing studies and the current report, comparing fatigue life for implants made from these two materials. Our results reveal that Ti-6Al-4V must be carefully treated because of sensitivity to notch, with special consideration given to screw-hub design.

Bone ingrowth fixation of artificial intervertebral disc consisting of bioceramic-coated three-dimensional fabric

STUDY DESIGN

The bone-bonding characteristic of the new artificial intervertebral disc consisting of bioceramic-coated three-dimensional fabric was evaluated mechanically and histologically in an in vivo sheep model.

OBJECTIVES

To investigate the mechanical properties and the histologic appearance of the interface between the three-dimensional fabric disc and the vertebral body, and to evaluate these alterations in vivo under a spinal segmentally mobile condition.

SUMMARY OF BACKGROUND DATA

Bone ingrowth to the bioceramic-coated three-dimensional fabric surface had been demonstrated already under a stable environment in preliminary animal studies.

METHODS

For this study, 20 sheep underwent two-level lumbar intervertebral disc replacement with three-dimensional fabric discs (Group I) or bioceramic spacers as a comparative material (Group II). All operative segments were stabilized temporarily with spinal instrumentation for the initial ingrown phase. Four animals each were killed at 4, 6, 15, and 24 months in Group I and at 6 months in Group II, and the operative segments were subjected to either a detachment test or histologic evaluation.

RESULTS

The interfacial tensile strength at 6 months was significantly higher in Group I than in Group II. No significant decrease in tensile strength was detected until 24 months after surgery in Group I. Histologically, bone ingrowth to the three-dimensional fabric surface was observed 4 months after surgery, and no aseptic loosening occurred until 24 months after surgery.

CONCLUSIONS

The findings show that the three-dimensional fabric disc was firmly fixed to the vertebral body by bone ingrowth, and that this biologic fixation was preserved even under the spinal segmentally mobile condition.

The pylon concept of pelvic anchorage for spinal instrumentation in the human cadaver

STUDY DESIGN

Human cadavera morphometric analysis of the iliac columns and biomechanical implant testing of traditional Galveston technique compared to intrailiac instrumentation of the entire iliac column.

OBJECTIVES

To describe the anatomy of the iliac columns and to evaluate the strength in forward flexion of a large implant spanning the entire column length compared to standard Galveston technique.

SUMMARY OF BACKGROUND DATA

We have observed substantial and straight columns of bone in the pelvis, connecting the acetabula to the sacrum, which may allow for improved spinopelvic instrumentation.

METHODS

Twenty adult cadaveric pelves were used. Each specimen was oriented in the computed tomography scanner to obtain a cross-section of the iliac columns, which begin from 2 cm caudal to the posterior iliac spines and end above the acetabula at the anterior inferior iliac spines. Two different instrumentation techniques were used. Standard Galveston pelvic fixation with paired 6.25-mm diameter rods extending 8 cm into the pelvis (Group 1) was compared to paired 8-mm diameter, 15-cm long custom implants, placed within the length of the entire iliac columns and connected to 6.25-mm spinal rods (Group 2). Both constructs had two rigid cross-links connecting the rods. Testing in forward flexion was performed for each construct with the MTS model 881 at 5 N/sec until failure occurred.

RESULTS

The rectangular shaped iliac columns averaged 15.2 (SD 0.8) cm in length, 2.5 (SD 0.3) cm in width and were consistently straight. The iliac column orientation as viewed in the transverse plane was 22 degrees laterally directed from the midsagittal plane. For the Galveston technique, failure with a flexion force occurred at a mean of 682 (SD 217) N. The iliac column implants failed at a mean of 2153 (SD 1370) N (P < 0.004).

CONCLUSION

The human adult pelvis has substantial and straight columns of bone extending from 2 cm below the posterior iliac spine, traversing above the sciatic notch, and ending at the anterior iliac spine. The shape resembles a weight-bearing long bone such as the tibia. Analogous to the architectural pylon, in this cadaver model, large implant instrumentation of the entire length of these pelvic columns provides at least three times stronger anchorage for spinal instrumentation compared to standard Galveston technique.

The effects of an interspinous implant on intervertebral disc pressures

STUDY DESIGN

Measurement of intradiscal pressure was performed after placement of an interspinous implant in a cadaver model.

OBJECTIVE

To understand the likelihood of accelerated adjacent-level disc degeneration as a result of the implant.

SUMMARY OF BACKGROUND DATA

An interspinous implant has been developed to treat lumbar neurogenic claudication secondary to spinal stenosis that places the stenotic segment in slight flexion and prevents extension. Previous biomechanical studies demonstrated that fusing one level may significantly increase the intradiscal pressures at adjacent levels. Moreover, clinical studies have reported an increased incidence of adjacent-level degeneration after lumbar spinal fusion.

METHODS

Eight cadaver lumbar specimens (L2-L5) were loaded in flexion, neutral, and extension. A pressure transducer measured intradiscal pressure and annular stresses during each of the three positions at each of the three disc levels. An appropriately sized implant was placed at L3-L4, and the pressure measurements were repeated.

RESULTS

The pressures at the adjacent discs were not significantly affected by the interspinous implant insertion. There was a significant decrease in intradiscal pressure at the L3-L4 disc in the posterior annulus and nucleus in the neutral and extended positions.

CONCLUSIONS

The implant does not significantly change the intradiscal pressures at the adjacent levels, yet it significantly unloads the intervertebral disc at the instrumented level in the neutral and extended positions. On the basis of the current findings, it does not appear that the implant causes accelerated disc degeneration at the adjacent levels.

The biologic response to particles from a lumbar disc prosthesis

STUDY DESIGN

Particles of a proprietary polyolefin rubber compound used in a lumbar disc prosthesis were generated in vitro and tested for biocompatibility in two animal models. OBJECTIVE To characterize any tissue response to polyolefin rubber particles.

SUMMARY OF BACKGROUND DATA

Intervertebral disc prostheses are emerging as alternatives to fusion techniques for the treatment of symptomatic disc degeneration. The biocompatibility of all novel components used in the construction of these devices must be verified before they can be considered for general use.

METHODS

Laboratory-generated polyolefin rubber particles were either injected into dorsal subcutaneous air pouches of 30 rats or placed directly onto the lumbosacral dura and nerve roots of 9 sheep. Histologic sections of tissues from, and remote from, the site of implantation were examined for evidence of inflammation and wound-healing responses.

RESULTS

Polyolefin rubber particle debris induced a tissue response that was consistent with a normal foreign body reaction to large particles. The response was not significantly greater than that seen with similar size particles of ultrahigh molecular weight polyethylene. There was no evidence of particle migration from the site of implantation, and there was no evidence of local or systemic toxic effects.

CONCLUSION

Polyolefin rubber particles induce only localized tissue response that is consistent with a normal foreign body reaction to large nontoxic particles.

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

STUDY DESIGN

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

OBJECTIVES

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

SUMMARY OF BACKGROUND DATA

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

METHODS

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

RESULTS

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

CONCLUSION

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

[A light-hardened polymeric material for metal-plastic whole-cast fixed dentures]

Self-hardening material for metal plastic permanent dentures, has been developed by the Central Research Institute of Dentistry for the first time in Russia. Its physicomechanical properties are described and the results of toxicological trials are presented. The new material is superior in wear resistance to foreign analogs Kulzer Dentacolor and ESPE Visio-Gem. Toxicological studies demonstrated the biological inertness of the material during long contact with the internal media of human organism and the absence of toxic, sensitizing, and mutagenic action.

Quantitative and qualitative investigations of surface enlarged titanium and titanium alloy implants

Screw shaped implants of commercially pure (c.p.) titanium and titanium-6aluminum-4vanadium (Ti6A14V) were blasted with particles of TiO2 of mean sizes of 25 microns (Group I) and 75 microns (Group II) and inserted in rabbit bone for 3 months. The surface roughness of the implants was examined and quantified with an optical scanning 3-dimensional instrument (TopScan 3D system), revealing the two alloy surfaces in each group had similar surface roughness. Biomechanical (removal torque) tests showed the c.p. titanium implants to be significantly more stable in the bone bed than those of Ti6A14V. In Group I, the c.p. titanium implants demonstrated a mean removal torque of 38 N cm while the Ti6A14V demonstrated a mean removal torque of 27 N cm (P = 0.004). Group II implants revealed a mean removal torque of 70 N cm for the c.p. ti and 50 N cm for the alloy samples (P = 0.003). The removal torque values were converted to shear forces/strengths by three calculation methods, based on (a) the entire length of the implant surface in the cortical region, (b) the thickness of the cortical bone measured in close vicinity to the thread peaks and (c) the bone-metal contact length measured on the non-unscrewed neighbouring implants. Group I: (a) the c.p. ti implants revealed a mean shear force of 4 vs a mean of 3 N/mm2 for the alloy samples. Shear strengths based on (b); were 8 for c.p. ti vs 6 N/mm2 for the alloy. The mean shear strength/force if calculated according to (c) revealed 23 for c.p. ti vs 18 N/mm2 for the alloy. Corresponding numbers for Group II; (a) c.p. ti 8 compared to 6 N/mm2 for the alloy, (b) c.p. ti demonstrated a mean value of 17 vs 11 N/mm2 for the alloy. According to method (c); c.p. ti had a mean shear strength of 26 vs 22 N/mm2 for the alloy samples. Histomorphometrical comparisons were performed on 10 microns thick undecalcified ground sections in the light microscope. In both Group I and Group II, the calculations of the mean bone-to-metal contact demonstrated more bone in contact to the c.p. titanium implants than to the Ti6A14V ones. Whereas comparisons of the bone volume inside the threads demonstrated slightly higher bone volumes around the alloy samples, no statistically significant difference was obtained between the two materials histomorphometrically.

A histomorphometric evaluation of screw-shaped implants each prepared with two surface roughnesses

Four different surface modifications were designed. Forty screw-shaped implants were divided into 4 groups, 10 screws in each. Every screw was prepared with 2 different surface topographies. The surface topography was measured with a confocal laser scanning profilometer and the surface roughness was characterized using 1 height, 1 spatial and 1 hybrid descriptive parameter. After 12 weeks in rabbit bone all screws were histomorphometrically evaluated. Blasted surfaces demonstrated more bone in contact to implant surface compared with turned surfaces. Most bone in close contact to implant surface was found for a surface blasted with 75 microns sized particles, numerically characterized with an average height deviation (Sa) of 1.4 microns, an average wavelength (Scx) of 11.6 microns and a developed surface area ratio (Sdr) of 1.5.