Radiodense structures resembling ossicles at the acetabular rim have received multiple names including “Os acetabuli, Os supertilii, Os marginale superius acetabuli, and Os coxae quartum”. Various theories regarding their origin have been postulated. These structures commonly are observed in dysplastic hips and hips suffering from femoro-acetabular impingement and represent fractures of the acetabular rim. In our series we observed acetabular rim fragments in 4.9% of the dysplastic hips and in 6.4% of the hips with femoro-acetabular impingement. Two different pathomechanics are responsible for the occurrence of these rim fragments. In dysplasia the short acetabular roof reduces the amount of available loading surface which leads to an overload on the lateral margin of the acetabulum, propagating the development of a fatigue fracture. However, as in all hips additional cysts were visible, it must be postulated, that cysts have to be present additionally and act as stress risers through which the rim bone eventually will fail. In hips with femoro-acetabular impingement the mode of failure is different. The relative anterior overcover in retroverted hips is subjected to stress during flexion of the hip, which is further increased by the frequent presence of an non-spheric extension of the femoral head as seen in cam impingement. The nonspheric femoral head-neck junction is jammed into the rim area. By repetitive traumatization the anterior rim eventually will fracture. The clinical importance of acetabular rim fractures in the dysplastic hip is readily understood even by an unexperienced observer. However, it has to be considered as a sign that the hip has decompensated and it usually goes with significant articular cartilage damage. Because the radiographic appearance of the hip with femoro-acetabular impingement seems normal at first sight, the mechanism leading to anterior rim fracture may be overlooked. However, recognition and adequate treatment is important to prevent further degeneration of the hip.
Directly molding IB, MG and AGC UHMWPE tibial inserts has provided excellent clinical performance. This performance may be related to the oxidation resistance and higher fracture toughness provided by the direct molding process. Directly molded UHMWPE components have been reported not to oxidize after either nine years post irradiation aging on the shelf or after 11 years of implantation. Retrievals show that molded IB inserts to have lower oxidation, better polyethylene quality and less surface damage than machined IB II inserts. However, the IB, MG and AGC products were directly molded from 1900 UHMWPE resin which is no longer available. The question remains if directly molding resins other than 1900 in a contemporary modular design will provide the same benefits. We report here on the first knee simulation wear of a contemporary total knee system comprised of a directly molded 1020 esin tibial insert. This result will be compared to the knee simulation result of an IBII machined from 4150 extruded ro 4 Optetrak tibial inserts made by directly molding 1020 resin were tested on a 4 station Instron/Stanmore simulator at 1.4 Hz with a 2279 N maximum load and right knee kinematics. The lubricant was bovine calf serum with EDTA and sodium azide. Axial loads were applied from 0 to 40&
#778; flexion and internal/external rotation was −3/+6 degrees. Location, type and area of surface damage, were evaluated every 1 million cycles (Mc). The wear rate of the directly molded inserts was 6X less than reported for machined IB II inserts (2 vs 12 mg/million cycles respectively). There were no signs of delamination or pitting with either design. The more conforming Optetrak provided 52% reduction in wear area over the IB II (21 vs 32 % respectively). This demonstrates that resins other than 1900 may be directly molded in a contemporary and provide the same historical advantages.
It is estimated that there will be over 12,000 total shoulder replacements implanted this year. In the best series, the survivorships of these devices are 90% at 7 years. However, there are radiographic indications that the long term success will be limited to wear and damage to the polyethylene glenoid components. Like tibial insert in total knee replacements, the glenoid is subjected to both rolling sliding motions of a metal counterface. Additionally, the compressive loads on the glenoid have been estimated to be as high as 2800N under ‘normal’ conditions. In contrast to tibial inserts, glenoid components are all typically less than 6 mm thick. In metal backed glenoid devices, the polyethylene thickness is often <
3 mm. The effect of these parameters and kinematics on polyethylene damage has not previously described. Although total shoulder replacements have been in use for over 25 years, there have been no reports describing the nature and extent of glenoid polyethylene wear and damage. We report the determination of polyethylene damage type and severity of 38 retrieved glenoid components of at least 4 different designs. Wear and damage were considered significant when either 80% of the glenoid surface was damaged or if over 25% of the component was worn away. Abrasion, burnishing and pitting were the main modes of damage. There were 2 fractured components. There was significant UHMWPE wear and damage in 17 (45%) components. In nine of these, the component was completely worn through.. These findings are consistent with high stress, high wear conditions and thin polyethylene components. These results indicate polyethylene wear and damage is expected to be a key factor in limiting the survivor-ship total shoulder replacements and that polyethylene damage and wear in total shoulder replacements may be higher than that found for either total hip or knee replacements.