Reliability of the Tönnis Classification and Its Correlation With Magnetic Resonance Imaging and Intraoperative Chondral Damage.

Purpose: To evaluate the reliability of the Tönnis classification in the setting of femoroacetabular impingement (FAI) hips without dysplasia.

Methods: Forty-nine patients with FAI underwent preoperative radiography and magnetic resonance imaging (MRI). Radiographs were evaluated in 2 separate settings by 5 observers and graded according to the Tönnis classification. Interobserver and intraobserver reliability was calculated using the κ coefficient. Intraoperative chondral damage was assessed, and chondral damage to the acetabulum (acetabular labrum articular disruption [ALAD] classification) and to the femur (Outerbridge classification) was graded. The Spearman coefficient was computed to quantify the degree of correlation between the Tönnis grade and MRI-detected chondral damage, as well as intraoperative chondral damage.

Results: The average intraobserver reliability of the Tönnis classification was moderate (κ = 0.472), and the interobserver reliability was fair (κ = 0.287). Statistically significant positive correlations were found between the Tönnis classification and the ALAD classification (P = .0087) and between the Tönnis classification and femoral chondral damage detected by MRI (P = .0247). A statistically significant correlation was not found between the Tönnis grade and the intraoperative Outerbridge classification of the femur (P = .4969), between the Tönnis grade and acetabular chondral damage on MRI (P = .4969), or between the Tönnis grade and the ability to detect a chondral flap on MRI (P = .2160). No statistically significant correlation was found between the ALAD classification and the presence or absence of a chondral flap on MRI (P = .3538), between the ALAD classification and MRI-detected chondral damage to the acetabulum (P = .103), or between the Outerbridge classification and the degree of chondral damage observed on MRI of the femur (P = .1922).

Conclusions: The Tönnis classification and MRI have substantial limitations when evaluating nondysplastic hips with FAI for the degree of chondral damage and arthritis.

Level Of Evidence: Level III, retrospective comparative study of prospective data.