Optimum component orientation in hip arthroplasty is vital in an effort to avoid dislocation and excessive wear. Computer navigation in hip arthroplasty surgery has the potential to improve accuracy in component placement. However, it has been slow to gain widespread acceptance. One of the major concerns surgeons have is the difficulty in registering pelvic landmarks.

We used a retrospective series of 200 pelvic CT scans to validate a new methodology to construct the anterior pelvic plane, using anatomical landmarks that are easily palpated with the patient positioned and draped in the lateral decubitus position. Analysis of the scans was also made in an effort to stimulate the inaccuracies of obtaining the anterior pelvic plane through soft tissue.

When comparing the new registration methodology to the anterior pelvic plane, the error in acetabular component inclination was 0.69° (SD 2.96) and anteversion was 1.17° (SD 3.53). This compares favourably to the error in acetabular component inclination of −0.92° (SD 0.26) and anteversion of −5.24° (SD 2.09) when the anterior pelvic plane is registered through soft tissue. The data also shows that using the new registration method in more than 99.6% of cases the acetabular placement is within the safe zone as described by Lewinnek.

This study appears to show that through the identification of anatomical constants we are able to construct the anterior pelvic plane from anatomical landmarks that are easily palpable in the lateral decubitus position during hip arthroplasty. These landmarks also appear to be more accurate in obese patients than registering the anterior pelvic plane.

Incorrect restoration of leg length (LL) and offset is a major source of patient dissatisfaction and dysfunction after total hip arthroplasties (THAs). Evaluations on anterior-posterior x-ray images are state-of-the-art to assess the accuracy of such techniques. However, x-ray based measurements of LL and offset are challenging and limited in terms of accuracy. Within this study, we evaluated the accuracy of such measurements by analysing a series of clinical data. We evaluated the results on the non-treated side, since we know that there should be no significant difference between pre- and postoperative measurements on this side.

A series of 44 consecutive patients was analysed regarding changes in the difference between pre- and post-operative LL and offset measurements. Anterior-posterior x-rays were taken pre- (pre-OP) and post-operatively (post-OP) with a calibration by a scaling ruler (pre-OP) or implant size (post-OP). The LL and offset measurements were performed with a digital planning software based on the teardrop and transischial line. The distance between the teardrop/transischial line and the trochanter minor was measured to assess LL differences. Femoral offset (FO) was calculated as the orthogonal distance between the centre of the femoral head and the proximal shaft axis. Global offset (GO) was calculated as the distance between the inferior aspect of the teardrop figure and the shaft axis along the teardrop line. Descriptive statistics (mean value ± standard deviation) were calculated for the different types of measurements. Statistically significant differences were checked according to a student's t-test (α = 0.05).

The differences between the pre-and post-operative situation was 0.8±3.2 mm for LL, 0.2±3.5 mm for GO, and −0.5±2.5 mm for FO when referencing to the teardrop line and 0.9±4.0 mm (LL) and −0.3±2.7 mm (FO) for the transischial line. The error distributions did not show statistically significant differences when referencing to the teardrop or transischial line. But high differences (0.1±6.6 mm) were found when comparing the LL values (teardrop vs. transischial) case-by-case.

Within this study we demonstrated that x-ray based offset and LL measurements show reasonable inaccuracies. X-ray based evaluations of navigation-based techniques to assist LL and offset restoration cannot produce significantly better results than these analysed limits. That is, even if the navigation technique would be perfectly accurate, the evaluation would not achieve better accuracies than approximately ±3.5 mm for LL, ±3.5 mm for GO, and ±2.5 mm for FO.

Limited postoperative range-of-motion (ROM) can lead to patient dissatisfaction and dislocation in total hip arthroplasties (THAs). To avoid this, femur first approaches have been developed which optimise particular aspects of ROM by using a virtual analysis of ROM. This study analysis whether it is possible to accurately assess ROM based on an intra-operative acquisition of anatomical structures by using an image-free navigation system. It compares the outcome of a collision detection algorithm when using 3d models from computerised tomography (CT) scans on the one side and intra-operatively acquired 3D models on the other side within a cadaver study. It focuses on peri-acetabular impingements.

During the cadaver session 14 hips (7 cadavers) were treated surgically by using press-fit implants. 3D models of the pelvis and femora were generated based on segmented pre-operative CT data sets. Intra-operative data acquisition was performed by using a CT-free navigation software. Beside standard landmarks, points at the acetabular rim and femoral resection plane were acquired. For assessing ROM, a 3D model of the pelvis was generated. The information about the femoral resection plane was directly entered into the collision detection algorithm. 3D Computer Aided Design (CAD) models provided by the implant manufacturer were used for the implants. Based on this setup, the ROM values for flexion (FLEX), external rotation at 0° flexion (EXT), and internal rotation at 90° flexion (INTROT90) were compared. Differences within intended ROM were considered relevant, since the goal was to enable the prevention of clinically relevant ROM limitations.

The average difference between the CT based and navigation data based ROM analysis was 2.13° ± 3.11° for FLEX, 3.33° ± 5.51° for EXT, and 1.6° ± 3.66° INTROT90. The values reduce to 1.58° ± 2.78° (FLEX) and 0.91° ± 3.77° (INTROT90) when only ROM values within the intended ROM are considered. For EXT all ROM values lied above the threshold for intended ROM. Thus, no relevant differences were found for this motion direction.

In this study, a real-time collision detection based approach was developed and evaluated, which allows to virtually detect prosthetic and bony impingements. It was shown that ROM can be assessed accurately based on an image-free navigation technique. This information can be used intra-operatively to adjust the position of the implants and thus avoid postoperative ROM limitations. In particular, it enables a comprehensive femur first approach which allows us to optimise the post-operative results regarding functional parameters like ROM.