Non-invasive expandable prostheses for limb salvage tumour surgery were first used in 2002. These implants allow ongoing lengthening of the operated limb to maintain limb-length equality and function while avoiding unnecessary repeat surgeries and the phenomenon of anniversary operations.

A large series of skeletally immature patients have been treated with these implants at the two leading orthopaedic oncology centres in England (Royal National Orthopaedic Hospital, Stanmore, and Royal Orthopaedic Hospital, Birmingham).

An up to date review of these patients has been made, documenting the relevant diagnoses, sites of tumour and types of implant used. 87 patients were assessed, with an age range of 5 to 17 years and follow up range of up to 88 months.

Primary diagnosis was osteosarcoma, followed by Ewing's sarcoma. We implanted distal femoral, proximal femoral, total femoral and proximal tibial prostheses. All implants involving the knee joint used a rotating hinge knee. 6 implants reached maximum length and were revised. 8 implants had issues with lengthening but only 4 of these were identified as being due to failure of the lengthening mechanism and were revised successfully. Deep infection was limited to 5% of patients.

Overall satisfaction was high with the patients avoiding operative lengthening and tolerating the non-invasive lengthenings well. Combined with satisfactory survivorship and functional outcome, we commend its use in the immature population of long bone tumour cases.

Infected mega-endoprostheses are difficult to treat with systemic antibiotics due to encapsulation of the implant by fibrous tissue, formation of biofilms and antibiotic resistant bacteria. Modifying the implant surface by incorporating a bactericidal agent may reduce infection. Infection rates are typically in the range of 8% to 30%. This study describes a novel process method of “stitching-in” ionic silver into the implant surface, in vitro testing and its early clinical usage.

A novel process has been developed to “stitch in” ionic silver into the upper surface of titanium alloy (Ti6Al4V). The process produces a modification by anodisation of the titanium alloy in dilute phosphoric acid, followed by absorption of silver from an aqueous solution. The engineered surface modification is therefore integral with the substrate and loaded with silver by an ion exchange reaction. Using this technique the maximum inventory of silver for typical a mega-prosthesis is 6mg and this is greater than 300 times lower than the No Observable Adverse Affects Level (NOAEL). Scanning electron microscopy revealed that the silver was concentrated in pits and forming reservoirs of ionic silver exposed to the body tissues.

Laboratory-based studies focusing on the safety and efficacy of silver as a bactericidal agent have included investigation into cytotoxicity using fibroblast and osteoblast cell lines, the impact of silver in reducing corrosion and laboratory testing to establish if the modified surface has an effect on the wear and mechanical characteristics. A range of fatigue, static, tensile pull off tests were performed. The silver elution profiles for both silver loaded and HA coated over a silver loaded surface have been examined. Histological studies were also performed to examine the impact of the silver on osseointegration.

The in vitro results confirm that silver is an effective antimicrobial agent. The mechanical characterization studies have identified that the surface treatment has no or minimal impact on the implant surface. Early results of the elution studies are encouraging showing that the HA coating of a silver loaded surface does not “seal” in the silver.

To date (May 10) 147 silver treated mega-prostheses have been implanted since March 2006. The majority of implants were distal femoral (29%), proximal tibial (23%) or hemiplevic (10%). The most common indication was revision of a failed limb salvage reconstruction (58%), with the dominant cause of failure being infection. The next most common indication was bone tumour (31%) and the large majority were used in the high risk skeletal locations of the tibia (44%) and the pelvis (27%). Early clinical results are encouraging indicating a significant reduction in the incidence of infection.

Three implants have been retrieved. An analysis of a proximal humeral replacement that had been in situ for 6 mths identified that there was 10-20% remaining on the implant surface.

This novel process of “stitching-in” silver appears to be a safe and effective surface treatment in helping to control infections of mega-prostheses. This technology has the potential to be transferred to other arthroplasty joints.

Background

Endoprosthetic reconstruction is an established method of treatment for primary bone tumours in children. Traditionally these were implanted with cemented intramedullary fixation. Hydroxyapatite collars at the shoulder of the implant are now standard on all extremity endoprostheses, but older cases were implanted without collars. Uncemented intramedullary fixation with hydroxyapatite collars has also been used in an attempt to reduce the incidence of problems such as aseptic loosening. Currently there are various indications that dictate which method is used.