Abstract
INTRODUCTION
Unlike current acetabular cups, this novel ceramic cup has a Ti/HA coating which removes the requirement for assembly into a metal shell which avoiding potential chipping/misalignment and reducing wall thickness [Figure 1]. This study examines the resistance of novel thin-walled, direct to bone fixation ceramic cups to critical impact loads.
METHODS
Samples of the smallest (Ø46mm) and largest (Ø70mm) diameter ReCerfTM acetabular cups and corresponding femoral head implants were implanted into Sawbones foam blocks considered representative of pelvic cancellous bone. Two different positional configurations were tested and were considered worst case and the extremes of surgical compromise; P1 simulates the cup fully supported by the acetabulum with a high inclination angle (70°) and a vertical impaction axis (worst case loading near the cup rim) and. P2 simulates the cup implanted with a lower inclination (55°) but with the superior section unsupported by acetabulum bone [Figure 2]. For each size, three acetabular cups were tested in each position. The impact fixture was positioned within a drop weight rig above a bed of sand and ≈22mm of pork belly representative of soft tissues damping effect and the implant components aligned to achieve the defined impact point on the cup [Figure 2]. Lateral falls were tested on all available samples applying impact energy of 140J [1] and 3m/s impact velocity [2]. After the lateral fall test, each sample was tested under impact conditions equivalent to a frontal car crash considering a peak impact force of 5.7kN occurring 40ms from initial contact (able to produce acetabular fracture)[3].
RESULTS
None of the testing simulating a lateral fall produced fracture or any other damage to the ceramic acetabular cup. In 7 of the 12 tests, the impact force was sufficient to fracture the foam block representing the periprosthetic bone. The cups showed a good stability within the blocks, with a maximum recorded cup spinning angle relative to the acetabulum of 4.5˚.
Subsequent testing simulating a car crash resulted in the fracture of two samples out of 12, one of the largest and one of the smallest ReCerfTM cups. In both instances, failure occurred very close to the inner edge. Of the remaining 10 samples no cup fractures were observed. All foam acetabulum blocks were severely damaged and 5 blocks fractured. The maximum recorded cup spinning angle following the car crash impact was 5.8˚.
SIGNIFICANCE
Extreme testing scenarios presented here are not a regulatory requirement for manufacturers and have not previously been considered for ceramic acetabular components. Fracture is a possible failure mode of ceramics but this testing has proven that modern ceramics can withstand lateral falls and the large majority can withstand subsequent loading equivalent to head on car-crash; loading under which pelvic bone fracture and significant injury is far more likely to occur than implant fracture.
