Abstract
Introduction
Improper acetabular component orientation has been shown to negatively affect the outcome of total hip arthroplasty through increasing dislocation rates, component impingement, bearing surface wear, and the rate of revision surgeries. The “Safe Zone” was defined by Lewinnek et al. in 1978 as 5 to 25 degrees of cup version and 30 to 50 degrees of cup inclination. Later, the inclination “Safe Zone” values were modified to 30 to 45 degrees.
Objectives
The primary purpose of this study was to assess whether the use of Mako robotic hip system improves cup positioning when compared to traditional THR.
Methods
Between June 2011 and February 2012, 23 hip replacements were performed by the senior author using the Mako robot. The radiographic cup positioning of those surgeries was compared to 59 consecutive posterior approach hip replacements by the same surgeon. Patients with tilted or rotated AP pelvis X-rays were excluded from the study. Anterior approach hip replacements during the study period were also excluded. Measurements of the cup inclination and version were performed by two observers in order to check the reliability of the measurement.
Results
After exclusions, a total of 23 Makoplasties and 56 traditional hip arthroplasties were included in the study. The average age of the patients in the study was 57 (range 35 to 85). A very high inter-observer correlation was found for the cup version and the inclination measurements (R values of 95% and 97% respectively, p<.0001 for both). The average cup version was 12.8 degrees (range, 6 to 22.5) in the Mako group and 12.6 degrees (range, 0 to 32.5) in the traditional THR group. The average cup inclination was 41 degrees (range, 30 to 48.5) in the Mako group and 43 degrees (range, 31 to 62) in the traditional THR group. Using Lewinnek's “Safe Zone” all Mako cases were found to be inside, while 10 cases (18%) of the traditional THR group were outside. Using the modified “Safe Zone,” 5 cases (22%) of the Mako group were outside, whereas, 26 cases (46%) of the traditional THR group were outside.
Conclusions
Posterior approach THR performed with the Mako robot had very consistent cup positioning, with all cups placed within the Lewinnek safe zone. When compared with posterior approach THR without the Mako robot, performed by the same surgeon, Makoplasty hips were significantly more likely to be in the safe zone.
Figure
A comparison of acetabular cup measurement of the inclination and version in Mako guided posterior approach and free hand posterior approach. The Lewinnek “Safe Zone” is shaded blue.
