The present study used data from an ongoing prospective registry in our hospital. We selected all patients who underwent radiographs and dynamic CT analysis for FAI diagnostics and who were operated on between 2012 and 2015. Inclusion criteria for the prospective registry were: diagnosed with FAI syndrome (i.e. evident clinical signs of femoroacetabular impingement [15], positive clinical assessment with positive tests specific for FAI [15] flowchart Fig. 1), age 18–65, managed conservatively first (with strengthening physiotherapy for at least three months, lifestyle changes and non-steroid anti-inflammatory drugs), suitable for surgery (after consultation of the anesthesiologist for any contra-indications for surgery) and patients have to be willing to participate. Exclusion criteria are age < 18 or > 65, prior hip arthroscopic surgery patient history and/or pathological fractures due to metastatic disease.
All patients were operated in our peripheral teaching hospital (location blinded). All patients signed informed consent to participate and to publish. The local Medical Ethics Committee decided that the study did not fall under the scope of the Medical Research Involving Human Subject Act because of the minimal burden for patients in comparison to regular care (METC nr 12–083). The data were retrospectively analyzed.
Radiographic measurements
Radiographic antero-posterior (AP) and Lauenstein images were made when patients were included. Radiographs were performed using standardized techniques in the supine position as described by Clohisy et al. [16]. AP pelvis radiographs were performed with the legs 15° internally rotated with the beam centered between the superior anterior iliac spine and symphysis pubis. The Lauenstein views were performed the hip in 30–40° of flexion and 45° of abduction with the heel a rest against to contralateral medial side of the knee.
A cam type morphology was measured on a Lauenstein radiograph by measuring the alpha angle. The angle is measured between two lines: a line from the center of the femoral head to the point where the radius of the femoral head exceeds a perfect circle drawn around the femoral head, and the line drawn from the center of the femoral head to the center of the femoral neck. An angle larger than 60° was considered an enlarged alpha angle and an indicator of cam morphology indicating FAI [6, 17, 18] Fig. 2.
A pincer type morphology is measured on an AP pelvic radiograph with the lateral center edge (LCE) angle. This is the angle between a line vertical to the center of rotation of the femoral head and the lateral edge of the acetabulum Fig. 4. An LCE angle larger than 33° was considered enlarged and an indicator of pincer morphology indicating FAI [10, 19]. All x-rays were interpreted by an independent radiologist who made a report in the patient file and by one of the researchers (MAR).
Dynamic CT analysis
The CT scans of the pelvis were performed with a standardized protocol. CT scans were performed at the department of Radiology using a second-generation dual source multi-detector spiral CT scanner (SOMATOM Definition Flash, Siemens Healthcare) AG, Erlange, Germany) with a tube voltage of 80 Kv and an effective mAs-value of 3140. Scan timer per CT scan was approximately 30 s. All patients were scanned in the standard anatomic axial plan orientation and were reconstructed with an effective slice thickness of 1.0 mm and a sharp reconstruction kernel (B75s). Multi-planar reconstruction was performed (image pixel size 0.265).
The dynamic analysis of the hip joints was made with proprietary software of Clinical Graphics® [20] which uses the coordinate systems as described in the recommendations of the International Society of Biomechanics and the equidistant method described by Puls et al. to simulate translation of the femoral head [21]. The software was previously validated in 2015 [11]. Figure 3 is an example of a cam type morphology causing impingement during simulated internal rotation as provided by the software. If kinematic motion is limited, the software reports the depth and location of the impingement and exact location of the type morphology, a dynamic movement analysis with exact impinging locations, an alpha angle (at seven positions from nine till three o’clock), a center edge angle (at three positions; 11, 12 and one o’clock positions) and a reproduction of the unlimited range of motion [11, 20]. An impingement is only detected within normal range of motion of the hip joint, according to relevant literature, which is discussed in the validation study [9].
The Dynamic CTs were interpreted by the software company who provides the software and dynamic analyses (Clinical Graphics), for which a detailed report of the analysis was made. Their reports and scans were also interpreted by one of the researchers (MAR).
Surgery
All patients underwent hip arthroscopy for treatment of the FAI. Intra-operative images with fluoroscopy were used to determine if the hip joint was adequately widened with traction and whether the impinging areas were adequately resected.
Examination of the joint and operative technique was performed in accordance with Bond et al. [22]. The intra-operative assessment contained documentation of intra-articular damage to cartilage or labrum, caused by impingement, as described by Beck et al. [23]. This contained inspection of the central and the peripheral compartment. Damage to the anterosuperior cartilage of the acetabulum, a chondral delamination, separation of the labrum and cartilage, degeneration of the labrum and chondral surface on the femoral head, pincer morphologies caused by a bony edge of the acetabulum, cam morphologies on the femoral head neck junction or signs of herniation pits on the femoral head neck junction were identified and recorded in the patient file. Locations and types of lesions were recorded in the patient file.
Impingement can be proven by identifying and recording such typical lesions to the hip joint.
These intra-operative signs of impingement were afterward used as the golden standard for impingement, to compare with the pre-operative diagnostics methods of x-rays and dynamic analyses.
Statistics
A cam type morphology suspect for FAI was defined as an alpha angle > 60° measured on Lauenstein radiographs.
The presence of a cam type morphology on dynamic CT analysis was defined as an osseous impinging area on the anterolateral side of the collum, due to the asphericity of the femoral head. This was highlighted by the software during the simulated range of motion within values of normal hip motion. Example Fig. 3.
A pincer type morphology was defined as an LCE angle > 33° measured on AP radiographs.
The presence of a pincer type morphology on the dynamic CT analysis was defined as an osseous impinging area on the anterior, lateral or posterior wall of the acetabulum, highlighted by the software during the simulated range of motion within values of normal hip motion.
The intra-operative assessments with identification of impinging cam and/or pincer type morphologies, were considered as the gold standard for impingement.
Sensitivity, specificity, positive-predictive-values (PPV) and negative-predictive-values (NPV) were calculated.
Software of Microsoft excel for MAC 2011, version 14.7.7. Was used for the calculations