INTRODUCTION

Modular knee implants are used to manage large bone defects in revision total knee arthroplasty. These implants are confronted with varying fixation characteristics, changes in load transfer or stiffen the bone. In spite of their current clinical use, the influence of modularity on the biomechanical implant-bone behavior (e.g. implant fixation, flexibility, etc.) still is inadequately investigated.

Aim of this study is to analyze, if the modularity of a tibial implant could change the biomechanical implant fixation behavior and the implant-bone flexibility.

Introduction:

Extensive bone defects of the proximal femur e.g. due to aseptic loosening might require the implantation of megaprostheses. In the literature high loosening rates of such megaprostheses have been reported. However, different fixation methods have been developed to achieve adequate implant stability, which is reflected by differing design characteristics of the commonly used implants. Yet, a biomechanical comparison of these designs has not been reported.

The aim of our study was to analyse potential differences in the biomechanical behaviour of three megaprostheses with different designs by measuring the primary rotational stability in vitro.

Introduction: During hip stem revisions osteotomies allow to remove well-fixed components. Once removal has been done, cerclage wires should secure the osteotomy and support primary stability of the new stem. Stability is important for a bony ingrowth and therefore the longevity of a cementless revision stem.

Tension wires seem to dominate revision surgery and studies only refer to the advantages of cable wires in general. This in-vitro study analyzed the infiuence of both, tension and cable wires on primary stability of cementless revision stems. We aimed to examine the effectivity of wiring a femoral osteotomy, differences achieved with each method, and whether one wire has advantages regarding the fixation concepts of revision stems (meta- and diaphyseal).

Methods: We studied a Ti-tension- and a CoCrWNi-cable-wire. The Helios-stem stood for the meta- and the Wagner-SL-stem for the diaphyseal fixation concept. Each stem was implanted into 3 synthetic femurs and a standardized extended proximal femoral osteotomy was performed. Spatial movements of bones and stems at several sites were explored under axial torques using a high-resolution measuring device. Movement graphs subjected to the sites defined relative movements RM = ΔαZ/TZ [mdeg/Nm]. The osteotomies were locked consecutively with both wires and all compounds were measured again. Wiring was done by a proximal figure 8 and a diaphyseal circular loop.

Results: Compared to the unlocked osteotomy the tension as well as the cable wires caused a changed RM for the stems (p=0.03). Both wires affect an increased stability within the proximal main fixation area of the Helios. Even for the Wagner-SL, usually fixating diaphyseally, a proximal fixation was reached with both wires. A significantly better stabilization could be observed for the Helios using cable wires (p=0.04). The overall RM reached with tension and cable wires was 16.6 and 11.1 mdeg/Nm. The Wagner-SL^® showed no difference in stability between tension and cable wire treatment (p=0.29).

Discussion: Both, the tension and the cable wires support the revision stems in bridging the artificial defect of an extended proximal femoral osteotomy. Especially for the proximal fixating stem, RMs could largely be reduced, while cable wires seem to be advantageous. Preventing a circular constriction leading to an osseous malnutrition, the use of cable wires, however, should be impeded with regard to diaphyseal fixating stems and proximal osteotomies. Comparable results with both wires were reached and none of the wires showed any advantage in this situation. In conclusion, the wires should be chosen depending on the fixation concept of the revision stem.

Following resection of primary malignant bone tumours of the humerus, limb salvage can be performed by vascularized fibula graft for reconstruction of large segmental defects.

In 12 patients with malignant bone tumour of the proximal humerus, tumour was resected and the bone defect reconstructed by vascularized fibula graft. Median age of the patients was 23 years. Median follow up was 114 months.

In 10 patients humeral head had to be resected and was replaced by fibular transplant including head and shaft of the ipsilateral fibula. Humeral head could be left in place in 2 patients. Median length of transplant was 17.2 cm. Radiographic union could be seen after 8 months in median. In 7 patients partial necrosis of the fibular head occurred, in 4 patients fracture of the transplant happened following trauma. In these 4 cases revision surgery was required. Partial necrosis of the head of fibula had no significant influence on shoulder function. One patient died of disease, the others are disease free. Enneking Index was 61% in median at time of last follow up. At donor side 3 cases of transient peroneal palsy could be seen.

We conclude that vascularized fibula graft is a successful surgical procedure for upper limb salvage especially for preservation of joint function also in long term follow up.