A retrospective study describing the acetabular consequences of Legg- Calve-Perthes disease

Abstract

Purpose

Legg Calve Perthes disease (LCPD) is a paediatric hip disorder caused by ischemia of the femoral epiphysis, causing femoral head deformity when untreated. This study aims to determine if previously validated pelvic obliquity radiographic parameters, used for assessing acetabular retroversion in developmental dysplasia of the hip, are applicable to patients with LCPD and its prognostic value.

Method

A retrospective study of patients with Legg Calve Perthes disease was carried out, analysing 4 pelvic parameters: Ilioischial Angle, Obturator Index, Sharp’s Angle and Acetabular Depth-Width Ratio (ADR). The differences between healthy and affected hips were studied, and subsequently, it was assessed whether these parameters have prognostic value in the disease outcome.

Results

Statistically significant differences have been obtained in the ilioischial angle, obturator index and ADR, between the affected and healthy hip. However, only the Acetabular Depth-Width Ratio showed predictive value for the disease outcome.

Conclusion

Although this study revealed differences in pelvic parameters between healthy and diseased hips, with only the ADR showing statistical significance in the disease's evolution and prognosis, further studies with larger sample sizes are necessary.

Introduction

In 1910, Arthur T. Legg, Jacques Calve, and Georg Clemenc Perthes independently documented a paediatric hip disorder resulting from ischemia affecting the proximal femoral epiphysis [1], mainly in male children between 4 and 8 years old. This leads to avascular necrosis in the affected region resulting in clinical symptoms characterized by pain, limited mobility, and claudication. The disease is primarily classified by radiographic findings, using the Waldenström classification. The four stages of Perthes are denoted as initial, fragmentation, reossification, and healed. Figure 1 [2]. The ultimate outcome varies depending on the extent of the femoral head deformity and the achieved articular congruence at the conclusion of the process.

Fig. 1

Evolution of LCPD according to Waldenström staging (with permission of Archives of Orthopaedic and Trauma Surgery (2023), 143:3945–3956

The femoral head changes resulting from necrosis have been extensively studied, with various classifications such as Catterall, Salter and Thomson, and more recently, Herring's lateral pillar classification. Additionally, concerning the long-term outcomes related to the femoral head's sphericity and the achieved articular congruence at the process's conclusion, Stulberg’s classification was developed [3] (see Tables 1 and 2).

Table 1 Statistiscal analysis summary of pelvic parameters. ADR acetabular width-to-depth ratio, CI Confidence Interval, Sign. Significance after Shapiro-Wilk test

Table 2 ANOVA of pelvic parameters according to Stulberg’s classification

In this classification, grade I represents a completely normal hip joint. In Grade II, there is spherical congruence, albeit with minimal deformity of the femoral head, measuring less than 2 mm. Grade III indicates non-spherical congruence, with a loss of femoral head shape exceeding 2 mm. Grade IV involves a flattened femoral head with abnormalities in the femoral head and neck but congruence in the acetabulum. Lastly, Grade V signifies non-spherical incongruence, presenting a flattened femoral head with acetabular incongruence. In addition to the previously mentioned femoral head deformity, radiographic studies have identified acetabular retroversion and pelvic obliquity. Many radiographic studies have also noted asymmetry in both obturator foramina, resembling a "Chinese eye" as illustrated in Fig. 2.

Fig. 2

Hip anteroposterior view RX image showing asimmetry of obturator foramen in both hips. The deformity of affected hip is evident

This is significant because the acetabulum plays a crucial role during the remodelling phase of the femoral head. The unchanged shape of the acetabulum facilitates the restoration of the sphericity of the proximal femoral epiphysis. However, alterations in the acetabulum's shape and inadequate coverage of the deformed femoral head can potentially lead to early onset osteoarthritis of the hip.

As LCPD typically manifests before reaching skeletal maturity, often between 4 and 8 years of age [4], accurately evaluating the shape of the acetabulum through radiographs during these ages is challenging due to incomplete ossification. Existing literature on acetabular retroversion in LCPD primarily focuses on skeletally mature hips with healed LCPD [5,6,7]. It is essential to note that acetabular retroversion involves not just the acetabulum but the entire hemipelvis [8, 9].

While CT or MRI can assess acetabular retroversion in the early stages of LCPD [10], these scans are not routine due to CT's radiation and the potential need for sedation or anaesthesia with MRI. Consequently, new indirect radiographic parameters have been proposed to evaluate acetabular retroversion before acetabular ossification [11].

This study aims to determine if previously validated radiographic parameters for assessing acetabular retroversion in developmental dysplasia of the hip (DDH) [12] are applicable to patients with LCPD. The objective is to identify potential variations between healthy hips and those affected by LCPD. Additionally, the study aims to investigate whether these parameters can serve as reliable factors in predicting the prognosis of LCPD and if they correlate with the outcome according to the Stulberg’s classification.

Material and methods

A retrospective observational study was conducted on patients diagnosed with LCPD who received care at a Children's Orthopaedic Unit of the Santiago University Clinical Hospital from 2014 to 2021. The study involved collecting demographic variables and reviewing radiographic studies in both anteroposterior and lateral view. The age range of the patients varied from two years and nine months to seven years and eight months, with a mean age of five years and three months (1933.52 days) and a standard deviation of one year, seven months, and seventeen days (592.26 days). Out of the patients, only two were female, constituting 9.52%.

Only patients with unilateral involvement were included, and they were followed up until reaching skeletal maturation. The affected hip was analysed, with the healthy hip serving as the control group.

All radiographs were consistently obtained following a standardized protocol. The patient assumed a supine decubitus position, with a film focusing distance of 1.2 m and the beam centred between the pubic symphyses. The pelvis was positioned in neutral rotation, confirmed by verifying correct longitudinal rotation when the tip of the coccyx aligned with the pubic symphysis.

In the radiographic assessments, the extent of femoral head involvement was evaluated using Herring's classification. Additionally, the presence of signs indicating a femoral head at risk was examined, and assessments of acetabular retroversion and pelvic obliquity were conducted by examining specific pelvic parameters. These parameters, previously validated by other authors in the context of DDH sequelae, include the ilioischial angle.

The ilioischial angle is determined by drawing two lines. The first line connects the base of each acetabular teardrop, forming the "interteardrop line." The second line extends from the intersection of the ilioischial line and the iliopectineal line to the outermost point of the ipsilateral obturator foramen. The measured angle is the one formed between these two lines, as illustrated in Fig. 3.

Fig. 3

Ilioischial angle, The ilioischial angle is determined by drawing two lines. The first line connects the base of each acetabular teardrop, forming the "interteardrop line." The second line extends from the intersection of the ilioischial line and the iliopectineal line to the outermost point of the ipsilateral obturator foramen. The measured angle is the one formed between these two lines

The obturator index is determined by delineating two lines. The first line extends across the maximum width of the obturator foramen (c), while the second is half the length of the interteardrop line (d). The index is computed by dividing the length of the first line by half the length of the interteardrop line, as illustrated in Fig. 4.

Fig. 4

Obturator index. The index is computed by dividing the length of the first line by half the length of the interteardrop line

Sharp's angle is formed by a line drawn between the two inferior points of both teardrops and another line connecting the inferior point of one teardrop to the lateral edge of the acetabulum, as depicted in Fig. 5.

Fig. 5

Sharp’s angle. Sharp's angle is formed by a line drawn between the two inferior points of both teardrops and another line connecting the inferior point of one teardrop to the lateral edge of the acetabulum

The acetabular depth-to-width ratio (ADR) is determined by a line connecting the superior bony margin of the acetabulum to the inferior margin of the ipsilateral teardrop (width measurement). Acetabular depth is defined by a line extending from the width line to the deepest point of the acetabular cavity. Once these measurements are acquired, the acetabular depth is divided by the width and then multiplied by 1000, following the formula: (depth/width) x 1000, as shown in Fig. 6.

Fig. 6

Acetabular depth-width ratio, ADR. The acetabular depth-to-width ratio (ADR) is determined by a line connecting the superior bony margin of the acetabulum to the inferior margin of the ipsilateral teardrop. Acetabular depth is defined by a line extending from the width line to the deepest point of the acetabular cavity. Once these measurements are acquired, the acetabular depth is divided by the width and then multiplied by a thousand, following the formula: (depth/width) x 1000

For consistency, these evaluations were conducted on all patients in Waldenström stage II, as acetabular retroversion is notably prevalent in this stage [11].

The radiographic measurements, anonymized for privacy, were initially taken in both hips by the same senior specialist in Orthopaedic Surgery. A reevaluation was carried out a month later by the same reviewer. Following the receipt of Ethics Committee approval and the acquisition of informed consents, the radiographic studies of 65 patients were examined. However, 21 were excluded after applying inclusion criteria—20 due to imperfectly centred radiographs or interference with measurements caused by the genital protector, and 1 due to discrepancies in measurements by the reviewer. Consequently, the study was completed with 44 patients.

Statistical analysis was conducted using IBM SPSS Statistics, version 22, and Microsoft Excel. A descriptive analysis of pelvic parameters was executed, encompassing mean, standard deviation, standard error, and a 95% confidence interval for the corresponding mean of the dependent variable for each studied parameter across both hips.

Initially, a comparison was made between the results obtained for each pelvic obliquity parameter in affected hips and those unaffected by the pathology. Subsequently, the suitability of these parameters in predicting the disease prognosis was assessed by comparing them with the final hip condition according to the Stulberg’s classification, utilizing the ANOVA statistical test.

A significance level of 0.05 was employed, signifying that differences were considered significant when the p-value fell below 0.05. The F values for each pelvic parameter were examined to determine whether statistical significance was greater or less than 0.05. Consequently, the null hypothesis of equality of means was accepted if the significance was greater, or conversely, rejected if the significance was less than 0.05, thereby accepting the alternative hypothesis. This study has been approved by the CEI-SL (Comité de ética de la Investigación Santiago Lugo) ethics committee (Code 2023/326) and have therefore been performed in accordance with the ethical standards laid down in the 1964 Declaration of Helsinki. Since subjects are minor, informed consents have been obtained from parents or legal representative. Details that might disclose the identity of the subjects under study have been omitted.

Results

Regarding the degree of involvement, as per the Herring classification for femoral head involvement, 10 corresponded to group A, 26 to type B, and 8 to type C.

Statistically significant differences in the ilioischial angle, obturator index, and ADR were observed when comparing pelvic parameter measurements between healthy hips and those affected by LCPD. However, no significant difference was found in Sharp's angle (Table 1).

An analysis was conducted to determine if the values of these pelvic parameters could impact the prognosis of the disease according to the Stulberg’s classification. Among the 44 hips, 4 were classified as Grade I, 5 as Grade II, 13 as Grade III, 15 as Grade IV, and 7 as Grade V.

Examining the results presented in Table 2, it is evident that there are no statistically significant differences, given that the p-value is greater than 0.05, except for the grades and ADR. This implies that the null hypothesis is accepted for the first three parameters in the table—namely, the ilioischial angle, obturator index, and Sharp angle have independent values concerning the assigned Stulberg’s grade of the affected hips. On the other hand, the acetabular width-to-depth ratio (ADR) exhibits statistical significance less than 0.05. Consequently, the values of this variable fluctuate concurrently with the Stulberg’s classification, suggesting prognostic value in the progression of LCPD.

Discussion

Clinical symptoms, disease onset age, and radiological signs play crucial roles in determining the final clinical and functional outcomes following hip joint treatment for Perthes disease [13]. Numerous imaging studies have focused on deformities in hips affected by Legg-Calvé-Perthes disease (LCPD), primarily emphasizing alterations in the femoral head [14, 15]. The risk head concept proposed by Catterall has been widely utilized to estimate hip joint form and function during LCPD treatment. Different classifications, such as those by Catterall, Salter, Thomson, and Herring, have been developed based on femoral head involvement, with Herring's classification currently being highly regarded as a prognostic factor [16, 17]. However, acetabular retroversion, present in 31–49% of LCPD cases, is also observed in these imaging studies [18, 19].

Acetabular retroversion further limits range of motion and exacerbates femoroacetabular impingement in LCPD hips. The biomechanics of the hip reveal an intimate relationship between the shape of the femoral head and pelvic morphology [20]. Acetabular retroversion significantly influences femoral head coverage and the orientation of the lateral wall of the acetabulum. Moreover, the acetabulum plays a pivotal role in the remodelling of the proximal femoral epiphysis and, consequently, the ultimate outcome of the disease.

Regarding the etiology of the aforementioned retroversion, the majority of authors consider that the initial changes originate in the shape of the femoral head with a normal acetabulum which secondarily becomes deformed. However, other authors [21] have previously speculated that in hips with LCPD, the primary deformity is the retroverted acetabulum which subsequently results in altered loading patterns of the joint and stress on the blood supply of the femoral head, causing secondary deformity of the femoral head. Notably, Larson’s study [22] also raises questions since it is unclear whether this is a result of the disease or whether acetabular retroversion predates femoral head involvement.

For the evaluation of acetabular retroversion, as far as imaging tests are concerned, the ossification of the acetabulum plays an important role, but complete closure of the triradiate cartilage usually occurs in women and men at the age of 15 and 16 years, respectively [22].

In Leibold's study [24], authors observed that the greater severity of the impairment of the femoral epiphysis (Herring B or C) was accompanied by greater acetabular retroversion. Likewise, in Larson's study [22], the author speculates on the hypothesis that acetabular retroversion, by causing a femoroacetabular impingement, could cause microtrauma in the retinacular vessels, which would have etiological importance in LCPD, although this speculation has several limitations due to the inclusion criteria. In addition, Larson et al. found that acetabular retroversion is more common in pediatric patients with LCPD than in pediatric control subjects, supporting the theory that retroversion may play a critical role in the pathogenesis of LCPD and that the acetabular abnormality should be taken into account during treatment of the disorder.

Assessing acetabular retroversion in skeletally immature hips with Perthes disease poses challenges due to the incomplete ossification of the acetabulum, limiting standard radiographic evaluation. Consequently, CT and MRI studies have been employed, albeit with concerns about radiation exposure and the potential need for sedation or anaesthesia in children.

Despite limited reliable studies on acetabular retroversion in skeletally immature hips with Perthes disease, contradictory findings in the early stage (Waldenström stage I) shift in prevalence and image homogeneity in stages II or III with fragmentation and new bone formation, as well as a greater homogeneity of the images, in these stages (Fig. 5). Various radiographic parameters, such as the crossover sign [23], retroversion index [24], and posterior wall sign [5], have been proposed to assess acetabular retroversion, considering the limitations of evaluating the ischial spine sign in immature hips.

Contrary to the affected hip, the contralateral side (unaffected hip) rarely exhibits changes in acetabular version over the same period.

Our study evaluates pelvic parameters in Perthes disease, focusing on their possible prognostic value rather than surgical treatments. The pelvis orientation during radiograph acquisition directly impacts radiographic anatomy, leading to the exclusion of improperly centered or oriented radiographs. Measurements, performed by a single observer, were repeated after one month to minimize inter-observer differences.

While inclination may affect radiographic measurements, studies indicate its similar impact on both hips. Despite the study's limitations, statistically significant differences were observed in the ilioischial angle, obturator index, and acetabular depth-width ratio between healthy and PCL-affected hips in the initial stages of Waldenström stage II disease, offering insights beyond studies requiring skeletal maturation for pelvic obliquity assessment.

Few studies have explored the relationship between final outcomes and pelvic and acetabular obliquity, making Grzegorzewski's [25] evaluation of acetabulum shape changes during the disease process particularly interesting. Notably, acetabula with a normal shape at the disease's conclusion were classified as Stulberg's group 1 and 2, while those with deformities in the acetabulum's external portion were classified as Stulberg's type 4 and 5, with no explicit reference to pelvic obliquity.

Conclusions

In our investigation, patients with LCPD exhibited distinct values for the mean obturator index, ilioischial angle, and ADR between the affected and healthy hips, whereas Sharp's angle showed no such difference. This suggests the presence of pelvic obliquity in the affected hip compared to the healthy hip in LCPD (when patients were evaluated in Waldenström phase II).

However, concerning the significance of these measurements and their impact on the disease prognosis, only the ADR has demonstrated a correlation with the final Stulberg’s classification. We believe that these results underscore the importance of a comprehensive approach to LCPD management, incorporating detailed anatomical assessments to inform treatment strategies and improve patient outcomes. In particular, our study highlights the prognostic significance of pelvic parameters. By integrating these measurements into prognostic models, clinicians can better predict disease outcomes and adjust treatment strategies accordingly. This approach might help in identifying patients at higher risk of poor outcomes and in intensifying monitoring and intervention efforts for these individuals.