The Hounsfield Units Algorithm Demonstrates Changes in Bone Density of the Distal Tibia in Patients with Talus Osteochondral Defect

Category: Ankle; Sports Introduction/Purpose: Osteochondral Defects (OCD) can be a painful condition that frequently affect the ankle joint, with talar OCD being more frequent than the tibia ones. Changes in bone density (BD) have been described around the lesion, particularly with sclerosis at the subchondral and cancellous bone of the talus. However, there is a paucity of data describing what happens with the distal tibia bone adjacent to a talar OCD. Weightbearing computed tomography (WBCT) offers a method for quantifying BD via calculation of tomographic Hounsfield units (HU), a quantitative scale for describing radiodensity. The aim of this study was to assess WBCT HU around talar OCD, investigating the pattern of BD distribution in the talus and adjacent tibia secondary to locally altered mechanics and stress concentration. Methods: In this retrospective comparative study, we included patients with talar OCD, either as primary diagnosis or as incidental finding, that underwent WBCT imaging of the foot and ankle. The Volume of Interest (VOI) represented a cube positioned around the talar OCD (width/depth) and length expanding 5mm below the OCD, the OCD, the joint space, and 5mm above the tibial plafond. The HU distribution was obtained along three parallel lines (anterior, central, and posterior aspects of the OCD) positioned inside the VOI, and aligned perpendicular to the joint surface. The same VOI and exact same process was then repeated on the opposite non-lesion side of the talus, that served as a control for normal HU distribution of talus, joint space and tibia. Graphical plots for HU distributions were generated for each line, separating the HU values and distributions in 3 control or 4 segments: talus, osteochondral lesion, joint space and tibia. Results: Thirty-two talar OCD patients (10 males, 22 females) were included. The mean age was 54 years old (range: 22-82 years). Fifty-two percent were symptomatic, and 48% had an incidental OCD finding. There was not significant difference in mean HU along the three lines (anterior, central, and posterior) in any segment (talus, OCL, joint space and tibia) when comparing the symptomatic and incidental OCD patients. However, when comparing talar OCD patients and controls, the mean HU followed a different distribution pattern. In controls, the talus had higher HU average than the tibia, but in OCD patients the tibia demonstrated higher HU than the talus. When comparing the HU in the tibia between OCD and controls, the OCD patients demonstrated significantly increased BD (p<.0001). Conclusion: Our study provides a novel understanding about BD changes in the subchondral bone of the distal tibia in talar OCD patients, with significantly increased bone density when compared to controls, even among asymptomatic cases. The observed increase in bone density in the distal tibia of OCD patients probably demonstrates the local mechanical derangement secondary to the talar OCD, and the tibial response to this derangement, what could potentially explain the pain symptoms in talar OCD patients, as well as the subsequent occurrence of concomitant mirror OCD in the tibia (kissing lesion). Future studies are necessary to further elucidate this issue.