Methods
Study Participants
This study follows the Declaration of Helsinki. All probands have voluntarily agreed to the study and gave their informed consent. All study persons have received and signed patient education for MRI. The study protocol was approved by the local ethical committee of the Ruhr-University Bochum (registration number 4370–12). Between January 2012 and July 2013, 14 male semiprofessional soccer players and 14 male amateur soccer players underwent a clinical examination of the mechanical leg axis, an MRI and a gait analysis. The MRI data was published in a previous study. The soccer players were semiprofessional athletes who played 4 training units per week for 2 hours with a seasonal duration of 10 months. The control group consisted of age-matched male amateur soccer players with a physical activity less than 5 hours a week. In all cases, the right leg was the kicking leg. For both groups, exclusion criteria were any kind of previous hip surgery in either hip joint, inflammatory or metabolic rheumatic disease or a history of haemophilia. Height and weight measurements were performed. Dorsal or knee pain was excluded by the clinical examination.
MRI Protocol
Each study volunteer underwent a nonarthrogram 1.5-T MRI (Magnetom, Siemens, Erlangen, Germany) of the right hip. All patients were examined in the supine position with a neutral position of the hip joint. The MRI sequences were obtained on each study test person by one specially-trained radiology technician. The MRI sequences parameters of the turbo spin-echo sequence were as follows: repetition times (TR) 637 ms, echo time (TE) 14 ms, a field of view (FOV) of 350 × 350 mm, a matrix of 512 × 256, a slice thickness of 6 mm and flip angle 150°. In addition, we used a coronal T1-weighted sequence (TR 530 ms, TE 14 ms, FOV 400 × 400 mm, slice thickness 5 mm, flip angle 150°), axial oblique T1-weighted sequence (TR 530 ms, TE 14 ms, FO 350 × 265 mm, slice thickness 5 mm, flip angle 150°) oriented along the axis of the femoral neck and fat-suppressed T1-weighted fast low angle shot (FLASH) sequences (TR 795 ms, TE 11 ms, FOV 400 × 400 mm, slice thickness 3 mm, flip angle 60°). For ethical reasons, neither intraarticular nor intravenous contrast was injected. The alpha angle was analyzed using the technique described by Nötzli. We considered an alpha angle >55° as cut-off value because an alpha angle >55° is associated with FAI. The alpha angle was subsequently measured by a radiologist (C.L.) and an orthopaedic surgeon (M.L.) both experienced in musculoskeletal imaging. The radiologist was blinded to the level of activity of the subjects.
Biomechanical Measurements
In a biomechanical laboratory setting, each participant of both groups ran in two shoe conditions (NW: regular running shoe; Crane, Isa Traesko, Germany; VW: same shoe with inserted valgus wedges, mediolateral height difference: 1 cm) at a speed of 3.3 m · s across a piezoelectric force platform (Kistler 9281 B). Running speed was controlled by two photocells at equal distances in front of and behind the force platform. Only running trials within ±3% of the target speed were accepted. Five successful trials were recorded in each condition. Simultaneously in-shoe pressure distribution, tibial acceleration, and rearfoot motion measurements of the right foot were performed. Seven anatomical locations of the foot (medial and lateral heel; lateral midfoot; first, third and fifth metatarsal heads; hallux) were palpated, and piezoceramic transducers (4×4×2 mm; Halm, Germany) were fastened under the foot with adhesive tape. The physical properties of the piezoceramic transducers were described by Hennig et al.. To measure tibial acceleration, an Entran EGAX-F-25 miniature accelerometer was glued to the skin above the medial aspect of the tibia at a location midway between medial malleolus and tibial plateau. The accelerometer was fastened by an elastic strapt to improve the mechanical coupling to the underlying bone. Rearfoot motion was measured by using an electrogoniometer (Megatron MP 10) that was attached to the heel counter of the shoe. Rearfoot angle was defined as the angle between rearfoot bisection and the direction of the achilles tendon. The detailed biomechanical setup was presented in the study of Milani et al..
Data Collection and Processing
The ground reaction force, axial tibial acceleration, pressure distribution, and rearfoot motion were collected simultaneously by a computer in a pretrigger mode. The data were sampled at a rate of 1 kHz per channel with a resolution of 12 bits. A threshold of 5 N of the vertical ground reaction force was chosen to determine the time onset of foot strike. The force and acceleration values were determined as multiples of body weight (bw) and gravitational acceleration (g), respectively. The maximum force rate was calculated as the highest differential quotient of adjoining vertical ground reaction force divided by the time resolution of 1 ms. The median power frequency of the vertical force signal was calculated from a 1,024-point FFT power spectrum analysis. A low frequency cut-off value of 10 Hz was chosen, since only the initial impact force was of interest. Under the seven anatomical locations, peak pressures were determined for all participants. Maximum range of rearfoot motion was chosen as a descriptor value for the pronation behaviour of the foot.
Statistical Analysis
The selected patient cohort was grouped in an Excel file (version 2003, Microsoft Corporation, Seattle, USA). Distribution of data was assessed by the D'Agostino-Pearson test. The arithmetic mean value, SD and 95% confidence intervals were calculated for the variables above and measured with Microsoft Excel. The values were recorded in IBM SPSS Statistics 14 (PASW 14, SPSS Inc., Chicago, IL, USA). The measurements on the alpha angles was compared using the Student's t-test. Inter-rater reliability was measured of the MRI readings. Statistical significance was defined as a P value <0.05.