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Nolan Taylor
Nolan Taylor

Bone Age

The X-ray image is black and white. Dense body parts, such as bones, block the passage of the X-ray beam through the body. These look white on the X-ray image. Softer body tissues, such as the skin and muscles, allow the X-ray beams to pass through them. They look darker on the image.

bone age

A bone age study helps doctors estimate the maturity of a child's skeletal system. They do this by taking a single X-ray of the left wrist, hand, and fingers. The bones on the X-ray image are compared with X-ray images in a standard atlas of bone development. The atlas is based on data from many other kids of the same gender and age. The bone age (also called the skeletal age) is measured in years.

The bone age study can help evaluate how fast or slowly a child's skeleton is maturing, which can help doctors diagnose conditions that slow down or speed up physical growth and development. This test is usually ordered by pediatricians or pediatric endocrinologists.

The hand and wrist bones consist of the radius, ulna, 19 short bones (5 metacarpals and 14phalanges) and 7 carpals. Bones are formed by endochondral ossification in the radius, ulnaand short bones and by intramembranous ossification in the carpal bones. The maturationrates of the carpals vary among individuals. The completion of maturation occurs earlier inthe carpals compared with the long and short bones, and intramembranous ossification is lessdependent on GH than endochondral ossification. Therefore, the carpals are not suitable forbone age assessment.

The TW2 method was developed using radiographs of average socioeconomic class children inthe United Kingdom, and the radiographs were collected in the 1950s and 1960s. TheTanner-Whitehouse 3 (TW3) method, which was developed to update the relationship betweenthe total bone maturity score and bone age based on secular trends, was published in 2001(6). Standardized Tanner-Whitehouse (TW) methodshave been reported in several countries, and these methods have changed the relationshipbetween the total bone maturity score and bone age to make the relationship suitable foreach generation and ethnic group (7, 8, 9).

Computerized method: Several computerized systems for bone age assessment have beenreported. Some of these systems were developed based on the TW method (15, 16, 17). Oneexample is the computer-assisted skeletal age score (CASAS) (15), in which an image is digitized and represented by severalmathematical coefficients. These coefficients are then compared to those generated by eachstage of the TW standards, and the closest match is determined. Although CASAS has beenreported to be more reliable than the manual methods (18, 19), a limitation of CASAS is that itcan take longer to estimate bone age than the manual methods because each bone has to belocated manually.

Alternatively, Sato et al. (20)reported a new system that can automatically evaluate the bone maturation of Japanesechildren, the computer-aided skeletal maturity system (CASMAS), which is not based on theTW method. In CASMAS, digital images of the third phalanges are automatically extractedfrom computerized scans of a hand and wrist radiograph, and the widths of the epiphyses,metaphyses and overlapping regions of the epiphysis and metaphysis in the third phalangesare automatically measured. The results are then used to calculate bone age by multipleregression analysis.

Another automated method has recently been developed, called BoneXpert (Visiana, Denmark)(21). In BoneXpert, the borders of 13 RUS bones(radius, ulna and 11 short bones in fingers 1, 3 and 5) are automatically determined froma digitized image (Layer A), and an intrinsic bone age is calculated from parameters suchas shape scores, bone density scores and features describing the texture of the fusion inthe growth plate (Layer B). Normal Danish children in 1966 participated in the developmentof Layer B. The intrinsic bone age is then transformed into the GP bone age or TW bone age(Layer C). This automated bone age determination system has been validated in Caucasianchildren with short statures (22) and precociouspuberty (23). Moreover, the usefulness of thissystem has been reported for various ethnic groups (24, 25), especially by calibration to thestandard for Japanese children (26, 27).

However, it has been reported that the variability in bone age at puberty onset in bothnormal boys and girls was not less than the variability in chronological age (42, 43, 44).Therefore, it may be difficult to predict the timing of pubertal onset in normal childrenusing bone age.

Satoh et al. (44) reported thatbone age at PHV evaluated by the Japanese TW2-RUS method was distributed at about 13 and11 yr in normal Japanese boys and girls, respectively; these ages were equivalent to theages at PHV on standard growth velocity curves for Japanese children. Pitlović etal. (45) reported that the olecranonapophysis maturity level assessed by ultrasound could predict the timing of PHV in healthychildren. Although further examinations are needed, bone age may be a predictor for thetiming of PHV.

As different modalities of bone age estimation provide different results and their applicability differs in different ethnicities, we need to design studies in order to compare them and select the method best suited to Pakistani children.

Calculation of bone age is also employed for estimation of chronological age in conditions were accurate birth records are not available. Absent birth data is a big problem in our part of the world. In South Asia, 65% of all births are not registered by the age of 5 years.3 Thus need for accurate estimation of age arises in conditions where the age of a child needs to be accurate, such as during immigration4, in law suits5 and in competitive sports.6 In these cases bone age is used to provide the closest estimate of chronological age.

The pattern of ossification in the hand and wrist bones is in a fairly predictable manner and age specific until end of adolescence when the elongation of bone is complete. Thus, the standards of bone age have been derived by comparing the level of maturation of hand and wrist bones with normal age levels.

Traditionally, the extent of growth and development of hand bones has been visualized by plain wrist radiographs, however newer methods such as ultrasound of hand bones are being tried but have yet not been validated.

Visualization by plain hand & wrist radiographs: There have been great advancements in radiological techniques over the past few decades but to date, plain radiographs of the hand are the investigation of choice for bone age assessment. A standard posterior-anterior (PA) view of the hand and wrist is ideal for visualization of features of hand bones.7

Various methods have been developed to compute bone age score from these radiographs by comparing the maturity of hand & wrist bones to idealized standards. A brief description of the commonly used methods is given below.

This method is simpler and faster than other radiograph based methods.1 GP atlas standards are considered applicable and reliable for children in Australia11 and Middle East.12 However, disparity between the calculated bone age and chronological age is noted when this method is applied to Asian children.9,13

2: TannerWhitehouse (TW2) Method: The Tanner &Whitehouse (TW) method in contrast is not based on the age, rather it is based on the level of maturity for 20 selected regions of interest (ROI) in specific bones of the wrist and hand in each age population. The development level of each ROI is categorized into specific stages labeled as (A, B, C, D, . . ., I). A numerical score is given to each stage of development for each bone individually. By summing up all these scores from the ROIs, a total maturity score is calculated. This score is correlated with the bone age separately for males and females. TW method is comparatively more complex and requires more time; however it is more accurate and reproducible when compared to GP method.14

It has been observed that both pediatric endocrinologists and radiologists showed nearly identical results in determining bone age from GP and GR atlas. However the GR atlas had an increased number of outliers. Still it can be used to replace the older GP atlas.16

4: Automatic Skeletal Bone Age Assessment: Manual estimations of bone age by the above mentioned methods do have some degree of inter rater variability. This creates problems in its clinical application, comparison between subjects and follow-up of patients. A computerized automatic system of bone age assessment would in theory be a solution18, but practically it is very difficult to generate an automated system that could accurately analyze the variations, size, shape and mineralization in multiple ossification centers in the hand and wrist bones.15 Computerized calculation of bone age from wrist radiographs has been around for the past 3 decades. Radiographs are either obtained by digital radiography or digitalized via a scanner and then undergo several steps. During pre-processing the image is normalized to grayscale so that important segments of the image can later be extracted, background is removed and orientation of the image is corrected. In the next step called segmentation, desired portions of the bones and soft tissue are separated from the background. Then the image is analyzed by taking account of selected regions of interest for calculating bone age by Tanner-Whitehouse method or by comparison with standard images for estimation by Greulich & Pyle Atlas.

Older methods of automated Image-processing that detect features of ossification in hand bones showed significant variation from bone age calculated by manual methods. However a newer method for automatic bone age assessment called Bone Xpert has been generated that rebuilds the edges of 15 bones of interest in hand radiographs and uses this information to compute bone age by both the Greulich Pyle (GP) and Tanner Whitehouse (TW) methods.19 The use of this software has been validated for various ethnicities.20 041b061a72


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