When is bone age considered abnormal?

When is Bone Age Considered Abnormal?

Bone age is considered abnormal when it differs from chronological age by more than 2 standard deviations, or specifically when Z-scores are ≤ -2.0 or ≥ +2.0 in patients younger than 50 years of age. 1

Diagnostic Criteria for Abnormal Bone Age

Bone age assessment is a critical tool in evaluating skeletal maturity and is used extensively in pediatric endocrinology and orthopedics. The determination of whether a bone age is abnormal depends on several factors:

Age-Specific Criteria

• For patients under 50 years of age:

  • Z-score ≤ -2.0 is defined as "below the expected range for age" 2
  • Z-score > -2.0 is "within the expected range for age" 2

• For patients 50 years and older:

  • T-scores are used instead of Z-scores
  • T-score between -1.0 and -2.4: Osteopenia 2, 1
  • T-score ≤ -2.5: Osteoporosis 2, 1

Clinical Considerations

When evaluating bone age, several important factors should be taken into account:

• Ethnicity: Significant variations exist in bone age among different ethnic groups. Studies have shown that:

  • Black and Hispanic girls tend to have advanced bone age (exceeding chronological age by approximately 10 months in late childhood) 3
  • Asian boys may show significant delays in bone age during middle childhood (lagging behind chronological age by nearly 15 months) 3
  • Hispanic adolescent boys may have bone age exceeding chronological age by about 11 months 3

• Sex differences: Bone age patterns differ between males and females:

  • In white preadolescent boys, bone age typically lags behind chronological age by 4-8 months 3
  • In adolescent white girls, bone age may exceed chronological age by an average of 4 months 3

Assessment Methods

The most widely used methods for bone age assessment include:

1. Greulich-Pyle method: Based on left hand and wrist radiographs, comparing to standard atlas images 4
2. Tanner-Whitehouse 2 method: More detailed scoring system using specific bones of the hand and wrist 4
3. BoneXpert: An automated method validated for children with growth disorders across various ethnic groups 4
4. Ultrasound methods: Less reliable in children with delayed or advanced bone age 4
5. MRI methods: Newer, non-invasive approach that requires further validation 4

Clinical Applications

Abnormal bone age has significant implications for:

• Growth disorders: Essential for diagnosing constitutional delay of growth, familial short stature, and other growth disorders 5
• Puberty disorders: Critical in evaluating precocious or delayed puberty 5
• Endocrine conditions: Helps interpret hormone tests during puberty and diagnose hyperandrogenism 5
• Height prediction: Used to predict adult height, though predictions may be less accurate in children with endocrine or bone pathologies 5
• Treatment decisions: Guides timing of hormone replacement therapy in hypogonadism 5

Common Pitfalls to Avoid

1. Overreliance on chronological standards: The Greulich-Pyle standards were derived from white children of upper socioeconomic class in 1931-1942 and may not apply equally to all ethnic groups 3

2. Ignoring ethnic variations: Using standardized bone age assessments without considering ethnic differences can lead to misdiagnosis 3

3. Improper DXA measurement: Common errors include not accounting for artifacts, failing to use the same DXA machine for follow-up measurements, and comparing follow-up measurements using T-scores rather than absolute BMD values 1

4. Overemphasizing bone age: Bone age should be interpreted in the context of clinical findings and not used in isolation 5

5. Using T-scores for young patients: For premenopausal women and men under 50, Z-scores should be used instead of T-scores 2, 1

By understanding these criteria and considerations, clinicians can more accurately determine when a bone age is truly abnormal and use this information to guide appropriate diagnosis and treatment decisions.