Principles of orthopedic hardware placement have remained largely unchanged during the past several decades.1 In standard drilling, advancement of the bit and revolution speed are controlled manually by the surgeon, increasing the opportunity for drill bit overpenetration, or plunge. Drill bit overpenetration can result in iatrogenic damage to soft tissue, which complicates surgical outcomes.2–6 Furthermore, drilling error and overpenetration occur regardless of experience or level of training.7
Depth measurement is routinely performed as a distinct process with a manual gauge and can be inaccurate. Assessment of screw length is often confirmed fluoroscopically, which contributes to an additional step and lengthens surgical time. Furthermore, fluoroscopy exposes the patient, surgeon, and assistant to ionizing radiation and its attendant risks.8 Combining accurate drilling and measurement into a single step would have several potential advantages in reducing plunge, improving measurement accuracy, optimizing screw length, shortening operative time, reducing implant waste, and limiting occupational exposure to ionizing radiation.
A dual-motor drill was created to simultaneously control revolutions per minute (rpm) and insertion rate while visually displaying drill bit depth and insertion torque in real time. The purpose of this study was to compare drill bit plunge, measurement accuracy, and drilling and measurement time between a dual-motor drill and standard drill. The authors' hypothesis was that the dual-motor drill would demonstrate less drill bit overpenetration and improved measurement accuracy in a shorter time than the standard drill.
Materials and Methods
A dual-motor drill was created consisting of a drill with 2 motors and a monitor (SMARTdrill; Smart Medical Devices Inc, Las Vegas, Nevada) (Figure 1). The first motor spins a chuck similar to a standard orthopedic drill but at a controlled rate (rpm). The second motor is activated by an independent second trigger. When engaged, it moves a “harp” and drill guide parallel to the axis of the drill bit, controlling advancement of the bit. The harp consists of 2 bars that are threaded on either side of the primary bit and can slide forward or backward, exposing the drill bit from the protected drill guide when the second motor is engaged. This design allows the drill to function like a handheld drill press where the harp and drill guide remain held against the bone during drilling to create a static reference point at the bone surface. Because the end of the drill guide and harp remain on the surface as the bit is advanced with the second motor, the distance to the tip of the drill bit is always known, providing the depth of drilling in real time. The depth of the bit relative to the end of the guide is recorded by the drill and displayed on a screen.
A dual-motor drill functions like a hand-held drill press. Depression of the chuck trigger (1) engages the first motor to spin the chuck (2) similar to any standard orthopedic drill. Depression of the harp and drill guide trigger (3) engages the second motor to move the threaded harp (4) and drill guide (5) toward the operator, exposing the drill bit (6) and allowing it to enter the drilling substance. During drilling, the harp (4) and drill guide (5) remain fixed against the bone by the operator as the drill bit (6) enters the bone. The distance from the tip of the drill bit to the drill guide represents the depth of drilling and is displayed on a monitor. Additional sensors within the drill relay data to the monitor in real time (A). A close-up view of the drill bit (6), drill guide (5), and harp (4) (B).
The drill also contains electronic sensors that provide feedback to a screen that visually displays information such as torque and drilling energy in addition to depth. During drilling, the dual-motor drill measures the work done by the drill bit by measuring torque and rpms as it is advanced. The calculated energy is plotted visually on a monitor with drill bit depth on the x-axis and drilling energy on the y-axis. A characteristic curve is created, making it possible to detect the far cortex and therefore measure screw length and prevent plunge (Figure 2).
Example of visual monitor display of a torque plot with torque (Nm) (y-axis) and depth (mm) (x-axis) for a 15-mm hole drilled in a bone block model. Bicortical drilling creates a characteristic torque plot where the first peak indicates the near cortex, the trough represents the intramedullary canal, and the second peak represents the far cortex. When the drill bit traverses the far cortex, the torque decreases and the surgeons stops drill bit advancement by releasing the harp and drill guide trigger. Screw length can then be determined by examining the x-axis at the apex of the second peak (15 mm in this example).
Dual-Motor Drilling Technique
Surgeon drilling technique is summarized in the Video. The drill starts in a “zeroed” position where the end of the drill bit, 3.2 mm, is flush with the end of the depth guide. The surgeon depresses the chuck trigger to begin spinning the bit. With the drill guide manually pressed against the drilling substance, the harp and drill guide trigger is depressed, allowing the bit to enter the bone. The surgeon then examines the monitor. The torque curve will rise as the drill bit traverses the near cortex and then fall as the bit enters the intramedullary canal. When the bit encounters the second cortex, the torque will increase again as the bit enters the denser cortical bone of the far cortex. When the surgeon recognizes the second apex of the torque curve on the screen, he or she stops the bit from advancing farther by releasing the harp and drill guide trigger. This prevents the plunge associated with standard drilling technique. The surgeon may then examine the screen and read the drill bit depth, which displays the appropriate length of screw needed. While the chuck trigger is still depressed, the drill bit can be removed in standard fashion.
Drill Testing Protocol
To compare dual-motor drilling with the standard technique, 5 orthopedic surgeons, each with more than 5 years in clinical trauma practice and no prior exposure to the dual-motor drill, were introduced to the drill and allowed to perform 5 test trials to familiarize themselves with the design using a bicortical bone model (Sawbones; Pacific Research Laboratories, Vashon, Washington). The composite bone block was selected to standardize the test among participants and limit variability associated with cadaveric bone. The use of bone block models for this type of testing has been previously reported and validated.9
After familiarization, each surgeon was provided randomized bone blocks of variable thickness from 15 to 27 mm. Blocks were randomized but paired for both groups such that similar depths were used for a single trial but not in sequential order, and no more than 4 holes on a given block of the same thickness were drilled to prevent development of motor learning during testing.
To evaluate the accuracy of drilling, bone models were placed on standard ballistic gels, which left a penetration defect to the maximal point of drill bit depth. Each surgeon performed 10 drilling trials with a control drill and 10 trials with the dual-motor drill.
The standard drill, a Small Frag (Synthes USA, West Chester, Pennsylvania), consisted of a battery-powered hand drill with a 3.2-mm drill bit typical of use in clinical practice. Drilling was performed by achieving bicortical penetration and then performing measurement with a manual depth gauge. Measurements of drill depth error (plunge) were obtained by a measuring rod calibrated to 0.5-mm increments, which was placed into the defect in the ballistic gel. This provided a quantitative description of the degree to which the drill bit tip passed the second cortex. Depth measurement was reported to the nearest 0.5 mm based on the surgeon's visual inspection of the manual depth gauge. Timing commenced from the time the drill trigger was initiated until the measurement was verbally reported.
For the dual-motor drill, the same bit type was used with the drilling technique described previously. Drilling commenced at a feed rate of 1 mm/s. Plunge was recorded using the same calibrated measuring rod described previously placed into the defect in the ballistic gel. Depth measurement was obtained by reading the monitor for drill bit depth and reported to the nearest 0.1 mm. Timing commenced on initiation of the primary motor and ended when the depth measurement was verbally reported.
Overpenetration values were compared directly between groups. Depth measurements were then compared with the actual block thickness as measured with a digital caliper accurate to 0.01 mm, and depth measurement errors were recorded and compared. Similarly, drilling and measurement times were recorded and compared.
The primary outcome measure was drill bit overpenetration (mm). Secondary outcome measures were accuracy of depth measurement (mm) and time (seconds) required to perform drilling and measurement.
For clinical testing, a priori power analysis testing with alpha of 0.05 and power of 0.80 confirmed 50 trials per group to be adequate to detect a statistically significant difference in plunge between groups. Data were recorded and analyzed using Excel (Microsoft Corp, Bellevue, Washington). Paired t tests were used for the statistical analyses between groups for all outcome measures. Subgroup analysis was performed to evaluate for differences between surgeons.
All test specimens were captured for final analysis.
The mean overpenetration (plunge) for the dual-motor drill group was 0.5±0.3 mm (95% confidence interval [CI], 0.35–0.57). This value was significantly less than for the standard drill group, 8.4±1.9 mm (95% CI, 7.81–9.05), indicating that the dual-motor drill nearly eliminated plunge (P<.0001) (Figure 3). The smallest plunge in the dual-motor drill group was 0 mm and the largest was 1.5 mm. The smallest plunge in the standard drill group was 4.5 mm and the largest was 14 mm.
Box and whisker plot demonstrating the relationship between drilling error for the standard drill and the dual-motor drill. The box represents the standard deviation. The whiskers represent the upper and lower limits of the 95% confidence interval. The dual-motor drill (0.6± 0.3 mm) showed significantly less overpenetration, or plunge, compared with the standard drill (8.4±1.9 mm) (P<.0001).
The mean depth measurement error for the dual-motor drill group was 0.6±0.3 mm (95% CI, 0.49–0.71). This value was significantly less than for the standard drill group, 2.6±0.5 mm (95% CI, 2.40–2.76), indicating increased accuracy for depth measurements with the dual-motor drill (P<.0001) (Figure 4).
Box and whisker plot demonstrating the relationship between measurement error for the standard drill and the dual-motor drill. The box represents the standard deviation. The whiskers represent the upper and lower limits of the 95% confidence interval. The dual-motor drill (0.5±0.3 mm) showed significantly less measurement error compared with the standard drill (2.6±0.5 mm), indicating more accurate depth measurement (P<.0001).
Considering a standard screw length increment of 2 mm, 0 (0%) of 50 screws from the dual-motor drill group had a measurement error greater than 2 mm and would potentially have benefited from screw exchange for incorrect length. Conversely, 31 (62%) of 50 screws in the standard drill group had a measurement error greater than 2 mm and would therefore potentially benefit from screw exchange for incorrect length.
The mean drilling and measurement time for the dual-motor drill group (6.0±2.2 seconds) was significantly less than for the standard drill group (13.4±3.9 seconds), indicating that the dual-motor drill performed nearly twice as fast as the standard drill (P<.0001).
Subgroup analysis demonstrated no significant variation among surgeons (P<.0001).
The primary finding of this study was that a dual-motor drill controlling revolution speed and feed rate significantly reduces overpenetration, provides more accurate depth measurement, and is faster than traditional drilling and measurement techniques. This is clinically relevant because of the potential to avoid local tissue damage, decrease the need for fluoroscopy, reduce implant waste from exchanged screws, and improve intraoperative efficiency.
Applications for placement of orthopedic hardware safely and accurately without dependence on fluoroscopy are far reaching. In the battlefield or mass casualty setting, use of external fixation can protect the soft tissue envelope, reduce the risk of infection, minimize fracture hemorrhage, provide pain control, and aid medical evacuation.10 However, blind placement of pins is not without risks. In a cadaveric study of external fixator application to a simulated femur fracture without fluoroscopy, the number of unicortical pins was up to 30%, the rate of observed plunges was up to 40%, and in one case the femoral artery was lacerated by a unicortical pin.11 In a similarly designed cadaveric study, the authors found the risk for neurovascular injury doubles with pin placement without fluoroscopic assistance, and the rate of pin overpenetration was 49%.12 In both cases, the studies were performed in a controlled experimental setting in an operating room, and one might reasonably assume in a combat situation the rates of complication increase.
In a retrospective review of pins placed in a combat environment, there was a 22% rate of overpenetration between 9 mm and 25 mm and a 5% rate of pins placed greater than 26 mm past the far cortex.13 The average distance to neurovascular structures has been shown to be as little as 10 mm in diaphyseal lower extremity fractures.12 Although these values may be acceptable in a combat environment, any improvement in accuracy and safety could reduce mortality and morbidity.
Even routine internal fixation procedures are affected by drill bit overpenetration, screw length errors, and the potential for iatrogenic injuries to vital structures.2–6,14 Average drill bit overpenetration has been reported at 6.31 mm regardless of experience or level of training.7 Interestingly, the current authors identified an average plunge of 8.4 mm in the standard drill group. The difference may have been attributable to differences in study or apparatus design. Currently, there is no previously described standard for clinically acceptable overpenetration, and this value may be expected to vary depending on the procedure and regional anatomy.
Distal radius fracture fixation is a common outpatient procedure; however, tendon irritation or rupture due to inappropriate drilling and screw placement has been frequently reported.15–18 Standard fluoroscopic views may be inadequate for identifying prominent screws, and additional views and fluoroscopy time may be required to avoid complications.19,20
In the spine, placement of pedicle screws has been reported to be as high as 90% accurate without the assistance of navigation guidance, but larger studies have concluded a benefit to fluoroscopic and computed tomography guidance.21,22 Furthermore, the allowance for error has been estimated as little as 0 mm at certain spinal levels for a given screw geometry and local anatomy.23
Even in the absence of adverse events, hardware complications related to inappropriate screw length and subsequent soft tissue injury have been well documented.24,25 Subsequently, the rate of elective hardware removal ranges from 15% to 30%.26,27 Hardware removal carries a significant risk with the need for further surgery and increased risk of fracture, infection, and poor wound healing.
In the current study, the authors identified improved accuracy of measurement with dual-motor drilling compared with a standard drilling technique and a potential screw error measurement error of greater than 2 mm in 62% of screws in the standard technique. Intraoperatively, inaccurate measurement can result in screw exchange, increased surgical time, and implant waste. Waste in spine procedures has been reported as high as 20%.28 Payne et al29 performed a review of waste at a single institution during the course of a year. They found that among all orthopedic subspecialties, trauma had the highest rate of implant waste (30% of all cases), with screws being among the most commonly wasted items. The total cost of implant waste was more than $600,000 during the course of the study.
Screw removal and exchange carries a biological cost as well. Multiple studies have examined the detrimental role of excessive bone removal and screw removal in fixation strength of a screw construct.30–32 Collinge et al33 reported that the reduction in fixation strength of a screw after exchange is as high as 80%. Therefore, improved accuracy in screw length measurement may reduce the rate of screw exchange, which removes additional bone, weakens the construct, and increases operative time.
Operative time was significantly less for the dual-motor group. Although the average difference of 7 seconds for placement of a single screw is modest, this excludes the time required for screw exchange if needed and demonstrates that dual-motor drilling is unlikely to take longer than the standard technique, despite the previously mentioned benefits. If a construct requires multiple screws and fluoroscopy is required to confirm screw length with several screws requiring exchange, then a clinically relevant decrease in operative time could be hypothetically achieved. The actual cost of time in the operating room is highly variable due to variations in complexity and regional variations between facilities, but it is reasonable to assume that decreasing the time for screw placement would decrease cost.34 Furthermore, decreased operative times have been linked to improved outcomes for certain procedures such as spinal fusion.35–38
Testing was performed in bicortical bone models instead of cadaveric bone. These models have been previously validated and were specifically chosen because they remove the regional variabilities inherent in cadaveric or animal bone. Nonetheless, regional changes in the density and cortical thickness of bone might reasonably be expected to increase the disparity between the groups because it would be more difficult for the surgeon to predict the variable cortical thickness compared with the standardized models. Testing was performed by operators with significant experience in placement of orthopedic hardware, and results may not be generalizable to other levels of skill or experience.
Use of a dual-motor drill reduced overpenetration, improved measurement accuracy, and reduced time spent placing orthopedic screws. This information has implications for decreasing adverse events, reducing implant waste, and improving efficiency in the operating room.
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