JBJS | Biomechanical Study of Screws in the Lateral Masses: Variables Affecting Pull-out Resistance*

The Journal of Bone & Joint Surgery, Volume 78, Issue 9

Articles | September 01, 1996

Biomechanical Study of Screws in the Lateral Masses: Variables Affecting Pull-out Resistance*

JOHN G. HELLER, M.D.†; BRADLEY T. ESTES, M.S.†, DECATUR, GEORGIA; MOUNIR ZAOUALI, PH.D.‡, RANG-DU-FLIERS; AMADOU DIOP, PH.D.§, PARIS, FRANCE

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Investigation performed at the Department of Biomechanical Engineering, École Nationale Supérieure des Arts et Métiers, Paris

The Journal of Bone & Joint Surgery. 1996; 78:1315-21

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Abstract

The purpose of this study was to investigate the effects of the design of the screw, the depth of insertion, the vertebral level, and the quality of the host bone on the pull-out resistance of screws used in the lateral masses. The study included twelve fresh cervical spines from human cadavera. Radiographs were made of each specimen to ensure the absence of defects, and then the cancellous-bone density of the vertebral bodies was measured at each level with quantitative computed tomography scanning. Six commercially available screws of various diameters and thread configurations (2.7, 3.2, 3.5, and 4.5-millimeter cortical-bone screws; a 3.5-millimeter cancellous-bone screw; and a 3.5-millimeter self-tapping screw) that are currently used for fixation of the cervical lateral masses were tested for axial load to failure. A twelve-by-twelve Latin square design was used to randomize the screws with regard to level (second through seventh cervical vertebrae), side (right and left), and depth of insertion (unicortical or bicortical purchase). Each screw was then subjected to uniaxial load to failure. The data were analyzed to determine if the diameter of the screw, the thread configuration, the number of cortices engaged, the cervical level, or the bone density was associated with the load to failure.Three major subgroups (greatest, intermediate, and lowest pull-out resistance) were identified. The subgroup with the greatest pull-out resistance included only screws with bicortical purchase; the 3.2, 3.5, and 4.5-millimeter cortical-bone screws and the 3.5-millimeter cancellous-bone screw were in this subgroup. Regardless of the thread configuration, no screw with unicortical purchase was in the group with the greatest pull-out resistance. Two of the three values in the subgroup with the lowest pull-out resistance were for the 3.5-millimeter self-tapping screw (with unicortical or bicortical purchase). The cancellous-bone density of the vertebral body was not associated with pull-out resistance and it did not vary significantly according to the cervical level, with the numbers available. However, the pull-out resistance of the screws varied significantly (p = 0.004) by level: it was the greatest at the fourth cervical level, decreasing cephalad and caudad to that level.CLINICAL RELEVANCE: Posterior plate fixation of the cervical spine with screws inserted into the lateral masses may be appropriate in certain circumstances. Previous studies on cadavera and clinical experience have attested to the potential pitfalls of fixation with screws in the lateral masses. Our data suggest that the surgeon should consider not only the type and size of the screws but also whether unicortical or bicortical purchase should be achieved. Bicortical purchase engenders a greater risk of injury to local anatomical structures, but this may be an acceptable compromise at the cephalad and caudad regions of the cervical spine, where the purchase of screws is relatively weaker.

Figures in this Article

Introduction

Introduction | Materials and Methods | Results | Discussion | References

The clinical use of posterior plate fixation of the cervical spine has been reported with increasing frequency^3,4,7,22. As with the evolution of pedicle-screw fixation, clinical experience has progressed more rapidly than has the understanding of the basic science underlying these procedures. The potential anatomical risks associated with screws inserted in the lateral masses have been explored in spines from cadavera^1,10-12. The biomechanical properties of plate fixation have been compared with those of wire fixation in in vitro studies of a limited group of instability patterns of the cervical spine^9,23. There is little available information concerning the variables that influence the mechanical failure of posterior plate constructs in the cervical spine⁶.

We are not aware of any clinical series in which a plate fixed with screws in the lateral masses broke. The few failures of such fixation have resulted from either the surgeon's error or failure at the bone-screw interface^3,4,7,22. The literature on pedicle-screw fixation attests to the importance of the bone-screw interface as the weak link in spinal fixation^14,25,26, as it is in other types of bone-implant fixation in humans. The variables that theoretically affect the purchase of pedicle screws have been investigated; resistance to failure has been clearly associated with the depth of insertion and the major (outer) diameter of the screw^14,16,26. The contribution of the minor (core) diameter to pull-out resistance is less predictable. In contrast to the data for fixation of long bones⁵, the thread pitch and configuration have no apparent influence on the load to failure of pedicle screws.

The variability of the host bone must also be considered. Not only does the quality of bone vary among individuals, but it may also vary among vertebral levels in the same person. The association between bone density and bone strength has been well established for lumbar vertebral bodies^2,8,17,19. Additionally, in vitro studies have shown that the resistance to failure of pedicle screws is proportional to the vertebral bone density^25,26. Osteopenia is an established risk factor associated with the failure of pedicle screws.

Whether the purchase of a screw in the lateral mass is affected in the same way and by the same variables as that of a pedicle screw elsewhere in the spine is a matter of conjecture. A screw in the lateral mass may engage one or both cortices and a highly variable amount of cancellous bone in between¹. Information about the bone quality of both the cervical vertebral bodies and the lateral masses is lacking. Therefore, the influence of the quality of the host bone on the purchase of a screw in the lateral mass can only be inferred from the data on pedicle screws. Also, to our knowledge, no data are available regarding the relationship between the design of the screw and its pull-out resistance after insertion into the lateral mass. The present study was undertaken to investigate the effects of the design of the screw, the depth of insertion, the vertebral level, and the quality of the host bone on the pull-out resistance of screws in the lateral masses.

*Although none of the authors has received or will receive benefits for personal or professional use from a commercial party related directly or indirectly to the subject of this article, benefits have been or will be received, but are directed solely to a research fund, foundation, educational institution, or other non-profit organization with which one or more of the authors is associated. Funds were received in total or partial support of the research or clinical study presented in this article. The funding source was Sofamor Danek, Memphis, Tennessee.

†The Emory Clinic Spine Center, 2165 North Decatur Road, Decatur, Georgia 30033.

‡Sofamor Danek Europe, B.P. 4, 62180 Rang-du-Fliers, France.

§Department of Biomechanical Engineering, École Nationale Supérieure des Arts et Métiers, 151, Boulevard de L'Hôpital, 75013 Paris, France.

†The Emory Clinic Spine Center, 2165 North Decatur Road, Decatur, Georgia 30033.

‡Sofamor Danek Europe, B.P. 4, 62180 Rang-du-Fliers, France.

§Department of Biomechanical Engineering, École Nationale Supérieure des Arts et Métiers, 151, Boulevard de L'Hôpital, 75013 Paris, France.

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Computed tomography scan of a specimen, demonstrating the region that was sampled to determine the bone density of the vertebral body. The mean density at each cervical level was calculated from three separate determinations.

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Schematic axial (left) and lateral (right) drawings of a cervical vertebra with a screw in the lateral mass. The arrows indicate that the applied load is along the axis of the screw.

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TABLE I TYPES OF SCREWS, GROUPED ACCORDING TO SIGNIFICANT DIFFERENCES IN PULL-OUT RESISTANCE

*Twelve specimens were tested with each type of screw, except for the 4.5-millimeter cortical-bone screw with bicortical purchase (eleven specimens) and the 3.5-millimeter self-tapping screw with bicortical purchase (five specimens).

		Axial Load to Failure (N)
Type of Screw*	Type of Purchase	Least Squares Mean	Mean and Stand. Dev.
Greatest pull-out resistance
3.5-mm cortical-bone	Bicortical	426	448 ± 253
3.2-mm cortical-bone	Bicortical	371	371 ± 155
3.5-mm cancellous-bone	Bicortical	368	368 ± 203
4.5-mm cortical-bone	Bicortical	367	369 ± 158

Intermediate pull-out resistance
2.7-mm cortical-bone	Bicortical	321	321 ± 223
3.5-mm cortical-bone	Unicortical	319	289 ± 231
3.2-mm cortical-bone	Unicortical	300	305 ± 207
3.5-mm cortical-bone	Unicortical	277	277 ± 126
2.7-mm cortical-bone	Unicortical	274	344 ± 383

Lowest pull-out resistance
3.5-mm self-tapping	Bicortical	244	236 ± 124
4.5-mm cortical-bone	Unicortical	239	231 ± 134
3.5-mm self-tapping	Unicortical	223	216 ± 166

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TABLE I TYPES OF SCREWS, GROUPED ACCORDING TO SIGNIFICANT DIFFERENCES IN PULL-OUT RESISTANCE

		Axial Load to Failure (N)
Type of Screw*	Type of Purchase	Least Squares Mean	Mean and Stand. Dev.
Greatest pull-out resistance
3.5-mm cortical-bone	Bicortical	426	448 ± 253
3.2-mm cortical-bone	Bicortical	371	371 ± 155
3.5-mm cancellous-bone	Bicortical	368	368 ± 203
4.5-mm cortical-bone	Bicortical	367	369 ± 158

Intermediate pull-out resistance
2.7-mm cortical-bone	Bicortical	321	321 ± 223
3.5-mm cortical-bone	Unicortical	319	289 ± 231
3.2-mm cortical-bone	Unicortical	300	305 ± 207
3.5-mm cortical-bone	Unicortical	277	277 ± 126
2.7-mm cortical-bone	Unicortical	274	344 ± 383

Lowest pull-out resistance
3.5-mm self-tapping	Bicortical	244	236 ± 124
4.5-mm cortical-bone	Unicortical	239	231 ± 134
3.5-mm self-tapping	Unicortical	223	216 ± 166

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TABLE II PULL-OUT RESISTANCE ACCORDING TO CERVICAL LEVEL

		Axial Load to Failure (N)
Level	No. of Screws	Least Squares Mean	Mean and Stand. Dev.
C2	22	278	365 ± 227
C3	24	355	290 ± 184
C4	24	391	286 ± 191
C5	23	313	309 ± 163
C6	22	262	288 ± 162
C7	21	266	381 ± 331

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TABLE II PULL-OUT RESISTANCE ACCORDING TO CERVICAL LEVEL

		Axial Load to Failure (N)
Level	No. of Screws	Least Squares Mean	Mean and Stand. Dev.
C2	22	278	365 ± 227
C3	24	355	290 ± 184
C4	24	391	286 ± 191
C5	23	313	309 ± 163
C6	22	262	288 ± 162
C7	21	266	381 ± 331

Materials and Methods

Introduction | Materials and Methods | Results | Discussion | References

Twelve fresh cervical spines from human cadavera were frozen in hermetically sealed plastic bags. Before testing, the specimens were thawed to room temperature and radiographs were made to ensure the absence of structural lesions that might affect the results of the investigation. A computed tomography scanner (model CGR.CE 12000; General Electric, Milwaukee, Wisconsin) was used to make images of each specimen at 1.5-millimeter intervals. This technique provided a more precise method of screening out possible defects and enabled us to perform quantitative bone densitometry (Fig. 1). The bone density (in Hounsfield units) of the vertebral body and the cancellous region of each lateral mass was determined, and the mean at each level was calculated from three separate determinations. The scanner was recalibrated with the same phantom before each scanning session. The entire study was performed within one month in order to minimize a so-called drift in the Hounsfield units².

Six different commercially available screws were used: the 2.7, 3.2 (so-called emergency), 3.5, and 4.5-millimeter cortical-bone screws (all from Synthes USA, Paoli, Pennsylvania); a 3.5-millimeter cancellous-bone screw (Synthes USA); and a 3.5-millimeter self-tapping bone screw (American Medical Electronics, Richardson, Texas). These sizes of screws spanned the practical range of the major diameters for screws used in the lateral masses and provided variation in thread configuration. Each screw was inserted according to the manufacturer's recommendations—that is, a drill and tap of appropriate size were used for each screw. Holes were not tapped before the self-tapping screws were inserted. In order to minimize variation in technique, the same one of us (B. T. E.) inserted all of the screws, according to the recommendations of An et al. The starting point was one millimeter medial to the center of the lateral mass. The drilling was done with the trajectory that was necessary for the drill-bit to exit at the junction of the lateral mass and the transverse process. Screws were inserted in both lateral masses of the second through the seventh cervical vertebrae.

A twelve-by-twelve Latin square design was used to randomize each type of screw according to the vertebral level, unicortical and bicortical purchase, and right and left sides. Bicortical purchase was defined as protrusion of the tip of the screw far enough beyond the anterior cortex such that this second cortex was fully engaged. For unicortical purchase, the tip of the screw was advanced as far as the inner edge of the second cortex, to engage as much cancellous bone as possible.

A special jig was designed to grip each disarticulated vertebra firmly. The head of each screw was then grasped with a slotted washer at the end of a cable and pulley; this allowed an axial pull-out force to be applied along the long axis of each screw. An axial load to failure was applied with use of a materials testing device (model 1185; Instron, Canton, Massachusetts) at a displacement rate of two millimeters per minute (Fig. 2).

Statistical analysis of the data was performed with use of a software package (Statistical Analysis Systems; SAS Institute, Cary, North Carolina) in which an analysis of variance was based on the twelve-by-twelve Latin square design. A Tukey pairwise comparison was then performed at the alpha = 0.05 level of significance to address specific comparisons when over-all significance was found. Significance was defined as a value of p = 0.05. Linear regression analysis was used to investigate the relationship between vertebral bone density and axial load to failure.

Results

Introduction | Materials and Methods | Results | Discussion | References

The axial loads to failure are reported as the least squares means—that is, the means were weighted according to the size of the sample. This was done to account for some inequality in the number of specimens because variations in local anatomy prohibited or modified the insertion of a few screws. One 4.5-millimeter cortical-bone screw could not be inserted, and seven 3.5-millimeter self-tapping screws, which are manufactured in only one length, were not long enough to engage the anterior cortex.

Testing revealed three significantly different subgroups (p < 0.05) (Table I). The subgroup with the greatest pull-out resistance included the 3.2, 3.5, and 4.5-millimeter cortical-bone screws as well as the 3.5-millimeter cancellous-bone screw, all with bicortical purchase. The 4.5-millimeter cortical-bone screw with unicortical purchase and the 3.5-millimeter self-tapping screw with either unicortical or bicortical purchase had the lowest pull-out resistance (Table I).

The least squares mean for the load to failure was 272 newtons for the screws with unicortical purchase and 350 newtons for the screws with bicortical purchase; this difference was significant (p = 0.001). The pull-out resistances of the screws inserted in the right lateral masses were equivalent to those of the screws inserted in the left lateral masses; however, the pull-out resistance differed significantly (p = 0.004) among the cervical levels (Table II). The greatest pull-out resistance (load to failure, 391 newtons) was found at the fourth cervical level, and two of the lowest pull-out resistances were found at the end vertebrae (load to failure, 278 newtons at the second cervical level and 266 newtons at the seventh cervical level).

The hypothesis that all of the levels had the same bone density was rejected (p = 0.03). The mean bone density of the seventh cervical vertebral body (295 Hounsfield units) was significantly different (p < 0.05) from that at the other levels. With the numbers available, no significant differences were found among the bone densities at the second through the sixth cervical levels (range, 350 to 406 Hounsfield units).

Linear regression analysis was performed for the load to failure for each type of screw, unicortical or bicortical purchase, and the bone density of the vertebral body. No significant association was found between the pull-out resistance and the bone density of either the vertebral body or the cancellous bone of the lateral mass.

Discussion

Introduction | Materials and Methods | Results | Discussion | References

This study was undertaken to investigate many of the variables that theoretically influence the purchase of screws in the lateral masses. We studied the variations in the design and dimensions of the screws, the depth of insertion (unicortical or bicortical), the quality of the host bone, and the vertebral levels. The identification of important factors related to the pull-out resistance of screws in the lateral masses could theoretically lead to the optimization of the design of screws and the techniques for their insertion. Also, determining the predictors of failure of the screw might permit better selection of patients or suggest a need for more restrictive bracing postoperatively. These data are available for pedicle-screw fixation in the thoracolumbar spine^{14-16,18,25,26}. However, the osseous anatomy of the cervical spine and the dimensions of the screws used in this region are so different that it would be unreasonable to extrapolate the data for pedicle screws to screws in the lateral masses.

Errico et al. are the only investigators, to our knowledge, who have isolated a variable that influences the pull-out resistance of screws in the lateral masses. They compared the uniaxial loads to failure for the same type of screw inserted with the method of either Roy-Camille or Magerl. They attributed the greater purchase strength of the screw that had been inserted with the latter method to the longer working length of the engaged thread. Their findings were consistent with those of Montesano et al., who had noted that the load to failure was greater for plates that had been fixed with screws inserted according to the method of Magerl. Errico et al. acknowledged that caution should be used when extrapolating their findings to humans, but Sutterlin et al. stated that the bovine cervical spine was a reasonable in vitro model of the human cervical spine. It should also be noted that Errico et al. did not specify either the thread configuration (they reported only the outer diameter [3.5 millimeters] of the screw) or whether bicortical purchase was obtained.

Our experiment demonstrated that the greatest variation in the axial load to failure was nearly 100 per cent among the different types of screws and the two depths of insertion. Three major subgroups were identifiable. The greatest pull-out resistance was seen with bicortical insertion of the 3.2 (so-called emergency), 3.5, and 4.5-millimeter cortical-bone screws as well as the 3.5-millimeter cancellous-bone screw. Apparently, a screw with a diameter of less than 3.2 millimeters, regardless of the depth of insertion (unicortical or bicortical), does not have the pull-out resistance of this superior group. With regard to a maximum diameter, the 4.5-millimeter cortical-bone screw had poor pull-out resistance with unicortical purchase (load to failure, 239 newtons); this value was almost one-half that obtained with bicortical purchase. A screw with an outer diameter between these two values (3.2 and 3.5 millimeters) appears to be the most effective. Furthermore, none of the screws, regardless of the diameter, in the group with the greatest pull-out resistance had unicortical purchase.

Variations in the thread pitch and configuration and the depth of insertion of pedicle screws have been investigated^{15,16,18,25,26}. The most systematic of these studies was done by Krag et al.¹⁶, who demonstrated that the thread pitch and configuration did not significantly alter the load to failure; their results were in contrast to the findings of studies of the fixation of long bones⁵. The minor diameter of the screw had a variable influence on the load to failure¹⁶, but Krag et al. suggested that the increased strength of the screw probably did not justify increasing the thread depth. They concluded that these findings were consistent with theories regarding failure of screws. According to these theories, since pull-out of the screw is caused by shear failure of the cylinder of bone immediately surrounding the outer diameter of the thread, differences in the thread configuration are not likely to be a factor in the failure of pedicle screws. Our data for screws in the lateral masses are consistent with the findings of Krag et al.¹⁶, with one exception. The 3.5-millimeter cortical-bone screw had the greatest pull-out resistance, with either unicortical or bicortical purchase, but the values did not differ significantly from those for the 3.5-millimeter cancellous-bone screw. Hence, this change in the thread configuration and the minor diameter of the screw did not significantly alter the pull-out resistance. In contrast, the 3.5-millimeter self-tapping screw had the lowest pull-out resistance, with either unicortical or bicortical purchase. We attribute the poor performance to the self-tapping design rather than to the thread configuration or the dimensions of the screw. The fluted conical tip of the screw decreases the thread available in the distal portion of the screw. For such a short screw, this small difference apparently has a large influence on pull-out resistance.

The risks of morbidity associated with screws in the lateral masses have been well characterized. Heller et al. demonstrated a finite residual risk of injury to the nerve roots and facet joints, even after considerable experience with the technique of insertion¹². Most of this theoretical risk could be eliminated by avoiding penetration of the anterior cortex. Jonsson and Rauschning used a cryoplaning technique to evaluate cervical spine specimens from patients who had had anterior or posterior plate fixation; on the basis of their findings, they cautioned against bicortical purchase. For pedicle screws in the thoracic and lumbar spine, the threat of injury to the great vessels has been deemed too important to justify the increased purchase strength contributed by the anterior cortex of the vertebral body. We found a 28 per cent increase (p = 0.001) in the pull-out resistance (from a load to failure of 272 newtons to one of 350 newtons) with bicortical purchase. This increase in purchase strength is similar to that observed by Zindrick et al. when they compared the results after pedicle screws had been inserted "to cortex" with the results after the screws had been inserted "through cortex." It remains to be determined whether this amount of enhanced purchase justifies the theoretical risks of morbidity associated with penetration of the second cortex.

The variations in the osseous anatomy of the cervical spine have been well characterized, most recently by An et al., who found that the length of screw required to engage the lateral mass ranged from six to sixteen millimeters. Because the transitional anatomy of the seventh cervical vertebra was so variable, they warned that the use of screws in the lateral masses should be considered carefully. This anatomical variability led us to evaluate the hypothesis that the load to failure of the screw varies according to the cervical level. The null hypothesis that the pull-out resistance is equivalent at all levels was rejected. The greatest pull-out resistance (load to failure, 391 newtons) was found at the fourth cervical level, decreasing approximately 30 per cent at the second and seventh cervical levels. Why the resistance was greater in the middle region of the cervical spine is a matter of speculation. However, it is known that the lateral masses transmit approximately two-thirds of the axial load on the cervical spine²¹. It is possible that this load is the greatest at the apex of the cervical lordosis and that, according to Wolff's law, there would be enhanced bone quality in the lateral masses of the fourth cervical vertebra. Unfortunately, we could not investigate this hypothesis, as it was not technically possible to quantify the bone quality of the lateral masses with our quantitative computed tomography technique.

The practical implications of the variability in the pull-out resistance among the cervical levels must be considered. We observed a significant increase (p = 0.001) in the pull-out resistance when bicortical purchase had been achieved. However, an increased risk of morbidity has been associated with this depth of insertion. Whether bicortical purchase is more strongly indicated at the cephalad and caudad ends of the cervical spine remains to be seen. An et al. suggested that pedicle screws might be more appropriate at the seventh cervical level, when the pedicles are large enough, but no clinical experience with this technique has been reported, to our knowledge. When anatomically possible, pedicle-screw fixation is probably the method of choice at the second cervical level. Unfortunately, there have been no reports, to our knowledge, in which the purchase strength of screws in the lateral masses was compared with that of screws in the pedicles of the second and seventh cervical vertebrae.

The association between vertebral bone quality and load to failure has been documented for pedicle screws used in the thoracolumbar region^25,26. Wittenberg et al. documented a high degree of correlation between the load to failure of pedicle screws and the findings of both quantitative computed tomography densitometry and direct bone densitometry. Their conclusion is further supported by the relationship between the depth of insertion and the load to failure for pedicle screws, as pedicle screws engage primarily cancellous bone. Osteopenia is an accepted risk factor associated with the failure of pedicle screws, and it has been suggested to be a risk factor associated with the failure of screws in the lateral masses. According to our data, the bone density of the vertebral bodies did not predict failure of screws in the lateral masses; linear regression analysis demonstrated no association between the pull-out resistance and the findings on quantitative bone densitometry. In fact, with the exception of that of the seventh cervical vertebra, the densities of the cancellous bone of the cervical vertebral bodies were uniform, whereas the load to failure varied significantly (p = 0.004) among the levels.

Bone adjacent to the tips of the screw threads fails in tension, compression, or shear, depending on the loading conditions. It cannot be denied that bone quality influences the load to failure of screws under these loading conditions. However, it is likely that there are more operatively related influences than influences related to vertebral bone density. The quality of cortical bone may play a more important role in the purchase of screws in the lateral masses, and the purchase strength may not be associated with cancellous-bone density in the vertebral body or the lateral masses themselves. Additional investigation is necessary to identify the predictors of failure of the screws in posterior plate fixation of the cervical spine.

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Introduction | Materials and Methods | Results | Discussion | References

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Schematic axial (left) and lateral (right) drawings of a cervical vertebra with a screw in the lateral mass. The arrows indicate that the applied load is along the axis of the screw.

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TABLE I TYPES OF SCREWS, GROUPED ACCORDING TO SIGNIFICANT DIFFERENCES IN PULL-OUT RESISTANCE

		Axial Load to Failure (N)
Type of Screw*	Type of Purchase	Least Squares Mean	Mean and Stand. Dev.
Greatest pull-out resistance
3.5-mm cortical-bone	Bicortical	426	448 ± 253
3.2-mm cortical-bone	Bicortical	371	371 ± 155
3.5-mm cancellous-bone	Bicortical	368	368 ± 203
4.5-mm cortical-bone	Bicortical	367	369 ± 158

Intermediate pull-out resistance
2.7-mm cortical-bone	Bicortical	321	321 ± 223
3.5-mm cortical-bone	Unicortical	319	289 ± 231
3.2-mm cortical-bone	Unicortical	300	305 ± 207
3.5-mm cortical-bone	Unicortical	277	277 ± 126
2.7-mm cortical-bone	Unicortical	274	344 ± 383

Lowest pull-out resistance
3.5-mm self-tapping	Bicortical	244	236 ± 124
4.5-mm cortical-bone	Unicortical	239	231 ± 134
3.5-mm self-tapping	Unicortical	223	216 ± 166

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TABLE I TYPES OF SCREWS, GROUPED ACCORDING TO SIGNIFICANT DIFFERENCES IN PULL-OUT RESISTANCE

		Axial Load to Failure (N)
Type of Screw*	Type of Purchase	Least Squares Mean	Mean and Stand. Dev.
Greatest pull-out resistance
3.5-mm cortical-bone	Bicortical	426	448 ± 253
3.2-mm cortical-bone	Bicortical	371	371 ± 155
3.5-mm cancellous-bone	Bicortical	368	368 ± 203
4.5-mm cortical-bone	Bicortical	367	369 ± 158

Intermediate pull-out resistance
2.7-mm cortical-bone	Bicortical	321	321 ± 223
3.5-mm cortical-bone	Unicortical	319	289 ± 231
3.2-mm cortical-bone	Unicortical	300	305 ± 207
3.5-mm cortical-bone	Unicortical	277	277 ± 126
2.7-mm cortical-bone	Unicortical	274	344 ± 383

Lowest pull-out resistance
3.5-mm self-tapping	Bicortical	244	236 ± 124
4.5-mm cortical-bone	Unicortical	239	231 ± 134
3.5-mm self-tapping	Unicortical	223	216 ± 166

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TABLE II PULL-OUT RESISTANCE ACCORDING TO CERVICAL LEVEL

		Axial Load to Failure (N)
Level	No. of Screws	Least Squares Mean	Mean and Stand. Dev.
C2	22	278	365 ± 227
C3	24	355	290 ± 184
C4	24	391	286 ± 191
C5	23	313	309 ± 163
C6	22	262	288 ± 162
C7	21	266	381 ± 331

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TABLE II PULL-OUT RESISTANCE ACCORDING TO CERVICAL LEVEL

		Axial Load to Failure (N)
Level	No. of Screws	Least Squares Mean	Mean and Stand. Dev.
C2	22	278	365 ± 227
C3	24	355	290 ± 184
C4	24	391	286 ± 191
C5	23	313	309 ± 163
C6	22	262	288 ± 162
C7	21	266	381 ± 331