The use of a robotic tibial rotation device and an electromagnetic tracking system to accurately reproduce the clinical dial test

The dial test was developed for the evaluation of a posterolateral corner (PLC) injury in the knee [5, 6, 15, 16]. The traditional description of the test includes rotation of the foot, while in dorsiflexion, from neutral to full external rotation and from neutral to full internal rotation while noting the angle created by the foot and the thigh. An increased amount of external rotation on the injured side is indicative of a potential PLC injury [6]. This simplistic view of the dial test does not reflect the sophisticated capture of information during the test [1]. The modern dial test involves the patient in the supine position or seated with the knee at 30° or 90° of flexion. The clinician rotates the tibia along its long axis both internally and externally while noting the position of the tibial tubercle and the force necessary to rotate the tibia. An instrumented dial test must reproduce these exact characteristics and must be able to quantify them.

An injury to the PLC is a complex injury that involves damage to multiple structures including but not limited to: the arcuate ligament, the popliteus tendon, the lateral meniscus, the lateral collateral ligament and the posterior cruciate ligament [19, 21, 22]. Only extreme damage creates biomechanical changes obvious to the clinician during a manual clinical knee examination [5]. In fact, isolated increased external tibial axial rotation may not identify the patient with a PLC injury without additional manoeuvres during the examination [13, 14]. A description of rotatory instability by measuring the relative translation of the lateral compartment of the knee with respect to the medial compartment appears to provide better accuracy in the diagnosis of a PLC injury [7, 25].

Instrumented devices have been developed in an attempt to better record the complex tibial motion during the dial test [4, 24, 27, 28]. However, these devices still have measurement error due to variability in the applied force, strain rate and patient setup. A robotic testing device has been previously developed that allows for consistent patient setup and consistent application of force [9, 11]. This device can generate load-deformation curves to describe tibial motion using the torque and rotation data collected through the motor. An improved version of this robotic testing device in which rotational data was collected in six degrees of freedom using an electromagnetic tracking system has been developed [8, 10]. To date, only long-axis rotation data has been reported using that system. However, long-axis rotation alone is not adequate to describe the true motion of the tibia during the dial test.

The purpose of this study was: (1) to determine whether a robotic tibial rotation device and an electromagnetic tracking system could accurately reproduce the clinical dial test at 30° of knee flexion; (2) to compare rotation data captured at the footplates of the robotic device to tibial rotation data measured using an electromagnetic sensor on the proximal tibia. The primary hypothesis of the study was that the robotic device and electromagnetic system would be able to provide data that would accurately represent the motion of the tibia during the dial test. A second hypothesis was that motion data measured using servomotors attached to footplates of the robotic device would not accurately represent the motion of the tibia as measured using an electromagnetic sensor on the proximal tibia. A final hypothesis was that the relationship between medial–lateral and anterior–posterior translation during tibial axial rotation could provide additional valuable information to the clinician.

After obtaining ethics approval, a subset of 68 patients from a cohort study following single-bundle (SB) and double-bundle (DB) anterior cruciate ligament (ACL) reconstruction using hamstring grafts were included in the study. Upon entrance into the study, patients signed human investigations consent for this IRB-approved study per the independent review board (Landesärztekammer Baden-Württemberg Ethics Commission: ID# 051-06-f). For the purpose of this paper, only the healthy knees of the patients were analysed. At follow-up, two patients were identified as having an injured opposite extremity through clinical examination, instrumented laxity testing or magnetic resonance imaging and were excluded, and another patient did not attend the follow-up. All of the remaining 65 patients had a unilateral knee injury with the opposite knee described as healthy with no history of injury or symptoms. Of these 65 patients, 33 had both knees instrumented with an electromagnetic motion tracking system (Flock of Birds, Ascension Technologies, a subdivision of NDI, Bakersfield, CA, USA) for analysis of proximal tibial motion. One patient had bilateral ACL reconstructions and was eliminated from the study group. The data from the healthy knees of the 32 remaining patients were included in this study.

Each patient was evaluated using a robotic tibial axial rotation testing device and an electromagnetic tracking system. The robotic testing device consisted of two servomotors designed to apply torque about the centre of rotation of the tibia (Fig. 1). The system and patient setup were identical to that used in previous studies [6, 8]. Patients were positioned supine in the device, with both knees flexed to 30° and with both feet attached to footplates that were mounted to the servomotor system. To limit ankle motion during rotational testing, the foot was maximally dorsiflexed using inflatable air bags at the level of the metatarsal heads.

The second toe was positioned perpendicular to the floor. The position of the toe was verified using a digital goniometer referencing the earth. This was considered 0 rotation for this study with internal rotation and external rotation defined as such.

An electromagnetic sensor was used to track the motion of the tibia during testing. The electromagnetic tracking system could record the position of the tibia with respect to the femur with an accuracy of 0.48 mm and 0.30° based on root mean square error (0.88 mm and 0.48° 95 % CI). The sensor was attached to a custom aluminium harness that did not interfere with the electromagnetic tracking system. The sensor was calibrated such that the x-axis was directed along the long axis of the harness, the y-axis was directed laterally and the z-axis was directed posteriorly on the right knee. In the left knee, the y-axis was directed medially (right-handed coordinate system). The harness was positioned on the leg with the sensor directly over the palpated tibial tubercle with its long axis pointing at the second toe and with the foot dorsiflexed. The harness was applied as the last step in the patient setup. Care was taken to position the y-axis of the harness/sensor parallel to the table and to the posterior condyles of the distal femur. While the sensor was not directly on the skin, it rested approximately 5 mm from the surface and was consistent side to side.

Both extremities were rotated at the same time into external rotation followed by internal rotation. The motors rotated each leg until the peak torque of 4 Nm was achieved, at which point the direction of rotation was reversed. Three preconditioning cycles were performed, followed by three test cycles with data recording. In addition to the motion measured using the electromagnetic tracking system, rotation of the lower leg as measured at the foot was also collected using an integrated optical encoder at the servomotor. Current was continuously measured and converted into torque (Nm) via an on-board computer program. Motor data was accurate to 0.01° and 0.01 Nm of torque as per the specifications of the motors. No filtering of any kind was performed.

A hysteresis curve was constructed of the three test cycles, with torque on the y-axis and rotation on the x-axis. Using the loaded portion of the hysteresis curve for each cycle, a third-order polynomial fit of the data was used for analysis. Once fitted, each curve was interpolated for a standard set of 500 torque points between −4 Nm (External Rotation) and +4 Nm (Internal Rotation). No averaging or registration was applied to the data. The position data from the electromagnetic tracking system did not require curve fitting.

The third full load-deformation curve was considered representative of each patient and was used as a single test. Within-test averaging of the multiple cycles was not utilized. Mean load-deformation curves between all patients were constructed using the pointwise mean (i.e. the mean for each of the 500 torque points) of each group along with the pointwise standard error of the mean (SEM). Using the means allowed for pointwise statistical comparison.

The most important finding in this study was that a robotic tibial rotation device and an electromagnetic tracking system could accurately mimic the dial test by capturing torque data in addition to the motion of the proximal tibia in all 6° of freedom. Anterior/posterior and medial/lateral translations of the tibia were of primary interest along with axial rotation of the tibia. Compression/distraction, varus/valgus rotation and flexion/extension rotation were measured, but not reported in this study. This system provides significantly more information than the traditional manual dial test that focused on long-axis rotation alone and provides numerical knee laxity data that can mimic the modern dial test. Capturing both the rotation of the tibia as represented by the tibial tubercle and its translation in the coronal (ML-AP) plane is critical in fully characterizing the motion of the proximal tibia. This information allows for increased accuracy in making a diagnosis of a knee injury.

The use of the dial test as a means of diagnosing rotational injuries to the knee has been discussed in the literature since it was developed from the posterolateral drawer test as described by Hughston and Norwood [19]. There are many ways in which this test has been reproduced with the main focus on increased external rotation on the injured side as the key measure of potential PLC injury. Pearle and others have abandoned the use of rotation alone and have incorporated lateral compartment translation as the key measure [12, 25]. The use of the electromagnetic tracking system during the dial test allows for a measure of proximal tibial translation. The combination of rotational and translational load-deformation curves describing the motion of the proximal tibia should add to the surgeon’s clinical database in determining the presence of rotational instability.

The second important finding in this study was that the use of the foot as a measure of tibial axial rotation, and thus, in the production of a tibial rotational load-deformation curve will lead to inaccuracies. The traditional dial test uses the foot–thigh angle to determine tibial axial rotation at the knee. Unfortunately, the position of the foot in internal/external rotation is not correlated with the position of the tibial tubercle. This finding matches the results of previous studies that have shown that the rotation of the foot is not an accurate measure tibial rotation [2, 3, 26]. This does not mean that lower leg rotation as measured by foot position is not valuable. Foot position and its concomitant load-deformation curve have value in studying the leg as a system [9, 11]. However, foot position cannot be used to indicate tibial position with respect to the femur.

The third important finding in this study is that the motion of the tibial tubercle in the coronal (ML-AP) plane helps define the location of a “centre of rotation” or a general “pivot point” within or near the proximal tibia during tibial axial rotation. The shape of the tibial tubercle pathway and its direction give insight into the condition of the ligaments influencing this pathway during rotation. For example, if the main ligament of rotational influence is the medial collateral ligament, then the proximal tibia is likely to rotate about a “medial pivot” (Fig. 5a). On the other hand, if the medial collateral ligament is damaged, perhaps the proximal tibia will rotate about a “lateral pivot” (Fig. 5b). The pathway data from the motion of the tubercle suggest that the proximal tibia does not rotate about a single axis.

When examining the knee for ligament injuries, the extent of motion in the injured knee is often compared to the extent of the opposite or healthy knee to determine if an injury has occurred. This dependence upon side-to-side comparison can be problematic for without a “healthy” knee the clinician has no frame of reference for analysis of the injured knee. Diagnosis in bilateral patients is extremely difficult, and in some patients who report one healthy knee, that knee may not truly be healthy but rather only asymptomatic. The knee itself is a system and has the capability of being stable or unstable like any other mechanical system. This stability can be measured in each limb individually. Currently, the pivot shift test is the best clinical indicator of a knee that is unstable or at least has the potential of being unstable during activity. It does not require comparison between knees for the test to be positive. Despite attempts to standardize the pivot shift test through either training or instrumentation, consistency and reproducibility of the in vivo test have remained low [18, 20, 23]. An understanding of the three-dimensional motion of the proximal tibia during application of tibial axial torque may be able to provide the consistency and reproducibility necessary to produce a single-limb test indicating potential knee instability. This could possibly be achieved through analysis of the shape of the pattern of motion seen in the coronal plane (ML-AP), which can be performed in a unilateral knee. This technique has not been validated, but may be explored in future studies.

This study included analysis of the healthy knees of a small sample of subjects from a population of patients requiring an ACL reconstruction. As a result, it may not be representative of the population as a whole. It has been reported that tibiofemoral kinematics can be altered in the opposite, healthy knee of ACL-reconstructed patients post-surgery [17]. In this study, the reported kinematics may not be representative of those seen in patients with bilateral healthy knees. The results in this study were limited by the setup of the patient’s foot in the device, the ability of the device to capture the distal femur and limit its rotation, and the measurement accuracy of the system. The electromagnetic tracking system was placed on the tibial tubercle and tibial crest in a consistent fashion side-to-side and from patient to patient, but its position was not calibrated to a world coordinate system nor was it calibrated to each individual anatomical malleolar axis. The x–y or coronal plane (ML-AP) translation was registered to the torque 0 position identified during tibial axial rotation. It was assumed that this position had subject-to-subject consistency in the healthy limb. The position of the tibial tubercle as measured in this study was approximately 5 mm anterior to its anatomical location.

The dial test performed by the clinician should focus observation at the tibial tubercle and not on the foot. During the clinical dial test, a mark should be made on the patient’s tibial tubercle with observations of that mark during axial rotational force application at the foot. The motion of the tubercle itself should give information as to increased rotation/translation of the proximal tibia. Certainly, during the test, motion of the medial and lateral tibial plateaus can be observed with additional information in regard to their positional change. The main point to be made is that clinical observation of the foot during the test tells the clinician something completely different than clinical observation of the proximal tibia. Pearle et al. and Branch et al. [7, 25] have shown that posterior translation of the lateral tibial plateau during the dial test is more sensitive for PLC instability than rotation alone.

The clinical information captured by a surgeon during a dial test includes both rotation of the tibia and proximal tibia translation during load application. All of this information can be captured using a robotic tibial axial rotation device with an electromagnetic tracking system and a sensor on the proximal tibia. The translation of the proximal tibia is important information that must be considered in addition to axial rotation of the tibia when performing a dial test whether done manually or with a robotic device. Instrumented foot position cannot provide the same information. While foot/ankle rotation is not a true representation of proximal tibial rotation during the dial test, it can provide valuable information for the analysis of the lower leg as a system. Finally, the pathway of the tibial tubercle during tibial axial rotation can provide additional information about knee instability without relying on side-to-side comparison between knees.