Background/Purpose: Ultrasound imaging is highlighting the prominent role of tendinitis in systemic rheumatic disease.  Because of challenging anatomy, limited literature exists on the sonographic appearance of the posterior tibialis (PTT) and the peroneus brevis (PBT) tendon entheses.  Our objectives were to determine the anatomic features and best imaging techniques of normal PTT and PBT using musculoskeletal ultrasound and to compare these findings to subjects with inflammatory arthritis. 

Methods:   Per IRB approval, subjects over the age of 18 without previous foot/ankle fracture or surgery were enrolled as: healthy controls (HC), rheumatoid arthritis (RA), and spondyloarthropathy (SpA).  Bilateral PTT and PBT entheses were imaged longitudinally to compare two angles of insonation: 1) perpendicular to the skin surface and 2) 45˚ cephalad.  A MyLab 70 Biosound Esaote Ultrasound machine with 6-18 MHz linear transducer was used with standardized presets.  Three sonographers, blinded to demographics, diagnosis and each other, scored the images on multiple semi-quantitative scales, including tendon visibility/irregularity, pre-insertional and insertional tendon diameter, Doppler signal, presence of erosions, and tenosynovial effusion. Statistical analysis was conducted by the chi squared test, Tukey multiple comparisons post hoc test, and a mixed-effects, ordered logistic regression analysis. 

Results:   Eighty-eight subjects were enrolled: HC (n=37), RA (n=21), and SpA (n=20).   Complete enthesis visualization was achieved more frequently in the perpendicular than in the cephalad view for the PBT (76.3% versus 58.7%), but more frequently in the cephalad view for the PTT (58.0% versus 19.6%).  The PTT partially inserted and traversed the navicular en route to the metatarsals in 87.3% of subjects.  Table 1 demonstrates the findings at the PBT and PTT entheses.  Insertional tendon diameters were not significantly different.  RA and SpA subjects had higher rates of PTT fiber disruption, tenosynovial effusion, and Doppler signal than HC.  No significant differences existed at the PBT enthesis.  Controlling for patient demographics, intra-observer variability, and idiosyncrasy of the grading system, RA and SpA subjects were 5.1 times (log-odds ratio 1.63, p<0.001) and 3.6 times (log-odds ratio 1.27, p<0.001) as likely as a HC to have a pathological sonographic result.

Conclusion:   This study is the first to describe PBT and PTT imaging techniques that are both feasible and useful for rheumatologists.  Perpendicular transducer aim is optimal for imaging the PBT, while cephalad transducer orientation was more effective for evaluation of the PTT.  Unlike distal PBT imaging, PTT imaging distinguished HC from disease state, with both RA and SpA patients showing features of PTT enthesopathy much more commonly. This study confirms PTT enthesopathy as a sonographic manifestation of inflammatory arthritis.