Faculty Bibliography

Osteonecrosis (ON) of the femoral head causes the bone to deteriorate, buckle, and collapse. As the vasculature is reportedly uniform in the femoral head, one would expect uniform susceptibility to ON; however, collapse typically occurs in the anterior region. We asked whether regional variations in bone quantity and/or quality could explain the bone's anterior susceptibility despite uniform vascularity. We examined seven femoral heads resected for primary osteoarthritis and three removed after femoral neck fracture. Each was cut into 4-mm-thick, 1.5 cm x 1.5-cm bone squares, processed for light microscopy, and sectioned twice. One section was stained with Gomori's trichrome and assessed by a computer-assisted microscope, which calculated trabecular area, a measure of bone quantity. The other was stained with hematoxylin and eosin and assessed by light microscopy to identify trabecular microfractures, a measure of bone quality. Bone quantity and quality were reduced in the fracture group as a whole; bone quantity was uniform in each femoral head, but the quality was reduced in the anterior portion. The quality was further reduced in the superior region of arthritic bone and in the lateral-inferior regions of the fractured bones. Our findings suggest the anterior susceptibility is the result of bone loading and, as such, reinforcement of the femoral head in ON should focus on the anterior hemisphere

The clinical results after total hip replacements using noncemented stems have shown considerable variability over the years; the design and characteristics of the implant seemed to play a role in explaining this fact. The purpose of this paper is to report the clinical, radiographic and densitometry results of total hip arthroplasties using a stem designed for noncemented implantation and to engage and load the femur proximally. Fifty-eight consecutive patients (62 hips) followed for an average of 4.3 years (range 36-70 months) were clinically and radiographically followed up at three weeks, three months, six months, one year, and yearly thereafter. The average pre-operative Harris hip score was 49 increasing to 98 at the latest follow-up. There were no cases of aseptic or septic loosening. The average subsidence at three years was 0.45 mm (SD +/- 0.36 mm). Radiographically all hips were classified as stable, and evident changes compatible with new bone apposition were observed in 64% of the cases. The extended proximal geometry of the device seems to favour initial and secondary stability as reflected by the low subsidence values over time. The maintenance of periprosthetic bone stock and the absence of stress shielding can be explained by the predominantly proximal loading pattern of the stem

Uncemented stems have been advocated for primary total hip arthroplasties in the young active population. We evaluated the clinical and radiographic results of total hip arthroplasties with a customized lateral flare cementless femoral stem in a prospective, consecutive series of 35 patients (40 hips) younger than 55 years at the time of surgery and who were followed up for an average of 9.2 years (range, 5.7-12.2 years). The study group comprised 20 men and 15 women with an average age of 45.2 years (range, 30-55 years). One patient had aseptic loosening of the stem and one patient had a deep infection. The mean preoperative Harris hip score was 47 and at the latest followup it was 97. The mean axial migration was 0.51 mm. Femoral osteolysis was found to be circumscribed to the proximal femur in Gruen Zones 1 (15%) and 7 (8%) in patients with accelerated polyethylene wear. Radiographic changes consistent with new bone apposition under the lateral flare of the stem in Zones 2, 6, and 7 were found in 73% of the cases. Our data suggest a custom lateral flare stem for primary arthroplasties in the younger patient population achieves excellent clinical results with a low rate of aseptic loosening. LEVEL OF EVIDENCE: Therapeutic study, Level IV (case series)

The femoral head deteriorates in osteonecrosis. As a consequence of that, the cortical shell of the femoral head can buckle into the cancellous bone supporting it. In order to examine the buckling scenario we performed numerical analysis of a realistic femoral head model. The analysis included a solution of the hip contact problem, which provided the contact pressure distribution, and subsequent buckling simulation based on the given contact pressure. The contact problem was solved iteratively by approximating the cartilage by a discrete set of unilateral linear springs. The buckling calculations were based on a finite element mesh with brick elements for the cancellous bone and shell elements for the cortical shell. Results of 144 simulations for a variety of geometrical, material, and loading parameters strengthen the buckling scenario. They, particularly, show that the normal cancellous bone serves as a strong supporting foundation for the cortical shell and prevents it from buckling. However, under the development of osteonecrosis the deteriorating cancellous bone is unable to prevent the cortical shell from buckling and the critical pressure decreases with the decreasing Young modulus of the cancellous bone. The local buckling of the cortical shell seems to be the driving force of the progressive fracturing of the femoral head leading to its entire collapse. The buckling analysis provides an additional criterion of the femoral head collapse, the critical contact pressure. The buckling scenario also suggests a new argument in speculating on the femoral head reinforcement. If the entire collapse of the femoral head starts with the buckling of the cortical shell then it is reasonable to place the reinforcement as close to the cortical shell as possible

Septic arthritis is a commonly reported complication of rheumatoid arthritis (RA). Tumor necrosis factor alpha (TNF-alpha) plays an important role in host defense against infection. Inhibition of its activity could therefore be anticipated to augment the risk of infection. Both opportunistic and bacterial infections have been described in patients with RA treated with anti-TNF-alpha therapy. We describe a patient who experienced 2 episodes of septic arthritis. Both occurred while the patient was on etanercept. Recurrence developed despite prolonged parenteral antibiotic. To our knowledge, this is the first report of relapsing oligoarticular methicillin-sensitive Staphylococcus aureus septic arthritis despite prolonged antibiotic treatment in a patient receiving etanercept therapy. Our case underscores the advisability of discontinuing TNF-alpha blockade in patients with septic arthritis during prolonged antimicrobial therapy

Estimation of the hip joint contact area and pressure distribution during activities of daily living is important in predicting joint degeneration mechanism, prosthetic implant wear, providing biomechanical rationales for preoperative planning and postoperative rehabilitation. These biomechanical data were estimated utilizing a generic hip model, the Discrete Element Analysis technique, and the in vivo hip joint contact force data. The three-dimensional joint potential contact area was obtained from the anteroposterior radiograph of a subject and the actual joint contact area and pressure distribution in eight activities of daily living were calculated. During fast, normal, and slow walking, the peak pressure of moderate magnitude was located at the lateral roof of the acetabulum during mid-stance. In standing up and sitting down, and during knee bending, the peak pressures were located at the edge of the posterior horn and the magnitude of the peak pressure during sitting down was 2.8 times that of normal walking. The peak pressure was found at the lateral roof in climbing up stairs which was higher than that in going down stairs. These results can be used to rationalize rehabilitation protocols, functional restrictions after complex acetabular reconstructions, and prosthetic component wear and fatigue test set up. The same model and analysis can provide further insight to soft tissue loading and pathology such as labral injury. When the pressure distribution on the acetabulum is inverted onto the femoral head, prediction of subchondral bone collapse associated with avascular necrosis can be achieved with improved accuracy.