Author : Dr. Zhu Chuangui
Last Updated : 1 October 2004

Dislocation of a total hip replacement causes significant distress to the patient and physician and is associated with significant additional costs in order to relocate the hip. Revision surgery is best avoided as it has a much higher chance of failure, and it often results in a leg that is weaker and shorter than the other. While dislocations can result from a variety of causes, such as soft tissue laxity and loosening of the implant, the most common cause is impingement of the femoral neck with either the rim of an acetabular cup implant, or the soft tissue or bone surrounding the implant. Impingement can lead to advanced wear of the acetabular rim resulting in polyethylene wear debris shown to accelerate loosening of implant bone interfaces. The position at which impingement occurs is determined by the design and geometry of the implants (such as the size of the femoral head, the width of the neck, and the design of the acetabular liner), and more importantly by the relative position of the femoral and acetabular implants. Malalignment of one or both implants is responsible for the majority of impingements with the acetabular cup being the component more likely to be malpositioned, and is the most sensitive variable in predisposing to dislocation. 

The Malposition Problem

Malposition of the acetabular component in THR increases the occurrence of impingement, reduces the "safe" range of motion and increases the risk of dislocation. The word "malposition" implies that there exists an optimal or neutral orientation for the acetabular cup and the deviation to this neutral position is malposition. However, currently there is still not a standard to define the optimal/neutral orientation of the acetabular cup given a patient. The commonly accepted standard of approximately 15° of anteversion and 45° of abduction should not be taken as the optimal/neutral orientation because it does not take much concern in individual anatomical variations of the patients. The relative position of the pelvis and leg when defining a "neutral" plane from which the angles of movement, anteversion, abduction, etc., are calculated will significantly influence the measured amount of motion permitted before impingement and dislocation occurs. As a result, there is no single optimal/neutral orientation of hip replacement components suitable to all the patients to minimize the impingement and dislocation propensity of the implant.

Intraoperatively, bony landmarks of the pelvis are somewhat obscured and most surgeons will admit that it is very difficult to know precisely how the patient's pelvis is oriented, despite many techniques of pelvis positioning and stabilization on the OR table. Mechanical intraoperative guides provided by implant manufacturers attempt to align the acetabular component with respect to the longitudinal and coronal planes of the patient. However, these devices assume that the patient's trunk and pelvis are aligned in a known orientation to the operating table. It has been proved that the pelvis true position relative to the operation table may change significantly.

The HipNav System

The Hip Navigation or HipNav system includes three components: a pre-operative planner, a range of motion simulator, and intra-operative tracking and guidance system. The pre-operative planner allows the surgeon to manually specify the position of the acetabular component within the pelvis based upon pre-operative CT images. The range of motion simulator estimates femoral range of motion based upon the implant placement parameters provided by the pre-operative planner. The feedback provided by the simulator can aid the surgeon in determining optimal, patient specific acetabular implant placement. The intra-operative tracking and guidance system is used to accurately place the implant in the determined optimal position regardless of the position of the patient on the operating room table. By accurately placing the acetabular component in an optimally selected position, the HipNav system has the potential to reduce the risk of dislocations and the generation of wear debris caused by impingement resulting from malpositioned components and increase the `safe' range of motion. An optimal implant alignment is defined as: maximizing stability while minimizing impingement for specific functional positions such as sitting in a chair or lying supine in bed.

To register the preoperative information (i.e., the CT scan and preoperative plan) to the position of the patient on the operating table, surface-based registration is used. Multiple points on the surface of the pelvis are collected with a digitizing probe during surgery. These intraoperative data create a cloud of points which is then matched to a geometric description of the bony surface derived from the CT images already used to plan the surgery. After registration, the position of the pelvis and any of the instrumented surgical tools can be continuously monitored during all phases of surgery. An optical tracking system (Optotrak) is used to track the positions of special targets. These targets are attached to a pointer, the wing of the ilium and the handle of an acetabular cup holder. One additional target is used to establish an operating room coordinate system at the beginning of the surgery.

The optimal/neutral orientation should also not be determined solely by the shape of the patient's pelvis due to the fact that the movement of the patient's leg after surgery is determined not only by the shape of the pelvis, but also by the activities of normal daily living. For example, the orientation of the leg to the pelvis can be greatly different depending or whether an individual is lying supine (as during a CT-scan or routine X-rays), in the lateral decubitis position (as during surgery) or in critical positions during activities (like bending over to tie shoes or during normal gait). Mechanical intraoperative guides provided by implant manufacturers attempt to align the acetabular component to an angle of 45 degrees of abduction and 15 to 20 degrees of anteversion. with respect to the saggital and coronal planes of the patient. The HipNav group accused that the current alignment strategies assume a known pelvic position on the operating room table, and inevitable variability in pelvic position contributes to the continued significant incidence of dislocation. However, they themselves rely this same assumption to establish the relationship between the preoperation plan and the introperative tracking system.

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