Bone mass, estimated strength and Kinetic changes in collegiate and post-collegiate runners with a history of stress fracture.

Abstract

Purpose: To determine differences in bone geometry, estimates of bone strength, muscle size, bone strength relative to load, and running kinetics, in female runners with and without a history of stress fractures. Methods: A total of 32 competitive distance runners aged 18-35, with (SFX, n = 16) or without (NSFX, n = 16) a history of stress fracture were recruited for this cross-sectional study. Peripheral Quantitative Computed Tomography (pQCT, Orthometrix XCT 2000) was used to assess volumetric bone mineral density (vBMD, mm3), bone area (ToA, mm2), and estimated compressive bone strength (bone strength index; BSI= ToA * ToD2) at the distal tibia (4%). Total (ToA, mm2) and cortical (CoA, mm2= ToA * ToD2) bone area, cortical vBMD, and estimated bending strength (strength strain index; SSIp, mm³) were measured at the 15%, 25%, 33%, 45%, 50% and 66% sites. Participants completed a fatigue run on an instrumented treadmill. GRF's were assessed throughout the run and used in conjunction with pQCT measurements to estimate bone strength relative (cm2) to load at all cortical sites (15%-55%). Results: Bone strength was significantly lower in the SFX group at the mid-upper (45-66% sites) tibia for SSIp (-7-10%). Differences in strength were due to smaller cortical bone area (-6-7%) while total bone area and vBMD were not different at any site. The SFX group also had lower bone strength relative to GRF at the 45% (-12%), 50% (-17%) and 66% (-11%) sites. Peak vertical (Z) GRF's in the SFX group were approximately 5% higher on both legs compared to the NSFX group. Peak breaking force also tended to be higher (0.5%-3.8%) in the SFX group. Propulsion forces were 8-14% in the SFX group compared to the NSFX group, while peak lateral forces were 14.9-32.5% higher. Conclusion: These findings suggest that bone CoA and strength are lower in the middle 1/3 of the tibia in runners with a history of stress fracture. Bone strength relative to load was also lower in this same region suggesting that strength deficits in the middle 1/3 of the tibia indicate higher stress fracture risk. Additionally, our trends indicate that there are several kinetic variables that may be important in predicting stress fracture risk. However, further research is needed to confirm these findings.