Mechanical Behaviour of Cam-type Femoroacetabular Impingement

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Abstract

Femoroacetabular impingement (FAI) is a hip condition which can limit hip motion and cause pain particularly in young and athletic patients. It is considered as a patho-mechanical process leading to progressive and degenerative damage of the joint. Surgical treatment for femoroacetabular impingement focuses on improving the clearance for hip motion, reducing the femoral impact against the acetabular labrum. The procedure involves the surgical resection of the cause of impingement which consists of trimming the acetabular rim and/or the femoral head-neck offset.Currently, there are no comprehensive tools available for pre-operative planning of FAI surgery and so the area and depth of bone resection are identified based on the skill and experience of the surgeon. This means that it is difficult to predict the degree to which the procedure will be successful, in terms of reducing pain and increasing the range of motion (RoM) of the hip, prior to surgery. In addition, resection can lead to increased stress in the remaining bone which in some cases can result in post-operative femoral neck fracture, a recognized risk of FAI surgery which is increased if the bone is osteoporotic.This thesis describes the development of a framework that will enable a tool to be created that can be used for the diagnosis, preoperative planning and selection of treatment for patients with cam-type FAI. The framework consists of a number of complementary 3-dimensional finite element (FE) models. The models are created from computer tomography (CT) data from actual patients with cam-type FAI. The first FE model was developed in order to predict the stress distribution in the head-neck region of the femur following resection surgery for FAI enabling the effect of resection depth to be investigated under loading conditions corresponding to typical daily activities. The model demonstrates that resection depth should be kept to less than one third of the diameter of the neck in order to ensure structural integrity. The second finite element model developed utilises a quasi-brittle damage plasticity material formulation to investigate the mechanism and risk of femoral neck fracture following femoral osteochondroplasty in osteoporotic and non osteoporotic hips. Predictions indicate that fracture can occur in osteoporotic hips during typical daily activities. Also, the likelihood of fracture increases when patients are subjected to high load conditions and activities, even in non-osteoporotic patients. The third FE model was developed to assess the reduction in the internal rotation movement in hips with cam-type FAI and identify and examine the areas where impingement occurs. The model shows that FAI can result in a significant reduction in hip motion and that impingement area and RoM are patient dependant. All three FE models were validated with results from experimental studies.The three models combined provide the framework for a virtual osteochondroplasty tool. The procedure for using the tool involves undertaking a virtual resection of a FAI hip based on the areas of impingement identified by the RoM analysis provided by the third FE model. Finite element models one and two are employed to ensure that the virtual resection remains within safe limits and stress does not elevate in the remaining bone to levels that would significantly increase the risk of femoral neck fracture. The framework was validated by comparing the RoM predicted following a virtual osteochondroplasty undertaken on a model of a hip from an actual patient with FAI with the results from a model of the same hip created from CT scan data taken after an actual osteochondroplasty had been performed on the patient using a resection area and depth identified in the conventional way by a surgeon.

Keyword(s)

Femoroacetabular Impingement; FiniteElement; Osteochondroplasty

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