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It is a well-known fact that jacket crowns sometimes fracture from apparently nor­ mal use. The types of fractures— the longitudinal split of the crown and the crescent-shaped fracture in the labiogingival or linguogingival regions ■— are familiar to all dentists. The purpose of the present study was to determine the causes of such fractures by analyzing the regions of strain within jacket crowns subjected to a load. The external shape of the jacket is determined by tooth form and physio­ logical requirements, but the core or sup­ porting tooth substance can be modified within certain limits to achieve two essen­ tial requirements: stability and strength. I f the crown is to have stability, it must not rotate, twist or deform excessively under tooth loads. Stresses in the crown must be low enough to prevent fracture due to tooth loads. Obviously, the core design must be a compromise between the shapes de­ manded by the two requirements of sta­ bility and strength. From the standpoint of stability, a cross section of the core should be rectangular with sharp corners; but from the standpoint of strength, a cross section of the core should be round or oval-shaped or at least rectangular with generously beveled or rounded line angles. It was decided, therefore, that if a large plastic prototype of a jacket was loaded on an appropriate core and then sectioned, it would be possible by view­ ing the sections under polarized light to find the precise regions of detrimental strains. Certain changes in the shape of the core could then be made to reduce these strains and thus reduce the inci­ dence of fracture of jacket crowns.

• The records of 886 patients who had appendectomy performed by the same surgeons within a five-year period were used to contrast appendiceal stump inversion vs simple ligation. Our analysis contrasted inversion vs simple ligation techniques as related to postoperative complications, hospital stay, and pathologic diagnosis. Adhesions requiring repeated operation to relieve bowel obstruction occurred in five of 87 patients with acute gangrenous appendicitis treated by inversion. Of 106 patients with acute gangrenous appendicitis treated with simple ligation, postoperative obstruction developed in none. No other statistically significant differences existed between the two techniques. These data suggest that simple ligation is at least as good as and probably better than inversion of the appendiceal stump. (Arch Surg1988;123:689-690)

To the Editor.— Rupture of pacemaker batteries is rare. Mercury-zinc batteries, used to power early pacemakers, have largely been replaced because their warranties lasted only three years. We believe ours is the first reported case of spontaneous rupture of an implanted mercury-zinc pacemaker battery. Report of a Case.— A 62-year-old man who had a ventricular-inhibited demand pacemaker placed in August 1978 for complete heart block arrived at the emergency department complaining of sudden swelling and pain over the pacemaker site. At 2 AM the patient was awakened by what he described as an "explosion" in his chest. Physical examination showed an elderly man in mild distress. His vital signs were normal except for a temperature of 38 °C (101 °F). The pacemaker site was swollen and erythematous, and the overlying skin was warm. Physical examination showed no other remarkable findings. A chest x-ray film showed fragments outside the epoxy casing