Abstract
Introduction
Total hip arthroplasty is considered to be one of the most successful orthopaedic interventions. Acetabular component positioning has been shown to affect dislocation rates, component impingement, bearing surface wear rates, and need for revision surgery. The safe zones of acetabular component positioning have previously been described by Lewinnek et al. as 5 to 25 degrees of cup version and 30 to 50 degrees of inclination. Callanan et al. later modified the inclination to 30 to 45 degrees. Our aim was to assess whether THA via robotic assisted posterior approach (PA) improves acetabular component positioning compared to fluoroscopic guided anterior approach THA (AA).
Methods
Subjects
This study is a matched-pair case-control study using prospectively collected data from THAs done between January 2012 and December 2013. Patients who underwent primary THA using the PA or AA by the senior surgeons (MH and JAR) were included in the study.
Ninety-six patients (of 176; 55%) underwent AA and 80 (of 176; 45%) underwent PA THAs. The matching process was performed by an observer blinded to the radiographic outcomes (EK). Patients were matched for sex and BMI +− 8 units. Seventy-nine patients who had AAs were manually matched to 79 patients who had PAs.
Surgical Techniques
For the AA THAs, the patient is supine and the approach is performed through a modified smith Peterson approach. Acetabular cup positioning is assessed intraoperatively with fluoroscopy.
For the PA THAs, the patient in the lateral position using the posterior approach. Acetabular cup positioning was guided by the MAKO robotic hip system using preoperative CT scans of the involved hip.
Radiographic Measurements
The radiographic measurements were done manually using a standardized technique by two observers blinded to the type of arthroplasty performed. Spearman's rank correlation coefficient was used to test user dependent variability. Means were used for final calculations.
Statistical Analysis
The average cup inclination and anteversion angles were calculated. Calculation of the number of hips that were in the safe zones of Lewinnek (inclination, 30°–50°; anteversion, 5°–25°) and Callanan (inclination, 30°–45°; anteversion, 5°–25°) regarding inclination, anteversion, and a combination of both were done for both groups. Independent t-tests were performed to compare both groups for sex, BMI, and inclination and anteversion angles. Fisher's exact test was used to compare both groups regarding the number of hips in the safe zones of Lewinnek and Callanan. Relative risk and absolute risk reduction were calculated.
Results
There was no significant difference in BMI between the two groups. Intraobserver agreement was found to be .92 and 0.82 for inclination and anteversion, respectively. Compared to fluoroscopic guided THAs, THAs performed with robot assistance were found to be more often in the safe zone of Lewinnek (90% vs. 75%, p=0.02, RR 0.40 [0.19–0.85] p=0.01). This pattern was observed in the zone of Callanan and approached statistical significance (80% vs. 68%, p=0.11, RR 0.64 [0.37–1.10] p=0.11).
Conclusion
Compared to fluoroscopic assisted THA, robot assisted THAs are more likely to be within the safe zone of Callanan and Lewinnek.
