In the past few years, the incidence of knee sprains, especially in children, has increased, mainly in exercise children [1]. These children develop important complications in adulthood such as instability, associated meniscal lesions, and gonarthrosis.

The diagnosis of rupture of the Anterior Cross Ligament (ACL) is mainly clinical by Lachman Tests and the pivot shift test [2-6]. However, these maneuvers are reviewer-dependent, imprecise, and not reproducible. The sensitivity of these tests for total breaks is excellent (96% for the Lachman and 86% for the pivot shift test, but poor for partial breaks (68% and 67%) [7,8]

Imaging examinations such as MRI have excellent sensitivity and specificity in children (97.9% and 98.6% for radiologists, 100% and 98.6% for orthopedists, respectively) in the diagnosis of LCA rupture. However, these examinations are costly and cannot be used for the measurement of anterior tibial laxity and for post-operative monitoring [9]. For these reasons, measurements objectifying the anterior tibial translation have been developed: Telos, KT-1000, Rollimeter and more recently GnrB. These devices are useful in adults for objectively quantifying anterior tibial translation in ACL rupture diagnosis and postoperative follow-up [3]. The GnRB system showed that it had a better intra and inter reviewer reproducibility compared to KT 1000 [4] and Telos [8], and was more efficient in diagnosis. Applied femoropatellar pressure can influence results.

However, there is limited data in children regarding GnRB measurements of normal 3 values of anterior tibial laxity. It would, therefore, be useful to obtain reproducible qualitative data in children using a validated adult measuring device. Knowledge of standard laxity would allow for an operative indication in case of clinical or radiological doubt, particularly for partial breaks. That would facilitate post-operative surveillance.

The main objective of this study is to collect physiological values in a normal knee of anterior tibial translation.

The secondary objectives are:

• Compare this translation by gender and age of children

• Locate these values in relation to adult data

This is a prospective monocentric study conducted from November 2017 to April 2018 at the Armand trousseau hospital-university center in Paris. Mandatory parental consent is signed for study participation.

The population of the study

60 children were included, 30 girls and 30 boys. The average age was 11.21 years ± 2.14. 3 age groups are formed: the first group of 8 to 10 years, the second group of 10 to 12 years, and the third group of 13 A15 years.

Each group contained 10 girls and 10 boys. The inclusion criteria were between 8 and 15 years of age and no prior pathology or knee surgery. Children with neuromotor disabilities were not included (Figure 1).

Figure 1: Measurement by GnrB.

The GNRB is a device used to diagnose total or partial ACL failures. The principle of the apparatus is to produce a push force on the calf using an articulated mechanical system and to record the movement of the tibia for each effort (from 0 to 300 N).

The test consists of three separate parts:

• The leg fastening on the device: The patient is in dorsal decubitus; the backrest incline must not be greater than 30° and placed in the GNRB axis. The body is released, the head rests on the examination table and the arms are extended. The knee must rest in the center of the support where the patella is tightened. The foot rests on the adjustable base of the heel. The knee is in neutral rotation so that the patella is in Zenith

• Patella and TTA positions are identified and marked. The hull is fastened to the patella in a horizontal position and centered on the patella. The foot is fastened to the support so that the heel stops on the pedestal, then the pedestal measure is lifted

• The displacement sensor is positioned on the TTA mark at 90° from the spin axis

• The data is saved and the test is launched

• The data are then analyzed and curved

• 4 measurements are made for both knees, at a push of 134 N, 150 N and two at 200 N, which makes 8 measurements for each child. The test is continued if it is painless. Significant pain causes the procedure to stop.

Pediatric specificities

The equipment must be adapted to the child’s corpulence. Two patellar fastening sizes were available and selected according to the child’s size. We would add a posterior reinforcement if necessary, to obtain sufficient clamping force (Figure 2).

Figure 2: Paediatric specificities.

Statistical analysis

Variables are distributed according to normal law. The Student test is used to show statistical differences between groups with a p<0.05

Results

• Four children of 8-10-year age group experienced pain at 200 N, forcing us to stop the measurements

• The average rotulian clamping forces were 35 for the 8-10-year group, 39 for the 10-12-year group and 46 for the 13-15-year group

• We made sure that there was no difference between the left and right knees (Table 1)

Table 1: Comparative degree left and right knee.

• The overall displacement for the right knees was 8.35 mm, 2.78 mm with a slope of 45.38 μm/N, 16.47 μm/N. The displacement for the left knees was 7.82 mm, 2.74 mm with a slope 41.59 μm/N, 14.48 μm/N

There was no significant difference between the right and left knees at each push (p=0.09; 0.11 and 0.31). The displacement values for all children were 134 N, 150 N and 200 N, respectively 7,01 mm ± 2,7 mm; 7,34 mm ± 2,59 mm and 9,03 mm ± 2,57 mm.

The av erag e s lo p e o f t h e t o t al g r o u p was 43.47 μm/N, 15.60 μm/N. We analysed the values of boys in relation to the values of girls of all ages (Table 2 and Figures 1 and 2).

Table 2: Displacement values by sex.

There is no significantly greater displacement for girls on the different 134 N, 150 N and 200 N push (respectively p=0.052; 0.146 and 0.337). The slope is significantly higher for girls than boys (P=0.006).

Analysis by age group was also performed (Table 3 and Figures 3 and 4).

Figure 3: Displacement according to the sex (in mm).

Figure 4: Slope according to the sex (in μm/N).

Table 3: Displacement values by age.

We compared the values found in these three age groups for each push

We found no significant difference in the movement between:

• Groups 13-15 years and 10-12 years (p=0.984; 0.627 and 0.564 for 134 N, 150 N and 200 N, p=0.933 on all three thrusts)

• Groups 13-15 and 8-10 years of age (p=0.775, 0661 and 0.437 for 134 N, 150 N and 200 N, p=0,476 on all three thrusts)

• Groups 8-10 and 10-12 years (p=0.796, 0.329 and 0, 849 for thrusts 134 N, 150 N and 200 N, p=0, on all three thrust)

We compared average slopes by age:

Grades 8-10 and 10-12 years of age are significantly higher than groups 13-15 years of age (p=0.007 and 0.006)

We found no difference between groups 8-10 and 10-12 years (p=0,700) (Figures 5 and 6).

Figure 5: Displacement according to the age (in mm).

Figure 6: Slope according to the age (in μm/N).

Numerous studies of anterior tibial translation exist in adults, involving different measuring devices (GnRB, Telos, K-1000).

Our study addresses the specific problem of laxity in children with the GnrB device.

The first difficulty encountered is practical. The GnrB is designed for lower limbs of adult size. As a result, particularly in category 8-10, the child’s knee size is less appropriate, and it is more difficult to adjust the rotulian clamping force as well as to continue the study to completion. We made adjustments (foam reinforcements in the popliteal hollow) to obtain a satisfactory clamping force. Indeed, for a displacement of 200 N, it has happened in 4 children that the test is painful and that one is obliged to stop the measures, thus resulting in a loss of data for this age category.

If we want in common practice to use the GnrB on the child, the device should be adapted to its corpulence in order to reduce the pain and ensure more accurate measurements.

Another limitation of the study is the lack of control by a second reviewer. Although the measurement technique is relatively simple and proven, a single reviewer is a measurement bias in this study.

In the literature, the values of anterior tibial laxity found in adults are not uniform:

Jenny and Arnt found a laxity of 8.6 mm; 2.8 mm for a thrust of 250 N. Sheep and all found a laxity of 4,7 mm; 0,7 mm to 200 N in 2014 [11] and 5 mm; 1.3 mm in 216 [12].

Vauhnik [13] found higher values with 6 mm to 134 N and 9 MM to 250N. Bouguennec [8] found a translation ranging from 4.86 to 5.67 mm 134 N. Colette [14] found values between 2.5 mm and 3 mm at 134 N. Other studies using GnrB do not show their gross results. However, by appreciating the average curves, the values found for 134 N thrusts are 3.5 mm and 5 mm for 200 N thrusts [6,15].

We find values of 7 mm to 134 N and 9 MM to 200 N, Which appear to be higher than adult values in the literature. This confirms the greater anterior tibial laxity in the child with this measuring device. This greater laxity was already known by the Breighton score measure [5] for which 55% of children aged 4 to 14 have a score greater than or equal to 4 (corresponding to hypermobility in adults), and 71% for children under 8 [16-20].

This difference must be considered when comparing the relaxation values of previous cruciate ligament surgery. We have not been able to identify a significant difference between ages, although the trend is more laxity among younger groups than among pre-teens. It is logical to think that the more the child grows, the closer his laxity is to that of the adult. The same applies to sex. Girls seem to have a greater laxity, but no significant difference except on average slope. A significant difference should be shown with larger groups of subjects in subsequent studies.

It would also be interesting to reproduce in children comparative studies of healthy knee laxity and cross ligament rupture in adults [6,15,16] in order to diagnose and follow with the Gnrb the children who are victims of rupture of the anterior crusader ligament and who benefit from restorative surgery.

Our results tend to show a greater anterior tibial translation in children than adults. It also suggests a decrease in this translation with age. This information shall be taken into account in the treatment of ruptures of the anterior cross ligament.