Skeletal maturity can evaluate the growth and development potential of children and provide a guide for the treatment of adolescent idiopathic scoliosis (AIS). A convenient and reliable method for assessing skeletal maturity is of great significance for guiding the treatment of spinal curvatures, especially in idiopathic scoliosis (1). By using skeletal maturity as a predictor, clinicians may determine the likelihood that a scoliosis curve will progress, which helps them decide how long to have their patients wear braces and whether to operate.

Currently, there are several commonly used methods to assess bone maturity: Greulich and Pyle (2, 3), Tanner–Whitehouse III (TW3) (4, 5), Sanders system (6, 7), Thumb Ossification Composite Index (TOCI) (8, 9), Humeral Head Ossification System (HHOS) (10, 11), Proximal Femur Maturity Index (PFMI) (12), and Risser's sign (13). Although the Risser sign can be used as an indicator of skeletal maturity evaluation, it has suboptimal reliability. These methods, for example, the Greulich and Pyle, Tanner–Whitehouse III, Thumb Ossification Composite Index, and the more acceptable Sanders system, evaluate skeletal maturity by hand radiographs. The Sanders system has substituted the Risser system to some extent. The TW3 score and Greulich and Pyle systems are clinically complicated, as they require access to atlas for individual bone scores. The TW3 score and Greulich and Pyle can be used reliably in idiopathic scoliosis patients with artificial intelligence (AI) model assistance (3, 5). Many studies (14, 15) indicate that the Sanders system and TOCI can be used as relatively simple and reliable methods in patients with idiopathic scoliosis. These methods of assessing bone maturity require x-rays of the hand. Many studies (16, 17) have shown a correlation between radiation exposure and cancer risk. However, scoliosis patients require long-term follow-up, usually a full-length spine film every 6 months. The adoption of a skeletal maturity assessment system without additional radiation is what is expected (18). HHOS and PFMI are used to assess bone growth and development by measuring the humerus head and proximal femur epiphysis included in standard spine radiographs.

The Humeral Head Ossification System was first proposed by Li et al. (11) to classify the proximal humerus in the radiographs of 94 scoliosis patients. In addition, developers have confirmed the extreme reliability of the HHOS (11). However, the system needs further confirmation of its reliability at other institutions not participating in the research and development. Lopyan et al. (18) repeated experiments based on the study by Li et al. to find fair to moderate interrater and intrarater reliability. This distinction requires further investigation. Cheung et al. (12) first described the Proximal Femur Maturity Index, and reliability testing of the PFMI revealed excellent intraobserver and interobserver agreement. Similarly, the Proximal Femur Maturity Index can be reproducible. This study assesses HHOS’s and PFMI’s reliability in the interrater and intrarater.

This study was a retrospective analysis of 80 patients from the Xi’an Honghui Hospital. After obtaining approval from the institutional ethics committee, we obtained the data of 38 patients with AIS by inclusion and exclusion criteria (Figure 1). These data included demographic information and x-rays of patients who met the inclusion and exclusion criteria. Study inclusion criteria: patients diagnosed with AIS, between the ages of 10 and 18, and with available posteroanterior (PA) scoliosis films that included the humeral head and the proximal femur. Exclusion criteria: non-adolescent idiopathic scoliosis (for example, early-onset scoliosis) and patients with incomplete films (excluding the humeral head and proximal femur). Demographic data consist of gender and age. Each rater received a PowerPoint (PPT) presentation consisting of radiographs. The PPT sequence was randomly changed and then reevaluated 2 weeks later. The raters were eight independent observers from the Xi’an Honghui Hospital and the Baoji Maternal and Child Healthcare Hospital. The seven raters from the Xi’an Honghui Hospital comprised one chief pediatric orthopedic surgeon, one deputy chief pediatric orthopedic surgeon, two attending pediatric orthopedic surgeons, two pediatric orthopedic surgeons, and one radiologist. The other rater was an attending pediatric orthopedic surgeon from another hospital. There are four stages of physician titles in the Chinese healthcare system: primary title (Resident Physician), middle title (Attending Physician), vice-senior title (Deputy Chief Physician), and senior title (Chief Physician). In general, each level of physician needs a 5-year interval of clinical accumulation to pass the qualification and examination before being promoted to the next level. The x-rays are required to be independently categorized by the raters. After a brief study, all raters who had not previously learned the two classification systems evaluated skeletal maturity using the HHOS and PFMI systems.

The HHOS classification (11) assesses skeleton maturity by epiphyseal morphology of the humeral head, using the developer's model as guidance. The HHOS classification is classified into five stages (1–5) (Figure 2). PFMI classification (12) assesses skeletal maturity by the comprehensive growth and development of the femoral head morphology, greater trochanter, and triangular cartilage, using the developer's model as guidance. The PFMI classification is classified into seven stages (0–6) (Figure 3).

We analyzed the data using IBM SPSS 26 statistical software. The intraclass correlation coefficient (ICC) was calculated using a random effects model with an absolute agreement to identify the intraobserver and interobserver reliability for each rater. The intraclass correlation coefficient of less than 0.20 denotes poor agreement, one between 0.20 and 0.40 denotes fair agreement, one between 0.40 and 0.60 denotes moderate agreement, one between 0.60 and 0.80 denotes good agreement, and one of more than 0.80 denotes excellent agreement. ICC was also counted for each grader to estimate intraobserver reliability.

The demographic data of 38 patients were analyzed statistically in this study (mean age, 12.55 ± 1.927 years; 76.3% female).

Assessing the skeleton maturity for AIS plays an important role in knowing the advancement of the disease and intervening effectively (14). The risk of AIS progression is substantially correlated with bone growth potential. The peak height velocity can be used to determine the remaining potential of bone growth to determine the most appropriate treatment. Skeletal maturity reliability assessment is still a significant procedure. Given multiple existing methods of assessing bone growth potential, this study provides data support for HHOS and PFMI systems.

While many methods have been proposed to evaluate skeletal maturity, all of these methods currently in use have disadvantages and advantages (18). Overall, Risser's sign has low interobserver and intraobserver reliability. The Greulich–Pyle and Tanner–Whitehouse classifications are more adapted for academic studies and have some difficulties in clinical work (14). The TOCI and Sanders systems, which are relatively simple to operate, have a fast learning curve and high reproducibility and reliability, but they need additional hand films. The HHOS and PFMI systems have multiple stages that also correlate with growth potential, and standard PA scoliosis radiographs usually obtain information about HHOS and PFMI (18).

The developer's study described interrater and intrarater reliability as excellent for HHOS among orthopedic surgeons with different experience (10, 11). The system used intraclass correlation coefficients to evaluate intraobserver and interobserver reliability. The developer's research suggested that the humerus’ location is not constrained (11). However, we discovered that numerous humeral heads, which had imaging overlays or occlusions on the humeral head, could not be used to assess skeletal maturity on standard scoliosis films. We concentrated on including optimal graphics to evaluate the reliability of the HHOS. The impact of different humeral head positions on the assessment of the HHOS system should be the subject of future study. It is crucial that evaluation systems be tested for reliability and repeatability by other institutions that do not take part in its development. Lopyan et al (18) repeated experiments based on the original study to find fair to moderate interrater and intrarater reliability. In light of the original study, we replicated the experiment and discovered good agreement in terms of both interobserver and intraobserver reliability for HHOS. This suggests that other institutions are required to confirm the value of this classification.

Cheung et al. originally described the Proximal Femur Maturity Index (12). The PFMI classification (12) assesses skeletal maturity by the comprehensive growth and development of the femoral head morphology, greater trochanter, and triangular cartilage. Furthermore, the PFMI showed fair to good agreement between interobserver, with excellent intraobserver reliability. The developers provided a flowchart and made the appropriate modifications to their classification. After 3 months, the rater reevaluated it and discovered excellent intraobserver and interobserver reliability. The kappa coefficient was employed to assess intraobserver and interobserver reliability for PFMI. This study expands the body of work of Cheung et al. proving excellent interrater and intrarater reliability. The PFMI classification appears to be more readily available on standard scoliosis radiographs. However, it is worth thinking about the effects of radiation on the gonads. In conclusion, we prefer to utilize the straightforward approach in clinical work (18). There are advantages and disadvantages to each classification. We should choose a personalized and reasonable strategy based on the situation of adolescent idiopathic scoliosis. There is currently a small amount of literature indicating the reliability of this classification scheme, which needs to be evaluated through additional studies.

We enumerate several limitations. First, this was a retrospective cohort study to assess the reliability of different classifications by collecting historical radiographs. This study was limited to adolescent idiopathic scoliosis, and further studies are needed for scoliosis in other age groups. Second, this study included only x-rays with the best images and did not include humeral head morphology in all positions because some of the humeral head was overlapping or obscured on the films. Future studies should assess the reliability of the HHOS in different humeral head morphologies. Finally, the relationship between HHOS and PFMI should be the subject of future study.

The HHOS and PFMI classifications had excellent reliability. These two methods are beneficial to reduce additional exposure to radiation and expense for AIS. There are advantages and disadvantages to each classification. Clinicians should choose a personalized and reasonable method to assess skeletal maturity, which will assist in the management of adolescent scoliosis patients.