BLU-DAT: a new reliable and accurate arthrometer for measuring anterior knee laxity

Diagnosis of ACL injury has been greatly improved in the last decades, thanks to the development of imaging modalities for the diagnosis of musculoskeletal diseases. Nonetheless, it is still strictly based on the clinical examination and functional tests that measure knee laxity and by assessing the functional competence of ACL. Among various tests, the Lachman test and the pivot shift test are the most accurate for diagnosing ACL insufficiency, both in acute and chronic conditions [23].

Quantification of ATT has not only diagnostic value in ACL injuries but also has significance in the outcome assessment of reconstructive surgery [11]. For this reason, various devices called “arthrometers” have been developed since the 1980s. These include their progenitor KT 1000 (MEDmetric Corporation, San Diego, CA, USA), Rolimeter (Aircast, Summit, New Jersey, US), GNRB (Genurob, Laval, France) and many others. Numerous studies evaluated the reliability and accuracy of these devices, both in clinical and experimental settings. A recent consensus [22] has encouraged their use to increase the reliability and validity of the assessment of anterior knee laxity, especially for follow-up evaluation of surgical treatments.

Indeed, the reliability and diagnostic accuracy of knee arthrometers can be affected by several factors, such as knee position, examiner’s experience and patient’s compliance [19]. Besides, some devices are unsuitable for the outpatient setting because of their size or costs. An instrumented laxity testing device should handle all these issues to be as accurate and reliable as possible and suitable for every condition of use.

The purpose of the present study was to evaluate intra- and inter-observer reliability and diagnostic accuracy of a new portable device for testing ATT, which, unlike the devices already on the market, is suitable for both clinical and experimental settings. The hypothesis of the study was that the new arthrometer has good validity in terms of both reproducibility and diagnostic accuracy.

The local IRB and Ethic Committee approved the study protocol (Prot. No. 4767, University of Brescia, ASST Spedali Civili). The study was conducted according to the principles of good clinical practice and of the Declaration of Helsinki and its updated version (Tokyo 2004). The study was designed as a prospective observational study with a control group. Guidelines established by the QAREL checklist for reliability study were followed [10]. Also, standards for reporting diagnostic accuracy studies (STARD) were adopted to assess diagnostic accuracy [3].

Seventy-eight subjects were consecutively recruited for the present study and divided into two groups of 39 patients each (Fig. 4). ACL-injury group consisted of 33 males and 6 females (mean age: 29.8 ± 13.2 years); the control group consisted of 25 males and 14 females (mean age: 38.3 ± 11.9 years).

Descriptive data for all measurement series on ACL-injured patients are reported in Table 1.

Intra-observer reliability was good for 7-kg and 9-kg measurements and excellent for MMT. Inter-observer reliability was moderate for 7-kg measurements ang good for 9-kg and MMT. Intraclass correlation coefficients for intra- and inter-observer reliability are shown in Table 2. SEMs are reported in Table 3.

Comparison between ACL-injured and control groups for the side-to-side difference in ATT showed a significant difference between the two groups at every loading condition. Mean difference between groups ranged from 3.38 mm for 7-kg measurement to 4.55 mm for MMT (Table 4).

Output of diagnostic accuracy estimates is reported in Table 5. As no false positives were reported at all loading conditions, PLR, AUC, DOR and Youden index could not be calculated. Overall accuracy ranged from 84.6% for 7-kg measurements to 98.7% for MMT. Detailed diagnostic accuracy measures are reported in Table 6.

The main findings of the present study are that the BLU-DAT has proven to be a valid instrument for testing anterior knee laxity and that the greater were loading conditions the better were results in terms of intra- and inter-observer reliability for measuring ATT.

Since the introduction of the KT-1000, the progenitor of arthrometers, numerous studies analyzed the reliability of these instruments to reproducibly diagnose ACL injury. KT-1000 and KT-2000, its successor, were certainly the most analyzed. The comparison of reliability results is not easy, as there are numerous studies in the literature that reported discordant reliability data, from poor to excellent, especially for inter-observer reliability with ICCs ranging from 0.41 to 0.92 [20, 27], but also often used different reliability assessment methods.

A recent study by Runer et al. [17] showed that the reproducibility analysis of different arthrometers (KT-1000, Rolimeter, KLT and Kira) was satisfactory for intra-observer reliability, but unsatisfactory for inter-observer reliability, thus concluding that anterior laxity analysis is best analyzed by the same operator. In another study, Murgier et al. [15] analyzed the agreement of measurements between different arthrometers (KT-1000, Rolimeter, GNRB and Telos) and found that different devices are difficult to compare in terms of side-to-side difference.

As mentioned above, there are several confounding factors that can undermine the reliability of these devices. These include the type of test used, experience of the examiner, applied force, knee flexion angle [9], tibial rotation and patient’s compliance. Those limitations led to create devices that reduce the role of the examiner, like the GNRB, whose reproducibility and validity has been tested with optimal results [4, 12]. This automated arthrometer exerts via a linear jack thrust forces chosen by the examinator with the lower limb positioned in a rigid leg support with the knee at 0° of rotation. In addition, surface electrodes are applied to the back of the thigh to control hamstring relaxation of the tested knee (feedback effect). Several studies in the past have evaluated its reproducibility, the most recent evaluation was performed by Smith et al.[21] who found moderate to good intra-rater reliability (ICC = 0.72–0.83) and good inter-rater reliability (ICC = 0.76–0.81). Unfortunately, automation required to minimize the role of the confounding factors results in a loss of handling and an increase in costs, making it difficult for routine outpatient use, and therefore, not comparable to portable devices like Rolimeter. Validity of Rolimeter was tested in several studies [5, 17]. Hatcher et al. [6], in a setting very similar to that of our study, analyzed its reliability and reported excellent intra- and inter-observer reliability (ICC = 0.912 and 0.945, respectively) during the Lachman test at 30° of knee flexion. However, extreme handling can prevent control of some variables involved and can impair the standardization of the testing setup.

The BLU-DAT can be a good compromise between manageable arthrometers like Rolimeter, and more complex and expensive devices such as GNRB and KT-1000, being able to keep excellent handiness and maintaining an objective control of the knee flexion and of the applied force, thus allowing standardization of the test also in the routine outpatient activity. To confirm this, the evaluation of its reliability in a cadaver study was recently published [14]. Specifically, inter-rater reliability was evaluated under the same loading conditions as in the present study with ICC for average measurements very good at all different loads (0.89, 0.85 and 0.88 at 7 kg, 9 kg and MMT, respectively). In addition, agreement of the BLU-DAT with a gold standard such as stress radiographs analyzed with the Bland–Altman method showed good agreement with a mean difference between the two methods of 0.83 mm ± 2.1 mm (95% CI 0.55–1.11).

The second outcome analyzed in our study was diagnostic accuracy. The BLU-DAT proved to be very good in terms of accuracy for diagnosis of ACL rupture, with a diagnostic accuracy of 98.7%. Also, for this outcome, the best results were observed when performing the test at MMT.

This agrees with previous results reported with other devices. In a meta-analysis by van Eck et al. [25], several arthrometers (KT 1000, Stryker Knee Laxity Tester and Genucom Knee Analysis System) showed better sensitivity, specificity and overall accuracy at MMT than at lower loads.

A similar finding was found by Klouche et al. [8] with GNRB, where accuracy was proportional to the force applied, except for the maximum force they applied (250 N) and, as explained by the authors, this may be caused by the patients exceeding their pain threshold. In our test series, this type of reduction in compliance due to excessive traction never occurred.

In terms of diagnostic accuracy, a new automatic knee arthrometer (AKA) recently introduced by Niu et al. [16] is noteworthy, which allows minimizing the examiner's bias by means of an automatically performed thrust. The authors found higher accuracy than KT 2000 (sensitivity: 86% vs 83%; specificity: 95.5% vs 88.5%), using a threshold of 3 mm as in the study. Wu et al. [28], recently tested a new automatic knee arthrometer (Ligs Innomotion). At a load of 150 N (comparable to the MMT in the present study), the authors found the maximum AUC (0.857) and a sensitivity and specificity of 0.87 and 0.73, respectively. The present study has some limitations. First, the study lacks a direct comparison with another arthrometer that has already been validated and studied. Second, we did not assess ATT in a postoperative setting, therefore external validity of the study cannot be extended to the use of the device for assessment of surgical treatment at clinical follow-up. Finally, the investigators were blinded to the measurements of the other investigator, but for practical matters, they were aware of the group to which the individual patients belonged. The Blu-DAT can be routinely used in the outpatient setting to confirm the suspicion of an ACL tear.

The BLU-DAT has proven to be an instrument with good intra- and inter-observer reliability and very good accuracy in the diagnosis of ACL injuries in the outpatient setting. MMT loading condition provided the best reliability and accuracy.