Dislocation is an early postoperative complication after total hip replacement, and one of the ways to decrease that is by doing the surgery more accurately and placing the implants very accurately. It involves preoperative planning, it involves preoperative radiographic templating, and robotic assistance and computer-navigated assistance has recently been of significant help in planning the hip replacement surgery and also recently knee replacement surgery.
In addition, there are also rigorous intraoperative tests for hip stability. A good repair of the soft tissues during closure also helps decrease the risk of hip dislocation. The hip joint is made of a cup and stem and there is a ball in between the cup and the stem. The cup has to be accurately positioned inside the pelvis and it has to be in the correct abduction and anteversion.
Correct abduction means that the cup inclination should be at around 40 degrees in the horizontal plane and anteversion means that in the vertical plane the cup needs to be placed at about 20 degrees. Intraoperatively we look at the preoperative template, look at the coverage of the cup, and that helps in deciding the abduction.
We also see the height of the cup intraoperatively based on how much higher it is located with reference to the teardrop in the pelvis. The anteversion in addition can be determined by accurately positioning the cup over the inferior acetabular ligament, which is the ligament inside the cup overlying its lower end. Also, I use a cup-angled guide which I place over the cup and that has two parts over it and it helps me decide how much horizontal the cup is in and how much incline the cup is.
On the stem side, I decide the anteversion of the stem, which means the inclination of the stem with reference to shaft of the lower leg when I place it inside, because the knees bend in 90 degrees of flexion during the placement of the stem at the time it is broached. Also, before making the neck resection we look at something called as the resection length of the neck, which means the length at which the neck is resected.
We also look at something called the center distance, which means we measure the distance between two points in the bone inside the hip and postoperatively we make sure that that length is in accordance with the preoperative template. If the height offset and the anteversion of the stem is restored, the possibility of a surgical hip dislocation is lower than when done with good soft tissue closure.