Complications of metal-on-metal hip resurfacing, leading to implant failure, include femoral notching, neck fracture, and avascular necrosis. Revision arthroplasty options include femoral-only revision with a head, however mis-matching radial clearance could accelerate metal ion release. Alternatively, revision of a well-fixed acetabular component could lead to further bone loss, complicating revision surgery. We have developed a ceramic hip resurfacing system with a titanium-ceramic taper junction; taking advantage of the low frictional torque and wear rates that ceramic affords. Taking a revision scenario into account, the ceramic head has a deep female taper for the resurfacing stem, but also a superficial tapered rim. Should revision to this resurfacing be required, any femoral stem with a 12/14 taper can be implanted, onto which a dual taper adaptor is attached. The outer diameter of the taper adaptor then becomes the male taper for the superficial taper of the ceramic head; ultimately allowing retention of the acetabular component. In an in-vitro model, we have compared the fretting corrosion of this taper adaptor to existing revision taper options: a titanium-cobalt chrome (Ti-CoCr) taper junction, and a titanium-titanium sleeve-ceramic (Ti-Ti-Cer) taper junction. To simulate gait, sinusoidal cyclical loads between 300N-2300N, at a frequency of 3Hz was applied to different neck offsets generating different bending moments and torques. Bending moment and frictional torque were tested separately. An electrochemical assessment using potentiostatic tests at an applied potential of 200mV, was used to measure the fretting current (μA) and current amplitude (μA). In a short term 1000 cycle test with bending moment, four neck lengths (short to x-long) were applied. For frictional torque, four increments of increasing torque (2-4-6-8Nm) were applied. In a long-term test using the taper adaptor, the combination of worst-case scenario of bending and torque were applied, and fretting currents measured every million cycles, up to 10 million cycles.Background
Methods
Hip resurfacing has advantages for the young active patient with arthritis; maintaining a large range of motion, preserving bone stock, and reduced dislocation risk. However high serum metal ion levels with metal-on-metal resurfacing, and their clinical implications, has led to a decline in the use of hip resurfacing. Ceramic bearing surfaces display the lowest frictional torque and excellent wear rates. Recent developments have enabled large, strong ceramic materials to be used as resurfacing components. Any wear debris that is generated from these articulations is inert. However an all-ceramic hip resurfacing could be at risk of fracture at the head-stem junction. A new ceramic hip resurfacing system with a titanium-ceramic modular taper junction has been developed. The introduction of a taper introduces the potential for fretting corrosion; we sought to determine the extent of this in an in-vitro model, and compared this prosthesis to the conventional 12/14 titanium-cobalt chrome (Ti6Al4V-CoCr) taper junction. To simulate the gait cycle, sinusoidal cyclical loads between 300N-2300N, at a frequency of 3Hz, were applied to different head-neck offsets generating different bending moments and torques. The effect of increasing the bending moment and frictional torque were tested separately. Furthermore, the resurfacing head was mounted in a fixture held with just the stem, thus representing complete bone resorption under the head. An electrochemical assessment using potentiostatic tests at an applied potential of 200mV, was used to measure the fretting current (μA) and current amplitude (μA). In a short-term 1000 cycle test, six neck lengths (short to xxx-long) of the Ti6Al4V-CoCr taper were compared to the standard neutral (concentric), and 3mm A/P offset stem options for the resurfacing design. To represent frictional torque, four increments of increasing torque (2-4-6-8Nm) were applied to both tapers. In a long term test with the resurfacing stem, the worst-case scenario of the eccentric offset option and 8Nm of torque were applied, and potentiostatic measurements were taken every million cycles, up to 10 million cycles.Background
Methods
Recent studies on large diameter femoral head hip replacements have implicated the modular taper junction as one of the significant sources of wear and corrosion products and this has been attributed to increased torque and bending on the taper interface. The aim of this study was to assess the effect of frictional torque and bending moment on fretting corrosion at the taper junction and to investigate whether different material combinations also had an effect. We examined 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 femoral stem in the anterior posterior plane in increments of 0 mm, 4 mm, 6 mm and 8 mm where the force generated was equivalent to 0Nm, 9Nm, 14Nm and 18Nm. In Test 2 we investigated the effect of increasing bending moment by offsetting the application of axial load from the midline in the medial-lateral (ML). Offset increments equivalent to +0, +7 and +14 heads were used. For each test we used n=3 for each different material combination.Introduction
Patients/Materials & Methods
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. Cite this article:
Due to absence of fusion in guided-growth devices for EOS (growing rods, Shilla, LSZ) movement of the rods against their attachment is possible resulting in wear debris formation. It is important to understand the wear resistance of materials used in these devices under appropriate conditions. The aim of our work was to investigate wear resistance of titanium alloy Ti-6Al-4V and superelastic Nitinol. Nitinol has been used recently for correcting scoliosis and may provide a better and more gradual correction than other materials.Introduction:
Aim:
There has been widespread concern regarding the adverse tissue reactions after metal-on-metal (MoM) total hip replacements (THR). Concerns have also been expressed with mechanical wear from micromotion and fretting corrosion at the head/stem taper junction in total hip replacements. In order to understand the interface mechanism a study was undertaken in order to investigate the effect of surface finish and contact area associated with modular tapers in total hip replacements with a single combination of materials of modular tapers. An inverted hip replacement setup was used (ASTM F1875-98). 28 mm Cobalt Chrome (CoCr) femoral heads were coupled with either full length (standard) or reduced length (mini) 12/14 Titanium (Ti) stem tapers. These Ti stem tapers had either a rough or smooth surface finish whilst all the head tapers had a smooth surface finish. Wear and corrosion of taper surfaces were compared after samples were sinusoidally loaded between 0.1 kN and 3.1 kN for 10 million cycles at 4 Hz. In test 1 rough mini stem tapers were compared with rough standard stem tapers whilst in test 2 rough mini stem tapers were compared with smooth mini stem tapers. Surface parameters and profiles were measured before and after testing. Electrochemical static and dynamic corrosion tests were performed between rough mini stem tapers and smooth mini stem tapers under loaded and non-loaded conditions.Introduction:
Methods:
High failure rates with large diameter, metal on metal hip replacements have highlighted a potential issue with the head/stem taper junction as one of the significant sources of metal ion release. Postulated reasons as to why this may be such a problem with large head metal on metal hip replacements is due to the increased torque achieved by the larger head size. This may be responsible for applying greater micromotion between the head and stem taper and consequently greater amounts of fretting corrosion. The aim of this study was to perform short term in vitro electrochemical tests to assess the effect of increasing head diameter and torque on the fretting corrosion susceptibility of the head/stem taper interface and to investigate its effect on different material combinations. 36 mm Cobalt Chrome (CoCr) femoral heads were coupled with either a CoCr or Titanium (Ti) stem with 12/14 tapers, all with a smooth surface finish. Increasing perpendicular horizontal offsets in the sagittal plane created incremental increases in torque. Offset increments of 0 mm, 5.4 mm and 7.5 mm were selected (Figure 1) to simulate the torque force equivalent to 9 Nm, 12 Nm and 17 Nm. An inverted hip replacement setup was used (ASTM F1875-98) (Figure 2). Components were statically loaded at 0 kN and 2.3 kN prior to sinusoidal cyclic loading and electrochemical testing. Mean & fretting currents were calculated every 50 cycles up to a maximum of 1000 cycles of sinusoidal cyclic loading at 3 Hz along with the Overall Mean Current (OMC), Overall Mean Fretting Current (OMFC) and Overall Current change (OCC).Introduction:
Methods:
The National Joint Registry has recently identified failure of large head metal on metal hip replacements. This failure is associated with the high torque at the interface of standard modular taper junction leading to fretting and corrosion. A number of manufacturers produce mini spigots, which in theory, provide a greater range of motion as the neck head junction is reduced. However, the relative torque to interface ratio at this junction is also increased. In this study we investigated hypothesis that the use of small spigots (minispigots) will increase wear and corrosion on modular tapers. Wear and corrosion of spigots were compared in-vitro when loaded with a force representative of the resultant force passing through the hip. The heads (female tapers) were made of cobalt-chrome-molybdenum (CoCrMo) and the stems (male tapers) of titanium alloy (Ti). Commercially available tapers and heads were used. The surface parameters & profiles were measured before & after testing. Electrochemical static and dynamic corrosion (pitting) tests were performed on minispigots under loaded and non-loaded conditions.Introduction
Methods
Following bone tumour resection, lower limb reconstruction results in leg-length discrepancy in skeletally immature patients. Previously, minimally invasive endoprostheses have been associated with a high risk of complications including joint stiffness, nerve injury, aseptic loosening and infection. The purpose of this study was to examine the outcome of the Stanmore non-invasive extendible endoprostheses used in our institution between 2002 and 2009 and compare them with implants used in the past. Fifty-five children with a mean age of 11.4 years (5 to 16) underwent limb reconstruction with thirty-three distal femoral, two total femoral, eight proximal femoral and twelve proximal tibial implants. Forty-six endoprostheses were lengthened in clinic without anaesthesia using the principle of electromagnetic induction. Patients were assessed using the Musculoskeletal Tumour Society Score (MSTS) and the Toronto Extremity Salvage Score (TESS).Introduction
Methods
Medial Displacement Osteotomy (MDO) of the os calcis is used to correct the hind foot valgus in a flat foot deformity. Screw fixation is commonly used although contemporary locking plate systems are now available. This study tested the hypothesis that a 10mm MDO would support a higher load to failure with a locked step plate than with a single cannulated screw. Eight pairs of embalmed cadaveric limbs harvested 10cm below the knee joint were axially loaded using a mechanical testing rig. Two pairs served as non-operated controls loaded to 4500N. The remaining limbs in pairs underwent a 10mm MDO of the os calcis and were stabilised with a locked step plate or a 7mm cannulated compression screw. One pair was loaded to 1600N (twice body weight) as a pilot study and the remaining 5 pairs were loaded to failure up to 4500N. The force-displacement curve and maximum force were correlated with observations of the mechanism of failure. In one pair of control limbs, failure occurred with fractures through both os calcis bones, whilst the other pair did not undergo mechanical failure to 4500N. In the pilot osteotomy, the plate did not fail whilst loss of fixation with the screw was observed below 1600N. For the remaining five pairs, the median (with 95% Confidence Intervals) of the maximum force under load to failure were 1778.81N (1099.39 – 2311.66) and 826.13N (287.52 – 1606.67) for the plate and screw respectively (Wilcoxon Signed Rank test p=0.043). In those with screw fixation loaded to 4500N, the tuberosity fragment consistently failed by rotation and angulation into varus.Materials and Methods
Results
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.
When performing limb salvage operations for malignant bone tumours in skeletally immature patients, it is desirable to reconstruct the limb with a prosthesis that can be lengthened without surgery at appropriate intervals to keep pace with growth of the contra-lateral side. We have developed a prosthesis that can be lengthened non-invasively. The lengthening is achieved on the principle of electromagnetic induction. The purpose of this study was to look at our early experience with the use of the Non Invasive Distal Femoral Expandable Endoprosthesis. A prospective study of 17 skeletally immature patients with osteosarcoma of the distal femur, implanted with the prosthesis, was performed at the Royal National Orthopaedic Hospital, Stanmore. The patients were aged between 9 and 15 years (mean 12.1 years) at the time of surgery. Patients were lengthened at appropriate intervals in outpatient clinics. Patients were functionally evaluated using the Musculoskeletal Tumour Society (MSTS) Scoring System and the Toronto Extremity Severity Score (TESS). Average time from the implantation to the last follow-up was 18.2 months (range 14-30 months). The patients have been lengthened by an average of 25mm (4.25-55mm). The mean amount of knee flexion is 125 degrees. The mean MSTS score is 77% (23/30; range 11-29) and the mean TESS score is 72%. There have been two complications: one patient developed a flexion deformity of 25 degrees at the knee joint and one patient died of disseminated metastatic malignancy. The early results from patients treated using this device have been encouraging. Using this implant avoids multiple surgical procedures and general anaesthesia. This results in low morbidity, cost savings and reduced psychological trauma. We do need additional data regarding the long-term structural integrity of the prosthesis.
For the treatment of malignant bone tumours in immature patients, extending prostheses are used to maintain growth in the affected limb. This new prosthesis allows the implant to be lengthened by using electromagnetic coupling that is simple and easy to use. Because of bone tumour, fourteen patients between the age of 8 and 15 years underwent bone replacement treatment and a further two patients, both male 18 and 61 years, received the same treatment to restore limb length discrepancy. These implants consisted of a telescoping shaft where the expansion is done by a power screw driven by a gearbox connected to a NdFeB magnet. This prosthesis is capable of being extended under an axial load of up to 1350N. This is in line with 76 distraction force measurements taken in 43 patients with growing prosthesis where extension was achieved by invasive procedure and where a force of up to 1513N for an extension of 6mm was recorded. Once implanted, the non-invasive prosthesis is extended by placing the limb through an external drive unit. As the drive unit is turned on, it produces a rotating magnetic field capturing the implant magnet causing it to rotate in synchronisation. At full speed, the implant grows at a rate of 0.23mm per minute. Of the sixteen patients, seven have been extended with one to its full capacity of 63mm. During extension, the patients have no sensations of vibration, heat, stretching or any other kind although the faint vibrations could be heard by placing a stethoscope on bony protrusions such as greater trochanter. At each sitting, the patients were extended by approximately 4mm during normal outpatient clinics and were able to walk as before immediately after the treatment. Patients with knee joint were functionally assessed before and after the treatment and showed approximately 10° to 15° of reduction in knee flexion/extension. This new extending mechanism in these prostheses has provided the patients a treatment, which reduces trauma infection and discomfort. The mechanism of extension is reliable and effective.
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° (100° to 120°). The mean Musculoskeletal Tumour Society score was 68% (11 to 29). Complications developed in two patients; one developed a flexion deformity of 25° 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.
When managing malignant long bone tumours in skeletally immature patients it is desirable, after resection, to reconstruct with a prosthesis that can be lengthened at appropriate intervals to keep pace with growth of the contralateral side. In an attempt to avoid multiple surgical procedures to achieve such lengthening we have recently developed a prosthesis that can be lengthened non-invasively.
