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
Restoration of normal hip biomechanics is vital for success of total hip arthroplasty (THA). This requires accurate placement of implants and restoration of limb length and offset. The purpose of this study was to assess the accuracy of computer navigation system in predicting cup placement and restoring limb length and offset. An analysis of 259 consecutive patients who had THA performed with imageless computer navigation system was carried out. Acetabular cup abduction and anteversion, medialisation or lateralisation of offset and limb length change were compared between navigation measurements and follow-up radiographs. Sensitivity, specificity, accuracy and PPV were calculated to assess navigation for cup orientation and student t-test used for evaluation of offset and limb length change.Introduction:
Material and Methods:
Limb length discrepancy (LLD) is a complication of total hip arthroplasty (THR). We reviewed the x-rays of patients who underwent THR in our unit to establish the incidence and magnitude of LLD, and try to identify reasons why a length discrepancy arose. Patients with abnormalities of the opposite hip (previous THR, significant osteoarthritis) were excluded, to allow comparison with a normal contralateral side. 100 consecutive patients who fulfilled these criteria were included. There were 38 male and 62 female patients. The implants used were Charnley (89 cases), Elite (4 cases), and Exeter (7 cases). The following measurements were made on pre-and post-operative films on the hospital PACS system: centre of lesser trochanter to ischial tuberosity; tip of greater trochanter to centre of femoral head; centre of head to base of teardrop. The distance from the osteotomy in the femoral neck to the centre of the lesser trochanter was also measured. The interval from the greater trochanter to the closest margin of the pelvis, and the interval from the lesser trochanter to the base of the teardrop (compared to the normal side) were recorded as indices on adduction. Surgery was performed via a direct lateral (Hardinge) approach (95 cases) or through transtrochanteric approach (5 cases). There was a radiographic difference between limbs of >
1cm in 43 cases; in 9 of these, the operated limb was longer, and in 34 cases it was shortened. In those cases where the operated side was lengthened, the cause was on the acetabular side in 2 patients, and on the femoral side in 25 cases, and on the femoral side in 9 cases. The shortened limb was noted to be adducted relative to the opposite side in 29 patients. There was difference noted in the incidence of discrepancy between different implants. The transtrochanteric approach was associated with significantly (p<
0.01) less length discrepancy. Our findings suggest that shortening is much more common than lengthening following THR, and that incorrect positioning of the acetabulum is the more likely cause. Persistence of an adduction contracture may also contribute to an apparent shortening postoperatively. The transtrochanteric approach appeared to make LLD less likely. Surgeons should be aware of these findings when performing THR. The clinical effect of differing degrees of LLD is till debatable.
