University Medical Centre, Department of Orthopaedic Surgery, Zalo_ka 9, 1000 Ljubljana;

_"J. Stefan" Institute, Jamova 39, 1000 Ljubljana, SLOVENIA

A new non-invasive quantitative method for evaluation of the anterior cruciate ligament function was tested in 24 asymptomatic subjects and in 58 patients (39 males and 19 females) with arthroscopically proven unilateral rupture of the anterior cruciate ligament. Anterior displacement of the tibia with the knee in 20 degrees flexion was achieved by using a redesigned device, constructed originally for stressradiography, and detected by a 3-D optical motion analysis system. In the normal group, the average difference (d) in anterior displacement of the tibia between the left and the right side under a displacement force of 250N was 0.56 mm (range 0.1mm to 1.9mm) or 17% (range 2% to 64 %), while in patients the average difference (d) in anterior displacement of the tibia between the injured and uninjured side under a displacement force of 250N was 4.02mm (range 0.6mm to 9.4mm) or 133.6% (range 26% to 451%). By the criteria for false negative results, i.e. the maximum value of the parameter d in normal subjects (1.9mm and 64%), 10% of the patients (6 patients) fell into the normal group. According to the criteria of false positive results, i.e. the minimum value of the parameter d in patients (0.6mm and 26%), 17% of the normal subjects (4 normal subjects) fell into the group of patients.

INTRODUCTION

Injury to the anterior cruciate ligament (ACL) is the most frequent cause of knee instability, and only in the USA approximately 70,000 ACL reconstructions are performed each year [16]. The ACL deficiency is detected clinically [10,14,22] and is highly dependant on the examiner’s skills. In order to obtain more objective and reliable information on the degree of instability, several measurement devices and techniques have been developed, including instrumented clinical testing [5,12,13], arthrometric measurement [3,21] and stressradiography of the knee [4,7,8,9,19].

In the present study we present a new measurement technique for the evaluation of the ACL deficiency using the ELITE 3-D optical motion measurement and analysis system [6].

Anterior laxity of the knee was measured in a standard way [1], with the patient’s knee placed in the Scheuba device (TELOS, Medizinisch- Technische GmbH, D-6103 Griesheim). In our testing, the TELOS device exerted force on the posterior part of the tibia, 8 cm distally to the joint line [1,17], where at the same time movements of the thigh and ankle were blocked mechanically. The experimental subject was placed in the lateral supine position with the knee in 20 degrees flexion [3,9,12]. A force of up to 250 N [1,17] was applied gradually over 10 seconds by a hand-controlled electromotor drive. A relative anterior displacement of the tibia was recorded by the ELITE 3-D motion analysis system [6]. The upper passive optical sensor was placed 2 cm above the patella, and the lower one on the tuberosity of tibia. For data acquisition two cameras were used. During testing the patient was instructed to relax the leg completely. The EMG recording of the hamstrings and quadriceps muscles was performed simultaneously in order to rule out any erroneous contractions of the thigh muscles. Five consecutive measurements were done for each knee of the tested subjects, and the average value and standard deviation of the five measurements were calculated. Anterior laxity of both knees was evaluated in all subjects. The parameter d represents the difference between the average values of the left and right side in controls and between the average values of the injured and uninjured side in patients. The parameter d is expressed in mm (d- absolute side-to-side difference) and in percentage (d%- percentage side-to-side difference).

The method was used in 24 asymptomatic subjects (14 females and 10 males; average age 23 years; range 12 to 53 years) (Graph I.) and in 83 patients. The diagnosis of ACL rupture was established by arthroscopy in all patients. The inclusion criteria were: unilateral involvement and no pain during testing. Twenty-five patients out of 83 patients were excluded from the study: 11 patients because of painful knees, 8 because of technical error, and 6 because of bilateral anterior knee instability. Finally, 58 patients with unilateral ACL deficiency were included in the study (39 males: average age at testing 30.2 years; range 18 to 45 years; 19 females: average age at testing 21.5 years; range 16 to 32 years) (Graph II.).

Before the testing all patients were examined clinically prior to the testing by the second author (V.P.). The following results: Lachman’s test was negative in 16 out of 58 patients (27%), the pivot shift test in 14 (24%), and both clinical tests (Lachman’s and pivot shift test) in 9 patients (16%).

In 58 patients (Fig. 3), the absolute difference (d) in anterior displacement between the average value of five consecutive measurements on the normal and the injured knee was 4.02 mm (min 0.6mm, max 9.4 mm) (Fig.3.). The absolute percentage difference (d%) between the injured and the uninjured side was 133.6% (min 26%, max 451%).

In the asymptomatic subjects (Graph I.), the difference in anterior displacement between the average value of 5 consecutive measurements on the left and the right knee (d) was 0.56 mm (min. 0.1mm, max. 1.9 mm). The percentage difference between the displacements of the left and the right knee (d%) was 17% (min. 2%, max. 64%).

The criteria of 1.9 mm (d) and 64% (d%) side-to-side difference (maximum values in normal subjects) gave false negative results in 10% of cases, meaning that 6 out of 58 patients fell into the normal range.

The criteria for false positive results are based on minumum values of the parameter d in patients, of 0.6mm (d) and 26% (d%). Thus 4 normal out of 24 fell into the group of patients, the rate of false positive results being 17%. Of the 3 patients (8%) assigned to the normal group, 2 had a negative Lachman’s test, and all 3 had a positive pivot shift test.

ACL absorbs 90% of forces tending to translate the tibia in the anterior direction, whatever the angle of knee flexion [2]. The described noninvasive method for measuring the anterior knee laxity due to cruciate ligament rupture is of particular value as a quantitative part of a combined qualitative-quantitative evaluation approach, well known in the assesment of abnormal gait [15,17]. The qualitative part of the experiment consists of Lachman's test, pivot shift test and arthroscopy. This clinical assessment is based primarily on a comparison of results obtained for the affected and the unaffected knee. If the movement of the reportedly affected knee is seen and felt to be superior to that on the normal side, we are inclined to consider the test as positive.

It was shown [3] that the average laxity of the unaffected knee was significantly greater in a group of subjects with involvement of the other knee than in a group of normal subjects. This finding, however, is of no particular value to the examiner testing individual patients. In both meneuvers the intrinsic deficiency of this qualitative method lies in the undefined time pattern of the force applied to the knee, which is true of the amplitude, direction and the grasp that defines the site of application. Using the pivot shift test, the examiner can typically obtain marked dislocation of the tibia only initially, as most patients, feeling marked discomfort or pain in the knee, unconsciously block the subsequent trials by contraction of quadriceps muscles because of marked discomfort or pain felt in the knee. Arthroscopy, as an invasive investigation technique, affords a detailed visualization of internal articular structures and injured segments, yet it sometimes fails to determine their direct impact on the functional behaviour of the knee.

The main advantage of the newly developed noninvasive optical measurement technique is that the direction of force and the ampitude and site of application are accuretely defined and documented, and that dislocation movement of the tibia towards the femur during the application of a growing force is continuously monitored with the 3D optical system using two sensors. The results obtained for normal knees are in accordance with those yielded by stressradiography [11,20], instrumented testing devices [5] and arthrometer measurements [21]. Unlike stressradiography, which measures only the initial and final stages of displacement, the optical motion system provides continuous measurement of displacement under an increasing force. Displacements of ACL-deficient knees obtained with our method are similar to those obtained by arthrometers and smaller than displacements determined by stressradiography [20] or arthrometers [11].

The above described non-invasive optical technique will be used to follow-up operated patients after the completion of rehabilitation programme, what is six to twelve months post surgery, to provide quantitative data on the effectiveness of patelar tendon operation employed for ACL reconstruction.

In conclusion, encouraged by the results of our study, and in view of the relevance of the provided data to accurate diagnosis, we have introduced the new diagnostic method combining Lachman's test, pivot shift test, arthroscopy and the new non-invasive 3-D optical motion analysis in the routine evaluation of all patients with ACL problems presenting at our Department .

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GRAPH I.

Absolute (d) and percentage differences (d%) in anterior displacement of the tibia

(average of five consecutive measurements) between the left and the right knee at a force of 250 N for 24 normal subjects.

GRAPH II.

Absolute (d) and percentage differences (d%) in anterior displacement of the tibia (average of five consecutive measurements) between the injured and the uninjured knee at a force of 250 N for 58 nonoperated injured subjects.