Malposition of the acetabular cup is associated with complications such as impingement, recurrent dislocation, increased ischial osteolysis and wear of prosthetic components [36–38]. Few studies have evaluated acetabular cup placement depending on the surgical approach [29, 39–41]. MIS techniques arose with the simultaneous effort to reduce periarticular tissue damage and achieve good clinical-radiographic results, the latter at least overlapping with those obtained with traditional surgical approaches [42–44]. In this regard, it would also be appropriate to clarify the correct definition of MIS, since the surgical approaches are manifold and different from each other. This is not the aim of our study, but it would at least provide a better understanding of SuperPath in that framework. Controversies exist over the precise definition of a minimally invasive approach, as no clear definition is reported [22, 37, 45]. The SuperPath technique fits into the group of minimally invasive muscle-sparing techniques, since it preserves the cutting of the extrarotator muscles, hip joint capsule, and avoids surgical dislocation of the femoral head [43]. Several studies report benefits related to this technique. Cost-saving benefits associated with the use of the SuperPath were also shown by Gofton et al., as a 28% reduction in in-hospital costs was reported compared with the standard lateral surgical approach [46]. Della Torre et al. reported good radiographic results within Lewinnek's safe zone, as a mean IA of 40,13° ± 6,30 was obtained from 66 postoperative radiographs [21]. Several criticisms were made towards different MIS regarding the correct placement of the acetabular cup, due to not always clear surgical visualization of the acetabulum [15, 18, 42, 47]. Our results claim that the minimally invasive SuperPath technique allows for satisfactory acetabular cup positioning, with an average IA of 42,7° ± 8.2 (an average of 82% cases over 4 years in the Lewinnek safe zone), and an average AA of 17,6° ± 3,2 (an average of 77% cases over 4 years in the Lewinnek safe zone) in a total of 756 radiographic cases. We inspected the radiographic results of acetabular cup placement reported in the literature by authors comparing SuperPath with traditional surgical approaches. In the systematic review and meta-analysis by Ramadanov et al., 80 patients operated by SuperPath technique and 80 patients operated by conventional approaches, collected from 4 randomized controlled trials, were evaluated. The results of this study showed no difference regarding the acetabular cup placement [22]. Specifically, Xie et al. compared the radiographic acetabular cup placement of 46 cases with primary hip osteoarthritis through the SuperPath technique with that obtained from 46 cases with the same disease through the conventional posterior approach, noting no statistically significant difference between them. In the SuperPath group, the mean IA was 43,6° ± 6,8, while the mean AA was 17,4° ± 1,6. In the conventional posterior approach group, the mean IA was 44,5° ± 6,5, while the mean AA was 18,5° ± 1,8 [23]. Ouyang et al. reported in a randomized controlled trial of 24 cases of THAs, 12 operated with the SuperPath and 12 operated with the posterolateral approach, the following radiographic findings of IA and AA: mean IA of 37,08° ± 6,53 and mean AA of 21,92° ± 5,78 in the SuperPath group, while mean IA of 39,67° ± 6,95 and mean AA of 21,75° ± 4,48 in the conventional group [48]. Meng et al. in their recent study of 2020, where they evaluated 4 patients with bilateral femoral head osteonecrosis operated with SuperPath technique on one side and with the postero-lateral approach on the other side, found lower values of IA in SuperPath (38,75° ± 8,21) than the control group (44,50° ± 3,64), and they are also lower than our values. Regarding AA, the authors obtained values of 15,00° ± 1,82, similar to those obtained by us [49]. A statistically lower result of IA with SuperPath compared to the use of posterolateral mini-incision approach was obtained by another work of Meng et al., where in 20 patients operated for THA with SuperPath they found an IA value of 36,94° ± 6,37, while in the control group it was 42,66° ± 3,58 (p-value = 0,004). On the other hand, there was no statistically significant difference in mean AA values (SuperPath, mean AA of 13,94° ± 4,73; posterolateral approach, mean AA value of 15,11° ± 4,06) [43]. However, from both papers by Meng et al., both IA and AA values were within the Lewinnek safe zone. On the other hand, Tottas et al. reported, in a group of 40 patients, IA values with SuperPath statistically higher than mean IA values obtained with the Hardinge approach (51,2° ± 4,8 vs 43,7° ± 4,4, respectively); while statistically similar values were found in the two groups regarding AA (20,5° ± 9,8 in the SuperPath group vs 25,0° ± 7,9 in the Hardinge approach group) [50]. With respect to the previous work cited, Kay et al. analyzed, in addition to the mean values of IA and AA, a 2-year follow-up regarding the evaluation of dislocations of THAs (zero dislocations reported). The mean IA value obtained was 43,6° ± 5,2 and the mean AA value was 20,9° ± 6,2 [51]. Evaluating the above IA and AA values of these authors, our results show substantially similarities. We obtained IA and AA values of a large case series (756 cases) satisfactory regarding the safe zone proposed by Lewinnek, with a low number of hip dislocations (0,3 %), moreover in the first year of the study. In addition, analyzing the averages of inclination angle across years, it was noted that there was no statistically significant improvement with increased surgical experience with the SuperPath technique. This finding can be interpreted in two ways: the learning curve for obtaining a good cup placement does not require few years to obtain satisfactory radiographic results; conversely, with the SuperPath technique, a statistically better result cannot be achieved in a 4-year period, even with increased surgical experience. This confuted the second hypothesis of our study.
Our study has a few limitations, such as the retrospective nature of the results, lack of randomization between Orthopaedic surgeons and operated patients, lack of a control group with traditional surgical approach, additional clinical information of patients (e.g., BMI, postoperative lower limb dysmetria). In addition, there is no correlation between radiographic and several clinical results in our study, but this was not the aim of our study, and the numerous benefits of SuperPath in clinical terms were already reported in the literature [21, 23, 24, 34, 52].