Age and motor score predict osteoprotegerin level in chronic spinal cord injury

L.R. Morse (1,2), H.P. Nguyen (3,4), N. Jain (3,4,8), S. Williams (5), C.G. Tun (3,6) ,R.A. Battaglino (2), P. Stashenko (2), E. Garshick (3,7,8)

(1) Department of Physical Medicine and Rehabilitation, Harvard Medical School and Spaulding Rehabilitation Hospital, Boston, Massachusetts, USA;
(2) Department of Cytokine Biology, Forsyth Institute, Boston, Massachusetts, USA;
(3) Research and Development Service, Department of Veterans Affairs, VA Boston Healthcare System Boston, MA;
(4) Programs in Research at VA Boston, Harvard Medical School, Boston, Massachusetts, USA;
(5) Department of Physical Medicine and Rehabilitation, Boston Medical Center, Boston, Massachusetts, USA;
(6) Rehabilitation Medicine Service, VA Boston Healthcare System and Harvard Medical School, Boston, Massachusetts, USA;
(7) Pulmonary and Critical Care Medicine Section, Medical Service, VA Boston Healthcare System, Boston, Massachusetts, USA;
(8) Channing Laboratory, Department of Medicine, Brigham and Women's Hospital, Harvard Medical School, Boston, Massachusetts, USA

Abstract
Objective: Individuals with spinal cord injury (SCI) develop a severe form of osteoporosis below the level of injury that is poorly understood. We conducted a preliminary investigation to assess whether circulating markers of bone turnover and circulating RANKL/OPG levels are related to the severity of SCI, aging, or to differences in mobility (i.e., walking or using a wheelchair). Methods: Sixty-four caucasian men .1.6 years since injury selected based on locomotive mode provided blood samples and completed a health questionnaire at the VA Boston Healthcare System from 10/2003 to 6/2005. Plasma sRANKL, osteoprotegerin (OPG), osteocalcin and carboxyterminal telopeptide of type I collagen (CTx) levels were determined. Results: Increasing age was significantly associated with increased OPG and CTx. Injury severity was predictive of OPG levels, and adjusting for age, participants with cervical motor complete and ASIA C SCI (n=11) had significantly lower mean OPG (46.1 pg/ml) levels than others (63.4 pg/ml). Locomotive mode was not associated with differences in bone markers. Conclusions: Severe cervical spinal cord injury is associated with decreased circulating OPG levels placing these patients at risk for accelerated bone loss that appears unrelated to locomotive mode.

Keywords: Osteoporosis, Osteoprotegerin, Aging, Spinal Cord Injury, Bone