Development of bones of the upper and lower limbs - stages and norms

Ossification of limb bones occurs through primary centers (usually formed in utero in the diaphyses of long bones) and secondary centers (usually appearing after birth in the epiphyses and apophyses). Growth in length is facilitated by growth plates, which close during adolescence, and some apophyses and individual epiphyses may complete fusion in young adulthood.

The timing of the appearance of ossification nuclei depends on gender and the overall rate of biological maturation. On average, the appearance and fusion of the epiphyses occurs earlier in girls than in boys, but this difference is not "fixed" and can vary across populations. [1]

Therefore, clinics rarely attempt to "determine age from a single bone." Instead, they assess the maturity pattern across several structures at once, most often using X-rays of the hand and wrist, comparing the image with atlases and bone age scales. [2]

It is important to distinguish normal ossification patterns from those due to trauma and disease. In children, individual ossification centers may appear fragmented and uneven, especially in the elbow and knee area, and this does not always indicate a fracture. Misrecognition of normal ossification centers is a common cause of misdiagnosis of a "fracture," especially in the elbow joint. [3]

Table 1. Basic terms and what is visible on an x-ray

Term	What is this	Where it is found	Why is it important?
Primary ossification center	the beginning of bone formation in the diaphysis	long bones of the limbs	"the basis" of the future bone, appears early
Secondary ossification center	nucleus in the epiphysis or apophysis	articular ends and attachment sites of tendons	helps assess bone age and growth plate injuries
Pineal gland	articular end of the bone	shoulder, hip, knee and others	involved in intra-articular injuries
Apophysis	tubercle or ridge for muscles	greater trochanter, calcaneal tuberosity, and others	a common area of "apophysites" in adolescents
Growth zone	cartilage between the epiphysis and metaphysis	long bones	damage may result in growth retardation or deformity

[4]

Upper limb: shoulder girdle and proximal humerus

The shoulder girdle is formed by the scapula and clavicle, and they are characterized by pronounced "multicentricity." For practice, it's more important not to memorize dozens of dates, but to understand that some centers appear relatively late and can fuse as early as after age 18, so "non-union" in adolescence is not the same as pathology.

The acromion of the scapula develops from multiple centers and may fuse late. If one of the centers fails to fuse, a normal variant known as an os acromiale develops. This sometimes becomes a clinically significant source of pain in adults, but in adolescents it may appear as a "suspicious line" without actual injury. [5]

The clavicle is unique in that its medial epiphysis is one of the latest maturing regions of the skeleton. This is why the medial end of the clavicle is used in forensic age assessments in late adolescents and young adults, and studies highlight significant variability between populations. [6]

The proximal humerus has three key secondary centers: the head, the greater tubercle, and the lesser tubercle. The timing of their emergence and fusion is important in traumatology because, in children, growth lines and fusion zones can mimic a fracture, and true proximal humeral fractures often pass through the growth plate. [7]

Table 2. Practical guidelines for ossification of the shoulder girdle and shoulder

Structure	What is important to remember	Typical age range as a guide
Acromion	several centers, late consolidation possible	the beginning of the merger in adolescence, completion sometimes before 25 years
Medial epiphysis of the clavicle	one of the latest in fusion	active maturation after 16 years, completion often at 20-26 years and later in some samples
Proximal epiphysis of the humerus	separate centers of the head and tubercles	The head can be visualized in the first months, the tubercles appear later in childhood

[8]

Elbow and forearm: why is normal and fracture often confused here?

The elbow is considered the "most insidious" joint in pediatric radiology because secondary centers appear in a specific sequence, but their appearance is very variable and often fragmented. Knowing the order in which the centers appear helps distinguish normal from traumatic lesions, especially when dislocations and fractures are suspected. [9]

A classic mnemonic for the elbow is CRITOE: capitellum, radial head, internal epicondyle, trochlea, olecranon, external epicondyle. Age numbers vary slightly between sources, so it's safer to think of them as guidelines rather than "passport dates." [10]

Trochlea often ossifies in multiple centers and appears "fragmented," which can mimic a fracture or aseptic necrosis. Reviews emphasize that fragmented trochlea in a child may be a normal variant and requires evaluation in conjunction with the clinical picture and other signs of injury. [11]

Growth plate injuries around the elbow and in the area of the radius and ulna pose a risk of premature closure of the growth plate and subsequent deformity. The incidence of complete growth arrest after growth plate fractures is low but clinically significant, so children with such injuries require ongoing monitoring. [12]

Table 3. CRITOE elbow: order of appearance of ossification centers

Order	Center	Why is it important?
1	Capitellum	the first center helps to "catch" the age norm
2	Radial head	important in radial head injuries
3	Medial epicondyle	a common area of avulsion injuries in children
4	Trochlea	may appear fragmented without a fracture
5	Olecranon	important for injuries of the olecranon
6	Lateral epicondyle	appears relatively late

[13]

Wrist: The Wrist Bones as the "Master Calendar" of Bone Age

Hand and wrist radiography is used to estimate bone age because the carpal bones and phalanges provide a rich, consistent picture of maturation. The most common methods are the Greulich and Pyle and Tanner-Whitehouse methods, both based on left hand and wrist radiography.[14]

According to modern reviews and studies, the first carpal bones to appear are the capitate and hamate, and they can appear as early as the first months of life. Next, the triquetrum and lunate are added, followed by the scaphoid, trapezium, and trapezoid, with the pisiform usually appearing last. [15]

This sequence has clinical nuances. Early or late visualization of individual bones may be a normal variant, but if ossification is significantly delayed, overall growth, nutrition, endocrine factors, and chronic diseases are assessed. Late pisiform bone formation alone, without other signs, is usually not a diagnosis. [16]

The metacarpal and phalangeal bones have primary centers in the diaphyses and secondary centers in the epiphyses, and the appearance of sesamoid bones in the thumb is considered a sign of approaching puberty. These landmarks assist in orthopedic planning and in assessing the correspondence between biological age and passport age.

Table 4. Wrist: typical sequence of bone appearance on X-ray

Group	Bones	Often the expected period of appearance as a guideline
The earliest	capitate, hook-shaped	the first months of life
Early preschool age	triangular, crescent	approximately 2-4 years
Late preschool age	scaphoid	approximately 4-6 years
School age	trapezoid, trapezoidal	approximately 5-7 years
The latest	pisiform	more often in adolescence

[17]

Pelvis and femur: triradiate cartilage and acetabular maturation

The pelvic bone is formed from three main components: the ilium, ischium, and pubis, which in children are separated by layers of cartilage. The triradiate cartilage remains in the acetabulum, which closes during adolescence and is critical for the proper formation of the acetabulum.

The timing of triradiate acetabular closure varies by gender: on average, it closes earlier in girls than in boys. Premature closure after injury can lead to secondary acetabular dysplasia, so such injuries require monitoring. [18]

The proximal femur also has a characteristic sequence of center appearance: the femoral head, the greater trochanter, and the lesser trochanter. These timings are important for interpreting images of congenital hip dysplasia and injuries, as well as for understanding when the "absence of a nucleus" is still within the normal range.

After the growth plates close, fusion of individual apophyses and the final formation of articular surfaces continues. During adolescence, this creates "windows of vulnerability" for apophysitis and avulsion injuries at tendon insertion sites, especially in active athletes. [19]

Table 5. Pelvis and hip: key age landmarks

Structure	What is being assessed?	Timeline
Triradiate acetabular cartilage	open or closed	closure usually occurs in adolescence, earlier in girls
Head of the femur	presence of an ossification nucleus	appears in infancy with variability
Large and small skewers	the appearance and fusion of apophyses	appear later than the head, fusion in adolescence

Knee, shin, and foot: what should be visible in a newborn and what appears later

In the knee region, large epiphyses of the distal femur and proximal tibia are usually present already in newborns, and then, as growth progresses, the epiphyses gradually fuse with the diaphyses during adolescence. In girls, fusion occurs, on average, earlier than in boys, which is confirmed by population studies of epiphyseal fusion. [20]

The patella ossifies from multiple centers, typically during preschool age, and variations in patellar formation and accessory sesamoid bones of the knee are relatively common. This is important because such variations can mimic a fracture following trauma if the cortical contours and clinical context are not considered.

In the foot, some of the tarsal bones are already visible at birth: the calcaneus and talus ossify in utero, and the cuboid often becomes visible at birth or shortly thereafter. The cuneiform bones appear later, and the navicular bone of the foot is usually among the latest to develop its nucleus, which fundamentally contradicts the notion of an "intrauterine navicular bone of the foot."

The calcaneal tuberosity zone has a separate apophysis, which forms during school age and fuses later. During this period, painful overload conditions and normal variations in the form of extra bones are possible. These are the result of non-union of the ossification centers and can be mistaken for a fracture.

Table 6. Foot: tarsal bones and their typical “order”

Group	Bone	Landmark for the appearance of the core
In utero and at birth	calcaneal, talus	in utero, visible early
Early age	cuboid	around birth and the first months
Later in early childhood	wedge-shaped	after birth with variability
The latest	navicular bone of the foot	usually preschool age, often 3-5 years, with gender differences

How to Use Standards Safely: Bone Age, Asymmetry, and Red Flags

In real-life practice, to assess the "normality" of limb development, hand bone age is most often used rather than individual bone dates. Guidelines emphasize that the Greulich and Pyle and Tanner-Whitehouse methods have limitations and may perform differently in different populations, so interpretation should always take into account clinical and growth curve data. [21]

Asymmetry in the appearance of nuclei or a marked delay in ossification in one area requires an explanation. The differential diagnosis typically includes endocrine causes, nutritional deficiencies, chronic inflammatory diseases, and local factors such as the consequences of trauma to the growth plate. [22]

Following injuries in children, the key risk is associated not only with fracture healing but also with potential growth arrest due to damage to the growth plate. Publications indicate that fractures through the growth plate constitute a significant proportion of childhood fractures and can lead to growth arrest in a significant proportion of cases, so follow-up is part of the standard of care. [23]

The imaging method is chosen based on the clinical need. Radiography remains the primary method for assessing ossification centers and growth plates, ultrasound is often used for hip joints in infants, and magnetic resonance imaging is used when cartilage, bone edema, or hidden growth plate injury need to be visualized. [24]

Table 7. Practical “red flags” that require re-checking of standards

Situation	Why is it important?	What do they usually do?
A sharp asymmetry of ossification on the left and right	there may be a local problem or injury	re-evaluation, clarifying visualization
Lack of expected nuclei combined with low growth rate	there may be a systemic cause for delayed maturation	growth assessment, nutrition, endocrine profile as indicated
Pain and swelling in the growth plate after injury	risk of damage to the growth zone	control images, observation, magnetic resonance imaging if necessary
"Fragmented" nuclei in the elbow without a clear fracture line	often a normal variant	comparison with age, clinical picture and sequence of centers

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