Rotational abnormalities in dysplastic hips and how to predict acetabular torsion

Developmental dysplasia of the hip (DDH) is an orthopedic disease with a variation in incidence among different ethnicities [1]. Even though Germany started screening infants in 1996 using ultrasonography at the latest at the age of 5 weeks, Partenheimer et al reported that up to 18% of infants with severe hip dysplasia are under- or misdiagnosed [2]. If conservative treatment fails, surgery might be required in adolescence or adulthood [3] in an effort to both improve function and also decrease the risk for future degenerative change [4]. Indeed, DDH is one of the leading causes of secondary osteoarthritis of the hip [5].

For diagnosis, plain radiographs are performed, including two conventional radiographs—one of the pelvis, in anteroposterior and one axial view of the affected hip in abduction [6, 7]. The relevant radiographic measurements include the lateral center edge angle, anterior center edge angle, acetabular hip index, and acetabular index angle (AIA) [8]. For more precise measurements, a computed tomography (CT) can be requested to assess the femoral and acetabular torsion [9]. However, this is not performed routinely and data on the rotational profile of the CT in correlation to plain radiography is lacking in the literature, especially in DDH or borderline hips [10‐14]. Some authors have provided a comparison between the lesser trochanter size on plain radiographs to the femoral version on a CT [15], although further description is largely lacking. Furthermore, there is no objective definition for the diagnosis of DDH based on these rotational values. This lack of a clear cut-off can sometimes cloud decision-making. Given the significance of the surgical intervention required, whether it be the triple osteotomy of Tönnis [16] or the Bernese periacetabular osteotomy (PAO) of Ganz [7] both of which correct the pathological torsion and improve femoral head coverage, more clarity in terms of decision-making would be advantageous.

Therefore, the aim of this study was to assess how torsion of the acetabulum and femur, as assessed by CT, correlated with and can be estimated from conventional measurements on plain radiography in dysplastic hips compared to borderline-dysplastic hips.

A retrospective trial was conducted after internal review board (IRB) approval was obtained from the local ethical committee. Between 2017 and 2019, all patients 18 years of age or older presenting to a major hip preservation center and diagnosed with hip dysplasia or borderline hip dysplasia who underwent CT for the assessment of rotational alignment and subsequent surgery were included in our study. Inclusion criteria consisted of full available medical reports as well as preoperative radiography and CT of the lower limb. Exclusion criteria were incomplete records as well as no accessible CT.

Data on demographics including age, gender, body weight, body height, and body mass index (BMI) were noted. On conventional radiography, common standard measurements were analyzed by a single orthopedic surgery–trained observer with a focus on hip preservation surgery (first author). These included the center-edge (CE) angle, the acetabular index angle, the Sharp angle, the hip lateralization and the acetabular hip index, and the centrum-collum-diaphyseal angle (CCD) on anteroposterior view as well as the alpha and beta angle on axial view. Furthermore, the presence of a crossing-over sign, CAM, or pincer femoroacetabular impingement was identified as illustrated in Fig. 1.

Between 2017 and 2019, 99 patients presented with borderline or dysplastic hips to our center. In 38 patients a rotational computed tomography was obtained of which 34 patients met inclusion criteria with 56 borderline or dysplastic hips. Females made up the majority of patients (89.3%, or 50/56) and the overall mean age was 28.9 ± 7.8 years (range 18 to 40 years). The average height was 170.2 cm (5’ 6”), at a mean body weight of 68.1 kg (150.1 lbs) and a mean BMI of 23.6 kg/m². Hereby, significant differences (p < 0.01) were observed in gender with a mean age of 34.5 ± 4.5 years in males vs. 28.2 ± 7.9 years in females, a body height of 184.8 ± 3.3 cm (6’ 0”) vs. 167.9 ± 6.8 cm (5’ 5”), a body weight of 83.5 ± 9.5 kg (184.1 lbs) vs. 66.2 ± 10.8 kg (145.9 lbs), and a mean BMI of 24.5 ± 3.0 kg/m² vs. 23.6 ± 4.1 kg/m².

Of the 56 hips, 43 were dysplastic and 13 were borderline-dysplastic. All individual demographics are summarized in Table 2.

For standard DDH measurements on plain radiography, the acetabular index angle and CE angle were more abnormal for the hip dysplasia group compared to the borderline-dysplasia group at 13.1° ± 4.80 compared to 6.3° ± 2.38 and 20.9° ± 5.37 vs. 27.6° ± 3.74 (all p < 0.001), respectively. Additionally, the acetabular hip index and sharp angle were significantly different. All individual findings are summarized in Table 2. For the rotational measurements, only significant differences in the femoral-tibial rotational difference were found with 6.7° ± 5.34 vs. 4.2° ± 3.09 (p = 0.04). Furthermore, Table 2 shows all findings measured on computed tomography.

Although the developmental dysplasia of the hip is one of the leading causes of secondary osteoarthritis of the hip, correlation between the rotational alignment of the lower limb (acetabular, femoral, and tibial torsion) and plain radiography of the pelvis is not well understood or described in the literature.

Until today, for rotational measurements of the lower limb, CT is the best diagnostic instrument according to Wissing et al [9]. It allows detailed examination of acetabular, femoral, and tibial torsion. Physical examination showed little correlation between external tibial torsion (57%) and hip rotation (14%) in comparison with the standard of CT [23].

In the diagnosis of hip dysplasia, an AP of the pelvis and an axial view of the affected hip are performed routinely and, in many cases, solely [6; 7]. For the center edge angle, the AIA, Sharp angle, and the beta angle measurements have shown significant correlations on plain radiography; only the alpha angle showed a poor correlation in one rater between the two diagnostic instruments. Therefore, the AIA, Sharp angle, and beta angle are as reliable and important as the center edge angle for the diagnosis of hip dysplasia. However, the correlation to rotational alignment remains unclear [14].

In the literature, rotational measurements of the lower extremity—femoral and tibial torsions—are described primarily in healthy patients [23‐26]. Hereby, good validity has been reported between CT and EOS scans [27].

In children, the femoral torsion measurements on 3D models based on biplanar radiographs (BPR) were comparable to MR images, although tibial torsion was not reliable [24].

These studies have not routinely assessed the acetabular version (normal range from 17° ± 6°) [28] or the correlation to plain radiography of the pelvis, especially in dysplastic hips. However, if a periacetabular osteotomy is indicated, not only the femoral head coverage but also the torsional abnormality need to be addressed and therefore be understood in detail [28].

In borderline-dysplastic hips, treatment is less uniform; it is possible that a better understanding of the rotational variation in these deformities may clarify the optimal treatment [29, 30]. A huge variation for the femoral antetorsion can be found in the literature ranging from 10 to 15° [25] with a mean of 12.7° ± 10° [31], including a [32] significant difference between the left and right femur (approx. 4° larger) and a gender difference with 17.8 and 22.7° for female and 15.3 and 21.4° for male healthy femurs [33] As patients age, femoral antetorsion decreases from a toddler to skeletal maturity from 31.1° ± 8.9 at age of 1 to 15.4° ± 7.6 at the age of 16 years. In our cohort, the mean age was 28.9 ± 7.8 years, and so the influence of age was likely not a factor [34].

In femoroacetabular impingement, a high prevalence of combined femoral and tibial torsional abnormalities has been already reported with a mean femoral torsion of 23° and tibial torsion of 29° [32, 35]. Furthermore, decreased femoral torsion showed less flexion and internal rotation in 90° flexion [36].

Even higher values for femoral torsions were observed in our study at 29.65° ± 13.44 compared to the reference values in healthy and FAI patients as reported in the literature [31, 32, 34, 35]. For tibial torsion, the mean was 37.18° ± 7.4 in our cohort. As with femoral torsion, this value was significantly higher compared with normal tibial torsion at 21.6° ± 7.6 (range 4.8 to 39.5) [37, 38]. In addition, a correlation between the femoral neck torsion and the beta angle was observed in all hips. This relates to its configuration, where a greater anteversion results in a higher CCD angle on radiography, causing an increasing beta angle. In contrast, the alpha angle, which expresses the angle of the anterior head-neck congruency between the head center and the acetabular rim, correlated significantly to the acetabular torsion. This correlation may result from an increased anteversion of the acetabulum in borderline- and dysplastic hips [39] and subsequent greater cartilage coverage.

In dysplastic hips, significant differences were found especially for the torsion of the acetabulum in correlation to the AIA and the hip lateralization index which likely results from increased anteversion. As a result, a shorter distance of the lateral edge of the acetabulum to the inferior point of the weight-bearing area of the acetabular sourcil can be observed, which then leads to a steeper angle on plain x-ray. Likewise, the higher the acetabular torsion, the smaller the hip lateralization index gets which originates from a decreased distance between the center of the femoral head to the pelvic teardrop due to both increased anteversion and a relatively ovoid (as opposed to round) femoral head [40]. If the AIA from plain radiography is used to predict the acetabular anteversion, the x-ray must be taken very precisely. The AIA is easy to misinterpret when the hip is rotated or tilted [41]. Based on these findings, we were able to develop a formula to estimate rotation from plain radiography with high accuracy in DDH of 70%. Other findings, included the alpha-angle which was only significant in the overall cohort, not on subgroup analysis (p = 0.070 in the dysplastic group, p = 0.068 in the borderline-dysplastic group).

When comparing the results of the dysplasia and the borderline dysplasia group, the femoral-tibial rotation difference showed significant correlations to the alpha angle. In dysplastic hips, a positive coefficient of regression was observed (0.312, p = 0.034), whereas for borderline-dysplastic hips, this was found to be negative (−0.434, p = 0.009).

Since periacetabular osteotomy remains an effective technique to treat symptomatic hip dysplasia with survival of up to 29% of hips 30 years postoperatively [42], a thorough examination is warranted to include the common standard measurements, as well as rotational alignment. An understanding of the correlations between the individual measurements to allow a 3-dimensional reorientation of the acetabulum can allow for more thorough deformity correction. For borderline-dysplastic hips, no significant correlations were found. When considering our results and the existing literature on the unclear indication for PAO or hip arthroscopy [29, 30], further diagnostics are essential including a computed tomography to assess the rotational alignment.

There are several limitations to this study. This retrospective study included 56 hips describing correlations between radiographic measurements and CT findings in dysplastic and borderline-dysplastic hips. Two different observers performed the measurements; the plain radiographs were analyzed by an orthopedic surgery–trained observer with a focus on hip preservation surgery (first author), whereas the computed tomography analysis was performed by a fellowship-trained musculoskeletal radiologist. No consensus reading was performed why we are unable to present the inter-/intraobserver reliability. To further validate the developed formula to estimate the torsion of the acetabulum based on the plain radiography, a lager cohort study is required.

To our knowledge, this study is the first to analyze the correlations between all torsions of the lower limb with rotational CT and plain radiography in patients with dysplastic and borderline-dysplastic hips. Significant correlations were found especially for the AIA and the hip lateralization index in relation to the rotation of the acetabulum. Based on our findings and the developed formula, this may help to estimate acetabular torsion. As the borderline-dysplastic hips had mostly normal standard measurements on plain radiography with no major differences in rotational CT, further diagnostics to assess the rotational alignment should be performed.