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Compared with MRI, Merchant-view radiographs resulted in smaller TT-TG distance
SAN DIEGO — Although researchers found a correlation between Merchant-view radiographs and MRI for measurement of the tibial tubercle to trochlear groove distance, measurements with Merchant radiographs were 5-mm to 8-mm smaller than those found with MRI, according to a presenter at the American Orthopaedic Society for Sports Medicine Annual Meeting.
“We demonstrated that Merchant radiographs allow for reliable measurement of the [tibial tubercle to trochlear groove distance] TT-TG,” Joseph D. Lamplot, MD, said during his presentation. “This correlates with but results in a lower average distance than MRI. Future studies should investigate the utility of the approach in predicting recurrent patellar dislocations and outcomes following soft tissue procedures.”
Lamplot and colleagues identified 41 patients who had standardized Merchant-view radiographs in 45° flexion which yielded 82 knee images. Based on palpation, investigators placed lead markers on the skin that were centered over the tibial tubercle. They also collected and analyzed radiographs. They added 16 patients to the study in order to associate Merchant TT-TG with MRI TT-T. Therefore, there were 30 patients with both a Merchant radiograph and MRI.
In 67 images, investigators were able to identify the tibial tubercle on the Merchant radiographs. On bony landmarks alone, Merchant TT-TG measured with a marker was a significantly associated measurement. The Merchant TT-TG was strongly associated with MRI TT-TG when measured with bony landmarks alone. TT-TG standardized by patellar width increased the strength of the correlation between Merchant TT-TG and MRI TT-TG.
According to researchers, there was an increase in MRI TT-TG for patients with a patellar instability at 13.9 mm vs. 10.5 mm. Patients with patellar instability at 9.1 mm vs. 1.9 mm had an increase in Merchant TT-TG. – by Monica Jaramillo
Reference:
Bernholt D, et al. Paper 119. Presented at: American Orthopaedic Society for Sports Medicine Annual Meeting; July 5-9, 2018; San Diego.
Disclosure: Lamplot reports no relevant financial disclosures.
Perspective
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This is a study evaluating the utility of a novel technique measuring the tibial tuberosity to trochlear groove distance (TT:TG) on plain radiographic axial (Merchant) images of the patellofemoral articulation. This study successfully shows that the TT:TG distance can be reliably measured on standardized Merchant views without radiographic markers. This method offers another useful tool in the diagnostic armamentarium for clinicians who treat patellofemoral conditions. With concerns of cumulative radiation exposure in our patients, this measurement can help to avoid additional radiation from CT images of the knee, the current gold standard for evaluation of TT:TG distance. This method can also be a viable option for measuring TT:TG distance when cost and/or availability preclude CT and/or MRI evaluation.
One of the largest utilities of this technique is for initial screening in concert with the initial evaluation of the patient with patellofemoral dysfunction. The clinician should be aware however that the technique can underestimate TT:TG by 5 mm to 8 mm when compared to TT:TG distance measurements obtained via MRI. When compared to CT scan, MRI has been shown by some authors to underestimate TT:TG distance by approximately 4 mm. While comparison to CT TT:TG measurement was not a part of this study, this plain film measurement technique may be expected to underestimate TT:TG by a centimeter. Additional studies are necessary to determine if measurements using this method can predict outcomes following nonoperative and operative treatment of patellofemoral dysfunction.