FE modeling was used to estimate the biomechanical effects of teriparatide and alendronate on lumbar vertebrae. Both treatments enhanced predicted vertebral strength by increasing average density. This effect was more pronounced for teriparatide, which further increased predicted vertebral strength by altering the distribution of density within the vertebra, preferentially increasing the strength of the trabecular compartment.

Introduction:​ Teriparatide 20 μg/day (TPTD) and alendronate 10 mg/day (ALN) increase areal, measured by DXA, and volumetric, measured by QCT, lumbar spine BMD through opposite effects on bone remodeling. Using finite element (FE) modeling of QCT scans, we sought to compare the vertebral strength characteristics in TPTD‐ and ALN‐treated patients.

Materials and Methods:​ A subset of patients (N = 28 TPTD;​ N = 25 ALN) from the Forteo Alendronate Comparator Trial who had QCT scans of the spine at baseline and postbaseline were analyzed. The QCT scans were analyzed for compressive strength of the L3 vertebra using FE modeling. In addition, using controlled parameter studies of the FE models, the effects of changes in density, density distribution, and geometry on strength were calculated, a strength:​density ratio was determined, and a response to bending was also quantified.

Results:​ Both treatments had positive effects on predicted vertebral strength characteristics. At least 75% of the patients in each treatment group had increased strength of the vertebra at 6 months compared with baseline. Patients in both treatment groups had increased average volumetric density and increased strength in the trabecular bone, but the median percentage increases for these parameters were 5‐ to 12‐fold greater for TPTD. Larger increases in the strength:​density ratio were also observed for TPTD, and these were primarily attributed to preferential increases in trabecular strength.

Conclusions:​ These results provide new insight into the effects of these treatments on estimated biomechanical properties of the vertebra. Both treatments positively affected predicted vertebral strength through their effects on average BMD, but the magnitudes of the effects were quite different. Teriparatide also affected vertebral strength by altering the distribution of density within the vertebra, so that overall, teriparatide had a 5‐fold greater percentage increase in the strength:​density ratio.