Abstract

Introduction: Conventional radiographs are not doing justice to the complexity of 3D hip pathologies. Some methods based on 3D technology did not find their way to clinical practice. The goal of this study was to develop a 3D measurement method (named spidermap) for the acetabular coverage that can be used for the diagnosis of DDH as well as for the quantification of correction after Periacetabular Osteotomy (PAO).

Methods: In a first step we defined the threshold between physiological and dysplastic hips using this spidermap and in a second step we compared physiological to surgically treated dysplastic hips to quantify the correction. The population included three groups: Group A consisted of 18 physiological, group B of 21 dysplastic and group C of 8 surgically corrected hips. CT scans were used to calculate femoral head coverage by using the newly developed 3D algorithm. The result is a 2D map, a circular diagram showing anterior coverage at 0^°, lateral coverage at 90^°, posterior coverage at 180^° and medial coverage at -90^°. In a first step, groups A and B were compared to determine in which areas there was a significant threshold regarding coverage. In a second step groups A and C were compared to quantify the surgical correction after PAO.

Results: A significant threshold between groups A and B was found in the areas from -30^° to 247.5^° (p < 0.05). In the areas from 0^° to 180^° the specificity was high (>95%). The comparison of groups A and C showed a significant improvement in the areas from of 37^° to 202.5^° after PAO (p < 0.05).

Conclusion: Our 3D measurement method was able to reliably distinguish between physiological und dysplastic hips which allows for diagnosis of DDH in three dimensions. Furthermore, the spidermap allows for assessment of successful correction after PAO.

Introduction

Developmental hip dysplasia is a common reason for hip pain in young adults and known to be a significant risk factor for secondary osteoarthritis [1]. Today the radiographic diagnosis of DDH in adults is based on multiple parameters that are measured on conventional radiographs. Each of those parameters focuses on a single aspect of the configuration of the hip joint. Summed up they allow an experienced orthopaedic surgeon to evaluate the hip joint as a whole. However, it is an attempt to describe a 3D problem with a 2D method. Furthermore, it is depending on multiple other variables such as the quality and technique of the radiograph and on the experience of the observer [2-4]. A 3D method to evaluate the configuration of a hip joint is highly desired. It would allow to see the hip joint as a whole, to carry out preoperative 3D error quantification, to serve as fundament for individual surgical planning and 3D computer assisted surgical guiding methods. A few studies using a similar 3D approach to our study already exist [5-9], but are rather two- than three-dimensional, are not covering acetabular deformities or did not look in the quantification of surgically corrected deformities. The goal of this study was to develop an illustrative 3D measurement method (spidermap) for the acetabular coverage that can be used for the diagnosis of DDH as well as for the quantification of correction after Periacetabular Osteotomy (PAO). Therefore, in a first step we defined the threshold between physiological and dysplastic hips using the novel spidermap and in a second step we compared physiological to surgically treated dysplastic hips to quantify the correction.

Methods

Patients

This study was approved by the local ethical committee (KEK ZH, BASEC Nr. 2018-01921). Three groups of patients were analyzed. Group A consisted of 18 hips in 10 patients (2 were excluded to previous surgery) with a physiological hip configuration. In all hips an anteroposterior (ap) pelvic radiograph and a CT scan of the pelvis was available. The patients were consecutive patients from our outpatient's clinic with the beforementioned imaging available. None of them showed radiographic signs of dysplasia (LCEA > 22^°, ACI < 13^°) [10] or had no history of hip pain). Group B consisted of young adults with dysplastic hips. In all hips an anteroposterior pelvic radiograph and a CT scan of the pelvis was available. Group C is a subset of Group B with patients who underwent PAO in out institution. In all hips an anteroposterior pelvic radiograph and a pre- and postoperative CT scan of the pelvis was available. All patients of Group B and C (consecutive series of patients) received the PAO in our institution. In Group C 3 hips were excluded after the surgery due to bad quality of the post-operative CT scan in certain areas of interest and consecutive processing error. Patient demographics and conventional radiographic parameters are available in Table 1.

Table 1: Demographic Data.

Data Acquisition and Processing

Plain ap pelvic radiographs and CT scans of the pelvis of all patients were carried out (Group A) or already available (Group B and C). The radiographs were used to confirm the absence (Group A) or presence (Group B) of a DDH with conventional radiographic parameters (LCEA, ACI). The CT scans were used to generate 3D models. All CT scans were performed in our institution, using Siemens Definition AS^® or Somatom Edge CT^® scanners. Slice thickness was 1.0 mm with an in-plane resolution (x-y) of 0.4 x 0.4 mm. The CT scans were segmented and smoothened using the global thresholding and region growing functionality of a standard segmentation software (Mimics Medical 19, Materialise NV, Leuven, Belgium) to generate 3D bone models (Figure 1).

 

Figure 1: 3D bone model of pelvic bone (blue) and both femurs (red and green).

Our in-house developed software was used to process these models. The algorithm is based on a Matlab (Version R2015b, MathWorks, Natick, MA, USA) script. The models are standardized in their spatial orientation by calculating the anterior pelvic plane (APP) with help of the surgeon indicating the most anterior points of the anterior superior iliac spines and the pubic tubercles. Then the surgeon marks random surface points on the femoral heads and the algorithm calculates a best fit sphere to define their centers (Figure 2). An axis through both centers is laid. Thereafter the limbus acetabuli is defined. Therefore, the surgeon marks random surface points on the acetabular rim. The algorithm uses these points to create an acetabular opening plane that is used to define the complete limbus acetabuli by minimal vertical distance to this predefined opening plane (Figure 3). The algorithm then calculates at total of 48 angles (comparable to the LCE angle) all around this axis. The angles measured are between the axis and a line drawn from the center of the femoral head of the joint assessed to its limbus acetabuli and each represents its local coverage according to Larson et al. [7] (Figure 4). This part of the processing is completely automated. These angles are then converted into percentage of coverage and presented on a spidermap. A 180^° angle measured by the algorithm for local coverage corresponds to 100% coverage. The spidermap is a circular diagram, able to show the 3D data in 2D. The x-axis represents the rotation around the femoral head in degrees and the y-axis shows the percentage of femoral head coverage. For the spatial orientation 0^° reflects anterior, 90^° lateral, 180^° posterior and -90^° medial.

 

Figure 2: 3D bone model of proximal femur (turquoise) with best-fit sphere (blue) of femoral head for determination of center of rotation