Mega prosthetic reconstruction
Mega prosthetic reconstruction:
Oncologic orthopedic surgery had long been confined to amputation in order to remove malignant tissue and avoid recurrence and metastases.
However, in the case of primary bone tumors, the advent of adjuvant therapies (radiation and chemotherapy) helped to dramatically improve patient survival and local tumor control so that limb salvage surgery could compare to amputation and become the gold standard of current treatment.
In the frame of radical excision surgery, special segmental bone and joint replacement systems were developed, that is usually referred to as tumor endoprostheses or Mega prosthetic reconstruction.
The rationale behind their use subsequently led to an application even in revision arthroplasty with great loss of bone stock, as well as in trauma surgery, for extremely comminuted fractures with poorly vascularized fragments, where traditional osteosynthesis cannot be applied.
Tumor endoprostheses in their beginning came usually in custom-made monoblock form made of cast steel alloys (eg vitallium). Material evolution continued to titanium and cobalt-chrome-molybdenum alloys while in the meantime experimenting with various acrylic polymers, which though failed prematurely and were abandoned.
In general, the requisites for a successful implant design are resistance to corrosion, high biocompatibility, and resistance to fatigue fractures as well as a potential for osteointegration and soft tissue ingrowth. Additional qualities like infection repelling are also more than desired.
Metallurgy and synthetic material industries are providing a variety of coatings (hydroxyapatite, porous tantalum, elemental silver) and soft tissue attachment materials (polyester, polypropylene, carbon fiber, expanded polytetrafluoroethylene) to address these requisites in an improving manner.
Prosthesis design has also evolved from the monoblock and fixed hinge models to modular endoprostheses and rotating platforms, with improved geometry to enhance fixation and stability.
Modular endoprostheses are currently and since the 1980s almost dominating surgical practice. They consist of a number of different components in readily available sets. These components can be assembled in various combinations to best address the specific bone defect of the patient in the operating theatre (Fig. 11). In this manner, surgery can proceed without the fabrication delay (4-6 weeks) of the custom-made models.
Importantly, more freedom of options is given to the surgeon in order to reconstruct defects that might prove to be different from what preoperative planning indicated. Literature supports the superiority of modular implants even in terms of limb survival, complication rate and functional outcome.
Fixation, as in regular prostheses, can be achieved with or without the use of polymethylmethacrylate cement. In the case of cemented models, the limitation in weight-bearing depends most on the soft-tissue parts, and restrictions are sometimes implemented until there are reliable healing and attachment of the soft tissues to the prosthesis.
Typically, orthoses are used for reconstructions around the knee joint and the shoulder. In the case of the hip, the patient can often be instructed immediate full-weight bearing. The main disadvantage of cemented implants is aseptic loosening, which has been reported to occur in up to one-third of reconstructions around the knee joint.
Recent advances in implant manufacturing are the use of hydroxyapatite-coated collars in order to achieve osteointegration and minimize the risk of loosening, silver-coated stems in order to further reduce the infection rate and highly cross-linked polyethylene cups in order to minimize wear.