In a prospective and randomised clinical study, acetabular cups were implanted free-hand (control group n=25) or with computer assistance using an image-free navigation system (study group n=25). Total hip replacement was performed in lateral position and through minimally invasive anterior approach (MicroHip). The cup position was measured postoperatively on pelvic CT using the CT-planning software.

An average inclination of 42.3° (range: 35°–56°; SD±8.0°) and an average anteversion of 24.0° (range: −5° to 54°; SD±16.0°) were found in the control group, and an average inclination of 45.0° (range: 40°–50°; SD±2.8°) and an average anteversion of 14.4° (range: 5°–25°; SS±5.0°) in the computer-assisted study group. The deviations from the desired cup position (45° inclination, 15° anteversion) were significantly lower in the computer-assisted study group (p< 0.001 each). While only 10/25 of the cups in the control group were within the Lewinnek safe zone, 18/25 of the cups in the study group were placed in this target region (p=0.003). We saw no disadvantage compared to previous studies in supine position with standard approach.

Background: During the past decade, there has been a resurgence of interest in hip resurfacing as a mode of treatment for the younger patient with hip disease since major disadvantages of previous resurfacing systems have been overcome. The purpose of the presented study was to clarify if an imageless navigation system will allow precise placement of the femoral component.

Methods: Between September 2004 and May 2006, 50 metal-on-metal surface arthroplasties each were performed either using an imageless navigation system or the conventional technique. The inclination and the axial alignment of the femoral component were determined by two independend examiners and compared to the values presented by the navigation system.

Results: In the preoperative x-rays a mean CCD-Angle of 129.2 degrees (Control group: 127.5°) was measured. The mean femoral shaft angle was 137.5 degrees (Control group 133°) postoperatively with a mean deviation of 2.1 degrees compared to the values shown by the system. In the computer assisted group the mean deviation from the ideal placement in the axial plane was 2.9 degrees compared to 4.8° when using the conventional technique.

Conclusion: The use of a navigation system was associated with only an average time loss of 7 minutes for surface data acquisition und mounting of the reference base. The computer assisted technique appears to be helpful to avoid notching during the femoral bone preparation and improve implant positioning which might improve durability.

The accuracy of component implantation is an important factor affecting long term results of unicompartmental knee replacement (UKR), particularly, since overcorrection of the leg axis has been associated with an inferior patients outcome. This problem is aggravated when using a minimally invasive approach with a limited view.

In a prospective study, two groups of 40 UKR each were operated either using a non-image-based navigation system or the conventional technique. Radiographic assessment of postoperative alignment was performed by postoperative long-leg coronal and lateral x-rays.

The results revealed a significant difference between the two groups in favour of navigation with regard to the mechanical axis, as well as the coronal femoral and tibial alignment. In the computer assisted group 38/40 (95%) of UKR were in a range of 4 Degree to 0 degree varus (mechanical axis) compared with 29/40 (72,5%) in the conventional group. There was no significant difference between the groups concerning postoperative range of motion, blood loss and pain score.

The only inconvenience was a lengthening of the operation time (20 min). Due to the limited exposure in minimal invasive unicompartmental TKA the navigation system is helpful in achieving a more precise component orientation. The danger of overcorrection is diminished by real time information about the leg axis at each step during the operation. This improvement could be related to a longer survival rate.