Background
Musculoskeletal tissues are in a constant state of turnover, with a dynamic equilibrium between tissue protein synthesis and breakdown rates. The synthesis of protein allows musculoskeletal tissues to heal following injury. However, impaired tissue healing is observed following certain injuries, such as geriatric hip fractures. Though these fractures are assumed to eliminate the regenerative capabilities of femoral bone tissue, the actual impact of a fracture on in vivo bone protein synthesis rates has never been determined.

Design
In the present study, 10 patients (age: 79±10 y, BMI: 24±4 kg/m²) with an acute (<24 h) intracapsular hip fracture received a primed continuous intravenous infusion of L-[ring-¹³C₆]-phenylalanine before and throughout their hip replacement surgery. Trabecular and cortical bone tissue from the femoral head and shaft were sampled during surgery to assess protein synthesis rates of affected (femoral head) and unaffected (shaft) bone tissue, respectively. In addition, tissue samples of gluteus maximus muscle, synovium, ligamentum teres, and femoral head cartilage were collected. Tissue-specific protein synthesis rates were assessed by measuring the incorporation of L-[ring-¹³C₆]-phenylalanine in tissue protein. 

Results
Femoral head trabecular bone protein synthesis rates (0.056 [0.024-0.086] %/h) were significantly lower when compared to femoral shaft trabecular bone protein synthesis rates (0.081 [0.056-0.118] %/h; P=0.043). Cortical bone protein synthesis rates did not differ between the femoral head and shaft (0.041 [0.021-0.078] and 0.045 [0.028-0.073] %/h, respectively; P>0.05). Skeletal muscle, synovium, ligamentum teres, and femoral head cartilage protein synthesis rates averaged 0.080 [0.048-0.089], 0.093 [0.051-0.130], 0.121 [0.110-0.167], and 0.023 [0.015-0.039] %/h, respectively.

Conclusion
Protein synthesis is still active in femoral head bone tissue during the early stages following an intracapsular hip fracture in older patients. Trabecular, but not cortical, bone protein synthesis rates are lower in the femoral head when compared to the shaft in older patients following an acute intracapsular hip fracture.