# Spine, Pelvis and Hip Kinematics--Characterizing the Axial Plane in Healthy and Osteoarthritic Hips
## 4 Discussion  
Spinopelvic assessments have been suggested to be included in the pre-operative algorithms by various authors [4; 5; 19]. However, assessments to date have been of the sagittal plane, and no information exists as to what happens in the axial/transverse plane, despite the appreciation of pelvic rotation having an equal effect on cup anteversion to pelvic tilt, thus influencing impingement and dislocation risk [22]. Given that axial assessments cannot be performed radiographically, we sought alternative methods of characterizing the range of movement taking place in this plane and tested the agreement between the measurements performed. Our findings show that assessments of spinopelvic characteristics in the motion analysis laboratory can produce reliable results, albeit with certain inherent limitations pertinent to the assessment of the pelvic, sagittal kinematics. The motion analysis laboratory has previously been used to study sagittal pelvic and hip range of motions [23]. The agreements illustrated between sagittal plane kinematics and radiographical measurements whilst performing the SBR task validates the ability to perform spinopelvic mobility and hip flexion assessments in the motion analysis laboratory. In particular, hip and spinal flexions showed strong correlations between motion laboratory and radiographic analyses. The pelvic maximum flexion motion illustrated the least agreement. This could be explained by the soft tissue movement artifacts due to the increased soft tissue envelope around the pelvis [24]. This occurred despite the creation of virtual markers associated with the pelvis segment to recreate the position of the missing markers minimize the effect of the soft tissue artifact and selecting participants with BMI \(<\) 35. The accurate assessment of the pelvis during a bend and reach assessment has certain inherent limitations that are difficult to overcome in the motion laboratory unless using dual fluoroscopy. Figure 6: (**A**) Range of motion (ROM) of the axial spine and pelvis assessments during the seated trunk rotation (STR) task; (**B**) maximal axial spine and pelvis rotation assessments during the seated trunk rotation (STR) task, for the control (dark grey), unilaterally affected osteoarthritis (OA) (white), and bilateral OA (dashed) participants. The weak correlation between axial and sagittal spinal kinematics is, in part, due to the type of assessments performed. Spinal flexion mostly occurs in the lumbar region, whilst spinal rotation is minimal in the lumbar spine and mostly occurs in the thoracic region. We thus feel that the weak correlation is primarily due to different segments of the spine responsible for the motion. Thus, patients who illustrate spinopelvic abnormalities in the sagittal plane, when assessed radiographically, are also likely to be illustrating abnormal movements in the axial plane. This is unsurprising, as the abnormal spinopelvic characteristics, particularly spinopelvic hypermobility, in pre-THA patients are, in part, due to the arthritis hip and reflect compensatory maneuvers of the pelvis to accommodate for the reduced range of movement the hip can perform [13]. Further study is required to characterize spine, pelvis, and hip ROM in the axial and sagittal plane during gait and for activities that require rotation in standing position. The STR task keeps the hip relatively static, as the subject is sitting, and thus the extent of rotation can be thoroughly tested. This study wanted to characterize the sagittal and axial plane kinematics in a spectrum of healthy to hip OA subjects. This allowed us to uncouple the association between the sagittal and axial plane kinematics in pre-THA patients. Patients exhibiting sagittal stiffness have been reported to be at risk of dislocation, due to increased flexion demands on the hip [25]. However, such patients may also be requiring extra rotation from the pelvis and hip for a given task, further increasing the impingement risk [26].