The Fitbone^® System is the only motor driven fully implantable system, which offers the ability to correct all deformities like shortening, bone defects, axis and torsion deviation [8]. Different designs of the Fitbone^® System cope with a wide spectrum of indications [12]:

The Telescopic Active Actuator (TAA) with a diameter of 11–13 mm is a compact system with a fixed integrated motor drive as a hermetic closed unit. The fixation options are easy and it is possible to use it even if the bone is initially very short (shortest implant 163.5 mm). This version is applicable to the following procedures:

The Slide Active Actuator (SAA) with a diameter of 12 or 13 mm provides the key advantage of a modular design and creates the possibility to fix even extremely short bone-segments (60 mm) by three angulated screws. The following procedures can be performed at the femur:

The Femur/Tibia Segment Actuator (FSA/FSA) was designed to combine the advantages of the TAA and the SAA in one system. Without further operation it is possible, to perform bone transport in cases of bone defect initially followed by lengthening as needed to compensate limb length discrepancy.

The Telescopic Actuator Mechanism (TAM) with a diameter of 16 mm is a very short variant of the Fitbone^® System to perform stump lengthening of the femur up to 100 mm.

The “Expandable” Tumor–Endoprothesis MUTARS Xpand^® is a new application of the Fitbone^® System in combination with a tumor endoprosthesis to perform lengthening after resection of malignant bone tumors. In comparison to other fully implantable systems like the KMFTR^®-System (Stryker) [13] or the Repiphysis^®-System (Wright) [14] the MUTARS Xpand system [15] combines a fully implantable motorized system for limb lengthening (Fitbone^®, Wittenstein-intens) and a modular tumor endoprosthesis system (MUTARS^® Implantcast). The articular component of the endoprosthesis and the stem which fixes the prosthesis to the diaphysis are nearly the same as in conventional modular devices. However, between these two components a telescopic intermediate component with a motor drive is inserted, which can be incrementally activated whenever a small leg length discrepancy is detected. The lengthening process can be done continuously or in small steps without any risk of injury to neurovascular structures or other soft tissues of the limb by excessive or acute distraction. The telescopic module with the motor drive remains in place until maturity, after which it should be replaced by a non-motorized component.

The “Bioexpandable” Tumor–Endoprothesis MUTARS Bio-Xpand^® (Fig. 29.3) is an innovative development of a growing endoprosthesis [16]. While limb lengthening with the Xpand^® device seems to be much easier and safer compared to other endoprosthetic devices, all these devices share a problem of increasing leverage between the endoprosthesis and bone segments. This can result in potentially problematic loss of fixation between the prosthesis and the host bone, aggravated with each lengthening step. This problem can be understood by appreciating the ever-changing (and worsening) length-ratio of the prosthesis to the remaining bone with sequential lengthening of the device, and little or no comparable growth in the host bone segment. The concept of incremental lengthening of the “bioexpandable” endoprosthesis and its relationship to the host bone is quite different. Based on the method of callus distraction after osteotomy, bone growth is activated and the remaining bone is lengthened, thus actually improving the endoprosthesis-host bone length ratio with time. The use of external fixators for limb lengthening in conjunction with an endoprosthesis is intolerable, since bacterial contamination of an endoprosthetic device almost always necessitates its removal with catastrophic implications for limb reconstruction. When the tumor has to be resected in the diaphysis, the implantation of an intramedullary lengthening nail only would be an excellent option, if the residual fragments are compatible with such a system. Malignant tumors are mostly epiphyseal [17] making the use of a simple intramedullary solution impossible. For children, the new “bio-expandable” Tumor Endoprosthesis MUTARS BioXpand^® combines the articular component of a tumor prosthesis with the fully implantable motorized Fitbone^® System for bone lengthening. It permits successful correction and lengthening after epiphyseal tumor resection with good long term results (Fig. 29.4a–g) [18]. The articular component is similar to a conventional tumor endoprosthesis and is coupled to the corresponding articular surfaces. The shaft assembly is available in different lengths to bridge the bone defect resulting from tumor resection. An exchangeable shaft is used as the bone-anchoring component and is placed in the central cavity of the two prosthesis components.

Immediately after tumor resection, a solid intramedullary stem is implanted as a preliminary fixation. To prevent bony ingrowth, the stem has a polished surface. A hexagonal cross section or two screws in a slotted hole prevent rotation of the implant. Once a leg length discrepancy of 3–4 cm resulting from normal contralateral limb growth is documented during postoperative follow-up, a cortical osteotomy is performed and the solid stem is exchanged for the active motorized lengthening nail. During distraction at a rate of 1 mm/day, the remaining bone is gradually lengthened using the principles of callus distraction and new bone is formed within the distraction gap. The energy which is necessary for the distraction process is supplied in the same way as described above by a small receiver placed subcutaneously.

The latest innovation is the second version of the BioXpand ^® endoprosthesis for the femur, the BioXpand II. The components are principally the same as used in the first version. With the second version, repeated invasive surgeries are reduced to only one at the time of tumor resection. All revisions to exchange the active component can be done from the opposite side of the bone in a minimal invasive way. If the tumor is located in the distal femur, the approach for the lengthening device will be done from the proximal side and if the tumor is located in the proximal femur the approach for the lengthening device will be done from the distal side. Even the final replacement of the Fitbone^® device to a solid stem with a coated surface can be done without an invasive procedure.

Evaluation of the function and entire geometry of the affected bone including all deformities – not only the shortening – have to be included into the preoperative analysis before correction with the Fitbone^® device [2]. A clinical examination of both legs including the range of motion (ROM) of all joints to verify for basic conditions and contractures is mandatory and has to be documented. In addition photo documentation and a gate video is recommended. Long standing radiograph (LSR) images are taken on patients with anteriorly faced patellae and full load bearing on both legs. Leg length differences are levelled with calibrated blocks. Spine orientation, length of each bone, the mechanical and the anatomical axis has to be measured as well as all joint angles. In addition, a lateral radiograph of each bone is mandatory to detect deformities in the sagittal plane, especially at the femur where antecurvature may be an obstacle for nailing, if the ostetomy will be done in the wrong location.