The evaluation of degeneration of posterior cruciate ligament using CT Hounsfield unit in knee osteoarthritis

BACKGROUND: Posterior cruciate ligament (PCL) degeneration is often seen in knee osteoarthritis (OA); however, there is no established method for its evaluation. The purpose of this study is to investigate whether the Hounsfield unit (HU) using computed tomography (CT) could be a useful scale to evaluate the degeneration of PCL in knee OA. METHODS: Knee OA treated with total knee arthroplasty (21 patients, 21 knees) and non-osteoarthritic knees (21 patients, 21 knees) were retrospectively observed and studied. All PCLs in the knees were analyzed using CT. The PCL in the sagittal section was divided into three regions: proximal, middle, and distal sections. The HU value of the PCL at each area was measured. In osteoarthritic knees, tissues from the PCL were collected and histologically graded. The correlation between the radiological classification by Kellgren and Lawrence and the histological grade was analyzed. The average CT HU values for each degenerative grade were also calculated and compared. RESULTS: The HU values in OA and non-OA were 70.7 and 88.4 HU (p < 0.05) at the proximal region, 75.7 and 85.3 HU (p < 0.05) in the central region, and 82.3 and 86.5 HU (p > 0.05) in the distal region, respectively. The degeneration of PCL was graded as follows: one, three, and 17 mild, moderate, and severe cases at the proximal portion, and 16, 4, and one mild, moderate, and severe cases at the distal portion, respectively. The radiological classification and the grade of degeneration were not correlated in either the proximal (r = 0.047, p = 0.84) or the distal (r = − 0.21, p = 0.35) portions. The HU value was 84.5, 72.1, and 70.6 HU for mild, moderate, and severe grades, respectively (mild versus moderate: p < 0.05, mild versus severe: p < 0.05, moderate versus severe: p > 0.05). CONCLUSIONS: In knee OA, a lower HU value in the PCL indicates the progression of degeneration. The CT HU value could be a useful measurement to predict the grade of PCL degeneration.