John K. Dowell

ORTHOPAEDIC BONE DRILLS – CAN THEY BE IMPROVED?

We studied the various drill bits available for engineering purposes, and compared them with standard orthopaedic drill bits, using continuous temperature recording at 0.5 mm, 1.0 mm and 1.5 mm from the edge of a 2.5 mm hole as it was drilled in fresh cadaver human tibia. We found that some commercially available drill bits performed better than their orthopaedic equivalents, producing significantly less thermal injury to the surrounding bone and halving the force required for cortical penetration. Our work suggests that the optimal bit for orthopaedic purposes should have a split point and a quick helix. Theoretical knowledge of cutting technology predicts that the addition of a parabolic flute will further reduce thermal damage. Further work is being done on other drill sizes used in orthopaedic practice and on new custom-designed bits.

Stresses in cement mantles of hip replacements: effect of femoral implant sizes, body mass index and bone quality

The effects of femoral prosthetic heads of diameters 22 and 28 mm were investigated on the stability of reconstructed hemi-pelves with cement mantles of thicknesses 1–4 mm and different bone qualities. Materialise medical imaging package and I-Deas finite element (FE) software were used to create accurate geometry of a hemi-pelvis from CT-scan images. Our FE results show an increase in cement mantle stresses associated with the larger femoral head. When a 22 mm femoral head is used on acetabulae of diameters 56 mm and above, the probability of survivorship can be increased by creating a cement mantle of at least 1 mm thick. However, when a 28 mm femoral head is used, a cement mantle thickness of at least 4 mm is needed. Poor bone quality resulted in an average 45% increase in the tensile stresses of the cement mantles, indicating resulting poor survivorship rate.

PERSISTENT SYNOVIAL FISTULA AFTER ARTHROSCOPY: IS TITANIUM SYNOVITIS A RISK FACTOR?

Synovial fistulae after arthroscopy and titanium-induced synovitis of the knee are both uncommon. We report a case in which a persistent fistula led to frank infection after a diagnostic arthroscopy during which a florid titaniuminduced synovitis of the knee had been discovered. We are not aware of any previous reports of this association. Case report. A 66-year-old man was admitted with pain and locking of his left knee two years after insertion of a Brigham unicondylar prosthesis for medial compartment osteoarthritis. The pain was worse around the patella and exacerbated by movement and weight-bearing. Radiographs were satisfactory apart from the presence of a cement loose body, and the WBC and ESR were normal. The provisional diagnosis was either mechanical impingement due to the loose body or patellar maltracking. Arthroscopy showed a florid black-stained titaniuminduced synovitis with a large quantity of metal debris in the synovial fluid. The polyethylene of the tibial component was completely worn, allowing metal-to-metal articulation at the tibiofemoral joint. The central part of the metal tibial tray had also worn through (Fig. 1). Synovial biopsy showed a giant-cell granulomatous reaction due to titanium oxide. A decision was made to revise the knee arthroplasty in a few weeks, but the medial parapatellar portal failed to heal and became a persistent and troublesome synovial fistula (Fig. 2). This discharged black-stained sterile fluid for over three months and was complicated by a frank staphylococcal septic arthritis. Treatment by joint lavage and longterm antibiotic therapy was required before the infection settled. The knee revision was eventually performed using antibiotic-loaded cement. A blood test one week later showed a markedly raised titanium level of 420 g/l (normal ≤8 g/l) indicating titanium metallosis. Discussion. The incidence of persistent synovial fistula after arthroscopy is very low. Bamford et al (1993) reported only one case in 8500 arthroscopies. In our patient a large amount of titanium debris had been generated by the wearing of the chrome-cobalt femoral component of the Brigham prosthesis through the titanium tibial tray. The chronic effusion caused by the titanium particles may have resulted in persistent leakage of fluid which prevented

Configuration of anchorage holes affects fixation of the acetabular component in cemented total hip replacement—a finite element study

Our survey of current practice among UK orthopaedic surgeons shows wide variations in fixation techniques. The aim of this study, is to investigate the effect of drilling different configurations of anchorage holes in the acetabulum on implant stability. To avoid variables that could incur during in vitro testing, we used commercially available COSMOS finite element analysis package to investigate the stress distributions, deformations, and strains on the cement mantle when drilling three large anchorage holes and six smaller ones, with straight and rounded cement pegs. The results, which are in line with our in vitro studies on simulated reconstructed acetabulae, indicate better stability of the acetabular component when three larger holes than six smaller holes are drilled and when the necks of the anchorage holes are rounded. The longevity of total hip replacements could be improved by drilling three large anchorage holes, rather than many smaller ones, as initially proposed by Charnley.

ACHIEVING UNIFORM CEMENT MANTLE OF OPTIMUM THICKNESS DURING ORTHOPAEDIC SURGERY

The stability of cemented hip replacements can be improved by achieving a uniform and optimum cement mantle thickness. However, in surgery, it is difficult to achieve this even thickness of cement mantle due the bottoming out of the cup. Some acetabular cups with 3 mm cement spacers exist on the market. However, the optimum cement thickness is not always 3mm [Lamvohee et. al., 2006]. We investigated the use of cement spacers to produce uniform cement mantles of pre-defined thicknesses. Although bone cement is reasonably strong in compression, it is a relatively brittle material, making it susceptible to fracture as a result of tensile loads [Harper et al., 2000].

KNEE JOINT CONTACT MECHANICS IN A MALALIGNED LIMB: AN INTEGRATED FINITE ELEMENT AND IN VITRO STUDY

Excessive joint stress, often caused by knee malalignment, contributes to osteoarthritis (OA) progression. High tibial osteotomy (HTO) is a conservative surgery that corrects lower limb malalignment to relieve damaged tissues from excessive loading. However, HTO outcome has been highly variable and the relationship between the degree of malalignment correction and knee joint contact stresses is not known. If this were known, HTO could be tailored to each patient to best restore joint stresses to normal levels. Therefore, the aim of this work is to create a three-dimensional (3D) finite element (FE) model of the knee joint to predict the effect of different malalignment corrections on knee joint contact stresses. In this study, we present the verification of our subject-specific 3D FE model.