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. 2019 Nov 1;477(11):2579–2585. doi: 10.1097/CORR.0000000000000922

Using Laser Range-finding to Measure Bore Depth in Surgical Drilling of Bone

Daniel Demsey 1,2,3,✉, Juan Pablo Gomez Arrunategui 1,2,3, Nicholas J Carr 1,2,3, Pierre Guy 1,2,3, Antony J Hodgson 1,2,3
PMCID: PMC6903841  PMID: 31464794

Abstract

Background

Measuring drilled bore depth in bone is an important part of osteosynthesis surgery. Current methods have substantial limitations in terms of reliability, leading to placement of incorrectly sized screws and unsatisfactory user experience.

Questions/purposes

(1) Can a prototype laser range-finder measure bore depth in bone as well as or better than a conventional depth gauge in terms of accuracy and precision (that is, variability)?

Methods

A conventional analog orthopaedic surgical depth gauge was compared with a laser range-finder-based prototype. Experiments were conducted on four pig hind limbs, with bicortical holes drilled in the femur and the tibia. Two surgeons alternated drilling bores in three different clinically relevant conditions: straight drilling through the diaphysis, angled drilling through the diaphysis, and straight drilling through the metaphysis. Depth measurements were taken with the laser range-finder-based prototype, and the conventional depth gauge and compared against depth measurements obtained from a CT image that served as the reference measurement.

Results

In straight diaphyseal drilling the laser range-finder-based prototype had a larger mean error of 1.34 mm (± 0.7 mm) compared with a mean error of -0.06 mm (± 1.38 mm) using the conventional gauge (95% CI 0.824 to 1.976; p < 0.001). In angled diaphyseal drilling, there was no difference in mean error between the laser range-finder-based prototype (1.66 ± 0.86 mm) and the conventional gauge (2.36 ± 3.79 mm [95% CI -2.338 to 0.938]; p = 0.393). In straight metaphyseal drilling, there was no difference in mean error between the laser range-finder-based prototype (2.11 ± 0.8 mm) and the conventional gauge (1.51 ± 3.19 mm [95% CI -0.500 to 1.700]; p = 0.280). The laser range-finder-based prototype had greater precision (smaller variance) than the conventional depth gauge in straight diaphyseal drilling (p < 0.001), angled diaphyseal drilling (p < 0.001), and straight metaphyseal drilling (p < 0.001).

Conclusions

A laser range-finder-based prototype mounted on a conventional surgical drill demonstrated overall similar accuracy and better precision in measuring drilled bore depth in bone compared with the conventional depth gauge.

Clinical Relevance

A device based on this concept could improve the reliability of bore depth measurement in surgical practice and could therefore reduce the frequency of screw replacement and associated complications.

Introduction

Surgeons who perform orthopaedic, plastic, and oralmaxillofacial surgery routinely need to measure the depth of holes drilled in bone [6]. Accurate measurements of the drilled bore depth are necessary to select the appropriate screw length for bone fixation in osteosynthesis surgery. Shorter screws are available in 1-mm to 2-mm increments, and the increments increase as the screw size increases [29]. The current measurement method for drilled holes uses an instrument called a depth gauge, which consists of a hooked wire and sliding component marked with graduations [11]. Using a depth gauge often results in placing screws of an incorrect length; the proportion of incorrect screw sizing in distal radius fixation is 9% [24]. These incorrectly sized screws can have important clinical consequences such as tendon rupture [7, 18]. Some authors have recognized the desirability of automatic depth measurement and have developed sophisticated surgical drills with built-in features for measuring bore depth [21, 27], often coupled with drilling-force measurement and anti-plunge features. However, these integrated designs require purchasing new drills, so the depth-measurement feature is not retrofittable to existing surgical drills.

Although these previously presented devices claim to improve the accuracy of depth measurement, few quantitative data have been shown to substantiate their claims. Our group has presented unpublished evidence that a mechanical linear transducer device could measure hole depth with better precision than the standard depth gauge, but we felt that this first-generation mechanical device would be relatively difficult to refine into a device that could be certified for use in the operating room as a retrofittable subsystem. In the current study, we present and evaluate a device based on laser range-finding technology.

Our specific research question is: (1) Can a prototype laser range-finder measure bore depth in bone as well as or better than a conventional depth gauge in terms of accuracy and precision (that is, variability)?

Materials and Methods

Our laser gauge prototype consisted of two commercially available laser displacement sensors mounted on a ConMed MPower2 (ConMed, Utica, NY, USA) surgical drill (Fig. 1) on either side of the drilling access in the horizontal plane (Fig. 2) and configured to send data to a microprocessor and personal computer. The sensors were a Keyence IL-300 (Keyence, Osaka, Japan) (range 160 mm to 450 mm; repeatability, 30 μm; sampling rate 0.33 ms to 5 ms) and a Panasonic HG-C1400 (Panasonic, Osaka, Japan) (range 200 mm to 600 mm; repeatability, 300 μm to 800 μm; sampling rate, 1.5 ms to 10 ms). We chose these two sensors to enable us to evaluate the influence of different levels of sensor repeatability and to evaluate a single versus dual-beam design. The sensors were oriented at a 5° angle relative to the drilling axis in the horizontal plane, and they measured displacement of the drill relative to a custom-fabricated drill guide with a flat, machined surface for reliable displacement measurement. The drill guide remained fixed relative to the bone, secured by manual pressure. The sensors were connected to an Arduino DUE microprocessor (Arduino, New York, NY, USA) and configured to send output to a personal computer with MATLAB software (Mathworks, Natick, MA, USA). The laser gauge provided continuous displacement measurements relative to the surface illuminated by a laser line. We converted the measured displacement to the distance of the sensor from the bone along the drilling axis by multiplying the measured value by cos 5° to account for the angled sensor’s position. After averaging the distance estimates produced by the two sensors, we estimated velocity and acceleration by numerical differentiation after filtering was performed using an acausal second-order Butterworth filter with a cutoff frequency of 10 Hz. The cutoff frequency was selected based on a residual spectral power analysis [26]. The bore depth was calculated post hoc by manually identifying breakthrough points on the averaged distance versus time plots, as was done in a previous unpublished study involving a mechanical sensor system. The difference in distance estimates between the breach of the second cortex (d2) and the initial position (d0) was taken as the bore depth, and the time of the breach was identified by noting a characteristic spike in velocity and acceleration (Fig. 3).

Fig. 1.

Fig. 1

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This photograph shows the surgical drill, equipped with two laser range-finding sensors.

Fig. 2.

Fig. 2

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This figure shows the laser gauge sensor arrangement from superior (left) and lateral (right) perspectives.

Fig. 3.

Fig. 3

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This figure shows plots of sample displacement, velocity, and acceleration versus time; profiles with periods of cortex drilling and breach points are indicated. A schematic view of the bone with starting position and breach points is included on the right.

To evaluate the accuracy and precision of our laser gauge prototype relative to the conventional analog orthopaedic surgical depth gauge (Stryker, Kalamazoo, MI, USA), we used it when drilling holes in fresh cadaver pig leg specimens. We chose pig bone because it is commonly used in orthopaedic surgical education [4, 19, 28], and the material properties of pig bones are known to be relatively similar to human bones [1]. To simulate surgical conditions, we made incisions in the thigh and foreleg of the pig legs and exposed the femur and tibia in a surgical fashion. A total of 125 drilling attempts were made in four pig hindlimbs. Two surgeons alternated testing the laser gauge: a staff orthopaedic surgeon (PG) and a resident in plastic surgery (DD). The surgeons drilled a series of bicortical holes in the exposed pig femurs and tibias under three different drilling conditions: straight drilling through the diaphysis, angled drilling through the diaphysis, and straight drilling through the metaphysis. The tested conditions were selected based on an analysis of images of orthopaedic fixation constructs [9], with particular attention paid to the alignment of screw placement relative to bone. During drilling, the laser gauge acquired continuous measurements of drill displacement relative to the position of the drill guide. Drill bores were made along the bone from proximal to distal. After drilling, the holes were then measured once using the conventional depth gauge by the primary author (DD).

Of the 125 attempts (65 in the femurs, 60 in the tibias), 95 holes were deemed suitable for analysis, with 30 attempts rejected for the following reasons: overlap with adjacent holes (3), drill bit exit into the joint space (10), technical problems with the drill (drill bit loosening or drained battery) (3), and technical problems with the prototype apparatus (clamp loosening and sensor shifting relative to the drill) (12). Of the analyzed attempts, seven measurements by the laser gauge were removed for uninterpretable velocity plots and 15 holes were too deep to be measured with the conventional surgical depth gauge.

After the drilling session, the legs were dissected to remove the femur and tibia, which were then sealed in a labelled bag and imaged with high-resolution peripheral quantitative CT (Sanco XTREMRE-CT, Brüttisellen, Switzerland), with an isotropic voxel resolution of 82 µm. We used the MIMICS software program (Materialise, Leuven, Belgium) to digitally identify the centers of the nominal ellipses forming the entry and exit points of each drill hole; these served as the reference standard for bore depth measurements. Our primary outcome of interest was the accuracy of the laser gauge and the conventional depth gauge measurements (that is, the difference, or error, between the depths reported by the laser gauge and conventional depth gauges and the CT bore depth measurement), along with the precision (variability of this difference) of each gauge type across the drilled holes. We also assessed the interobserver variability of the measurements by the two users (see Appendix, Supplemental Digital Content 1, http://links.lww.com/CORR/A216).

We analyzed data using Microsoft Excel (Microsoft, Redman, WA, USA). Mean error was compared using Student’s t-test for unequal variance (two-sided; p < 0.05). We used the F test for equality of two variances (one-sided, p < 0.05) to compare the variances between the laser gauge and conventional depth gauge. We created Bland Altman plots to depict the precision and accuracy of the laser gauge and the conventional depth gauge across the range of drilled depths.

Results

Neither measurement method showed consistently better accuracy in measuring bore depth across the tested conditions (Fig. 4). The laser gauge showed better precision in measuring drilled bore depth in bone than the conventional depth gauge under all three testing conditions. In straight diaphyseal drilling, the laser gauge had a larger mean error of 1.34 ± 0.7 mm compared with a mean error of -0.06 ± 1.38 mm using the conventional gauge (95% CI 0.824 to 1.976; p < 0.001). In angled diaphyseal drilling, there was no difference in mean error between the laser gauge (1.66 ± 0.86 mm) compared with the conventional gauge (2.36 ± 3.79 mm [95% CI -2.338 to 0.938]; p = 0.393). In straight metaphyseal drilling, there was no difference in mean error between the laser gauge (2.11 ± 0.8 mm) compared with the conventional gauge (1.51 ± 3.19 mm [95% CI -0.500 to 1.700]; p = 0.280). The laser gauge had greater precision (smaller variance) than the conventional depth gauge in straight diaphyseal drilling (p < 0.001), angled diaphyseal drilling (p < 0.001), and straight metaphyseal drilling (p < 0.001) (Fig. 5). The Bland-Altman plots showed that accuracy and precision of the laser gauge was consistent among different depths of drilled bore (Fig. 6).

Fig. 4.

Fig. 4

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This image shows the mean error and SD of the error for the laser gauge and conventional depth gauge under the three tested conditions. The mean error of the two measurement methods was different in straight diaphyseal drilling, but not in the other two conditions.

Fig. 5.

Fig. 5

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This figure shows the cumulative frequency distribution of the mean error for the laser gauge prototype and conventional depth gauge for the three tested drilling conditions. The horizontal bars show 95% CIs.

Fig. 6.

Fig. 6

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This figure shows Bland-Altman plots comparing the variability in error between the laser gauge and conventional depth gauge under the three tested conditions. The x-axis shows the mean of the tested measurement method (laser gauge or conventional surgical depth gauge) and the reference standard (CT). The y-axis shows the difference between the tested measurement method and the reference standard. The solid line shows the mean error, and the dashed line shows the limits of agreement (95% CI of the error).

Discussion

Measurement of drilled bore depth in bone is an essential part of modern surgical practice in multiple disciplines. We investigated whether a novel device based on laser range-finding technology could provide superior accuracy and precision compared with a conventional orthopaedic surgical depth gauge. The laser gauge prototype showed better precision and overall comparable accuracy and could be the basis for a new method for measuring drilled bore depth in bone. Further design refinements, including making the device sterilizable and automating the depth calculations, are necessary to bring the device into practice.

One limitation of this study is that it was performed using fresh pig bone; therefore, the device may perform slightly differently in human bone. However, the use of pig bone for surgical simulation is common [4, 15, 22], and there is evidence that the material properties of pig and human bone are similar [1]. In addition, only two users performed the experimental tasks: one resident (DD) and one attending surgeon (PG). This allowed us to assess whether users with markedly different experience levels obtained different results with the laser gauge. Although no differences were found (see Appendix, Supplemental Digital Content 1, http://links.lww.com/CORR/A216), it is possible that more or different users may obtain different results; it will be important for future studies with larger numbers of users to be performed. Further, the prototype makes measurements during the drilling process; thus, only one measurement can be made per hole, and therefore no direct comparison could be made for multiple users for the same hole. There were also some issues with reliability of the laser gauge prototype leading to occasionally lost or erroneous measurements, which is not unexpected given the early stage of the design.

At the current level of device development, the signal analysis is not yet automated, and the bone depth is computed post facto; thus, the device is not ready to be used intraoperatively. An intraoperative test would be required to demonstrate whether there are clinical benefits such as a reduction in the number of inaccurately sized screws or a reduction in surgical time. Although not reported here, we found that the device produced more variable results if a drill guide was not used (presumably because of changes in the estimated distance to the bone owing to changes in the drill’s angle of approach during the drilling process); consequently, we are not prepared to claim that it would produce accurate results if used without a drill guide. Fortunately, for most of the potential surgical applications we anticipate, it is standard practice to use a drill guide; therefore, we do not anticipate that this will be a practical limitation.

The laser gauge demonstrated lower accuracy under straight diaphyseal drilling conditions, and equivocal accuracy under angled diaphyseal and straight metaphyseal drilling. The laser gauge showed greater precision compared with the conventional depth gauge in measuring bore depth in bone, as indicated by decreased (sub-millimetric) variability in the bore depth measurements relative to the CT reference standard (Fig. 4). Although the accuracy of the laser gauge was not better in the tests performed, the error was relatively consistent (1.34 mm to 2.11 mm). This can be interpreted as bias in the sensor and accounted for in device calibration and thus does not limit the viability of the device concept. Additionally, the magnitude of the errors are close to the interval difference in screw size and may not be clinically significant in most use cases. Previous studies have attempted to incorporate sensors into surgical bone drilling. Some have examined incorporating sensors into a hand-held surgical drill to prevent the drill bit from plunging beyond the distal drilled cortex [3, 5, 10, 30]. Other studies have described developing a drilling system mounted on a robotic arm to allow precise control of drilled depth [2, 8, 12–14, 16, 17, 23]. Generally, these systems relied on force and torque sensors, which must be integrated into the drill’s transmission assembly. If they are to be used in practice, existing surgical drills would have to be replaced. The change in drilling forces between cortical bone and the medullary canal or adjacent soft tissues has been shown to be effective in identifying layer transitions, which has often been used in conjunction with an automatic stopping mechanism on the drilling device to prevent the drill bit from plunging into adjacent structures. Investigators have proposed using range sensing principally to replace the existing depth gauge [20]. Laser triangulation has also been described in an application involving laser ablation of bone for dental implant insertion [25], but in that application, the laser source was fixed and the depth of the resulting hole in the tooth was measured based on the distance to the hole’s bottom. Because we had a through-hole in our study, this approach was not relevant. The accuracy and repeatability of the laser-ranging systems mentioned above that are designed for through-hole depth measurement have not yet, to our knowledge, been reported.

Three main challenges remain to be solved before this concept can be used in the operating room. First, a software algorithm needs to be developed to analyze the sensor output in real time so that screws may be selected immediately after the bore is drilled. Because of the presence of spikes in the velocity and acceleration profiles, we feel that developing this algorithm will be relatively straightforward. Second, the device needs to be sterilized, either through using a disposable cover or sterilizing the device itself. In recent related unpublished work, we demonstrated that we can operate a depth camera (Intel RealSense, Santa Clara, CA, USA) through a transparent sterile cover. Therefore, we believe that such a cover would likely work with the laser gauge without impairing its accuracy, although this presumption must be evaluated. Lastly, the device must be made more reliable because the prototype device malfunctioned approximately 10% of the time. Causes of malfunction included both hardware issues (device loosening from the surgical drill and shifting out of position, so the measurements were no longer accurate) and software problems (computer program experiencing an error in data collection). We believe these concerns could be addressed with straightforward refinement of the prototype and software design.

In summary, we have demonstrated that using a laser range-finding device can improve the reliability of measuring hole depth relative to the existing depth gauge under three realistic surgical drilling conditions. In all cases, accuracy relative to a CT-based measurement was between 1 mm and 2 mm, repeatability was better than 1 mm. A device based on laser-displacement sensing may be a promising alternative to the conventional depth gauge for osteosynthesis surgery.

Acknowledgments

We thank the Division of Plastic and Reconstructive Surgery at the University of British Columbia for their time during the “needs assessment” component of the project. We also thank the Centre for Hip Health and Mobility and the Clinician Investigator Program at the University of British Columbia.

Footnotes

One author (PG) certifies that he received grants from the Canadian Institutes for Health Research and Natural Sciences and Engineering Research Council (Ottawa, ON, Canada) during the study; has received or may receive payments or benefits, during the study period, in an amount of less than 10,000 USD from Traumis Surgical Systems Inc (Vancouver, BC, Canada); has received or may receive payments or benefits, during the study period, in an amount of 10,000 USD to 100,000 USD from Stryker (Kalamazoo, MI, USA). Each remaining author certifies that neither he, nor any member of his immediate family, has funding or commercial associations (consultancies, stock ownership, equity interest, patent/licensing arrangements, etc.) that might pose a conflict of interest in connection with the submitted article.

All ICMJE Conflict of Interest Forms for authors and Clinical Orthopaedics and Related Research® editors and board members are on file with the publication and can be viewed on request.

Clinical Orthopaedics and Related Research® neither advocates nor endorses the use of any treatment, drug, or device. Readers are encouraged to always seek additional information, including FDA approval status, of any drug or device before clinical use.

Each author certifies that his institution waived approval for the animal protocol for this investigation and that all investigations were conducted in conformity with ethical principles of research.

This work was performed at Vancouver General Hospital, Vancouver, BC, Canada.

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