The implantation of structural allografts after great tumor resections is still being recognized as a good reconstructive strategy, because it is considered that the allograft has a good capability of integration to the host bone and to the biomechanics of the reconstructed limb . Nonetheless, all of the reconstructions with structural allografts performed in our series of young patients, failed because of aseptic loosening of the megaprosthesis because none of the structural allografts integrated to the native bone.
Megaprosthesis are big constrained implants with long lever arms that confer a high stress at the interface “implant-cement-bone”, reason why they can be considered with propensity to eventual mechanical failure . Regarding the reasons for failure of these implants, a retrospective multicentric revision published by Henderson et al., in 2011 describe 5 types of failure . They describe that aseptic loosening is the second reason for failure in terms of frequency, and infection is the main one. According to this classification, all of the AMC of our series belongs to the type II (aseptic loosening).
Aseptic loosening is considered as the final consequence of a series of mechanical factors common to all the megaprosthesis. It has been attributed to: a long interface “implant-cement-bone” , to the stresses generated by the important lever arms of the constrained hinges , by an altered transmission of the axial loads when walking , and high functional demands of young patients . Besides these reasons, we believe that the fact that the structural allograft did not integrate in any of the cases of our series had a determinant role in the development of the aseptic loosening and the subsequent failure of the megaprosthesis.
In our series the mean age of the patients at the time of revision surgery was 29 year-old, while in other published series of megaprosthesis revision surgery, the mean age of patients at the moment of implantation of the primary megaprosthesis was 41  and 27 year-old . Despite our series have a low number of cases, it is confirmed by patients in which revision surgery was performed with a mean age lower than the mean age at the moment of the primary surgery of other published series. We believe that the fact that the structural allograft had to be removed in all of the cases of our series (despite the theoretical biological advantages of the young patient in terms of bone consolidation and integration, and despite the five-year survival of massive allograft reconstructions has been described to be 80.8% ) denotes that the reconstructive strategy with AMC could be at least questionable.
One of the theoretical advantages of an AMC in those cases of resection of the proximal tibia, is that the structural allograft allows a reinsertion of the extensor mechanism when the resection compromises the Anterior Tibial Tuberosity (ATT). Donati et al., reported a survival of 78% at five years in cases of resection of the proximal tibia managed with AMC as a primary Surgery , reporting that 12.9% of their patients underwent additional surgery to promote union of the graft. They also describe that union of the graft could be negatively conditioned by the use of radiotherapy. Our experience has led us to believe that in this kind of patients, structural allografts behave as a temporary spacer, so union to the native bone is something that rarely occurs. In case number 6 of our series, the proximal segment of the tibia had to be completely resected at the moment of revision surgery, but a bone block with the ATT could be preserved. The primary reconstruction strategy consisted in an AMC in order pretend integration of the bone block with the ATT to the structural allograft. In this case, neither the structural allograft integrated to the host-bone nor the ATT integrated to the structural allograft.
In regards to the mechanical failures and the mean survival of the AMC and the MMP, Biau et al., described in their series of 91 megaprosthesis that mechanical failures were more frequent in AMC (10/33) than in MMP (10/58), and that the mean survival of AMC was 117 months and of MMP 138 months . In another series of 75 knee megaprosthesis, Wunder et al., described that the failure proportion was greater in the group of AMC when compared to the group of MMP (6/10 vs 10/64 respectively) . It could be considered that nowadays there is enough evidence to establish that a MMP is a superior reconstruction strategy than an AMC. Based on the mentioned premises, actually in our unit all of the primary and revision megaprosthesis are MMP.
Our study has the limitation of being a retrospective revision with a low number of cases, with a heterogeneous pathology and with several anatomic locations. However, our revision has the strength of being conformed by a series of young patients in which all of them had a revision of the megaprosthesis only because of mechanical failures, when they were younger than 40 year-old.
The results of our revision allow us to conclude that the use of structural allografts in limb salvage surgery after massive tumor resection, may not be the best reconstructive option because the high probability that the grafted bone segment might not integrate to the native bone, even in young patients, may lead to prosthetic failure because of aseptic loosening.
The study was performed in accordance with the ethical standards of the Declaration of Helsinki (amended in October 2013); and the level of confidentiality concerning the protection of personal data was as required by the Spanish laws (LOPD 15/1999). All patients gave their written informed consent, and they accepted that data from their electronic medical file could be used for purposes of this scientific research.