Clinical tibiofemoral angle in south Indian children

Article focus

Development of the tibiofemoral angle (TFA) in south Indian children aged between two and 18 years

Key messages

The reversal of physiological varus in our population occurs by the age of two years

Peak valgus angulation is seen at five to six years of age, after which the valgus angulation gradually decreases until maturity

Girls show higher valgus angulation than boys

Strengths and limitations

Demonstrates the use of a standardised custom-made goniometer as an easy and reliable technique of measurement of the TFA

Values serve as guidelines for deformity correction in the paediatric age group and for designing joint prostheses and implants, while the standard deviations help distinguish physiological variations from pathological ones

Limitations include lack of data for children aged < two years

Introduction

Bow legs and knock knees are commonly encountered paediatric orthopaedic problems.¹ During development the knee undergoes sequential physiological changes in its axial alignment from varus to valgus. Knowledge of the normal limits of the tibiofemoral angle (TFA) in children is an aid in distinguishing physiological from pathological variations. Many children undergo treatment for physiological genu varum or valgum, which, in some cases, may prove harmful.^2-4 This knowledge also helps the surgeon determine the acceptable limits for deformity.

Physiological variations have been studied measuring the TFA and the intercondylar (ICD) and intermalleolar (IMD) distances. These parameters have been evaluated using radiological, photographic and clinical measurement with a goniometer.^2-7 Although each method has its advantages and disadvantages, the clinical method is widely used as it is non-invasive, easy, reliable and reproducible.^3-6

The existing studies have shown variations in the range of the normal values in different ethnic groups.^8,9 No normal values have been established so far in the south Indian (Dravidian) paediatric and adolescent population. This study aimed to establish such values for the south Indian population and to define differences that might exist between different ethnic groups.

Materials and Methods

A cross-sectional study was undertaken on 360 normal south Indian children, ranging in age from two to 18 years. The approval of the Institutional Review Board and the local education authorities were obtained. Children aged < five years were selected from those attending the outreach vaccination clinics of the institution, and also from two childcare centres in the district. Children aged ≥ five years were selected from six schools in the same locality. The children were divided into six interrupted age groups: two to three years; five to six years; eight to nine years; 11 to 12 years; 14 to 15 years; and 17 to 18 years. A total of 30 boys and 30 girls were recruited into each of these age groups. A sample size of 30 per age group was deemed to be adequate to obtain information on central tendency and dispersion for this normative study.¹⁰

Children with orthopaedic disorders (such as developmental dislocations of the hip, various skeletal dysplasias, cerebral palsy, neuromuscular disorders and foot deformities), those who had undergone surgery of the lower limbs and those who had a leg-length discrepancy of > 2 cm were excluded. The left lower limb of each child was measured by a single examiner (SEM). Children aged < two years were not included in this study. Age in years was verified by documents from the schools or immunisation cards in younger children.

All measurements were made by a single researcher (SEM). Standing and sitting heights were measured using a standardised height rod and weight was measured using a standardised weighing scale. The length of the thigh was measured from the anterior superior iliac spine (ASIS) to the medial joint line of the knee. The leg length was measured from the medial joint line of the knee to the tip of the medial malleolus. The sum of the above two was designated as the length of the lower limb. These measurements were undertaken using standardised measuring tape.

The tibiofemoral angle and the intercondylar and intermalleolar distance were measured as described. The children were made to stand with the hips and knees in full extension and neutral rotation, with the knees or ankles touching each other. The ASIS was marked with a skin marker pen. The centre of patella was identified with the aid of concentric circles of increasing diameters and then marked with the pen (Fig. 1). The centre of the ankle was marked as the midpoint between the medial and the lateral malleoli with the help of a standardised vernier calliper (Fig. 2). We customised standardised goniometers, validated by the Biomedical Department, with expandable arms for higher accuracy (Fig. 3). The goniometer was placed with its hinge at the centre of the patella. Each axis of the goniometer was adjusted such that the tip of the proximal limb touched the ASIS and the tip of the distal limb touched the midpoint of the ankle (Fig. 4). The TFA was measured using the goniometer to the nearest degree. This angle corresponded to the angle suspended by the anatomical axis of the femur with the anatomical axis of the tibia. A valgus TFA was given a positive value, while a varus TFA was given a negative value.

Figs. 1a - 1b

Clinical photographs showing the determination of the centre of the patella using a concentric circle.

Fig. 2

Clinical photograph showing the determination of centre of the ankle using a vernier calliper.

Fig. 3

Photograph showing the customised standardised goniometers with expandable arms.

Fig. 4

Clinical photograph showing the determination of the tibiofemoral angle using customised standardised goniometer.

The ICD and IMD were measured with a standardised vernier calliper. The IMD was measured as the distance between the medial malleoli with the medial femoral condyles in contact. In cases where the IMD was 0, the ICD was measured as the distance between the medial femoral condyles with the medial malleoli touching.

Statistical analysis

This was conducted using STATA 10.0 (StataCorp, College Station, Texas). The mean differences of the TFA between the boys and girls were assessed using the independent t-test. Any possible correlation between the TFA, ICD or IMD with any of the following variables: age, standing and sitting heights, weight, thigh length, leg length and length of the lower extremity, was assessed by performing the Pearson’s correlation test. A p value < 0.05 was considered to be statistically significant.

The left lower limb was measured in each child by a single examiner (SEM). In order to assess intra-observer variability, the TFA of 20 children (aged between five and 15 years) who presented to the Paediatric Orthopaedic outpatient clinic on a single day were measured, and then measured a second time after six hours in random order. The difference between the two readings for any angle measured (R1 – R2) was plotted against the mean between the two angles measured on a single individual (R1 + R2)/2 (Fig. 5).

Fig. 5

Scatter plot showing the intra-observer variability. The graph is plotted between the difference in angle measured (R1 – R2) and the mean angle ([R1 + R2]/2) measured by a single observer. Most of the points lie between the –1° and +1° axes, indicating minimal intra-observer variation.

Results

The mean TFA in boys and girls of different age groups is shown in Table Iand Figure 6. Children between two and three years of age were found to have a valgus angulation. Thus the reversal from varus to valgus alignment occurred under the age of two years. Peak valgus angulation was achieved at five to six years, with a mean valgus of 6.7° (sd 1.3) in boys and 7.25° (sd 0.64) in girls. Thereafter the angle decreased with age until it reached a mean of 3.18° (sd 1.74) in boys and 4.43° (sd 0.68) in girls aged 17 to 18 years. The overall mean valgus angulation was 4.05° (sd 2.32) in boys and 4.90° (sd 2.11) in girls. Girls had a greater valgus angle than boys, which was statistically significant (p = 0.0004).

Fig. 6

Graph showing the mean tibiofemoral angle (TFA) in age group and gender.

Discussion

In 1975, Salenius and Vankka² published their data on the development of the TFA in healthy Caucasian children. They found a pronounced varus alignment of the knee before the age of one year, which changed to valgus between 18 months and three years. Thereafter the valgus corrected spontaneously to between 5° and 6°.² Following this radiological study, the photographic technique was adopted by Heath and Staheli⁴ and clinical measurement with a goniometer by many investigators,^3,5,6,8 with the advantages of low price, reliability, reproducibility and being radiation-free. We therefore chose the clinical measurement technique for the measurement of the TFA in this study.

The clinical technique using a standardised goniometer has the advantage of using fixed bony points over photographic and radiological techniques. In the previous studies the femoral and tibial axes were either marked with skin dots, or the goniometers were placed on imaginary lines.^3,5,6,8 We believe that this was subject to error as the marking of imaginary lines may not be reliable and skin dots can stretch and get displaced. With the use of bony anatomical landmarks and also expandable rigid goniometers whose tips reach long enough to touch the landmarks, the measurement error is minimised. The reliability and minimal intra-observer variability of the measurement of the TFA in this study with custom-made extensions for a standardised goniometer has led us to propose its use in actual clinical practice.

The normal ranges of the TFA have been defined in Caucasian, Chinese, Korean, Nigerian, Turkish, Indian and Saudi children (Table II). However, these ranges have been shown to differ with respect to age, time to transition from infantile varus to valgus, peak valgus and gender variation in children of different ethnic groups.^3-5,8,11-14