Volume VII, Number 2 | Summer 2023

How Pilot Hole Size Affects Screw Pullout Strength in Normal and Osteoporotic Bone Models: A Biomechanical Study

  1. N. Reyes DO – VCME Orthopedic Surgery Residency 
  2. R. Saito DO – VCME Orthopedic Surgery Residency
  3. C. Beckett DO – VCME Orthopedic Surgery Residency
  4. J. Steimel PhD – University of the Pacific School of Engineering and Computer Science
  5. E. Huish DO – VCME Orthopedic Surgery Residency
  6. W. Holmes MD – VCME Orthopedic Surgery Residency

Failure of screw purchase is a major concern in both normal and osteoporotic bone that can lead to many complications. The goal of this study was to assess how an under-drilled pilot hole diameter affected screw pullout strength in both normal density and osteoporotic bone.

Seventy-two Synthes stainless steel self-tapping cortical screws (50 mm length and 3.5 mm diameter) were inserted into pilot holes of s 2.5 mm, 2.0 mm and 1.5 mm in both normal and osteoporotic synthetic bone blocks. Maximum axial pullout strength was then tested using an Instron Universal Testing System.

Screw pullout strength was increased by 9.4% and 18.5% (p < 0.05) when inserted into 2.0 mm and 1.5 mm pilot hole diameters respectively, as compared to screws inserted into the standard 2.5 mm pilot hole in a normal density bone block model. There was no significant increase in screw pullout strength when under-drilling with 2.0 mm and 1.5 mm pilot holes as compared to the standard 2.5 mm pilot hole in an osteoporotic bone block model.

In general, cortical screws demonstrated a significantly (p < 0.05) greater hold by under-drilling the pilot hole compared to the recommended pilot hole diameter in normal bone block models. This trend was not seen with the osteoporotic bone block models as there was no significant increase in pullout strength with under-drilling the pilot hole.

Keywords: Biomechanics, Pilot Hole, Screw Fixation

The use of cortical screws for fracture stabilization is commonplace in orthopedic surgery. They can be used independently or combined with other stability constructs such as plates or nails. The major purpose of the screw is to fixate via compression.  A screw must resist loosening and pullout to maintain the fixation of the construct it is applied to. Screw loosening may result in secondary loss of reduction, leading to delayed union, malunion, nonunion, and implant failure with screw pullout(1-10). Since this is a major concern, there have been numerous studies looking at the various characteristics of what can lead to greater pullout strength.

Two common types of screws are cortical and cancellous screws. These can be further subdivided into fully threaded or partially threaded. The type of screw applied is dictated by its design and goal of fixation. Cortical screws have fine, shallow threads with an inner core diameter closer in to the outer diameter to allow fixation in denser cortical bone, while cancellous screws are relatively coarser in thread design with deeper threads for better fixation in less dense cancellous bone(11). Fully threaded screws are often used as positional screws, whereas partially threaded screws can be used by design as lag screws for fractured bone. Furthermore, screws can either be self-tapping, which allows them to be directly drilled into a pilot hole, or non-self-tapping

Cortical bone screws used in fracture fixation are commonly self-tapping and can play an essential role in osteosynthesis. Although the screws have cutting flutes and are self-tapping, according to the AO/ASIF manual, screws require a pilot hole to be drilled before insertion(11). A pilot hole is drilled to create a path to guide the screw and ease its insertion(12). The size of the pilot hole can potentially affect the overall hold of the screw.  Studies show that pilot holes too small create resistance to screw insertion that may result in screw fracture, inaccurate screw insertion, or fracture of surrounding bone(12). Pilot holes that are too large decrease contact with the screw’s threads, thereby decreasing pullout strength(12). This decrease in pullout strength compromises the stability of the construct the screw is fixating and can lead to failure.

Another factor when dealing with screw loosening and pullout strength is bone quality. Once bone becomes osteoporotic, it loses density which reduces the interconnectivity of the trabeculae. This can become clinically challenging when dealing with screw failure as these changes may compromise the hardware’s ability to maintain thread contact with bone that can apply equal force back. Prior studies investigating screw pullout strength in osteoporotic bone have been variable due to the difficulty in replicating the various densities of osteoporotic bone. Some studies have attempted using chemical demineralizing processes whose results were more similar to bone affected by osteomalacia instead of osteoporosis(10).  For this reason, synthetic bone models are often selected because there is low intra- and interspecimen variability, the cost is low, and it can be easily obtained and stored(13).

Screw purchase into bone is pivotal for fixation. The purpose of this study was to investigate how the diameter of a drilled pilot hole affects cortical screw pullout strength in regular and osteoporotic bone block models. We hypothesize that a decreased pilot hole diameter will yield a higher force needed to pullout the screw. 

Materials and Methods
To replicate bone material properties synthetic bone blocks (SawBones Corporation, Vashon, WA) were utilized. To characterize the behavior of normal and osteoporotic bone the PF (density measurement for SawBones products) value of normal bone was 20 PF laminated with 3mm 40 PF and 20 PF without a laminate for osteoporotic bone. The sawbone blocks were cut into samples that were 40mm x 40mm x 43mm. A total of 36 samples were utilized for each bone type: normal and osteoporotic. Pilot hole diameters of 1.5, 2.0, and 2.5mm were prepared and 12 experiments were conducted for each pilot hole for both bone types. The pilot holes were drilled by hand using a surgical drill to mimic typical surgical operations at the center of each sawbone sample. Stainless steel self-tapping cortical screws (DePuy Synthes, West Chester, PA), 50 mm length and 3.5 mm diameter, were then screwed by hand into the pilot holes until approximately 4 mm of the screw head extended above the sawbone sample to allow the screw to be gripped by the materials testing systems (MTS) apparatus.

The MTS apparatus utilized in this study was an Instron 3369 Universal testing system (MTS Systems, Eden Prairie, MN) that was modified to allow for measurement of screw pullout strength as depicted in Figure 1. The bottom grips of the Instron were fitted with grips to grab flat tensile specimens with thickness up to 0.25 inches.

A custom machined structure (figure 1) was developed with a flat rectangular extruding end that was secured by the bottom grip of the Instron apparatus. The bottom structure was designed such that the sawbone samples could be placed and secured by two chains that fit within groves in the aluminum sawbone support. This apparatus was then secured with locking wingnuts. Chains were placed on top of a 40mm x 40mm piece of 6061 aluminum alloy with a hole cut out at the center. This was placed on top of the sawbone sample with a cortical screw where the chains would rest on top of the piece of aluminum. This was done to ensure the sawbone sample would not undergo plastic deformation by the chains while the screw was pulled by the upper grips and apparatus. The upper grips were designed to grip cylindrical samples. They were utilized to secure a cylindrical low carbon steel component that was machined with a groove to allow a 3.5mm screw head. The two systems were then aligned.

The upper apparatus was positioned such that the head of the screw was secured with the bottom apparatus securing the bone sample, thus ensuring the applied force was directly applied on the screw. Once secured, the load and displacement were normalized, and the experiment was run at a strain rate of 15mm/min as is typical to measure pullout force strength. The experiment continued until displacement was approximately 10mm, which is well beyond the displacement where pullout strength was achieved. The pullout strength was measured for the 12 experimental repetitions and statistical analysis was performed to estimate error in the experiment. Specifically, small sample statistics using the student-t distribution was utilized to estimate error. ANOVA analysis was performed for these measurements to determine the effect of pilot hole on cortical screw pullout strength. 

Data Analysis
The pullout strength was measured for 3.5 mm cortical screws for different pilot holes in the normal and osteoporotic samples. The pullout strength is determined to be the value of maximum force. An example of this measurement can be seen in Figure 2 which demonstrates a force vs. displacement curve for the pullout force for a 3.5 mm cortical screw in a 2.0 mm pilot hole.

The maximum pullout strength is indicated by the dashed red circle in Figure 2 for these experimental conditions. The inset shows the variation in pullout strength due to the scholastic nature of plastic deformation of the sawbone and fracture as the screw is pulled from the sawbone material. The pullout strength was measured for each experiment and the average value was calculated for the 12 trials per experimental condition.  The error bar was determined using small sample Student t-test distribution statistics. The confidence interval can be seen in the Appendix in Table 1 (Figure 4). As seen in Figure 3, there appears to be a correlation between pilot hole and pullout strength. 

The results of the ANOVA analysis comparing the normal bone block model samples’ pull-out strengths noted a mean pullout strength of 1671.67 N (95% CI, 1490.25-1853.09 N),  158 N (95% CI, 1745.78-1913.55) and 309.33 N (95% CI, 1922.7-2039.3) when utilizing a standard 2.5 mm pilot hole, 2.0 mm, and 1.5 mm pilot hole, respectively. These results demonstrated a significant increase in pull out strength as the pilot hole was decreased (p=0.00145) in normal bone block samples. In the osteoporotic bone block model, the pullout strengths noted were 640 N (95%, CI, 611.32-668.68), 664.08 (95% CI, 641.95-686.21), and 660.09 (95% CI, 639.61-680.57) when utilizing a standard 2.5 mm, 2.0 mm, and 1.5 mm pilot hole respectively. These values for osteoporotic bone did not demonstrate a statistical difference in pullout strength p=0.259. 

Our results demonstrate an increase in pullout strength when under drilling pilot hole in normal bone Sawbone samples with the average screw pullout strength increasing by 9.4% and 18.5% (p < 0.05) when inserted into 2.0 mm and 1.5 mm pilot hole diameters, respectively. These findings provide a biomechanically  effective method to gain purchase and resist screw loosening and pullout in normal bone.

In contrast, there was no significant difference found in the screw pullout strength when under-drilling with 2.0 mm and 1.5 mm pilot holes compared to the standard 2.5 mm in osteoporotic bone block models. This lack of statistical significance supports previous articles that show under-drilled pilot holes being ineffective at improving pullout strength of cortical screws in osteoporotic bone. This may be due to the lower density and quality of osteoporotic bone compared to normal bone effecting the screw thread’s ability to engage into the bone. Without the screw finding purchase, it can be assumed that increasing the surface area interacting with the screw past the threads (smaller pilot hole) would not have a significant effect in osteoporotic bone.

When discussing normal bone, diminishing pilot hole does not come without any risks. Pilot holes that are too small can create increased insertional torque which may lead to complications such as screw failure, inaccurate screw insertion, or fracture of the surrounding bone(14). Tapping of the pilot hole prior to inserting the screw may help mitigate these risks. This is achieved by decreasing insertional torques of the screw threads and creating thread channels. It is important to note that our study did not have any failed trials due to decreasing pilot hole and suggest further studies on the incidence and risk of this type of failure could be beneficial.

Osteoporotic bone samples did not show any gain in cortical screw purchase with decreasing pilot holes. This could be due to the different bone density values and the resulting mechanical forces exerted on the inserted cortical screw in the osteoporotic bone. The lower density material exerted lower initial forces and adhesion of the cortical screw to the bone and did not lead to the same gain in pullout strength. This demonstrates the ability of osteoporosis to dramatically affect the bone screw interface which is relied on heavily for fixation(15). Failure of fixation leads to loss of screw purchase which may result in a multitude of complications ranging from instability to nonunion or malunion(16).

The findings of this study provide insight for surgeons when considering drill bit when drilling pilot holes for cortical screws in normal bone. This is shown to be significant when considering smaller diameter pilot holes in normal bone block models to increase the force necessary to remove the screw. However, the question remains if this improves clinical outcomes when other patient factors are present such as implant design, operative technique, and patient bone quality.

This study has several limitations. First and foremost, our study is a biomechanical study utilizing synthetic bone models. Synthetic material cannot fully emulate human bone despite being accepted as a traditional substitute for cadaveric bone(17). Values of PF are given as appropriate representation of bone density, 20 PF for cancellous and osteoporotic and 40 PF for normal, bovine samples, cadaveric specimens, and in-vivo bone would better serve this study. It was also difficult to estimate and replicate the change in of the laminated layer of 40 PF sawbone from the normal to osteoporotic samples.

However, the use of saw bone constructs has shown to be clinically relevant in past studies (17). They were effective for the purpose of this study but do not completely represent the clinical effects on actual bone influence by other patient factors of varying bone quality. However, demonstrating the biomechanics effects with this study method provide the foundation for appropriate cadaveric evaluation to improve upon. 

The results of this study emphasize appropriate drilling of pilot hole diameter for cortical screws and its effect on screw fixation. A smaller pilot hole diameter can increase cortical screw pullout strength in normal bone block models. This increase in pull out strength is maximized . However, decreasing pilot hole does not increase pullout strength in osteoporotic bone models tested. Further studies are needed to elucidate the clinical implications of these findings and what other parameters (max screw torque, cancellous screws, plate and screw fixation, surgical technique etc.) may affect fixation in relation to screw purchase and patient outcomes.

Figure 1 | Figure 2 | Figure 3 | Figure 4


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The Journal of the American Osteopathic Academy of Orthopedics

Steven J. Heithoff, DO, FAOAO

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