MRI of perianal fistulae: a pictorial kaleidoscope

Perianal fistulae are an abnormal communication between the anorectum and the perianal skin. A seemingly benign condition, it can be a cause of considerable distress to the patient if it is not mapped out adequately before embarking upon surgical correction. The persistence of residual disease complicates and up-stages the grade of the remnant fistula with increased risk of anal incontinence following surgery secondary to damage to the anal sphincter complex. Magnetic resonance imaging (MRI) can play a critical role in mapping the fistulae tract in relation to the anal sphincter complex and hence, act as a reliable guide for the surgeon to chart the optimised management of perianal fistulae. This review illustrates the role of MRI in the imaging evaluation of perianal fistulae, to facilitate a well-planned surgical course.

Finite element assessment of block-augmented total knee arthroplasty

Loosening and migration of tibial prostheses have been identified as causes of early total knee replacement (TKR) failure. The problem is made more complex when defects occur in the proximal tibia compromising fixation and alignment. Clinical studies using metal augments have shown these to be an alternative to other means of defect treatment. Finite element (FE) analysis can be used to identify regions that may be prone to loosening and migration. In the current work, 3D FE models of TKR uncontained type-2 defects treated with block augments have been constructed and analysed. It has been shown that a metal augment is the most suitable. The use of bone cement (PMMA) to fill proximal defects is not considered suitable as stresses carried by the cement block exceed those of the fatigue limit of bone cement. It has been shown that the stresses in the proximal cancellous bone of block-augmented models are significantly below levels likely to cause damage due to overloading. Furthermore, the use of stem extensions has been shown to reduce the cancellous bone stresses in the proximal region thus increasing the likelihood of bone resorption. Given this, it is recommended that stem extensions are not required unless necessary to mitigate some other problem.

Stress-shielding induced bone remodeling in cementless shoulder resurfacing arthroplasty: a finite element analysis and in vivo results

Cementless surface replacement arthroplasty (CSRA) of the shoulder was designed to preserve the individual anatomy and humeral bone stock. A matter of concern in resurfacing implants remains the stress shielding and bone remodeling processes. The bone remodeling processes of two different CSRA fixation designs, conical-crown (Epoca RH) and central-stem (Copeland), were studied by three-dimensional (3-D) finite element analysis (FEA) as well as evaluation of contact radiographs from human CSRA retrievals. FEA included one native humerus model with a normal and one with a reduced bone stock quality. Compressive strains were evaluated before and after virtual CSRA implantation and the results were then compared to the bone remodeling and stress-shielding pattern of eight human CSRA retrievals (Epoca RH n=4 and Copeland n=4). FEA revealed for both bone stock models increased compressive strains at the stem and outer implant rim for both CSRA designs indicating an increased bone formation at those locations. Unloading of the bone was seen for both designs under the central implant shell (conical-crown 50-85%, central-stem 31-93%) indicating high bone resorption. Those effects appeared more pronounced for the reduced than for the normal bone stock model. The assumptions of the FEA were confirmed in the CSRA retrieval analysis which showed bone apposition at the outer implant rim and stems with highly reduced bone stock below the central implant shell. Overall, clear signs of stress shielding were observed for both CSRAs designs in the in vitro FEA and human retrieval analysis. Especially in the central part of both implant designs the bone stock was highly resorbed. The impact of these bone remodeling processes on the clinical outcome as well as long-term stability requires further evaluation.

Initial stability of type-2 tibial defect treatments

Treatment of proximal tibial defects is important to the survival of tibial prosthesis after total knee replacement. The objective of this finite element study was to determine a better understanding of the stresses produced by different treatment options for moderate uncontained type-2 defects. Methods analysed were the use of metal wedges, metal blocks, cement wedges, and cement blocks for the two defect angles 15° and 30°. The effect of a stem extension on the stress profiles was also analysed for each defect treatment and angle to establish the necessity of these extensions and consequent bone removal on the stability of the augments. Equivalent stresses in two regions of interest (ROIs) adjacent to the augments and shear stresses along the bone—cement interface of the defect were investigated. The lowest equivalent stresses were found in the metal block augment for both defect angles and ROIs. The highest equivalent stress in the ROIs and shear stress values along the bone—cement interface of the defect were found in the cement wedge augment model for both defect angles. Stem extensions were shown to increase equivalent stresses in the bone closer to the tibial stem but to decrease equivalent stresses closer to the cortical bone. The use of a stem extension significantly increased the shear stresses in the cement in all cases except in the metal block model. It is recommended that metal block augments are used without a stem extension in small-defect (i.e. peripheral defect angle of 15°) total knee replacement procedures.

Safety and efficacy of outpatient transseptal radiofrequency ablation of atrioventricular accessory pathways

A retrospective analysis of 60 consecutive patients who underwent outpatient transseptal radiofrequency ablation of left sided accessory pathways at Westchester County Medical Center/New York Medical College from September 1994 to December 1997 was performed. Patients were followed for a mean duration of 22 months. No complications either local or related to the transseptal method were observed. All patients had successful ablation of the accessory pathway. One patient had a recurrence of symptoms. This study suggests transseptal radiofrequency ablation of the left sided accessory pathways to be safe, feasible and an effective procedure when performed in an outpatient setting. These results were obtained at a high volume center with experience using the transseptal technique.

Use of prostacyclin and its analogues in the treatment of cardiovascular disease

Prostacyclin is found in all body tissues and body fluids, and is the major metabolite of arachidonic acid in the vasculature. It is a potent vasodilator that affects both the systemic and pulmonary circulations. Prostacyclin also prevents vascular smooth muscle proliferation and inhibits platelet adhesion and aggregation. These features have made it a very attractive substance for treatment of many different cardiovascular diseases. Epoprostenol, a synthetic prostacyclin, is currently available in parenteral form for the treatment of primary pulmonary hypertension, and has been shown to be a valuable palliative therapy. The drug appears to have limited effectiveness for treating patients with congestive heart failure and coronary artery disease, but has shown some utility in patients with peripheral vascular disease, including Raynaud's phenomenon. Analogues of prostacyclin are now in clinical trials and include iloprost, a stable analogue that has been used intravenously to treat patients with peripheral vascular disease. Other parenteral formulations under investigation include taprostene, ciprostene, and UT-15. Beraprost and cicaprost, are two prostacyclin analogues that can be used in oral form, are being studied in clinical trials.