The medium-term results of the Stanmore non-invasive extendible endoprosthesis in the treatment of paediatric bone tumours.

In skeletally immature patients, resection of bone tumours and reconstruction of the lower limb often results in leg-length discrepancy. The Stanmore non-invasive extendible endoprosthesis, which uses electromagnetic induction, allows post-operative lengthening without anaesthesia. Between 2002 and 2009, 55 children with a mean age of 11.4 years (5 to 16) underwent reconstruction with this prosthesis; ten patients (18.2%) died of disseminated disease and one child underwent amputation due to infection. We reviewed 44 patients after a mean follow-up of 41.2 months (22 to 104). The mean Musculoskeletal Tumor Society score was 24.7 (8 to 30) and the Toronto Extremity Salvage score was 92.3% (55.2% to 99.0%). There was no local recurrence of tumour. Complications developed in 16 patients (29.1%) and ten (18.2%) underwent revision. The mean length gained per patient was 38.6 mm (3.5 to 161.5), requiring a mean of 11.3 extensions (1 to 40), and ten component exchanges were performed in nine patients (16.4%) after attaining the maximum lengthening capacity of the implant. There were 11 patients (20%) who were skeletally mature at follow-up, ten of whom had equal leg lengths and nine had a full range of movement of the hip and knee. This is the largest reported series using non-invasive extendible endoprostheses after excision of primary bone tumours in skeletally immature patients. The technique produces a good functional outcome, with prevention of limb-length discrepancy at skeletal maturity.

Non-invasive extendible endoprostheses for limb reconstruction in skeletally-mature patients.

We describe the application of a non-invasive extendible endoprosthetic replacement in skeletally-mature patients undergoing revision for failed joint replacement with resultant limb-length inequality after malignant or non-malignant disease. This prosthesis was developed for tumour surgery in skeletally-immature patients but has now been adapted for use in revision procedures to reconstruct the joint or facilitate an arthrodesis, replace bony defects and allow limb length to be restored gradually in the post-operative period. We record the short-term results in nine patients who have had this procedure after multiple previous reconstructive operations. In six, the initial reconstruction had been performed with either allograft or endoprosthetic replacement for neoplastic disease and in three for non-neoplastic disease. The essential components of the prosthesis are a magnetic disc, a gearbox and a drive screw which allows painless lengthening of the prosthesis using the principle of electromagnetic induction. The mean age of the patients was 37 years (18 to 68) with a mean follow-up of 34 months (12 to 62). They had previously undergone a mean of six (2 to 14) open procedures on the affected limb before revision with the non-invasive extendible endoprosthesis. The mean length gained was 56 mm (19 to 107) requiring a mean of nine (3 to 20) lengthening episodes performed in the outpatient department. There was one case of recurrent infection after revision of a previously infected implant and one fracture of the prosthesis after a fall. No amputations were performed. Planned exchange of the prosthesis was required in three patients after attainment of the maximum lengthening capacity of the implant. There was no failure of the lengthening mechanism. The Mean Musculoskeletal Tumour Society rating score was 22 of 30 available points (18 to 28). The use of a non-invasive extendible endoprosthesis in this manner provided patients with good functional results and restoration of leg-length equality, without the need for multiple open lengthening procedures.

Design and characterization of a novel permanent magnet synchronous motor used in a growing prosthesis for young patients with bone cancer.

Approximately 50 child sarcomas are treated with limb salvage surgery each year in the United Kingdom. These children need an extendable implant that can be lengthened periodically to keep pace with the growth in the opposite limb. Surgically, invasive devices have been used for the past 30 years with intrinsic problems of infection and long-term recurrent trauma to the patient. To eliminate problems associated with the invasive device, a noninvasive extendable prosthesis was developed. The magnetically coupled drive technology used for this prosthesis was a synchronous motor with a gear-driven telescoping shaft. In this design the motor configuration was in two parts: a rotating magnet (rotor) that fitted inside the prosthesis where space was limited and the stator, which was an external device used to extend the prosthesis remotely as the patient grew. This compact external drive produced a focused magnetic flux that required no cooling and operated on a single-phase power supply. The extending mechanism in the implant was able to overcome up to 1300 N force, which is the tension force exerted by the soft tissues during the lengthening procedure. The device has been successfully implanted in 50 patients.

Non-invasive distal femoral expandable endoprosthesis for limb-salvage surgery in paediatric tumours.

We report our early experience with the use of a non-invasive distal femoral expandable endoprosthesis in seven skeletally immature patients with osteosarcoma of the distal femur. The patients had a mean age of 12.1 years (9 to 15) at the time of surgery. The prosthesis was lengthened at appropriate intervals in outpatient clinics, without anaesthesia, using the principle of electromagnetic induction. The patients were functionally evaluated using the Musculoskeletal Tumour Society scoring system. The mean follow-up was 20.2 months (14 to 30). The prostheses were lengthened by a mean of 25 mm (4.25 to 55) and maintained a mean knee flexion of 110 degrees (100 degrees to 120 degrees ). The mean Musculoskeletal Tumour Society score was 68% (11 to 29). Complications developed in two patients; one developed a flexion deformity of 25 degrees at the knee joint, which was subsequently overcome and one died of disseminated disease. The early results from patients treated with this device have been encouraging. The implant avoids multiple surgical procedures, general anaesthesia and assists in maintaining leg-length equality.

Stanmore non-invasive growing arthrodesis endoprosthesis in the reconstruction of complicated total knee arthroplasty: A case report.

As the number of primary and revision arthroplasties performed each year increases, the complexity of the reconstructive efforts also increases. A case of a patient with a total knee arthroplasty complicated with infection, deficient extensor mechanism, bone loss and limb shortening of 5.5 cm is reported. We describe an alternative surgical technique of reconstruction of the knee and lengthening of the limb using the Stanmore Non-Invasive Growing-Distal Femoral Prosthesis. The prosthesis had a magnetic disc attached to a gearbox. The generation of an external electromagnetic field caused the magnetic disc to rotate and lengthen the prosthesis. This reconstructive technique has not been described in the literature.

In vivo distraction forces in extendible endoprosthetic replacements--a study of 34 patients.

Using an instrumented distraction tool 34 extendible endoprosthetic replacements were lengthened to measure total resistance to extension. The study consisted of 19 distal femoral, 2 proximal femoral, 1 total femoral, 8 proximal tibial, 1 distal humeral, 2 proximal humeral and 1 total humeral replacements. Among these prostheses a total of 76 measurements were made. The initial load on the prosthesis due to soft tissues and muscles varied between 0 and 578 N with an overall mean of 128 N. There was a linear relationship between extension and load. At 6 mm extension the load varied between 42 and 1513 N with a mean of 476 N. There was very little difference between the means of femoral and tibial measurements. There was very little resistance to extension in the total humeral replacement due to the absence of glenoid and muscle attachment. In the distal humeral replacement all three measurements were comparable with the forces in proximal tibial replacements. Two cases showed very high resistance to extension, which was mainly attributed to the presence of large amounts of scar tissues observed at surgery. During one lengthening operation a 30 s pause at 1 mm increments from 0 to 9 mm showed load recovery of approximately 10 per cent for up to 3 mm, beyond which there appeared to be stiffening of tissues.

Telemetry of forces from proximal femoral replacements and relevance to fixation.

Two proximal femoral replacements were instrumented to enable axial forces to be determined at two sites within the prosthesis: in the main shaft and near the tip of the intramedullary stem. The goal was to measure the changes in force distribution over time, as indicated by the ratio of the two forces. Inductive coupling between a coil worn around the leg and a small implanted coil was used, both to supply power to electronic circuits sealed into a welded cavity in the prosthesis and to telemeter data from the prosthesis. Data from both subjects were recorded over the first two years following surgery. For the first subject, there was an increase in mean shaft force excursions (peak force minus resting force) during level walking from 0.53 x BW after 1 week 2.77 x BW after 23 months. The corresponding mean tip force excursions were 0.13 x BW and 1.74 x BW, respectively. The ratio of mean tip force excursions to shaft force excursions steadily increased over the same period from 25 to 63%. Similar increases over time in the tip/shaft ratio were found during treadmill walking, stair climbing and stair descending. Data from the second subject were obtained for the shaft forces only, and were consistent with those from the first subject. The progressive transfer of axial load from the proximal to the distal part of the IM stem recorded telemetrically, together with radiographic observations, suggested that bone remodelling had taken place together with a less stable interface around the proximal part of the stem. This process evidently began soon after implantation.