The present study accepted the null hypothesis showing that the acetabular reconstruction with TM-coated cups and augments provides a statistically significant improvement of hip ROMs and HHS values, and reduction of LLD and cranial migration of the hip COR in patients affected by Paprosky type III defects without pelvic discontinuity.
Over the next twenty years, the number of primary and revision total hip arthroplasties (THAs) in the United States has been estimated to increase by 174% and 137% respectively . Since aseptic loosening, bearing surface wear, and osteolysis are the most common causes of THA failure in both Europe 26. and 27. and United States , revision surgery can be technically demanding whether severe acetabular bone defects need to be addressed at the time of the reconstruction. The management of Paprosky type III defects should include the anatomical reconstruction of the lesion with the restoration of the hip COR and host bone stock. Although the structural bone grafting may address such issues, some authors reported high rate of failure, ranging from 22% to 45%, in the long term 29. and 30.. During the revascularization and remodeling, the allograft is weak and unable to transfer the load bearing from the implant to the host bone leading to the collapse of the graft and the loosening of the prosthesis .
In literature, reconstruction cages associated with structural allograft reported controversial results. Although the technique provides a good primary stability, high rates of mechanical failure (from 9% to 64%) have been reported in type III acetabular defects 32., 33. and 34.. Although structural allograft can be remodeled and gradually transformed into normal living bone , the lack of porous surface of the cage determines the inability to promote a secondary biologic fixation at the implant-graft interface . Moreover, when the liner is cemented into the cage, a reduced amount of cement penetrates through the screw holes and bonds the pelvic bone. For these reasons, a fatigue failure of the screws or flanges of the cages can occur in the long term.
Bilobed oblong components have been also proposed for the management of superior acetabular bone defects. Although they may fill the lesion maximizing the contact between host bone and implant and restore the hip COR, they reduce the bone stock and could not fit the defect . Moreover, some authors reported a failure rate greater than 20% with a follow-up of 41 months .
The use of TM cups and augments has recently increased for the management of severe bony defects 3., 8., 9., 10., 11., 12., 13., 14., 15., 16. and 17.. However, previous studies are affected by some biases. First of all, the sample included patients with different preoperative diagnosis, from loosening to wear and infection, and different types of bone defects, from Paprosky grade IIA to grade IIIB with pelvic discontinuity. Second, the revision surgery was performed using TM acetabular cups with or without TM augments.
To our knowledge, only two previous studies assessed a small case series including patients affected by Paprosky type IIIA defect managed with the association of TM acetabular components and augments 10. and 11.. Sporer et al  reviewed 28 acetabular revisions at an average follow-up of 36 months (from 12 to 48) and reported only one case of revision for recurrent instability (3.5%). On the other hand, Del Gaizo et al  reviewed 37 acetabular revisions at an average follow-up of 60 months (from 26 to 106), reporting an overall revision rate of 21.6%. Because both studies included patients with Paprosky type IIIA defect with loosening as more frequent preoperative diagnosis, we could hypothesize that the length of follow-up plays a role in determining such difference in the overall revision rate. However, despite the higher overall revision rate, Del Gaizo et al reported only one case of aseptic loosening (2.7%) and two cases of recurrent instability (5.4%).
In our series, we included only patients with aseptic loosening to minimize bias related with different preoperative diagnoses. We also excluded patients who underwent primary hip arthroplasty, because of the better quality of the host bone that has not been previously aggressed. Compared with two previous studies 10. and 11., we included only patients with Paprosky type IIIA and IIIB defects without pelvic discontinuity. At an average follow-up of 53.7 months (from 36 to 91), we reported three cases of loosening (5.4%), and only one case of recurrent instability (1.8%). These findings confirm the results of previous studies also in patients with Paprosky type IIIB defect without pelvic discontinuity. Moreover, we reported a lower overall revision rate (9.1% versus 21.6%).
In literature, the survival rate of TM cup and augment constructs has been estimated from 92% to 99% in the mid-long term 12. and 15.. However, these findings are referred to a sample including patients with mild to severe bone acetabular defects. In this study, we reported a survival rate at 2 and 5 years of 96.4% and 92.8% respectively in a population affected only by severe bony lesions. Moreover, these results are better than those reported for structural bone grafting, reconstruction cages and oblong cups.
The augments represent a modular system allowing the surgeon to perform a customized acetabular reconstruction. Regardless of the size and shape of the bony lesion, the combination of TM-coated cups and augments provides a modular construct filling the defect and maximizing the contact with the host bone. In this series, we were able to manage severe bony lesions, such as Paprosky type IIIA and IIIB defects without pelvic discontinuity. Moreover, we found that this system allows the surgeon to achieve a significant reduction of the cranial migration of hip COR.
In our practice, we aim to restore the anatomical position of the COR to achieve an effective function of the abductors . During the surgical procedure, we try to achieve the best acetabular trial position in terms of COR height, cup anteversion and abduction even though there is partial stability of the construct. Then, we use the TM augments to fill the defect and stabilize the cup. Finally, despite the quite large size of bone defects, we were usually able to use medium-size cups. In our opinion, the use of non-large cups could allow to prevent impingement between the acetabular construct and soft tissues around the hip.
Major limitations of the study are the retrospective design and the lack of a control group. The uncontrolled design prevents us to prove that the reconstruction with augments is superior to other techniques described to manage these patients, including reconstruction cages, structural allografts, and bilobed or oblong components. However, according to our results, we believe that the use of TM augments associated with TM cups approach could be considered an effective management of Paprosky type III defects without pelvic discontinuity.
Another limitation is that we did not perform an a priori power analysis and sample size calculation; we planned to include in the study all the eligible patients who underwent acetabular reconstruction with TM cups and augments during the index period. However, we performed a post hoc power analysis on our results showing that the study had a power of 0.90 to detect a significant difference between preoperative and postoperative values of HHS, ROMs of the hip, and LLD by using the two tails Wilcoxon signed ranks test for paired sample, with an alfa error (the probability of yielding a type I error) equal to 0.05 and an effect size equal to 0.4.