Dose estimation and surveillance of mechanical loading interventions for bone loss after spinal cord injury

Abstract

Background and purpose: The interpretation of the results of previous anti-osteoporosis interventions after spinal cord injury (SCI) is undermined by incomplete information about the intervention dose or patient adherence to dose requirements. Rehabilitation research as a whole traditionally has struggled with these same issues. The purpose of this case report is to offer proof of the concepts that careful dose selection and surveillance of patient adherence should be integral components in rehabilitation interventions.

Case description: A 21-year-old man with T4 complete paraplegia (7 weeks) enrolled in a unilateral soleus muscle electrical stimulation protocol. Compressive loads applied to the tibia approximated 1.4 times body weight. Over 4.8 years of home-based training, data logging software provided surveillance of adherence. Soleus muscle torque and fatigue index adaptations to training as well as bone mineral density (BMD) adaptations in the distal tibia were measured.

Outcomes: The patient performed nearly 8,000 soleus muscle contractions per month, with occasional fluctuations. Adherence tracking permitted intervention when adherence fell below acceptable values. The soleus muscle torque and fatigue index increased rapidly in response to training. The BMD of the untrained tibia declined approximately 14% per year. The BMD of the trained tibia declined only approximately 7% per year. The BMD was preferentially preserved in the posterior half of the tibia; this region experienced only a 2.6% annual decline.

Discussion: Early administration of a load intervention, careful estimation of the loading dose, and detailed surveillance of patient adherence aided in the interpretation of a patient's adaptations to a mechanical load protocol. These concepts possess wider applicability to rehabilitation research and should be emphasized in future physical therapy investigations.

Figure 1

Percentage of adherence, fatigue index (FI), and torque during the patient’s training history. SCI=spinal cord injury.

Figure 2

Longitudinal peripheral quantitative computed tomography scans of the patient’s distal tibia at various times after spinal cord injury. The apparent size difference in the panels at the far right is an image processing artifact (the scan was collected with a larger image matrix; resolution did not differ). Scans are normalized to the density of fat (0 mg/cm³), depicted as bright blue; higher densities are represented by brighter colors (see color scale at right). Fully mineralized cortical bone appears white (~1,200 mg/cm³), and trabecular bone appears orange (~300 –500 mg/cm³). Over time, large areas of marrow fat (blue) replaced trabecular bone within the trabecular envelope. This destruction of the trabecular lattice was substantially minimized in the posterior region of the trained tibia.

Figure 3

Bone mineral density (BMD) data corresponding to the scan images depicted in Figure 2. Data are shown for the whole tibia cross section and for the posterior subregions (delineated in Fig. 2 by a green line transecting the cross section). SCI=spinal cord injury.