Biomaterials, Volume 24, Issue 5, February 2003, Pages 701-710
An in vivo study of bone response to implants topographically modified by laser micromachining
Carin Hallgren, , a, Henrik Reimersb, Dinko Chakarovb, Julie Goldb and Ann Wennerberga, c
a Department of Biomaterials/Handicap Research, Institute for Surgical Sciences, Göteborg University, Box 412, 405 30, Göteborg, Sweden
b Department of Applied Physics, Chalmers University of Technology, 412 96, Göteborg, Sweden
c Department of Prosthetic Dentistry/Dental Materials Science, Faculty of Odontology, Göteborg University, Box 450, 405 30, Göteborg, Sweden
Received 17 April 2001; accepted 24 June 2002. ; Available online 9 December 2002.
Abstract
Dental implants topographically modified by laser ablation of periodic arrays of micron-sized craters, were studied in a two-part laboratory investigation. The patterned and control (turned) implants were inserted in rabbit femur and tibia. After 12 weeks the fixation in the bone was evaluated mechanically or by histomorphometry (all threads along the implant and the three best consecutive threads were analysed).
In the pilot study no difference was found with respect to bone-to-implant contact and peak removal torque. Significantly more bone was found for the control implants when measuring the bone area inside the threads in the tibia.
In the second part of the study, the pattern was improved and significantly more bone-to-implant contact was found for the laser-machined implants. The second part of the study also demonstrated significantly greater peak removal torque values in the tibia with the test implants than the control implants.
Author Keywords: Laser machining; Dental titanium implants; Microstructure; In vivo study
Article Outline
1. Introduction
Surface structure is one of several factors proposed to influence the bone response to an implant material [1]. Earlier animal studies [2] have shown that an implant prepared by abrasive blasting to an average height deviation of about 1.5
m, an average spacing of irregularities of 11
m and a developed surface area ratio of 1.5 gave firmer fixation in the bone than smoother turned and blasted implants and even coarser blasted surfaces. In those studies a Gaussian filter sized 50×50
m was used to eliminate errors of form and waviness in the topographical evaluation.
A blasted surface represents a stochastic surface modification and so do other commercial surface modifications [3, 4 and 5] claimed to be of clinical importance especially for low quality bone [6]. However, to investigate the importance of different aspects of surface roughness to bone response (e.g. surface feature dimensions, spacings, etc.), requires a surface with a controlled, non-random structure [7].
The technique most commonly used today in micro-systems technology is photolithography. Photolithography is a technique, which allows the formation and patterning of well-controlled micron-sized features (chemical, topographical), such as in microelectronic circuits. It is relatively easily employed, and is widely used on flat surfaces (e.g. silicon wafers). It was used in a previous study [8] to produce micron-sized features by acid etching on the flanks of threaded dental implants (with adaptations for the special geometry of the screws). However, the photolithography method with acid etching had problems with reproducibility and achieving a distinct hemispherical shape of pits. It is also not suited for large-scale production of screw shaped dental implants.
In the present paper, a laser ablation technique [9] was used to produce controlled, micron-sized, topographical features on the flanks of threaded, dental titanium implants.
The pattern achieved with the laser technique was intended to produce a surface finish having values of average height deviation, average spacing of irregularities, and a developed surface area ratio in the same range as the blasted surfaces showing optimal bone fixation (as mentioned above).
The aim of the present study was to produce and characterise a well controlled surface texture (roughness and waviness) and characterise surface roughness alone on threaded implants: the pattern controlled with respect to height/depth and spacing of the surface irregularities. The fixation of the patterned implants was to be evaluated in the femora and tibiae of rabbits, with conventionally turned screws as control implants.
2. Material and methods
2.1. Implant surface modification
One hundred and twenty commercially pure, titanium, dental implants (Brånemark System®, Nobel Biocare AB, Sweden) were used in this study. The implants were 7 mm long, 3.75 mm in outer diameter and the pitch height was 0.6 mm. Sixty of the implants were left turned (as received from the manufacturer) as controls, and the remaining 60 implants were patterned on the entire flanks, throughout the threaded area, by a laser ablation process.
The screws were taken directly from the sterile package, without any additional preparation prior to laser machining.
The laser micromachining was carried out at 532 nm wavelength irradiation, at a pulse frequency of 10 Hz, and an energy of 3.5 mJ/pulse using a pulsed Nd:YAG laser (Continuum Powerlite 8010). Lenses, mirrors, beam attenuators and an aperture were used for beam shaping and focussing of the 9 mm-diameter beam. The beam was scattered onto the implant using a kinoform [10] that enables advanced beam patterns to be generated by controlled splitting of the beam [11] (see Fig. 1).
Fig. 1. The laser beam is split by the kinoform into an array of 290 spots. The array area covers 30° (1/12) of a full thread turn of a screw.
The design, experimental set-up and assessment of the kinoform (Radians Innova, Sweden) and the machining conditions have been described previously [9 and 12]. The laser beam diffracted by the kinoform produces an array of 290 beam spots with a pitch of 30
m, covering 30° of a full thread turn and the entire flank (see Fig. 1 and Fig. 2). The size of the ablated pits was chosen to be 10
m (ideally a hemisphere with 10-
m diameter, see Fig. 3).
Fig. 2. Left: SEM image of a micropatterned implant. The pattern is projected onto the flanks of the 3.75×7 mm titanium screw. Right: SEM image of the flanks of the screw with the generated pattern. The dark area around the pits is a heat-affected zone (HAZ). During the ablation process, heat is transported to the surroundings results in a thicker oxide layer and high thermal-gradient tension that can cause cracks [28] around the pits.
Fig. 3. Left: Side-view of a laser ablated pit (SEM). The shape of the pit is due to melting during the ablation process. The ablated material is melted and then expulsed out of the pit by gas pressure, forming a ridge around the pit. Right: Material is thrown tens of
m s away from the pit. The grooves on the surface are machining marks from thread turning.
A pilot study was first carried out, then a second study was designed based on indications found in the results from the pilot study. In the pilot study, the pattern produced on the implants differed somewhat between the implants. Therefore, the implants were divided into five groups depending on the pattern quality. Each implant, the group it belonged to, and its insertion site was recorded to enable information on possible influence of different pattern qualities on bone formation.
The difference between the two studies was an improvement in the laser machining via changes in the optical set-up in the second study. During the processing of the first round of screws, it was realised that the method showed instability when machining multiple samples (see Fig. 4). This was improved by using an optic-mount rail (assuring linearity) and using an in situ stereomicroscope to monitor the process. The microscope was also used to ensure that two pattern arrays never overlapped. To further ensure that the pattern was placed correctly on the flank, the pattern incidence was tested before machining with reduced intensity by decreasing the aperture diameter to minimum (2.5 mm).
Fig. 4. Left: Overview of processed implant from the pilot study. Middle and right: SEM images from in situ AES. The digits 1 and 2 indicate the centres of the two 200-
m diameter investigation spots. Middle: An implant from the second study. The placement of the pattern is good with respect to distance from bottom and top of flank. The pattern is also well focussed over the entire array. Right: An implant from the pilot study. Note the incomplete pattern coverage of the flank compared to the implant in the left picture.
After patterning the screws were sequentially cleaned in successive ultra-sonic baths of methanol, acetone (both of PA quality), EXTRAN MA-01 (Merck, a strong water-based tenside) diluted with ultra pure (MilliQ, 18 M
) water (2.5%), and finally pure MilliQ-water.
2.2. Surface composition analysis
Surface composition was assessed by auger electron spectroscopy (AES) with a scanning auger microprobe (SAM PHI 660, Perkin-Elmer). Two screws from the control implants (one in each study) and 15 implants (six in the pilot study and nine in the second study) with patterned surfaces were analysed. The AES parameters were: 3.0 keV electron energy, ca. 350-400 nA beam current and 200
m beam diameter. Each screw was analysed in two spots, including several ablation pits each, on the second thread flank (see Fig. 4). The sample was tilted 30°, so that the electron beam had perpendicular incidence onto the surface of the flank.
2.3. Surface topographical characterisation
A total of 30 screws were analysed with a confocal laser scanning profilometer, TopScan 3DTM (Heidelberg Instruments GmbH, Heidelberg, Germany), which has a maximum horizontal resolution of 0.5
m and a maximum vertical resolution of 0.6 nm, properties that make it suitable to measure the surface modification in the present study. This instrument has previously been described [13, 14 and 15]. Eighteen implants were analysed before implant insertion and 12 after retrieval of the implants used for peak removal torque analysis. Since all of the implants were received from the manufacturer at the same time, three control implants were measured for the pilot study and the topographical values measured were also used for the second study. In the second study, a more consistent pattern was achieved and only three patterned implants were measured and characterised as recommended by Wennerberg and Albrektsson [16]. Three thread tops, three valleys and three flanks were analysed on each implant. Each area measured was 245×245
m, consisting of a series of line scans, where the distance between each line scan was 3
m. Data evaluation included a Gaussian high pass filter to separate form, waviness and roughness from each other. The filtering was carried out in the frequency domain. A Gaussian filter, size 50×50
m was used to exclude shape and waviness from the roughness evaluation, and a filter size 250×250
m was chosen to include the waviness but remove errors of form. Three surface roughness parameters were determined:
Sa—the arithmetic mean of the absolute values of the surface departures from a mean plane within the sampling area, measured in
m,
Scx—the average mean spacing in the x-direction of profile irregularities which cross the mean plane, measured in
m,
Sdr—the ratio of the developed surface area from a flat reference area.
Digital images from the measurements were recorded to enable the visual description of the surface structure. Further qualitative characterisation was recorded by scanning electron microscopy (SEM) (JEOL JSM-6301F).
Of the implants in the best group in the pilot study the surface roughness was analysed in six pits and six areas between the pits of five flanks; a total of 30 measurements of pits and 30 measurement between the pits. These analyses were performed on measurements made by the confocal laser scanning profilometer and analysed using a Gaussian filter sized 50×50
m.
The implants removed during peak removal torque analysis for post-implantation topographical analysis (three patterned and three control implants from the femur, and three patterned and three control implants from the tibia) were immersed in an EDTA-solution until any traces of bone left on the surface were easily removed. Test and control implant came from the same rabbit. The implants were then cleaned in Extran MA01 diluted with deionised water in an ultrasonic bath, rinsed twice in deionised water and were ultrasonicated in deionised water, rinsed twice in absolute ethanol and ultrasonically cleaned in absolute ethanol. Then the implants were analysed by a confocal laser scanning profilometer.
2.4. Animal and anaesthesia
In each of the pilot and second studies, 10 adult New Zealand White rabbits were used. In the pilot study they were all females and in the second study they were all males. The rabbits in the pilot study were housed communally in a large open room but places to hide, while in the second study the rabbits were kept in separate cages. They had free access to tap water and were fed with a standard pellet diet. The surgery was performed under aseptic conditions. Surgical anaesthesia consisted of an intra muscular injection of 0.9 ml fentanyl citrat 0.315 mg/ml and fluanisone 10 mg/ml (Hypnorm®, Janssen-Cilag Ltd., Saunderton, High Wycombe, Buckinghamshire, England), and an intra peritoneal injection of 0.5 ml Diazepam 5 mg/ml (Stesolid Novum®, A/S Dumex, Denmark). If necessary during surgery, additional doses of Hypnorm® and Stesolid Novum® were administered.
The hind legs were shaved and washed with a solution of ethanol 70% and iodine solution and the feet were wrapped in a bandage. At each insertion site 1 ml of lidocain 10 mg/ml (Xylocain®, Astra, Sweden) was infiltrated. Prophylactic antibiotics were administrated as a 0.75 ml intramuscular injection of Penovet®vet 300 mg/ml (Boehringer Ingelheim Agrovet A/S, Hellerup, Denmark) at the day of surgery and the two following days. Analgesia by Buprenorphinum 0.3 mg/ml (Temgesic®, Reckit and Colman, England) was given after surgery and if necessary the following 2 days. Fifteen milliliters of saline at a concentration of 9 mg/ml was delivered subcutaneously after the surgery to prevent desiccation. After 12 weeks, at sacrifice, the rabbits were anaesthetised as described above and received an overdose of pentobarbitalum natricum (Apoteksbolaget, Malmö, Sweden) intravenously.
2.5. Surgical technique and implant insertion
The surgical technique, animal care and performance of resonance frequency analysis (RFA) were identical in the pilot and the second study. Therefore, these sections are only described once. Before insertion, all test implants were autoclaved. The control implants were supplied sterile from the manufacturer. Three implants were inserted through the first cortical layer in each hind leg; one in the distal femur and two in the proximal tibia. Half of the rabbits received the laser machined implants in the right leg and the control implants in the left leg. The other half of the rabbits got the control implants in the right leg and the laser machined implants in the left leg. The implantation holes were drilled with low speed, profuse saline irrigation, and successively increased drill diameters. Finally, the holes were pre-tapped.
2.6. Resonance frequency analysis
RFA is a non-invasive test method designed to make quantitative measurements of the implant stability as a function of the bone/implant interface as described by Meredith [17]. In brief, a small transducer was attached to the implant. The transducer comprised a modified cantilever beam, to which Piezoceramic elements had been attached to excite the beam and measure its response.
RFA measurements were performed using a prototype equipment. RFA was performed on all implants immediately after insertion and after sacrifice. Each RFA measurement resulted in a resonance frequency value and a frequency/amplitude plot, which was saved in a computer. When retrospectively analysing all graphs, only measurements with one clear peak were accepted and the rest were excluded.
Insertion values were compared to the values at sacrifice, each implant site separately.
2.7. Peak removal torque
The peak torque necessary to loosen the implant from the bone bed was measured with a purpose-designed electrical torque transducer [18]: removal torque being expressed in Newton centimetre (N cm).
In the pilot study, the proximal implant in the tibiae were submitted for removal torque evaluation immediately after sacrifice. In the second study, implants in the femur as well as the proximal implant in the tibiae were evaluated for peak removal torque.
2.8. Preparation of specimens and histomorphometric evaluation
The femoral and distal tibial implants were used for histomorphometrical evaluations in the pilot study. In the second study the distal tibial implants alone were used for histomorphometrical analyses. After sacrifice the implants with their surrounding tissue were removed en bloc and fixed in 4% neutral buffered formaldehyde. The samples were then processed to be embedded in a light curing resin (Technovit® 7200 VLC Heraeus Kulzer GmbH, Germany). Cutting and grinding were performed as described by Donath [19] until a specimen thickness of approximately one cell layer was achieved. The ground sections were stained with toluidine-blue mixed with pyronin G. Histomorphometric evaluation was performed using a Leitz Aristoplan light microscope equipped with a Leitz Microvid unit connected to a computer. Bone-to-implant contact and bone area inside the threads were measured. Measurements were made along the whole sides of the implants and also just the three best consecutive threads [20]. The calculations used 10× magnification objective and an eye piece of 10× magnification. Calculations of means for each implant, followed by computation of group means and standard deviations were carried out for all measurements.
2.9. Statistical methods
The Wilcoxon signed rank test was used for the peak removal torque and histomorphometric evaluation with p
0.05 as level of statistical significance.
Student's t-test was used when comparing the surface topography.
3. Results
In the second study one rabbit had to be sacrificed due to a broken leg, 1 week after surgery, so only nine rabbits were included in the evaluation.
3.1. Micropattern surface composition
The AES data showed no great differences between processed samples and controls for the pilot and the second study (see Table 1). For both studies, the carbon content tended to be somewhat higher for processed implants than for controls but this did not achieve statistical significance. Typical trace elements found in titanium dental implant materials were also detected on the implant surfaces [21].
Table 1. Surface chemical composition of the implants used in this study
The data is both studies, and from respective controls.
3.2. Surface topography characterisation
Surface topographic analysis showed that implants used in the second study had a much more successful pattern placement and beams ablated pits with the anticipated topography. The array of pits was centred on the slope of the flank and the individual pits had all the same and distinct crater shape and desired diameter. The pits were approximately 8
m deep, 19
m in diameter (including the ridge) and the space between the pits was 20
m (pit edge to pit edge). The pits had a near hemispherical shape in the second study, while for the pilot study the pits had more often an elongated, elliptic and narrow shape. The pits in the pilot study were approximately 7
m deep, 11
m in diameter (including the ridge) and the space between the pits was 8
m (pit edge to pit edge), on average.
The topographical description of implants measured before insertion with a 50×50
m Gaussian filter showed that control implants were smoother than the laser machined in both studies, when evaluated over the entire implant or just the flanks, evidenced by the surface roughness parameters used. In the second study, comparing just the flanks of control and test implants, the patterned implants had significantly higher values for all the three parameters used p<0.001. In the second study, when analysing the patterned flanks, the 250×250
m Gaussian filter returned higher mean values for both Sa and Scx values than that were measured with the 50×50
m filter. With the 250×250
m filter in the second study the flanks of the test implants had a significantly higher Sa value (p=0.0009) than the flanks of the control implants. However, due to the fact that longer wavelengths were now included in the evaluation, the parameter Sdr of the flanks was significantly higher (p<0.001) for the patterned implants than the controls.
The measurements of the pits in the best group in the pilot study showed that the pits had an Sa-value of 2.79
m (SD=0.62), Scx-value of 1.60
m (SD=0.84) and an Sdr-value of 2.23 (SD=0.31). The areas between the pits had a Sa-value of 0.86
m (SD=0.24), Scx-value of 3.03
m (SD=2.45) and an Sdr-value of 1.29 (SD=0.13).
The screws measured after sacrifice and harvest were of similar roughness to the pre-insertion values.
The mean values from the TopScan 3D measurements are presented in Table 2, Table 3, Table 4 and Table 5.
Table 2. Second study, before insertion. Evaluation of surface roughness
Numerical values and SDs from the roughness analysis of three implants. Three tops, three valleys and three flanks have been measured on each analysed screw. A Gaussian filter size 50×50
m was used.
SD: Standard deviation.
Table 3. Second study
Numerical values and SDs from the roughness analysis of the flanks of three pattered and three control implants. Three flanks have been measured on each analysed screw. A Gaussian filter size 250×250
m was used.
SD: Standard deviation.
Table 4. Second study. Evaluation of the surface roughness on the retrieved implants
After evaluation by removal torque and after removal of bone and cleaning. A Gaussian filter size 50×50
m was used.
SD: Standard deviation.
Table 5. Second study. Evaluation of surface roughness and waviness on the retrieved implants
After evaluation by removal torque and after removal of bone and cleaning. Gaussian filter size 250×250
m was used.
SD: Standard deviation.
3.3. Resonance frequency
3.3.1. Pilot study
Comparing insertion and end-point RFA values end-point values were significantly greater for the control femoral implants (p=0.009), the proximal tibial (patterned, p=0.007; control, p=0.03) and the distal tibial implants (pattered, p=0.02; control p=0.04). The laser treated implants in the femur tended to higher end-point values, but did not attain statistical significance.
3.3.2. Second study
Due to measurement errors, one rabbit with laser treated implants in the femur and two rabbits with control implants in the femur were excluded in the RFA. There were significantly higher end-point values than insertion values, for the laser treated implants in the femur (p=0.05), for the control (p=0.01) and test (p=0.04) implants in the proximal tibia and for the distal, tibial control implants (p=0.02). There was no statistically significant difference between insertion and end-point values for the control femoral implants or the distal, tibial test implants but the mean values at end-point tended to be higher (Fig. 5).
Fig. 5. Mean values of the RFA (second study) at surgery and sacrifice. The bars show the standard deviation.
3.4. Removal torque
3.4.1. Pilot study
There was no statistically significant difference between test implants with a mean value of 43 N cm, standard deviation (SD) 9 N cm and control implants (mean=45 N cm, SD=12 N cm) in the pilot study.
3.4.2. Second study
Peak removal torque values were not statistically different in the femur (means=63 N cm, SD=13 N cm for the test; MEAN=55 N cm, SD=15 N cm for the control; p=0.23), but were in the tibia (mean=52 N cm, SD=10 N cm for the test; MEAN=35 N cm, SD=15 N cm for the control; p=0.05).
3.5. Histomorphometry
3.5.1. Pilot study
No statistically significant difference was found when measuring the bone-to-implant contact for all threads along the implant or only the three best consecutive threads. Tibial control implants had significantly more bone area inside the threads, whether all threads (p=0.007) or only the three best consecutive threads (p=0.02) were evaluated. All mean values and SDs are presented in Table 6.
Table 6. Pilot study. Histomorphometry results
Presented is the mean value from the mean value of each implant.
Mean (SD), %; BIC=bone-to-implant contact; Area=bone area inside the threads; SD=standard deviation.
3.5.2. Second study
Patterned test implants showed significantly greater bone-to-implant contact for all threads (p=0.01), and the three best consecutive threads (p=0.02). There were no statistically significant differences when comparing the bone area in the threads, but the mean values were higher for the control implants (Table 7, Fig. 6 and Fig. 7).
Table 7. Second study. Histomorphometry results
Mean (SD), %; BIC: bone-to-implant contact; Area: bone area inside the threads; SD: standard deviation.
Fig. 6. Micrograph of a ground section on a patterned implant. The distance between the thread tops is 0.6 mm.
Fig. 7. Micrograph of a ground section on a control implant. The distance between the thread tops is 0.6 mm.
4. Discussion
Laser machining has here been demonstrated to be a better method to micropattern threaded dental implants than photolithography as described by Hallgren et al. [8]. It is in principle a quicker method, cleaner, and easier to adapt to implants with complex 3D geometries. Problems encountered during laser machining of the threaded dental implants were the need for specialised sample holders and sample manipulators, and a method to rapidly machine the array of pits over the entire threaded area of the implants. In order to simplify the process of patterning the thread flanks, our approach was to focus the laser pattern at one point on the thread and then rotate the threaded implant until the entire thread was patterned. However, this process relies on finding the proper axis of rotation used during machining of the threads at the manufacturer in order to maintain alignment and focus on the thread flank. We used the axis of a threaded abutment hole, which we assumed to be centred with the outer threads of the implant. However, the two were not centred. The variation in centre between the internally threaded abutment hole and the external threads meant that pattern placement was too high or too low or even out of focus on the flanks as the implant was rotated during laser micromachining (See Fig. 4). This resulted in a lower yield than expected of high quality samples for the pilot study. A stereo microscope was installed to survey the machining process, which enabled much better control of the pattern placement and improved implant surface quality in the second study. The pattern placement was tested intermittently, with reduced beam energy (below ablation threshold) by decreasing aperture diameter, and could be adjusted if necessary. However, the machining process was slowed significantly. The optimal solution to the centre of rotation problem is to laser micromachine the implants on the same tool as they are turned during the manufacturing process.
SEM images of the screws showed that the flank area on the screws from the second study were more successfully covered with pits and had improved quality of individual pits compared to screws from the pilot study (see Fig. 4).
However, in general in the second study higher standard deviations were found for Sa values, which could be because the flanks were better covered by patterning in the second study. Presumably, a patterned area has greater variations in roughness than a non-patterned area.
Furthermore, a turned surface has a maximum peak-to-valley height (St) of about 10-15
m [22], i.e. slightly more than the depth of the laser machined pits, so superposition of a pit on a machined ridge removes the high machined ridge and replaces it with a milder depression. Surface composition analysis showed expected results when taking into account the conditions of surface treatment, sterilisation and storage. Normally, for a titanium surface in ambient atmosphere, the surface is covered with a thin (~100 Ĺ) layer of titanium oxides of different stoichiometries and this explains the high oxygen content on the surface. The carbon content was somewhat higher for laser machined implants than for controls, but this was not statistically significant. Possible reasons for this could be residues from the cleaning procedure with organic solvents and surfactants, and the fact that the micropatterned samples have been exposed longer to ambient atmosphere.
For nanosecond and longer pulse lengths of laser light interacting with titanium, ablation occurs by ejection of melted substrate material. The extreme local conditions, such as large temperature gradients and plasma formation of evaporated material, cause expulsion of melted metal cracks on the surface [23]. The process leaves a ridge of re-deposited material around the pit [23].
The microtopography of the laser machined implants can be altered by changing, for example the ablated pit size and depth, which is controlled by the pulse energy and/or the number of pulses during laser ablation. Furthermore, spacing and shape of the pits could be changed by using a different kinoform designed to generate a different pattern. The kinoform method needs improvements through stability of the photoresist relief pattern on the kinoform itself (i.e. the diffraction grating). This can be achieved by etching the relief into the glass or quartz substrate. The need for automation of laser micromachining is obvious if this patterning method should be employed to routinely make large number of samples, e.g. industrial scale manufacturing of dental implants. Automated laser systems have previously been used for patterning via ablation of self-assembled monolayers (SAM) and other biomolecular coatings [24]. A basic set-up for producing the surfaces used in this study, would include a motorised, computer controlled sample stage, an electronic shutter for the laser beam, and an autofocus device to ensure good focus when moving along a complex 3D surface. It is also possible to ablate with several laser set-ups simultaneously, thus speeding up the process.
The pilot study demonstrated no difference between peak removal torque or bone-to-implant contact for the test or control implants. More bone was found for the control implants inside the threaded area than for the patterned screws. However, in the pilot study the pattern was not identical on all screws. When analysing the different screws and selecting only those with an optimal pattern, there was firmer fixation in the bone bed than the controls. This was why the study was repeated with implants with improved surface pattern. The second study demonstrated higher peak removal torques for the patterned implants; significantly higher in the tibia but in the femur the difference was not statistically significant. Also bone-to-implant contact in the second study was significantly greater for the patterned implants, but bone area inside the threads were lower than for the controls without attaining statistical significance. The difference seen in the bone-to-implant contact but no difference of the amount of bone in the thread area is not fully understood and in several other studies it has been seen a lack of positive correlation [25 and 26] between the bone-to-implant contact and bone area inside the thread. It may be speculated if this finding is due to a response from the bone due to biomechanical stimulus. Vercaigne [27] discussed the possibility that less firmly fixed implants could result in a persistent stimulation of the surrounding bone resulting in more bone around the implants.
In conclusion, the pattern produced in the second study was controlled with respect to height/depth and spacing of the surface irregularities, and was well placed on the flanks. The implants with this pattern gave a firmer fixation in the bone bed than the controls, indicating the importance of both spatial and height dimension of surface topography for implant incorporation in bone. Further investigations are needed to find which of these factors are most important for an optimal surface structure in the bone and also to develop a method to produce the optimal structure on threaded implants, with a high reproducibility, in a large scale.
Acknowledgements
This study has been supported by grants from the Swedish Foundation for Strategic Research, the Wilhelm and Martina Lundgren Foundation, the Hjalmar Svensson Foundation and the Royal Society of Arts and Sciences in Göteborg.
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