The orientation of an acetabulum or an acetabular prosthesis may be described by its inclination and anteversion. Orientation can be assessed anatomically, radiographically, and by direct observation at operation. The angles of inclination and anteversion determined by these three methods differ because they have different spatial arrangements. There are therefore three distinct definitions of inclination and anteversion. This paper analyses the differences between the definitions and provides nomograms to convert from one to another. It is recommended that the operative definitions be used to describe the orientation of prostheses and that the anatomical definitions be used for dysplastic acetabula.

The definition and measurement of acetabular orientation.

Dislocation remains the leading early complication of total hip arthroplasty; surgical approach and implant positioning have been recognized as factors influencing total hip arthroplasty stability. We describe a total hip arthroplasty technique done through a single, tissue sparing anterior approach

Single-incision anterior approach for total hip arthroplasty on an orthopaedic table.

https://onlinelibrary.wiley.com/doi/pdf/10.1016/j.orthres.2003.11.001

Compliant positioning of total hip components for optimal range of motion.

Malposition of the acetabular component is a risk factor for post-operative dislocation after total hip replacement (THR). We have investigated the influence of the orientation of the acetabular component on the probability of dislocation. Radiological anteversion and abduction of the component of 127 hips which dislocated post-operatively were measured by Einzel-Bild-Roentgen-Analysis and compared with those in a control group of 342 patients. In the control group, the mean value of anteversion was 15 degrees and of abduction 44 degrees. Patients with anterior dislocation after primary THR showed significant differences in the mean angle of anteversion (17 degrees), and abduction (48 degrees) as did patients with posterior dislocation (anteversion 11 degrees, abduction 42 degrees). After revision patients with posterior dislocation showed significant differences in anteversion (12 degrees) and abduction (40 degrees). Our results demonstrate the importance of accurate positioning of the acetabular component in order to reduce the frequency of subsequent dislocations. Radiological anteversion of 15 degrees and abduction of 45 degrees are the lowest at-risk values for dislocation.

Reducing the risk of dislocation after total hip arthroplasty: THE EFFECT OF ORIENTATION OF THE ACETABULAR COMPONENT

Apparatuses and methods are disclosed for determining an implant position for at least one artificial component in a joint and facilitating the implantation thereof. The apparatuses and methods include creating a joint model of a patient's joint into which an artificial component is to be implanted and creating a component model of the artificial component. The joint and artificial component models are used to simulate movement in the patient's joint with the artificial component in a test position. The component model and the joint model are used to calculate a range of motion in the joint for at least one test position based on the simulated motion. An implant position, including angular orientation, in the patient's joint is determined based on a predetermined range of motion and the calculated range of motion. In a preferred embodiment, the implant position can be identified in the joint model and the joint model aligned with the joint by registering positional data from discrete points on the joint with the joint model. Such registration also allows for tracking of the joint during surgical procedures. A current preferred application of the invention is for determining the implant position and sizing of an acetabular cup and femoral implant for use in total hip replacement surgery.

Apparatus and method for facilitating the implantation of artificial components in joints

In a series of 300 total hip replacements, nine (3 per cent) dislocated. Precise measurements of the orientation of the acetabular cup were made and it was found that anterior dislocations were associated with increased acetabular-component anteversion. There was no significant correlation between cup-orientation angle and posterior dislocation. The dislocation rate for cup orientation with anteversion of 15 +/- 10 degrees and lateral opening of 40 +/- 10 degrees was 1.5 per cent, while outside this "safe" range the dislocation rate was 6.1 per cent. Other factors that were documented include time after surgery (with the greatest risk in the first thirty days) and surgical history (with a greater risk in hips that have had prior surgery).

G E Lewinnek

Papers

Dislocations after total hip-replacement arthroplasties.