In this prospective comparative study, eligible participants were gravidas older than 18 years and > 32 weeks’ gestation, based on dating criteria of last menstrual period or first trimester ultrasound. Exclusion criteria included a body mass index > 40, a maternal medical condition that prevented maternal hip hyperflexion, inflammatory joint diseases, or joint hypermobility syndrome, such as Marfan’s syndrome. The study protocol was approved by the Ethics Committee of Poitiers Hospital (Comité de Protection des Personnes: 2013-1203-42) and by the French National Agency of Drug Safety (Agence Nationale de Sécurité du Médicament: B131-460-22). All women provided written informed consent. No financial incentive was offered for participation. This biomechanical study took place in an experimental setting (i.e., not during labour). Medical students in their 4th year of training, who had undergone pre-study briefing and training, performed the manoeuvres. A birthing bed (Maquet®) was used with an angle of inclination of the headboard of 30° (typical placement of the parturient at Poitiers’s hospital).
A full protocol description about this innovative methodology is available in a recent publication (6). A traditional three-dimensional motion analysis was performed to analyse the position of the markers in space. It was based on an optoelectronic motion capture system consisting of 12 infrared cameras cadenced at 100 Hz (VICON, Oxford Metrics, UK). Thirty-three reflective markers were affixed using double-sided tape on anatomical landmarks according to an adapted version of the Helen Hayes’s marker set (7). To assess the position of the pelvis, we placed additional markers on the pelvis. An antenna fitted with three markers was positioned on the top of each iliac crest to provide a technical coordinate system, allowing the reconstruction of the pelvic markers if they were to be hidden during the experimentation. Marker trajectories were low-pass filtered using a double-pass Butterworth filter with a cut-off frequency of 10 Hz.
The lumbar curve was assessed by measuring the lordosis according to the Epionics SPINE system (Epionics Medical GmbH, Potsdam, Germany). This system consists of two flexible sensor strips that use strain gauge sensors located alongside flexible circuit board strips. The positioning of the system is standardized. According to this measure, a lordosis of 0° corresponds to a back perfectly flattened. The data acquisition (50 Hz) was transmitted in real time via Bluetooth to a local PC (6).
Subjects were positioned in the lithotomy position, with the thighs lying on the stirrups, with a flexion of 90°. Two medical students performed the McRoberts manoeuvre as initially described by Gonik (1) ((i) McRoberts). Afterward, the subjects were placed with their legs outside the stirrups, with the feet lying on the bed and the thighs in a neutral position in terms of flexion but with a maximal abduction and femoral external rotation. The same students again applied simultaneous maximal flexion of the thighs at the request of the investigator. This modified McRoberts’ manoeuvre was referred to as (m)McRoberts. The sequence (i)McRoberts then (m)McRoberts, which corresponds in fact to two different starting positions (Figure 1), was carried out 3 times for each subject. The medical students used to employ the manoeuvres were different for each patient, in order to avoid any training effect. Each manoeuver was performed under the supervision of a senior obstetrician (DD) to check the appropriate achievement of the manoeuvre. Examples of the manoeuvres after reconstruction are available in supplementary files added to this article.
A custom Matlab code (MathWorks Inc., Natick, MA) was used to merge data from Epionics and optoelectronic systems and to extract the required data. We defined a plane following the external conjugate diameter using the two markers placed on the posterosuperior iliac spines and the marker placed on the superior edge of the pubic symphysis. The hip joints angles (flexion and abduction) were obtained as defined by the conventional gait model (7). The flexion of the plane of the external conjugate on the spine (ANGce) was defined in the sagittal plane as the angle between the external conjugate and the line defined by the markers placed on the 7th cervical and the 10th thoracic vertebrae (Figure 2). The lumbar curvature was measured during the manoeuvre for each subject. For each angle, the initial and maximum values were noted. To provide a better description of the manoeuvre, the maximum angular acceleration of the flexion of the external conjugate on the spine. The angular acceleration was obtained by a double differentiation (numerical method by differentiation decentred on the right) of the angle ANGce. The total duration of the movement was also calculated. The beginning of the movement was defined as the time at which the angular velocity of the thigh flexion exceeded 5% of the maximal angular velocity reached during the manoeuvre. The end of the movement was defined as the moment when the angular velocity of a segment (thigh or pelvis) became less than 5% of the maximal angular velocity reached during the manoeuvre. The beginning of the external conjugate diameter flexion, the maximum flexion acceleration were determined. Durations were expressed relative to the moment of the beginning of the manoeuvre. The values obtained during each manoeuvre were recorded and averaged over the three repetitions of each manoeuvre.
Based on the previous results of Gherman et al (3), we considered as significant, a 4° (5%) increasing of ANGce. With this hypothesis, 22 participants are necessary with of power of 90% and a risk of type I error of 5%. All values obtained for the two types of manoeuvres ((i)McRoberts and (m)McRoberts) were compared using a Wilcoxon matched-pairs signed-ranks test. The significance level was defined as p < 0.05.