Minimizing Electromagnetic Interference from Surgical Instruments on Electromagnetic Surgical Navigation

Background

Electromagnetic computer-assisted surgery (EM-CAS) can be affected by various metallic or ferromagnetic factors.

Questions/purposes

We determined to what extent metals interfere with accuracy and identified measures to prevent interference from occurring.

Methods

Using an EM-CAS system, we made six standard measurements of tibiofemoral position and alignment on a surrogate knee. A stainless steel mallet was positioned 10 cm from the stylus, and then 10 cm from the localizer to create errors attributable to electromagnetic interference. The experiment was repeated with bars of different metals placed 10 cm from the stylus.

Results

The maximum errors recorded with a mallet were: varus/valgus alignment, −2.7° and 2.4°; flexion/extension, −5.8° and 3.0°; lateral resection level, −3.1 and 7.5 mm; and medial resection level, −4.0 and 2.3 mm, respectively. The smallest errors were recorded with cylinders of titanium, cobalt-chrome alloy, and stainless steels. When moved more than 10 cm away from the stylus, errors became negligible.

Conclusions

The accuracy of EM navigation systems is affected substantially by the size, type, proximity, and shape of metal objects.

Clinical Relevance

Stainless steel objects, such as cutting blocks and trial prostheses, should be kept more than 10 cm from EM-CAS instruments to minimize error.     

Ewing's sarcoma of the calcaneus with metastases to the tibia and fibula

Ewing's Sarcoma of the calcaneus has been infrequently reported in literature. Metastases to the adjoining tibia and fibula have been reported even more rarely. We report a patient who, at the time of presentation, showed gross destruction of the calcaneus and metastases to the ipsilateral tibia and fibula.     

Irritant potential of some constituents from the seeds of Caesalpinia bonducella (L.) Fleming

The irritant potential of four triterpenoids, isolated for the first time from the seeds of Caesalpinia bonducella, identified as f -amyrin [12-ursen-3 g -ol], g -amyrin [12-oleanex-3 g -ol], lupeol [lup-20(29)-en-3 g -ol] and lupeol acetate [lup-20(29)-en-3 g -yl acetate] was investigated by open mouse ear assay, evaluating their ID₅₀ (irritant dose in 50% animals) after acute effects and by irritant units (IU) after chronic effects. f -Amyrin, lupeol acetate and g -amyrin were the most potent and persistent irritant compounds with red weals of 1.5-2.1 cm diameter areas of the animal skin and with lowest ID₅₀ =0.078, 0.186 and 0.190 mg/10 w l after 1.5, 2.10 and 3.5 h, respectively. Their reactions lasted for 24 h with IU=2.5; 0.312 and 1.25 mg/10 w l, respectively. Lupeol was the least irritant and least persistent compound with ID₅₀ =0.603 mg/10 w l after 4.5 h. Its reaction subsided before 24 h.     

Management of iatrogenic facial nerve palsy and labyrinthine fistula in mastoid surgery

A 6-year review of complications of mastoid surgery between June 1995 and June 2001 revealed five cases with serious iatrogenic complications from mastoid surgery, of which four were facial nerve palsy and two were labyrinthine fistula. One of these patients had concomitant facial nerve palsy and labyrinthine fistula. There were two cases of complete facial nerve palsy (House Brackmann grade VI) and two cases of incomplete palsy (House Brackmann grades IV and V). The second genu was the site of injury in three of the four cases. Of the four cases with facial nerve palsy, two patients had full recovery (House Brackmann grade I), one recovered only to House Brackmann grade III, and one was lost to follow-up. Both patients with labyrinthine fistula had postoperative vertigo and profound sensorineural hearing loss. The site of iatrogenic fenestration was the lateral semicircular canal in both cases.     

Nephrotic proteinuria associated with high-dose pamidronate in multiple myeloma

Five patients receiving increased dose or frequency of pamidronate beyond the recommended dose (90 mg/monthly) exhibited nephrotic proteinuria (range 3.96-24 g/24 h). On dose reduction or discontinuation, three of these patients showed decreased proteinuria to normal levels (< 1 g/24 h), and proteinuria decreased to 4.5 g/24 h from a peak of 24 g/24 h in one patient. One patient on haemodialysis (hence not evaluable) had proteinuria of 2 g/24 h and elevated creatinine levels. One other patient continued to show elevated creatinine levels (272.8 micro mol/l). Renal biopsies obtained in two patients revealed focal segmental glomerulosclerosis.