DXA evaluation of femoral bone mineral density and cortical width in patients with prior total knee arthroplasty

Periprosthetic fractures after total knee arthroplasty (TKA) have devastating consequences. Osteoporosis increases periprosthetic fracture risk, but distal femur bone mineral density (BMD) is not measured post-TKA. This study measured distal femur BMD and cortical width; both were lower in the TKA compared to the non-operated leg. BMD measurement reproducibility was good. Standardized DXA regions of interest are proposed.

INTRODUCTION

Periprosthetic fractures following total knee arthroplasty (TKA) are not rare. We hypothesized that TKA is associated with low BMD, potentially increasing periprosthetic fracture risk. However, distal femur dual energy x-ray (DXA) measurement is virtually never performed after TKA due to lack of standardized approaches. Thus, this study's aims were to develop standard DXA femur regions of interest (ROIs), assess cortical width, and determine measurement reproducibility in TKA patients.

METHODS

Thirty adults (15 M/15 F) age 59-80 years with unilateral, primary TKA within 2-5 years had femoral DXA scans performed in duplicate using a Lunar iDXA densitometer. In prior work, we established that femur BMD was lowest in the distal metaphysis and highest in mid-shaft. Thus, BMD and cortical width were measured at 15%, 25%, and 60% of the femur length measured from the distal notch. Femur BMD and cortical width were compared between limbs (TKA vs. non-operated side) by paired t test.

RESULTS

BMD was 3.2-9.9% lower (p < 0.001) in the operated femur at all custom ROIs; substantial between individual differences existed with some up to 30% lower. Cortical width was lower (p < 0.05) at the 25% ROI on the TKA side. BMD reproducibility was excellent; CV 0.85-1.33%.

CONCLUSIONS

Distal femur BMD can be reproducibly measured using DXA and is ~ 10% lower on the TKA leg. Similarly, medial and lateral cortices are thinner at the 25% ROI. These bone changes likely increase periprosthetic fracture risk. Further work to define and mitigate periprosthetic fracture risk after TKA is needed.

Reproducibility of jumping mechanography and traditional measures of physical and muscle function in older adults

SUMMARY

Improved approaches to assess functional change over time are needed to optimally reduce fall/fracture risk; jumping mechanography (JM) may be one such methodology. In this study, JM parameters were more reproducible than traditional functional tests. JM may be better able to demonstrate efficacy of interventions to mitigate sarcopenia.

INTRODUCTION

Jumping mechanography (JM), a tool using maximal countermovement jumps performed on a force plate, may more reliably assess muscle function than traditional methods. The purpose of this study was to examine JM retest reliability in older adults compared with commonly used muscle and physical function assessments.

METHODS

Community-dwelling individuals age≥70 years performed physical and muscle function assessments including the short physical performance battery (SPPB), grip strength, and JM on multiple occasions over 3 months. JM parameters included body weight-corrected peak power and jump height. Appendicular lean mass was measured by dual energy x-ray (DXA). Mixed effects linear regression models were used to estimate between- and within-person variability summarized as intra-class correlation coefficients (ICC).

RESULTS

Ninety-seven individuals (49 females, 48 males, mean age 80.7 years) participated. All testing was well tolerated; no participant sustained injury. Jump power, height, and grip strength were greater (p<0.0001) in men than women. Grip strength, jump power, and height had excellent ICCs (0.95, 0.93, and 0.88, respectively); chair rise, SPPB score, and gait speed had lower ICCs (0.81, 0.77, and 0.76, respectively).

CONCLUSION

In older adults, JM has excellent retest reliability, is stable over time, and can be performed safely. JM retest reliability was comparable to grip strength and possibly better than SPPB and gait speed. JM is a promising tool for muscle function assessment in older adults. Comparison of this approach with traditional assessment tools in longitudinal interventional studies is needed.