Binary Tönnis classification: simplified modification demonstrates better inter- and intra-observer reliability as well as agreement in surgical management of hip pathology

For hip joint pathologies, two major operative treatments exist: hip preservation and hip replacement. The presence of osteoarthritis is a critical factor in a surgeon’s decision between the two options [1]. Efforts to preserve the hip joint are hindered by the presence of osteoarthritis [2]. Therefore, a reliable evaluation of the degree of osteoarthritis is necessary for optimizing patient outcomes. Radiographic assessment provides essential information concerning the diagnosis and treatment of osteoarthritis [3]. The traditional Tönnis Classification System is commonly used to classify the severity of osteoarthritis. The literature generally supports hip preservation for hips graded as Tönnis 0 and 1, and replacement for hips graded Tönnis 2 and 3 [2, 4]. However, despite its extensive use in clinical practice and medical literature, the traditional Tönnis Classification System has some drawbacks [5]. First, several studies have reported questionable inter-observer and intra-observer reliability [3, 6, 7]. A cardinal drawback of the traditional Tönnis Classification System is it’s subjectivity. It has been criticized for being unclear and having overlapping parameters. Yet, another difficulty may rise when parameters from different grades are found in a single radiograph e.g. moderate loss of head sphericity and slight narrowing of the joint space, which pretrain to grade 2 and 1, respectively [5]. The pitfalls associated with the traditional Tönnis Classification System reach beyond the boundaries of orthopedics and may have multidisciplinary manifestations that impair the cross talk between radiologists, general practitioners, and rheumatologists. Similar to the traditional Tönnis Classification System, the Garden Classification for femoral neck fractures also demonstrated poor reliability derived from the challenging radiographic distinctions between the grades. Based on the clinical relevancy of the Garden Classification, a simplified binary classification was developed that demonstrated higher reliability compared to the original classification [8‐11]. Given the binary nature of available surgical interventions (i.e. hip preservation versus reconstruction) derived from the traditional Tönnis Classification System, a two-level classification could be more reliable and reproducible without compromising the clinical relevance. Taking into consideration Occam’s Razor [12], which states that the simplest answer is typically the correct answer, a two-level classification for surgical treatment options seems most appropriate. The goal of this study is to validate a simplified Binary Tönnis Classification System to reduce excessive complexity and better capture the diagnostic essence of having a certain classification. Specifically, this study (1) compares the inter-observer and intra-observer reliability of the traditional Tönnis Classification System and a new simplified Binary Tönnis Classification System for hip osteoarthritis and (2) evaluates the clinical applicability of both systems, notably its agreement with the clinician’s decision for either preservation or replacement. Our hypothesis is that a binary system will have better reliability and agreement for surgical decision-making.

The study sample contained 40 anterosuperior hip radiographs, 19 of which received hip preservation and 21 of which received hip replacement. There were 15 males and 25 females (age 35.05–59.25 years). The demographics of the overall group and subgroups are presented in Table 3.

The traditional Tönnis Classification System showed fair reliability for the inter-observer reliability, (κ = 0.474) and excellent reliability for the intra-observer reliability (κ_mean = 0.866, range = 0.780–0.907), as calculated by the weighted κ agreement.

The inter-observer and intra-observer reliability showed improvement with the simplified Binary Tönnis Classification System. The inter-observer reliability was (κ = 0.858) and intra-observer reliability was (κ _mean = 0.928, range = 0.892–0.948). Both inter-observer and intra-observer reliability were deemed excellent (Table 4).

The Tönnis grading based on both systems and their agreement with the ultimate surgical management were calculated and are represented in Table 5. On average, the simplified Binary Tönnis Classification System correctly captured 87% of cases. When the traditional Tönnis Classification System was dichotomized (0 and 1 as hip preservation and 2 and 3 as hip replacement), the capture rate was 84%. The confusion matrices for the capture rates are depicted in Tables 6 and 7.

The aim of this study was to validate a simplified Binary Tönnis Classification System. In this study, 40 radiographs of consecutive patients were analyzed by five fellowship-trained orthopedic surgeons. Overall, the Binary Tönnis Classification System reported better inter-observer and intra-observer reliability and demonstrated higher agreement rate with the ultimate surgical treatment, as recommended by the treating surgeon, compared to the traditional Tönnis Classification System.

In their study, Clohisy et al. [3] (Table 8) evaluated the ability of hip specialists to reliably indicate the correct diagnosis based on plain radiographs alone. Five hip specialists and one fellow performed a blinded radiographic review of 25 hips with developmental dysplasia, 27 hips with femoroacetabular impingement, and 25 control hips. The readers assessed a variety of radiographic parameters including osteoarthritis using the traditional Tönnis Classification System. The combined κ for intra- and inter-observer reliability for the traditional Tönnis Classification System were 0.60 (95% CI: 0.54–0.66) and 0.59, respectively. Furthermore, Steppacher et al. [6] had two readers assess the Tönnis grade for a set of 50 radiographs illustrating dysplastic hips. The range of reported κ for intra-observer were 0.73 to 0.74. The Fleiss κ for interobserver reliability was 0.74. Clohisy et al. attributed the difference between their results and Steppacher’s to their inclusion of a non-dysplastic cohort, in contrast to a dysplastic only cohort in Steppacher’s study. The higher radiographic variability may have contributed to a decrease in reliability, especially in cases with none or mild arthritis. Troelsen et al. [7] aimed to investigate the variability of diagnostic assessment of the hip joint. In their study, four observers independently assessed the level of osteoarthritis in 25 radiographs. Treolsen dichotomized Tönnis grades. They assessed the dichotomized inter-observer reliability, of a quaternary classification, as well as its agreement with CT scan. The κ for inter-observer agreement was 0.54 for the Tönnis classification and 0.66 for the dichotomized version. Furthermore, the observed agreement with the CT scan was 70% for the traditional Tönnis Classification System and 88% for the dichotomized alternative. In this present study, κ for intra- and inter-observer reliability for the traditional Tönnis Classification System were 0.86 and 0.47, respectively. In contrast to the evidence reported for the traditional Tönnis Classification System, the simplified Binary Tönnis Classification System demonstrated excellent inter- and intra-observer κ (i.e. 0.86 and 0.85, respectively). Additionally, this study supports Troelsen’s findings from dichotomizing the traditional Tönnis Classification System. Adopting a true binary classification will better serve the clinician as it would eliminate the need for a preliminary low-reliable four level classification which requires further dichotomization for determining treatment.

The second step in validating the simplified Binary Tönnis Classification System was to assess its reliability in indicating the surgeon-recommended treatment. Valera et al. [1] evaluated the reliability of the traditional Tönnis Classification System as a reference for hip preservation. Three orthopedic surgeons examined 117 hip x-rays for hip joint osteoarthritis according to the traditional Tönnis Classification System. The κ value for interobserver reliability were slight or fair (0.173–0.379) and the κ value for intra-observer reliability were fair (0.364–0.379). Variance in classifying low grade osteoarthritis was the major cause for disagreement between observers. In contrast, experience did not play a significant role in grading reproducibility. The authors concluded that the traditional Tönnis Classification System is a poor method of assessing early hip osteoarthritis and that routine use in clinical decision-making for preservative surgery should be reconsidered. In this study, the traditional Tönnis Classification System correctly overlapped with actual surgical treatment in 85.2% of cases. The simplified Binary Tönnis Classification System had a higher overlap, correctly capturing 86.5% of the cases. While the binary classification did show a slightly better correlation with the indicated treatment, it should be emphasized that radiographic evaluation is only part of the overall patient assessment and thus a discrepancy between both classification systems and the actual performed treatment should be expected.

In summary, the simplified Binary Tönnis Classification System addresses the drawbacks of the traditional Tönnis Classification System without compromising clinical relevance. Adoption of a binary system would allow for more consistent data collection, thus improving the quality of studies. Practically for the clinician, a two-grade classification is more appropriate for a two-way treatment.

A simplified Binary Tönnis Classification System demonstrates better reliability and clinical implementation than the Traditional Tönnis Classification System.