ROLE OF RADIOLOGY IN DIAGNOSIS OF OSTEOPOROSIS
The WHO ( world health organization) describes Osteoporosis as a systemic skeletal disease characterized by low bone mass and microarchitectural deterioration of bone tissue leading to enhanced bone fragility . Osteoporosis is the result of protein matrix (osteoid) deficiency in bone with reduced calcium . Osteoporosis predisposes to fractures, particularly of vertebral bodies in spine and the Femoral neck at hip joints. The causes of osteoporosis are - Type 1 ( postmenopausal osteoporosis), Type 2 ( senile osteoporosis , patients over 70 years age) ,Type 3 ( steroid therapy , Cushing’s syndrome , malabsorption syndromes , hypogonadism ) , other causes like idiopathic , disuse osteoporosis.
Plain x-ray film requires atleast 30 % to 50% reduction in bone mass to detect osteoporosis – the x-ray film may show increased radiolucency of vertebrae and long bones.
In osteoporosis involving the vertebral spine xray films may show - picture frame appearance of vertebrae due to cortex of vertebrae appearing prominent , wedge fracture of vertebra, biconcave compression of vertebrae ,high grade compression fracture of vertebrae ( vertebra plana, pancake vertebra). Grading of vertebral fractures in osteoporosis on xray films – Grade 0 (normal), Grade 1 ( mild deformity less than 25% loss of vertebral body height), Grade 2 ( moderate 25% to 40% loss of vertebral body height) , Grade 3 ( severe deformity with over 40% loss of vertebral body height ).
In osteoporosis of long bones typically most fractures occur with minimal trauma in fractures of neck of Femur or distal Radius shaft fractures at the wrist.
Screening for osteoporosis is done by DEXA scan ( dual energy x-ray absorption ) . The patients which may be referred for DEXA scan include – patients with osteopenia on plain x-ray films , patients on steroid therapy , fractures due to minimal trauma ,smoking history . The bone density is measured in grams per square centimeter in lumbar spine ( L1 to L4 vertebrae) and at the Femur neck .The T- score compares the patient with healthy individuals of 30 years age . The Z- score compares patient with age adjusted healthy population.
WHO ( world health organization) osteoporosis criteria for T-scores on DEXA scan: Normal bone density T-score up to -1 SD (standard deviation) , Osteopenia refers to T-score between -1 and -2.5 SD , osteoporosis refers to T- score less than -2.5 SD , established osteoporosis refers to case with fracture. About 95% of all patients who suffer from fracture have a T- score below – 2.5 SD and for practical purposes -2.5 SD is a reference value for osteoporosis.
Falsely elevated BMD levels on DEXA scans in osteoporotic patients can be due to degenerative changes in lumbar spine ( osteophytes , degenerative discs) , hyperostosis of ligaments and joints and aortic calcifications.
Quantitative CT scan ( QCT) is a technique used to measure bone density in milligrams per millimeter , the target area to scan is between L1 – L3 vertebrae . QCT maybe used as an alternative to DEXA scan . The WHO osteoporosis criteria do not apply to QCT . QCT bone density is based on the calcium hydroxyapatite content in bone mass , normal bone density on QCT is calcium hydroxyapatite > 120mg/ml, osteopenia on QCT is calcium hydroxyapatite 80 to 120mg/ml , osteoporosis on QCT is calcium hydroxyapatite < 80mg/ml.
The radiological differential diagnosis of Osteoporosis is – osteomalacia , renal osteodystrophy , metastasis from cancer , multiple myeloma ( plasma cell cancer).