Margaret A. McGee

Biodegradable injectable polyurethanes: Synthesis and evaluation for orthopaedic applications

Biodegradable polyurethanes offer advantages in the design of injectable or preformed scaffolds for tissue engineering and other medical implant applications. We have developed two-part injectable prepolymer systems (prepolymer A and B) consisting of lactic acid and glycolic acid based polyester star polyols, pentaerythritol (PE) and ethyl lysine diisocyanate (ELDI). This study reports on the formulation and properties of a series of cross linked polyurethanes specifically developed for orthopaedic applications. Prepolymer A was based on PE and ELDI. Polyester polyols (prepolymer B) were based on PE and dl-lactic acid (PEDLLA) or PE and glycolic acid (PEGA) with molecular weights 456 and 453, respectively. Several cross linked porous and non-porous polyurethanes were prepared by mixing and curing prepolymers A and B and their mechanical and thermal properties, in vitro (PBS/37 degrees C/pH 7.4) and in vivo (sheep bi-lateral) degradation evaluated. The effect of incorporating beta-tricalcium phosphate (beta-TCP, 5 microns, 10 wt.%) was also investigated. The cured polymers exhibited high compressive strength (100-190 MPa) and modulus (1600-2300 MPa). beta-TCP improved mechanical properties in PEDLLA based polyurethanes and retarded the onset of in vitro and in vivo degradation. Sheep study results demonstrated that the polymers in both injectable and precured forms did not cause any surgical difficulties or any adverse tissue response. Evidence of new bone growth and the gradual degradation of the polymers were observed with increased implant time up to 6 months.

Wagner resurfacing hip arthroplasty. The results of one hundred consecutive arthroplasties after eight to ten years.

In a prospective study of 100 consecutive Wagner resurfacing hip arthroplasties in ninety-three patients, the outcomes for all hips were determined for an eight to ten-year follow-up period. By survivorship analysis, the rate of survival of the arthroplasty was calculated to be 70 per cent at five years, but only 40 per cent at eight years. The major cause of failure was aseptic loosening of the acetabular or femoral component, or both. Fracture of the neck of the femur occurred in three hips. Although the medium-term results (at fifty-six to eighty-three months) were better than those in most comparable studies of resurfacing arthroplasty, the poor long-term results (at ninety-one to 118 months) show that meaningful studies of new prosthetic designs must continue for at least eight years, and, if at all possible, must include 100 per cent follow-up. The survival curve for the resurfacing arthroplasties in this study can serve as the basis for comparison of the early, medium, and long-term results of future designs of resurfacing hip prostheses.

Biologic effects of cobalt chrome in cell and animal models.

The literature on animal and cellular models used to study the response to cobalt chrome alloy implants and wear and corrosion products is reviewed. Animal studies show that in solid form cobalt chrome alloy is relatively well tolerated. Injections of large numbers of particles in a single bolus lead to acute inflammation and necrosis, followed by a chronic inflammatory response. Macrophages are the predominant cell type and may persist in the tissues for years. Long term studies have failed to confirm the induction of tumors. In vitro studies confirm the toxic effects of cobalt chrome alloy corrosion products and wear particles, especially cobalt, and show that intracellular corrosion is an important mechanism for early release of cobalt ions. In vitro studies show that cobalt chrome alloy particles induce the release of inflammatory mediators from macrophages before causing cell death. These mediators have significant effects on osteoblastlike cells, as well as inducing bone resorption. Variations in the cell types, implantation site, and characteristics of the particles used in experimental models make interpretation of the results difficult. Standardized methods to control for size, shape, and number of particles for testing are proposed. It is important that in vitro and in vivo findings not be taken in isolation, but be compared with the results of human studies.

Implant retrieval studies of the wear and loosening of prosthetic joints: a review

Joint replacement surgery is established as a cost-effective treatment for degenerative joint disorders, but periprosthetic bone loss and aseptic loosening of prostheses is the major surgical complication of this procedure. Early loosening is related to poor initial fixation and design. Late loosening, which is the major problem and the principal cause of failure of this procedure, is related to wear of the prosthetic components. From the retrieval and analysis of failed implants, hypotheses regarding the mechanism of wear-induced loosening and other causes for implant failure may be developed. The criteria and methods, which we have developed for undertaking analysis, and the potentially valuable information available from these analyses, are reviewed.

Progression of acetabular periprosthetic osteolytic lesions measured with computed tomography.

BACKGROUND A better understanding of the factors associated with the size and/or progression of osteolytic lesions has been hampered by a lack of sensitivity of radiographic measurement techniques. METHODS We retrospectively analyzed quantitative computed tomography scans that had been made with use of a high-resolution multi-slice scanner with a metal artifact-suppression protocol. The scans had been made to determine the volume of osteolytic lesions around thirty-five cementless Harris-Galante acetabular components that had been in situ for at least ten years. Repeat scans of thirty hips allowed for the measurement of progression in the size of osteolytic lesions over a one-year period. Associations between the volume of osteolytic lesions, progression in the size of the lesions, polyethylene wear since the time of implantation, change in component position, and patient-related variables (age, gender, body mass index, activity level, walking limitations, joint pain, and function) were determined. RESULTS In sixteen of the thirty hips that had repeat computed tomography scans, the lesions progressed in size during the study period. The median size of the lesions in these sixteen hips was 10.3 cm(3) at the time of the initial scan, compared with 13.3 cm(3) at a median of fifteen months later (p = 0.001). Osteolytic lesions measuring >10 cm(3) in volume on the initial scan were 2.5 times (95% confidence interval 1.3 to 4.8 times) more likely to progress in size over one year than smaller lesions were. Patients with greater polyethylene wear rates, higher activity levels, no walking limitations, and larger prosthetic femoral head dimensions (26 or 28 mm) had significantly larger osteolytic lesions (p < 0.0001, p = 0.009, p = 0.006, and p = 0.028, respectively). Progression in the size of the osteolytic lesions over one year was significantly associated with larger initial osteolytic lesions (p = 0.002), greater polyethylene wear rates (p = 0.009), and larger (26 or 28-mm) prosthetic femoral head dimensions (p = 0.019). CONCLUSIONS There is considerable variation in the rates of progression of the size of osteolytic lesions around stable acetabular components. Lesion size and the progression of lesion size are generally related to polyethylene wear rates, higher patient activity levels, and larger-diameter femoral heads. Osteolytic lesions measuring >10 cm(3) in volume are associated with a high rate of progression.

The efficacy of allogeneic mesenchymal precursor cells for the repair of an ovine tibial segmental defect

INTRODUCTION Synthetic void-fillers offer an alternative to autograft or allograft bone in the repair of segmental defects. However, the reparative process is delayed as only osteoconductive elements are present. The inclusion of pluripotential cells may resolve this limitation, and the use of allogeneic tissue provides the opportunity for an off-the-shelf remedy. The current study evaluated the utilisation of mesenchymal precursor cells (MPC) for the repair of an ovine critical-size tibial segmental defect. METHODS Twenty-four, mature female sheep underwent surgery for the creation of a 3 cm tibial diaphyseal defect. In one group of 12 sheep the scaffold was used alone, and in the second group the scaffold was seeded with MPC. The defect was stabilised using a locking intramedullary nail and allowed to heal over a nine-month-period. Outcome assessments of healing included radiology of callus formation, computed tomography, assessment of new-bone volume, mechanical attributes, and histological evaluation of linear bone apposition rate and tissue response. RESULTS The MPC-treated group displayed a significantly greater level of callus formation and rate of bone apposition in the defect. DISCUSSION The incorporation of allogeneic MPC to a synthetic void filler stimulated early repair of critical-size diaphyseal segmental defects and holds potential as an off-the-shelf therapy for augmenting bone regeneration.

Measurement of bone defects adjacent to acetabular components of hip replacement

Computed tomography can assist in the detection of periprosthetic osteolysis, but it has not been used to measure the actual volume of bone defects adjacent to hip replacement components because of the scanning artifact caused by metal. The aim of the current study was to develop a spiral computed tomography technique that provides precise and reliable volumetric measurement of bone defects adjacent to uncemented metal-backed acetabular components. Computed tomography scans were taken of small and large defects of known volume created in the ilium in a bovine hemipelvis and a pelvis from a cadaver. Scans were analyzed by two independent observers. The computed tomography operating conditions were determined that enabled volumetric measurements and that were accurate to within 96% for small and large defects and precise to greater than 98% for small and large defects. This computed tomography technique has the capability to measure accurately and precisely the volume of bone defects in the ilium adjacent to metal-backed acetabular components. This technique has clear advantages over plain radiographs. It will allow investigation of the natural history of osteolytic lesions, enhance preoperative planning, and improve monitoring of the outcomes of treatments of osteolysis.

Revision total hip replacement using cemented collarless double-taper femoral components

We investigated 219 revisions of total hip replacement (THR) in 211 patients using a collarless double-taper cemented femoral component. The mean age of the patients was 72 years (30 to 90). The 137 long and 82 standard length stems were analysed separately. The mean follow-up was six years (2 to 18), and no patient was lost to follow-up. Survival of the long stems to re-revision for aseptic loosening at nine years was 98% (95% confidence interval (CI) 94 to 100), and for the standard stems was 93% (95% CI 85 to 100). At five years, one long stem was definitely loose radiologically and one standard stem was probably loose. Pre-operative femoral bone deficiency did not influence the results for the long stems, and corrective femoral osteotomy was avoided, as were significant subsidence, major stress shielding and persistent thigh pain. Because of these reliable results, cemented long collarless double-taper femoral components are recommended for routine revision THR in older patients.

Peripheral wear of Wagner resurfacing hip arthroplasty acetabular components.

One hundred nine Wagner resurfacing arthroplasty acetabular components retrieved at revision surgery were examined for type, depth, and extent of wear at the periphery of the components. Ninety-two components were found to have evidence of peripheral wear caused by impingement of the femoral neck on the edge of the component. The thickness of the wall loss due to this impingement wear was found to be less the farther that measurements were taken from the edge of the component. Thus, when those components with any evidence of impingement wear were analyzed, it was found that significantly fewer components had evidence of impingement at depths of 3 mm and 5 mm in from the edge of the component. It is suggested that future designs of resurfacing hip arthroplasty acetabular components should be at least 3 mm and possibly 5 mm less than a hemisphere. This is likely to lead to less impingement on the femoral neck and therefore less cause for mechanical loosening and less production of wear debris.

Weight-bearing-induced displacement and migration over time of fracture fragments following split depression fractures of the lateral tibial plateau: A CASE SERIES WITH RADIOSTEREOMETRIC ANALYSIS

We investigated the stability of seven Schatzker type II fractures of the lateral tibial plateau treated by subchondral screws and a buttress plate followed by immediate partial weight-bearing. In order to assess the stability of the fracture, weight-bearing inducible displacements of the fracture fragments and their migration over a one-year period were measured by differentially loaded radiostereometric analysis and standard radiostereometric analysis, respectively. The mean inducible craniocaudal fracture fragment displacements measured -0.30 mm (-0.73 to 0.02) at two weeks and 0.00 mm (-0.12 to 0.15) at 52 weeks. All inducible displacements were elastic in nature under all loads at each examination during follow-up. At one year, the mean craniocaudal migration of the fracture fragments was -0.34 mm (-1.64 to 1.51). Using radiostereometric methods, this case series has shown that in the Schatzker type II fractures investigated, internal fixation with subchondral screws and a buttress plate provided adequate stability to allow immediate post-operative partial weight-bearing, without harmful consequences.