Cementless femoral stems with lower canal fill ratio have similar mid-term to long-term outcomes to those with adequate fill ratio in Dorr type C femurs.

Abstract

Lower canal fill ratio was reported to correlate with aseptic loosening in many studies. However, the most widely used standard of fill ratio seemed inapplicable to Dorr type C femurs. We aimed to adapt the method of measuring the fill ratio in Dorr type C femurs and compare the outcomes among patients with different fill ratios. Twenty patients with Corail stems implanted in their Dorr type C femurs received spectrum CT to evaluate the whole-stem's fill ratio. Pearson Correlation Coefficient was calculated to assess the correlation between the fill ratio in X-ray film and spectrum CT. Then 87 THAs were involved in this study, divided into the fill ratio ≤ 80% group and the fill ratio > 80% group. Clinical and radiological outcomes were evaluated with a mean follow-up of 8.2years. Fill ratio at 2cm below the lesser trochanter in anterior-posterior X-ray film correlated with the whole-stem's fill ratio (r = 0.50, P = 0.02). Survival rate of stem, function scores, and radiological outcomes between the two groups showed no significant difference. In the fill ratio > 80% group, intraoperative fracture was significantly higher (19% VS 5%, P < 0.05). Patients with lower fill ratios at 2cm below the lesser trochanter did not have poorer functional scores or more subsidence, but had a lower intraoperative fracture rate. The revision rates of the two groups presented no significant difference, but this result need to be confirmed in larger cohort in the future. In Dorr type C femurs, risk of fracture and the special morphology of the femur should be noted, and high fill ratio is not the most decisive factor for stem size selecting.