Cam type impingement mechanics in the hip

Cam-type femoroacetabular impingement syndrome is defined by additional bony morphology on the femoral neck, hip pain and reduced mobility of the hip joint. Through motion of the joint, cam morphology can impinge with the soft tissues of the labrum and acetabular cartilage leading to soft tissue damage. The bone beneath damaged acetabular cartilage has also been reported to change in structure and biomechanical properties, however the damage mechanism of impingement and how this impacts bone is not well understood. Additionally, anatomical variations characterised as cam morphology vary in shape, size and location making predicting the severity of impingement in a patient-specific manner challenging. This is further complicated by patient-specific variations in hip joint orientation which can alter the relative positions of the cam morphology in relation to the acetabulum. The aim in this thesis was to computationally investigate the hip shape and orientation factors involved in the cam-impingement mechanism and determine potential impacts of cam-type impingement on the subchondral bone of the acetabulum.

Twenty subject-specific hip shapes were computationally assessed, through the development of a novel three-dimensional shape description method for the femoral head-neck junction, using hip models derived from CT scans. The variation in cam morphology was captured by describing cam height using ‘contours’ equivalent to topographical mapping. Subject-specific shape descriptions of the cam and acetabular coverage were used to predict and compare impingement risk and severity using a computational model capable of simulating cam-type impingement across a range of hip joint motions (the “HipMoSh” model).

The shape capture method provided a way to comparatively assess the three-dimensional shape profile of the cam. The cams defined by the shape capture of 20 subjects-specific hips were shown to be within the typical ranges of the cam-type hip population.

From the computational assessment of a cohort of real hip shapes, the precision of the source imaging, shape capture and modelling process was sufficient to separate subjects using predicted impingement outcomes from the HipMoSh model.

Quantitative shape, size and location measures of cams and acetabular coverage were assessed for their relationship with predicted impingement which highlighted the importance of assessing the full three-dimensional morphology of individuals.

Using the shape capture method to assess hip shapes with the HipMoSh model also highlighted the sections of the cam typically involved in impingement. Impingement typically occurred at cam bump heights of < 2 mm when subjects were simulated with patient average orientation.

Hip orientation measures were investigated for their effect on subject-specific impingement predictions in five subject-specific hip shapes. Subject-specific hip orientation changed the predicted impingement measures in all cases. In one patient, changing from population average hip orientation to subject specific orientation changed the location of the impingement from posterior to anterior. Additionally, increases in the height at which the cam impinged (up to 4 mm) were observed when using subject-specific hip orientation. This highlights the importance of assessing subject-specific hip orientation for predicting individual impingement patterns.

To conduct a preliminary investigation into how the change in radius of the femoral head, due to the cam morphology, might impact the subchondral bone, the over-filling of porcine acetabula was mimicked experimentally using a surrogate femoral head. Porcine acetabular samples were CT scanned unloaded and loaded (to mimic “over-filling”) to assess for deformation and structural changes in acetabular subchondral bone. Small amounts deformation of the subchondral bone of the acetabulum were observed. However, definitive evidence of new microdamage in the subchondral bone was not observed, and further investigation with higher resolution imaging is required for conclusive assessments of the subchondral bone.

Overall, findings suggested that a combination of subject-specific shape and orientation measures are key for prediction of subject-specific impingement. This highlights the complex interplay between shape, motion and orientation on the cam-type impingement mechanism.