Jay Meswania

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.

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.

Enhanced wear and corrosion in modular tapers in total hip replacement is associated with the contact area and surface topography

Widespread concern exists about adverse tissue reactions after metal‐on‐metal (MoM) total hip replacement (THR). Concerns have also been expressed with wear and corrosion of taper junctions in THR. We report the effect of surface finish and contact area associated with a single combination of materials of modular tapers. In an in vitro test, we investigated the head/neck (CoCrMo/Ti) interface of modular THRs using commercially available heads. Wear and corrosion of taper surfaces was compared following a 10 million loading cycle. Surface parameters and profiles were measured before and after testing. Electrochemical static and dynamic corrosion tests were performed under loaded and non‐loaded conditions. After the load test, the surface roughness parameters on the head taper were significantly increased where the head/neck contact area was reduced. Similarly, the surface roughness parameters on the head taper were significantly increased where rough neck tapers were used. Corrosion testing showed breaching of the passive film on the rough but not the smooth neck tapers. Thus, surface area and surface finish are important factors in wear and corrosion at modular interfaces.

The effect of frictional torque and bending moment on corrosion at the taper interface: an in vitro study

The aim of this study was to assess the effect of frictional torque and bending moment on fretting corrosion at the taper interface of a modular femoral component and to investigate whether different combinations of material also had an effect. The combinations we examined were 1) cobalt-chromium (CoCr) heads on CoCr stems 2) CoCr heads on titanium alloy (Ti) stems and 3) ceramic heads on CoCr stems. In test 1 increasing torque was imposed by offsetting the stem in the anteroposterior plane in increments of 0 mm, 4 mm, 6 mm and 8 mm when the torque generated was equivalent to 0 Nm, 9 Nm, 14 Nm and 18 Nm. In test 2 we investigated the effect of increasing the bending moment by offsetting the application of axial load from the midline in the mediolateral plane. Increments of offset equivalent to head + 0 mm, head + 7 mm and head + 14 mm were used. Significantly higher currents and amplitudes were seen with increasing torque for all combinations of material. However, Ti stems showed the highest corrosion currents. Increased bending moments associated with using larger offset heads produced more corrosion: Ti stems generally performed worse than CoCr stems. Using ceramic heads did not prevent corrosion, but reduced it significantly in all loading configurations.

Influence of stem type on material loss at the metal-on-metal pinnacle taper junction

The clinical importance of material loss at the head–stem junction is unknown. Comparison of retrievals with different stem types can provide the opportunity to understand the importance of the taper junction. This was a case–control study involving 20 retrieved 36 mm metal-on-metal Pinnacle (DePuy) hips that were paired with either a Corail (n = 10) or S-ROM (n = 10) stem. The median head taper material loss rate for the Corail group was 0.238 (0.0002–2.178) mm3/year and was significantly greater than the S-ROM group (p = 0.042), which had a median material loss rate of 0.132 (0.015–0.518) mm3/year. The only significant difference between the groups was the stem taper roughness and length: this was rougher and shorter for the Corails. Long and smooth stem taper designs are preferred when used in conjunction with metal heads.

Mechanical stability of a locked step-plate versus single compression screw fixation for medial displacement calcaneal osteotomy.

Background: Reconstruction of a flatfoot commonly involves a calcaneal Medial Displacement Osteotomy (MDO) to correct hindfoot valgus in combination with soft tissue procedures. We compared fixation of an MDO using either a single, large cannulated screw versus a locking step-plate in load to failure in a cadaveric model. Methods: Eight matched pairs of cadaveric limbs were loaded using a mechanical testing rig. Two pairs served as non-operated controls. The remaining paired limbs underwent a 10-mm MDO stabilized either with a single 7-mm screw or a step-plate with four locking screws. One pair was used as a pilot study and the remaining five pairs were loaded up to 4500 N to failure. Results: In the five pairs loaded to failure, the median (with 95% CI) maximum force were 1779 N (1099-2312) and 826 N (288-1607) for the plate and screw, respectively (p = 0.043). With single screw fixation, the tuberosity fragment consistently failed by rotation and angulation into varus. With plate fixation, failure occurred as the screws cut through the internal surfaces of the tuberosity and body with no failure at the screw-plate interface. Conclusion: In this cadaveric model, a locked step-plate supported a significantly higher maximum force than a single large cannulated screw. Clinical Relevance: The magnitude of the load supported by the locking step-plate suggests that allowing early weightbearing post-operation may be safe in clinical practice before union of the osteotomy.

Clinical relevance of corrosion patterns attributed to inflammatory cell-induced corrosion: A retrieval study

In vitro studies have shown that human osteoclasts can corrode stainless steel and titanium leading to the production of metal ions responsible for inflammatory reactions. Moreover, traces of cellular activities on metal orthopaedic explants have recently been reported as inflammatory cell-induced (ICI) corrosion being the result of the cells sealing on the metal surfaces and releasing reactive oxygen species (ROS) through Fenton-like reactions. The extent and clinical relevance of this phenomenon has yet to be understood. We analysed a cohort of 100 CoCr alloy hips collected at our retrieval centre; we performed macroscopic and microscopic screening and used statistical analysis to correlate our findings with implant and clinical variables. We found that 59% of our implants had evidence of surface damage consistent with what has previously been described as cell-induced corrosion. There was a significant association between the patterns and aseptic loosening for the ASR modular (r = -0.488, p = 0.016) and the Durom modular (r = 0.454, p = 0.026). This is the largest implant retrieval study to examine the phenomena of so-called ICI corrosion and is the first to investigate its clinical relevance. We recommend further work to determine the role of cells in the damage patterns observed.

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.

The effect of using components from different manufacturers on the rate of wear and corrosion of the head–stem taper junction of metal-on-metal hip arthroplasties

AIMS Surgeons have commonly used modular femoral heads and stems from different manufacturers, although this is not recommended by orthopaedic companies due to the different manufacturing processes. We compared the rate of corrosion and rate of wear at the trunnion/head taper junction in two groups of retrieved hips; those with mixed manufacturers (MM) and those from the same manufacturer (SM). MATERIALS AND METHODS We identified 151 retrieved hips with large-diameter cobalt-chromium heads; 51 of two designs that had been paired with stems from different manufacturers (MM) and 100 of seven designs paired with stems from the same manufacturer (SM). We determined the severity of corrosion with the Goldberg corrosion score and the volume of material loss at the head/stem junction. We used multivariable statistical analysis to determine if there was a significant difference between the two groups. RESULTS We found no significant difference in the corrosion scores of the two groups. The median rate of material loss at the head/stem junction for the MM and SM groups were 0.39 mm(3)/year (0.00 to 4.73) and 0.46 mm(3)/year (0.00 to 6.71) respectively; this difference was not significant after controlling for confounding factors (p = 0.06). CONCLUSION The use of stems with heads of another manufacturer does not appear to affect the amount of metal lost from the surfaces between these two components at total hip arthroplasty. Other surgical, implant and patient factors should be considered when determining the mechanisms of failure of large diameter metal-on-metal hip arthroplasties. Cite this article: Bone Joint J 2016;98-B:917-24.

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 non-invasive 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.