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Fibular Tip Periostitis: New Radiographic Sign, Predicting Chronic Peroneal Tendon Subluxation/Dislocation in the Setting of Pes Planovalgus.

Background: Tearing of the superior peroneal retinaculum (SPR) is a known cause of peroneal tendon subluxation/dislocation (PTS). However, with the exception of cortical avulsions at its fibular attachment, SPR injury and subsequent PTS are typically radiographically occult. Objective: Evaluate the previously undescribed association between radiographic fibular tip periostitis and MRI evidence of PTS, in patients with hindfoot valgus. Methods: 35 patients with radiographic fibular tip periostitis and 35 age- and sex-matched controls without periostitis, were selected out of 220 consecutive patients with hindfoot valgus and both ankle radiographs and MRIs. Studies were retrospectively assessed by two musculoskeletal radiologists in consensus, and two additional blinded, independent radiologists for presence of PTS, sub-fibular impingement, and hindfoot valgus angle measurement. Inter-observer agreement and accuracy, sensitivity, and specificity for detecting fibular periostitis, PTS, and sub-fibular impingement were measured. Fischer-exact, Mann-Whitney, kappa coefficients, and intra-class correlation tests were performed. Results: Both consensus and two independent readers' interpretations showed that PTS was significantly greater in subjects with periostitis (62.9%, 65.7% and 85.7%, respectively) compared to those without (5.7%, 0% and 14.3%, respectively) (p<0.001); periostitis was specific and highly sensitive for predicting PTS. Sub-fibular impingement was also statistically higher in the periostitis group compared to the one without (p<0.001). Hindfoot valgus angle was statistically higher in the periostitis group compared to the control group, (p = 0.010 to 0.002) and among subjects with PTS compared to those without (p = 0.002 to <0.001). The blinded readers reached substantial or near perfect agreement for all imaging interpretations (82.9-95.7% concordance rate, κ= 0.66-0.91). Conclusion: Radiographic fibular tip periostitis, in patients with hindfoot valgus, can be a predictor of PTS, and may also suggest advanced hindfoot valgus and sub-fibular impingement. These radiographic associations should be recognized by the radiologist and MRI may be recommended as clinically indicated. Clinical Impact: Chronic, undiagnosed PTS can be a persistent cause of lateral ankle pain, leading to further degeneration and the possibility of complete peroneal tendon tears. Distal fibular periostitis in patients with hindfoot valgus can be a reliable radiographic indicator of this entity and may suggest the presence of sub-fibular impingement.