Changes in Osteocalcin after Femoral Neck Fracture

Abstract

SUMMARY.In a group ofelderly patients with osteoporosis and femoral neck fracture, osteocalcin concentrations rose significantly in the first week after fracture fixation. The changes in osteocalcin correlated well with the changes in plasma 1,25-dihydroxycholecalciferol (P < 0'001).Osteomalacia is an important':" but uncommon] treatable accompaniment of femoral neck fracture. Current biochemical screening tests which rely on measurements of serum calcium and alkaline phosphatase have a high false positive rate." This is largely because alkaline phosphate arises from both bone and liver: hepatic dysfunction is common in the elderly fracture po pulation. ' The recent availability of radioimmunoassays for osteocalcin, a bone specific protein that is released during osteoblastic activity-7 and is stimulated by 1,25-dihydroxyvitamin D (1,25-(OH)2D)8.9 raises the possibility that it may be a more specific test for osteomalacia than serum alkaline phosphatase. The problem is that there is no information about changes in osteocalcin concentration during the immediate post-fracture period when it might be used for screening.In the present study we describe the changes in osteocalcin concentration which occur in the first week after a femoral neck fracture and its fixation. We have also examined its relationship to alkaline phosphatase, 1,25-(OHhD and bone histomorphometry. Patients and methodsThe study was based on 26 consecutive patients (3 male, 23 female; mean age 78·2 years, range 58-92 years) who had sustained a traumatic fracture of the femoral neck. Perioperative iliac crest bone biopsies had excluded osteomalacia in all cases: osteoid area < 3'5%; surface extent of osteoid < 25%, osteoid seam thickness index Fasting blood samples were taken pre-operatively, within 24 h of the fracture in all patients, and were repeated one week later by which time all fractures had been treated by internal fixation.Blood samples were collected into lithium heparin tubes and the plasma stored at -20°C until assay. Plasma calcium, phosphate, creatinine, albumin concentrations and alkaline phosphatase were measured by standard autoanalyser techniques. 10 Calcium concentrations were adjusted to a plasma albumin of 40 g/L.12Plasma osteocalcin concentrations were measured by radioimmunoassay (CIS(UK) Ltd, London, UK) using a rabbit antibody raised against bovine osteocalcin. 1,25-(OH)2D was extracted from plasma and purified by straight phase high performance liquid chromatography on a Zorbax-Sil column. I] The 1,25-(OH)2D peak was collected and quantitated by a competitive protein binding assay using calf thymus cytosol." Vitamin D binding protein (DBP) concentrations were measured by radial immunodiffusion.tv's The free 1,25-(OH)2D index was calculated as the molar ratio of 1,25-(OHhD/DBP x 10 5 • This gives a good estimate of the free 1,25-(OH)2D concentration." Parathyroid hormone (PTH) concentrations were measured using a C-terminal assay."Reference values for osteocalcin, 1,25-(OH)2D and DBP were derived from a group of health...

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