Background: In order to achieve a minimized need for tissue dissection resulting in a faster rehabilitation, minimally invasive surgery (MIS) in Total Hip Arthroplasty (THA) was developed. In this small incision technique the skin and musle dissection has been reduced with respect to the classical approach. Literature shows ambiguous results comparing the posterolateral minimally incisive with the classical approach. As the anterolateral approach is also a routine procedure, and to test how minimally invasive MIS is, we hypothesized that patients treated with a THA using a posterolateral or anterolateral MIS would experience improved clinical results compared with a standard incision after six weeks and no clinical differences after one year. This was tested in a double-blind randomized controlled trial with the Harris Hip Score (HHS) as a primary endpoint.

Methods: One hundred and twenty consecutive primary uncemented THAs were randomized into one of four groups of 30 patients each. Either standard posterolateral or anterolateral approaches (PL- or AL-CLASS), or minimal invasive posterolateral or anterolateral approaches (PL- or AL-MIS) were performed. CLASS incisions were 18 cm. To avoid postoperative bias, MIS incisions were extended at skin level to 18 cm at the end of the procedure. The HHS as well as patient-centered questionnaires (SF-36, WOMAC and OHS) was obtained preoperatively, at six weeks and one year after the index operation. Preoperative data, blood loss, hemoglobin, muscle damage parameters and radiological parameters were analyzed. In order to detect a minimal clinically important difference of five points or more between the MIS or CLASS groups with respect to the Harris Hip Score at the 0.05 alpha level with 80% power, 120 patients were enrolled in the study.

Results: Mean incision length of the THAs performed by MIS was 7.8 (SD = 1.6). In the patients of the MIS group a significant increased mean HHS was observed compared with the CLASS (p = 0.03) after six weeks and one year. This difference was caused by the favorable results of the PL-MIS (p = 0.009). Of the three patient-centered questionnaires, the SF-36 results were also favourable in the PL-MIS group after six weeks (p = 0.04). In the MIS group operation time was longer (p < 0.001) and a learning curve was observed based on operation time and complication rate. Peri-operative complications rates were not significantly different between the groups. Blood loss, hemoglobin, muscle damage parameters and radiological parameters also showed no difference.

Conclusions: This double-blind, randomized study reveals an improved clinical outcome of the PL-MIS compared with the AL-MIS, PL-CLASS and AL-CLASS after six weeks and one year follow-up with the Harris Hip Score as primary endpoint.