New sonographically-guided test for anterior knee instability - preliminary report.

AIM OF THE STUDY: Ultrasound examination is widely used in orthopedic diagnostics, however sonographic evaluation of traumatic anterior cruciate ligament insufficiency is still inadequate. Aim of this study is to evaluate diagnostic capability of a new sonographically-guided test for diagnosing complete anterior cruciate ligament insufficiency.
MATERIAL AND METHODS: In 47 patients, with suspicion of unilateral anterior cruciate ligament injury (based on magnetic resonance imaging), the sonographically-guided test for anterior instability was performed. The translation of the intercondylar eminence against the patellar tendon was measured in both knees. Afterwards all patients underwent arthroscopy.
RESULTS: In 37 patients, with arthroscopically confirmed complete anterior cruciate ligament insufficiency, the mean anterior knee translation was 8.3 mm (SD = 2.8) in affected knee vs. 3 mm (SD = 1.1) in uninjured knee (p < 0.001). In 10 patients with no anterior cruciate ligament insufficiency the difference between body sides was not significant (2.6 mm, SD = 1.4 in injured knee vs. 2.5 mm, SD = 1.1 in uninjured joint; p < 0.7753).
CONCLUSIONS: The proposed test supports the clinician with fast and non-invasive examination that can facilitate evaluation of anterior knee instability.

Introduction

In order to avoid damage to the meniscus, secondary degenerative disorders and proprioceptive gonarthrosis, accurate and non-invasive diagnosis of anterior cruciate ligament (ACL) injury followed by the proper treatment are essential(. Complete ACL rupture results in anterior knee instability, however tests evaluating this parameter depends on the subjective opinion and the experience of the examiner. Mechanical devices evaluating translation between the femur and tibia (ex. arthrometr KT-1000, Stryker Knee Laxity Tester etc.)( might aid the diagnosis but they are not widely used by clinicians. Magnetic resonance imaging (MRI) is the gold standard modality for diagnosing knee pathologies(, however ultrasound allows for dynamic clinical tests with visualization in real time. Due to its wide availability, it is indicated to introduce new method for ACL evaluation. It should allow for quantitative and objective instability assessment and it should characterise with steep learning curve. Such a test might become an ideal first-line imaging technique when ACL insufficiency is suspected. The aim of study is to present and assess the usefulness of a new, sonographically-guided examination of ACL insufficiency.

Materials and methods

Examination design

Between the years 2008–2010 a sonographically-guided, dynamic test assessing anterior knee laxity was performed in 47 patients who: experienced acute knee trauma; had a suspicion of ACL injury (based on clinical assessment and MR examination); and were planned to undergo arthroscopy. In all patients it was the first-time injury of the knee that required medical attention. There was no history of the contra-lateral knee trauma. Ultrasonography was performed between 10–365 days after trauma (average 42 days). Written informed consent has been obtained from all patients. Examination protocol complies with the Declaration of Helsinki.

Ultrasound examination Protocol

The group consisted of 31 men and 16 women with an average age of 32 years (SD = 1). The GE Vivid 7 ultrasound machine, with a 6–14 MHz linear transducer (12 L), was used for the study.

To prepare the start position for further examination, several push-pull movements of the lower leg were performed in the patient laying in a supine position with bent legs. This manoeuvre insured proper muscle relaxation(, which allowed for further, more precise measurements.

Afterwards, the patient was seated and a elastic roll (a diameter of about 20 centimeters) was placed beneath the distal part of the thigh so that the lower leg of the patient hung freely from the edge of the examination couch. In this start-position the examiner sat opposite to the patient with his lower leg close to the patient's shin. The transducer was placed onto the anterior aspect of the knee parallel to the patellar tendon. After identification of the anatomical landmarks (intercondylar eminence, tibial tuberosity and patellar tendon), the examiner pushed backwards the lower leg of the patient with his foot, flexing the tibia in the knee joint (a leverage mechanism) (fig. 1). The translation of the intercondylar eminence with respect to the patellar tendon (fig. 2) was evaluated. Force was applied till no further displacement of the tibia relative to the femur occurred. The uninjured knee was also examined and served as a control.

Fig. 1

Manoeuvre made by the examiner. Arrows indicate the direction of the force applied in moving the shin from a start (A) to an end (B) position

Fig. 2

Anterior knee translation from the start position (A, C) to the end point (B, D). Representative ultrasonographs of patients with an intact ACL (A, B) and the insufficient ACL (C, D). The intercondylar eminence – IE; the patellar tendon – PT

Procedure was repeated three times and stored as a cine loop. Further analyses were performed on a workstation (EchoPack, GE). The translation was measured and given in millimetres as a mean of three repetitions.

Statistical analysis

The values of knee joint laxity are presented as the mean and standard deviation. The normality of data distribution was checked by the Shapiro–Wilk test. To compare the difference of knee joint laxity between body sides, the Wilcoxon signed-rank test was applied. Statistical analysis was performed using Statistica for Windows (version 10.0, StatSoft, Tulsa, OK, USA). A p < 0.05 was regarded as statistically significant.

Results

Arthroscopy revealed the total ACL insufficiency in 37 patients. In these patients the mean value of the anterior knee translation was 8.3 mm (SD = 2.8). In the not affected knees the mean translation was significantly lower (3 mm, SD = 1.1; p < 0.001) (fig. 3). In 10 patients with no signs of complete ACL insufficiency in MRI, arthroscopy confirmed single-bundle injury or scare-like appearance of the ligament. Difference in the anterior knee translation between injured and uninjured knees was not significant (2.6 mm, SD = 1.4 vs. 2.5 mm, SD = 1.1, respectively; p < 0.7753).

Fig. 3

A box-and-whiskers diagram displaying the mean difference of the anterior knee translation between body sides in patients with positive and negative arthroscopy confirmation

Discussion

We have presented a sonographically-guided test that may aid the diagnosis of the anterior knee instability in patients with ACL injury.

All clinical tests, proposed to evaluate anterior knee instability, have some limitations: they are subjective, imprecise, rarely reproducible, and pain in a swollen joint or a muscle spasm can interfere with proper examination(. The anterior drawer test has a good specificity (up to 91%), however its sensitivity is lower (68–71%)(. The presented test is a modification of the anterior drawer test, however due to applied leverage mechanism and evaluation of a quantitative parameter (anterior translation) may overreach diagnostic capability of classic method.

Imaging techniques are widely used to assess the ACL injury(. The specificity and sensitivity of MRI in detecting ACL injuries is 94.4% (95% CI: 92.3–96.6) and 94.3% (95% CI: 92.7–95.9) respectively(. Nevertheless, MRI evaluates only the structure of the ligament and a static subluxiation of the tibia plateau against the femur condyles, which do not correlate fully with the mechanical and functional stability of the knee. In addition, MRI produces artefacts due to metallic implant placement; is contraindicated in patients with heart stimulators and metallic foreign bodies in the eyeball; it is expensive; and sometimes unavailable as a routine diagnostic tool.

On the contrary, ultrasound examination gives the ability to assess the dynamic range of motion in a quantitative manner. Many tests for sonographically-guided examination have been proposed. Those techniques differ in: dorsal or ventral probe placement, force applied due to gravity or external source, and a number of required operators(. The presented test has several advantages over those examinations. Due to a leverage mechanism we eliminated the influence of the examiners posture and physical strength.

Thus the test can be performed by a single physician. Moreover, the test depends on the application of constantly increased force rather than on the dynamic pull. Hence, the pain symptoms due to examination can be diminished, and the displacement of the US probe during manoeuvre is reduced, enabling more precise diagnosis and improving test repeatability.

The limitation of presented test concern its decreased diagnostic ability in acute phase after trauma. However, even the MRI sensitivity is reduced in acute phase due to the presence of haematoma and/or oedema in the injured joint(.

Conclusions

The presented test in a quick and non-invasive manner allow for evaluation of complete ACL insufficiency. Due to its objectivity it holds the potential to become the first-line imaging technique when ACL insufficiency is suspected. However, further research is required to determine the test reliability and a cut-off value of the anterior translation in patients with insufficient ACL and also other knee pathologies.