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Specific Aim

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  1. Specific Aim • Design a new apparatus for measuring the cross-sectional area of mouse patellar tendons and other small, soft connective tissues Why is this important? …

  2. Cross-Sectional Area • Indicator of scar tissue formation • Important for calculation of mechanical properties • Stress • Elastic modulus • Area of mouse patellar tendon challenging to measure • Small • Easily compressed • Can dry out somewhat rapidly

  3. tendon tibia patella stain lines Mouse Tendon Prepared for Testing • Tendon dissected with tibia and patella • Stamped in standard dumbbell shape • Stain lines placed on surface for optical strain measurement • Tendons typically ~1 mm wide and <1 mm thick mm

  4. Indenter probe Previous Area Measurement Method • Our previous technique: • Thickness measured with indenter probe attached to LVDT • Width obtained optically from digital images • Cross-sectional area = thickness x width Limitations: • Not non-contact compression of tissue • Rectangle is not true representation of cross-sectional shape

  5. Existing Non-Contact Methods • Non-contact area measurement techniques: • Shadow amplitude method (Ellis, 1968) • Rotating microscope (Gupta, 1971) • Collimated laser beams (Lee, 1988) • Limitation: Unable to detect concavities  overestimation of area

  6. Existing Non-Contact Methods • Other techniques: • Replica molding (Race, 1996; Goodship, 2005) • Ultrasound (Noguchi, 2002; Ying, 2003) • MRI (Magnusson, 2003) • Costly and labor-intensive • Need for a new approach still exists …

  7. Specimen Laser Reflectance • Charge-coupled device (CCD) laser reflectance system • Laser transmitter projects light spot onto surface below • Light is reflected off surface and focused onto receiving element Laser transmitter Receiving element x

  8. Specimen thickness Specimen Laser Reflectance • Charge-coupled device (CCD) laser reflectance system • When specimen introduced, position of projected spot on receiving element changes • Changes in spot position output as changes in voltage proportional to thickness of specimen Laser transmitter Receiving element x x

  9. Specimen thickness Specimen Laser Reflectance • Charge-coupled device (CCD) laser reflectance system • Can detect surface concavities • Newer lasers offer improved accuracy and resolution • Suitable for small specimens Laser transmitter Receiving element x x

  10. Laser sensor New Device – Configuration • Specimen placed on moveable stage • Laser measures thickness • Two LVDTs measure stage motion • Data output to LabVIEW Stage Two LVDTs

  11. New Device – User Interface

  12. Custom fixture Setup for Mouse Patellar Tendon

  13. Custom fixture Setup for Mouse Patellar Tendon Tibia Patella Patellar tendon

  14. Start data collection Measurement Protocol • Fixture placed on stage • Tendon translates beneath laser • Five passes (“slices”) • <1 minute To tibia Stain line Stain line Patella

  15. Average area Cross-Sectional Area Calculation • Software plots “slices” • Creates interpolated mesh to reconstruct volume • Calculates area of all slices in mesh and returns average • Validation …

  16. Validation – Range of Thicknesses • Measured thickness of 4 strip gauges of varying sizes • Measured each 10 times with laser • Accuracy: Compared to digital caliper measurements • Repeatability: Coefficient of variation (COV)

  17. Validation – Range of Thicknesses • Measured thickness of 4 strip gauges of varying sizes • Measured each 10 times with laser • Accuracy: Compared to digital caliper measurements • Repeatability: Coefficient of variation (COV) Strip Gauge 1 Strip Gauge 2 Strip Gauge 3 Strip Gauge 4 (mm) (mm) (mm) (mm) Mean Laser Measurement 0.46 0.81 2.16 3.18 Standard Deviation 0.01 0.01 0.01 0.01 COV(%) 1.60 0.87 0.33 0.33 Caliper Measurement (mm) 0.46 0.79 2.16 3.15 Percent Difference (%) 0.15 2.69 0.05 0.87

  18. Validation – Range of Thicknesses • Measured thickness of 4 strip gauges of varying sizes • Measured each 10 times with laser • Accuracy: Compared to digital caliper measurements • Repeatability: Coefficient of variation (COV) Strip Gauge 1 Strip Gauge 2 Strip Gauge 3 Strip Gauge 4 (mm) (mm) (mm) (mm) Mean Laser Measurement 0.46 0.81 2.16 3.18 Standard Deviation 0.01 0.01 0.01 0.01 COV(%) 1.60 0.87 0.33 0.33 Caliper Measurement (mm) 0.46 0.79 2.16 3.15 Percent Difference (%) 0.15 2.69 0.05 0.87

  19. Validation – Area of Known Geometry • Measured cross-sectional area of thin metal strip 10 times with laser system • Accuracy: Compared to digital caliper measurements (height x width) • Differed by 3.4% • Repeatability: COV = 2.4%

  20. Final Validation – Mouse Tendon • Measured cross-sectional area of same mouse patellar tendon 10 times with laser system • Repeatable results (COV = 4.1%) • Unable to test accuracy in this tissue • No “gold standard” for comparison

  21. Comparison with Old Method • Measured area of 10 mouse patellar tendons using both methods and compared results • Average was 0.41±0.11 mm when measured with the laser system and 0.08±0.02 when measured using previous method • Non-contact vs. contact method • Repeatability of previous method • Assessed by taking 10 consecutive measurements at same location on a single specimen using probe • COV = 8.9% • Less repeatable than laser system (4.1%)

  22. www.uphs.upenn.edu/orl McKay Orthopaedic Research Laboratory

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