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Comparison of MRI vs Ultrasound for Gap Assessment After Flexor Tendon Repair: A Cadaveric Study
Patricia Drace, MD1; Kevin Renfree, MD2; Karan Patel, MD1; Mark Kransdorf, MD2; Nirvikar Dahiya, MD2; Ryan Mclemore, PhD1
1Banner University Medical Center- Phoenix, Phoenix, AZ; 2Mayo Clinic, Phoenix, AZ

Introduction: Clinical exam is important in the assessment of tendon repair however adhesions, callous at the repair site, and recurrent rupture may be difficult to differentiate clinically. Following tendon repair, the ideal imaging modality for accurate detection of repair failure has not been determined. It is unknown how various suture materials and gapping at the tendon repair site interfere with MRI and ultrasound imaging.
Methods: 48 digits (thumbs excluded) from 12 matched fresh-frozen cadaver upper extremities were used. The FDP was transected in Zone 2, and a 4-strand locked cruciate repair was performed. Repairs were randomized to 3 gap sizes (0mm/no gap, 2mm, and 4 mm) and to suture type (4-0 Fiberwire, 4-0 Ethibond, or 4-0 Prolene). Specimens were evaluated by 1.5T MRI, 3 T MRI, and high-resolution ultrasound with the digits and wrist maintained in full extension. Images were interpreted by a board certified musculoskeletal radiologist and ultrasonographer both blinded to gap and suture material used in the repair. Gap measurements were compared using regression and GEE models. Gaps not measured confidently due to artifact were excluded. Generalized estimating equations (GEE) were used to compare predictors of error and measurability. McNemarís test for paired data compared predictability between groups. Error was plotted by imaging technique, gap, and suture type using boxplots. Linís concordance correlation coefficient determined interobserver agreement between imaging techniques.
Results: Multivariate regression indicates that imaging method and gap are responsible for observed differences in accuracy and number of measurements that could be made. 1.5T MRI had less artifact and thus less images had to be excluded from measurement when compared to 3T MRI (p=0.11). Ultrasound had significantly less artifact than both MRI modalities (p=0.001 and 0.006 respectively), but accuracy in determining smaller gap sizes (0 and 2mm) was sacrificed and less than both MRI types. For larger gap sizes (4 mm), all 3 modalities underestimated the gap. Suture type did not have a significant impact on measurement error between imaging modalities.
Conclusions:
1. Suture type does not appear to affect measurement error as much as gap size and imaging modality.
2. Measurement of large gaps has more error (underestimation) for all 3 imaging modalities, with ultrasound being inferior to MRI.
3. Due to artifact, MRI renders fewer analyzable images compared to ultrasound, but accuracy of MRI is superior to ultrasound for smaller gaps (<4mm)
4. Further clinical work in this area is ongoing including investigation of MR protocols and sequencing.


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