The effect of patient gait on the material properties of UHMWPE in hip replacements.

The wear of ultra-high molecular weight polyethylene (UHMWPE) acetabular components in total hip replacements (THRs) has been shown to be highly dependent on the direction of shear. Greatly reduced wear rates have been reported for unidirectional, compared to multidirectional, articulation in vitro. This work for the first time enables investigation of a relationship between clinical wear conditions, as determined by patient gait path, and the mechanical and structural changes that occur within the UHMWPE acetabular component. Individual patients' wear paths were determined prior to revision operation from hip joint kinematics measured by clinical gait analysis. The material properties of the acetabular components removed during the revision operation were subsequently analysed. A technique using Fourier transform infra- red analysis (FTIR) was developed to quantify the orientation of the individual UHMWPE lamellae. This study shows that there is a direct relationship between a patient's clinical gait path and the molecular properties of their UHMWPE acetabular socket. Patient kinematics are an important factor affecting the wear and long-term biocompatibility of UHMWPE used as a bearing surface in THR.

Formation of Propionibacterium acnes biofilms on orthopaedic biomaterials and their susceptibility to antimicrobials.

Failure to treat and eradicate prosthetic hip infection with systemic antibiotic regimens is usually due to the fact that the infection is associated with biofilm formation and that bacterial cells growing within a biofilm exhibit increased resistance to antimicrobial agents. In this in vitro study, we investigated the susceptibility of prosthetic hip Propionibacterium acnes and Staphylococcus spp. isolates growing within biofilms on polymethylmethacrylate (PMMA) bone cement to a range of antibiotics. All P. acnes isolates in the biofilm mode of growth demonstrated considerably greater resistance to cefamandole, ciprofloxacin and vancomycin. In contrast, only four of the eight P. acnes isolates demonstrated an increase in resistance to gentamicin. All ten Staphylococcus spp. isolates in the biofilm mode of growth exhibited large increases in resistance to gentamicin and cefamandole with eight of the ten isolates also exhibiting an increase in resistance to vancomycin. However, only three of the ten Staphylococcus spp. isolates exhibited an increase in resistance to ciprofloxacin. Biofilms were also formed on three different titanium alloys and on PMMA bone cement using P. acnes, Staphylococcus epidermidis and Staphylococcus aureus strains to determine if the underlying biomaterial surface had an effect on biofilm formation and the antimicrobial susceptibility of the bacteria growing within biofilms. Although differences in the rate at which the three strains adhered to the different biomaterials were apparent, no differences in biofilm antibiotic resistance between the biomaterials were observed. In the light of these results, it is important that the efficacy of other antibiotics against P. acnes and Staphylococcus spp. prosthetic hip isolates growing within biofilms on orthopaedic biomaterials be determined to ensure optimal treatment of orthopaedic implant infection.

Recurrent hip arthroplasty dislocation: good outcome after cup augmentation in 20 patients followed for 2 years.

Recurrent posterior dislocation occurs after primary total hip arthroplasty at rates of up to 7%. Component revision is regarded as standard management, but this major surgery may be unsuitable for elderly patients. We have developed a congruent polyethylene acetabular posterior lip augmentation device (PLAD) with a stainless steel backing plate. This can be used when there is no gross malalignment, wear or loosening of the primary components. We retrospectively compared 20 patients who had revision surgery with twenty patients who had been treated with the PLAD for recurrent posterior dislocation after primary Charnley total hip arthroplasty. In the PLAD group, the mean operative time, intraoperative blood loss, time spent in the high-dependency unit (HDU), transfusion requirements and the duration of hospital stay were all less than that in the revision group. There was no difference in the Oxford Hip Score between the groups at latest review 2 years after surgery. None of the patients in either group had suffered another dislocation.