The purpose of the study was to reduce peak cement mantle stresses occurring at the tip of the keel for an all-polyethylene cemented glenoid component using finite element (FE) techniques. Loosening of the glenoid component remains to be one of the most determinant factors in the outcome of total shoulder arthroplasty. Due to the off-centre loading that occurs, there is bending of the glenoid component with high shearing forces. These forces are transmitted to the underlying cement mantle and bone. It has been reported in previous FE studies that high cement mantle stresses occurs at the tip of the keel and at the edges of the cement flange. These stresses at the bone-cement interface can exceed the fatigue life of the cement, therefore initiating crack formation and damage accumulation. This results in loosening of the component and thus failure. A three-dimensional (3D) model of the scapula was developed using CT data. Surfaces of the inner and outer contours of the cortical shell were created within commercially available software, using a threshold algorithm. The glenoid bone geometry was then produced. Material properties for the reconstructed glenoid were taken from literature, using four differing material properties. The articulating surface of the keeled glenoid component was modelled with a 3mm radial mismatch. This was positioned in the glenoid bone with a uniform cement mantle thickness of 2mm. The resulting FE mesh consisted of solid parabolic tetrahedral elements. The effect of varying the angle on the keel of the component in the superior/inferior (S/I) direction was studied with uniform cement mantle thickness. The S/I length of the keel at the lateral end where it meets the back face of the component was maintained (juncture with flange), whilst the S/I length of the keel at the medial end (tip of the keel) was reduced as the change in angle increased. Two load cases were studied, involving a physiological load for 90 degrees of abduction and a central load of same magnitude. It was found that by increasing the angle of the keel, where the S/I length at the tip of the keel was reduced, resulted in lower cement mantle stresses in this area of interest. This can be attributed to it being further away from the stiffer cortical bone where high tensile stresses exist due to inherent bending of the glenoid construct under loading. Therefore by reducing these high cement mantle stresses at the tip of the keel, fatigue failure of the cement mantle could be reduced.
For one year (July 1999-July 2000), the rate of post-operative infection in patients undergoing joint arthroplasty was recorded (including wound, chest UTI etc). Standard precautions against infection used in most orthopaedic units in the UK were employed. In July 2000 elective orthopaedic beds were ‘ring-fenced’. Only elective orthopaedic patients who had negative swabs for MRSA in the community were admitted. Eradication therapy was commenced in the community if appropriate. Trauma and other specialties’ patients were excluded. In addition to standard precautions, nurses wore a disposable apron and gloves for each intervention. Antibacterial hand cleanser was installed by each bed, and staff expected to use it after each consultation. Doctors left jackets at the door and donned clean white coats for ward rounds. These were left on the ward and laundered daily. New cleaning regimes were adopted. Pre ring-fencing, 417 joint replacements were performed and 60 patients were cancelled due to no bed. There were 43 post-op infections, 9 of which were MRSA. In the year post ring-fencing, 488 joint replacements were performed; there were no cancellations due to bed shortage. There were 15 post-op infections and no MRSA. Eight patients swabbed positive for MRSA in the community, and were admitted after eradication therapy with no infections post-op. We concluded that ‘ring-fencing’ of elective orthopaedic beds reduced cancellations, reduced the overall infection rate and abolished MRSA. We have continued to ring-fence elective beds following this study, and recommend these precautions be employed in all units dealing with elective orthopaedic patients.
We studied 56 patients with fractures of the tibial shaft in a multicentre prospective randomised trial of three methods of external fixation. Group I was treated with a fixator which was unlocked at 4 to 6 weeks to allow free axial compression (axial dynamisation) with weight-bearing. Group II was treated with a fixator that was similarly unlocked at 4 to 6 weeks but included a small silicone spring which on weight-bearing could be compressed by up to 2 mm. this spring returns to its original length on cessation of weight-bearing thus allowing cycles of motion of up to 2 mm. Group III had a spring fixator like group II, but it was unlocked from the start to allow cyclical micromovement as soon as weight-bearing began. Fracture healing was monitored by the measurement of fracture stiffness. We defined healing as achieving a stiffness of 15 Nm per degree. The mean time was 14.1 weeks in group I, 15.9 weeks in group II, and 19.3 weeks in group III. The difference between groups was statistically significant (p = 0.004). The 95% confidence intervals for the average delay in healing with early cyclical micromovement (group III) as compared with later axial dynamisation (group I) was 1.8 to 8.7 weeks. The healing time in patients whose cyclical micromovement was delayed for 4 to 6 weeks (group II) was between these two extremes, but the differences from either of the other groups could have been due to patient selection. In the patients who completed the full trial, there were pin-track infections in over 60% of those in the cyclical micromovement groups compared with 20% in the axial dynamisation group (p = 0.03).(ABSTRACT TRUNCATED AT 250 WORDS)
